U.S. patent application number 14/612018 was filed with the patent office on 2015-07-23 for method and apparatus for control of non-invasive parameter measurements.
The applicant listed for this patent is TENSYS MEDICAL, INC.. Invention is credited to Manouchehr Goharlaee, Gregory J. Martin, Gregory I. Voss.
Application Number | 20150201845 14/612018 |
Document ID | / |
Family ID | 31990289 |
Filed Date | 2015-07-23 |
United States Patent
Application |
20150201845 |
Kind Code |
A1 |
Voss; Gregory I. ; et
al. |
July 23, 2015 |
METHOD AND APPARATUS FOR CONTROL OF NON-INVASIVE PARAMETER
MEASUREMENTS
Abstract
Improved methods and apparatus for non-invasively assessing one
or more parameters associated with fluidic systems such as the
circulatory system of a living organism. In a first aspect, an
improved method of continuously measuring pressure from a
compressible vessel is disclosed, wherein a substantially optimal
level of compression for the vessel is achieved and maintained
using perturbations (e.g., modulation) of the compression level of
the vessel. In one exemplary embodiment, the modulation is
conducted according to a pseudo-random binary sequence (PBRS). In a
second aspect, an improved apparatus for determining the blood
pressure of a living subject is disclosed, the apparatus generally
comprising a pressure sensor and associated processor with a
computer program defining a plurality of operating states related
to the sensed pressure data. Methods for pressure waveform
correction and reacquisition, as well as treatment using the
present invention, are also disclosed.
Inventors: |
Voss; Gregory I.; (Solana
Beach, CA) ; Martin; Gregory J.; (Carlsbad, CA)
; Goharlaee; Manouchehr; (Encinitas, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TENSYS MEDICAL, INC. |
San Diego |
CA |
US |
|
|
Family ID: |
31990289 |
Appl. No.: |
14/612018 |
Filed: |
February 2, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13154315 |
Jun 6, 2011 |
8945016 |
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14612018 |
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11300019 |
Dec 13, 2005 |
7955267 |
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13154315 |
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10211115 |
Aug 1, 2002 |
6974419 |
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11300019 |
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Current U.S.
Class: |
600/485 ;
600/481; 600/500; 600/508 |
Current CPC
Class: |
A61B 5/7203 20130101;
A61B 5/7207 20130101; A61B 5/684 20130101; A61B 5/024 20130101;
A61B 5/02028 20130101; A61B 5/022 20130101; A61B 5/02007 20130101;
A61B 5/6843 20130101; A61B 5/7264 20130101 |
International
Class: |
A61B 5/022 20060101
A61B005/022; A61B 5/02 20060101 A61B005/02; A61B 5/00 20060101
A61B005/00; A61B 5/024 20060101 A61B005/024 |
Claims
1-68. (canceled)
69. A method of operating a hemodynamic parameter measurement
apparatus adapted for use on a living subject and comprising one or
more sensors in contact with tissue overlying a blood vessel of
said living subject, said method comprising: operating said
apparatus in a first non-transient mode such that data relating to
one or more hemodynamic parameters is obtained using said one or
more sensors; detecting at least one transient event via said
obtained data; and operating said apparatus in a second transient
recovery mode subsequent to said act of detecting said transient
event so as to compensate therefor.
70. The method of claim 69, wherein said transient event changes
said contact of at least one of said one or more sensors to said
tissue, thereby potentially affecting an accuracy of said obtained
data.
71. The method of claim 70, wherein said act of detecting comprises
detecting a reduction in a magnitude of one or more pressure
values.
72. The method of claim 70, wherein said act of detecting comprises
detecting an absence of one or more cardiac beats in said obtained
data.
73. The method of claim 70, wherein said hemodynamic parameter
comprises blood pressure, and at least one of said one or more
sensors comprises a tonometric or non-invasive pressure sensor.
74. The method of claim 70, wherein said second transient recovery
mode comprises temporarily suspending use of said obtained
data.
75. The method of claim 74, wherein said second transient recovery
mode further comprises adjusting a position of said at least one of
said one or more sensors obtaining said data.
76. The method of claim 75, wherein said blood vessel comprises a
radial artery, and said act of adjusting said position of said at
least one of said one or more sensors obtaining said data comprises
adjusting at least one of a lateral and/or proximal position of
said at least one sensor relative to said artery.
77. The method of claim 75, wherein said blood vessel comprises a
radial artery, and said act of adjusting said position of said at
least one of said one or more sensors obtaining said data comprises
adjusting an applanation or compression level of said at least one
sensor relative to said tissue.
78. The method of claim 69, wherein said act of detecting comprises
detecting a reduction in a magnitude of a parameter represented by
said data.
79. The method of claim 78, wherein said act of detecting said
reduction comprises detecting a reduction in a magnitude of a pulse
pressure.
80. The method of claim 69, wherein said act of detecting comprises
detecting an absence of one or more cardiac beats in said data.
81. An apparatus for use in transient event compensation during
hemodynamic parameter measurement from a living subject, said
apparatus comprising: first apparatus configured to establish an
initial substantially optimal state of mechanical coupling to said
living subject; a sensor configured to obtain data relating to said
hemodynamic parameter from said living subject; and a processor
apparatus configured to execute a computer program thereon, said
computer program comprising a plurality of instructions configured
to, when executed: cause said data to be received from said sensor;
detect an occurrence of said transient event by analyzing changes
in said data, said transient event alters said mechanical coupling
between said living subject and said sensor; cease use of said data
from said sensor; identify a second substantially optimal state of
mechanical coupling of said sensor to said living subject; and
resume using said data from said sensor after said second state has
been achieved.
82. The apparatus of claim 81, wherein said plurality of
instructions are further configured to initiate a sweep through a
plurality of different states to identify said second substantially
optimal state of mechanical coupling.
83. The apparatus of claim 82, wherein said plurality of
instructions are further configured to cause said sweep to begin
only after said transient event is determined to have substantially
subsided.
84. The apparatus of claim 81, wherein said plurality of
instructions are further configured to control said first apparatus
to establish said second substantially optimal state.
85. The apparatus of claim 81, wherein said initial substantially
optimal state and said second substantially optimal state each
comprise states of compression of tissue overlying a blood
vessel.
86. The apparatus of claim 81, wherein said detection of said
occurrence of said transient event comprises a comparison of at
least one of (i) a measured pressure, (ii) a measured velocity
and/or (iii) a measured acceleration, with one or more
predetermined values.
87. A non-invasive blood pressure measurement apparatus comprising:
a pressure sensor; a data storage medium configured to store
pressure data in a non-transient mode from said pressure sensor;
and a processor configured to detect an occurrence of at least one
transient event based at least in part on an evaluation of said
data; wherein said apparatus is placed in a transient recovery mode
subsequent to said detection of said at least one transient
event.
88. The apparatus of claim 87, wherein said apparatus is configured
to operate in a second non-transient mode when non-transient
induced changes occur, said non-transient changes being detected by
said processor via evaluation of a second set of non-transient
pressure data, said second set of non-transient pressure data being
obtained at least in part by a modulation of a compression level of
a blood vessel with an applanation mechanism from which said
non-transient data is obtained.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to methods and apparatus
for monitoring parameters associated with circulating fluid
systems, and specifically in one aspect to the non-invasive
monitoring of arterial blood pressure in a living subject.
[0003] 2. Description of Related Technology
[0004] The accurate, continuous, non-invasive measurement of blood
pressure has long been sought by medical science. The availability
of such measurement techniques would allow the caregiver to
continuously monitor a subject's blood pressure accurately and in
repeatable fashion without the use of invasive arterial catheters
(commonly known as "A-lines") in any number of settings including,
for example, surgical operating rooms where continuous, accurate
indications of true blood pressure are often essential.
[0005] Several well known techniques have heretofore been used to
non-invasively monitor a subject's arterial blood pressure
waveform, namely, auscultation, oscillometry, and tonometry. Both
the auscultation and oscillometry techniques use a standard
inflatable arm cuff that occludes the subject's peripheral
(predominately brachial) artery. The auscultatory technique
determines the subject's systolic and diastolic pressures by
monitoring certain Korotkoff sounds that occur as the cuff is
slowly deflated. The oscillometric technique, on the other hand,
determines these pressures, as well as the subject's mean pressure,
by measuring actual pressure changes that occur in the cuff as the
cuff is deflated. Both techniques determine pressure values only
intermittently, because of the need to alternately inflate and
deflate the cuff, and they cannot replicate the subject's actual
blood pressure waveform. Thus, continuous, beat-to-beat blood
pressure monitoring cannot be achieved using these techniques.
[0006] Occlusive cuff instruments of the kind described briefly
above have generally been somewhat effective in sensing long-term
trends in a subject's blood pressure. However, such instruments
generally have been ineffective in sensing short-term blood
pressure variations, which are of critical importance in many
medical applications, including surgery.
[0007] The technique of arterial tonometry is also well known in
the medical arts. According to the theory of arterial tonometry,
the pressure in a superficial artery with sufficient bony support,
such as the radial artery, may be accurately recorded during an
applanation sweep when the transmural pressure equals zero. The
term "applanation" refers to the process of varying the pressure
applied to the artery. An applanation sweep refers to a time period
during which pressure over the artery is varied from
overcompression to undercompression or vice versa. At the onset of
a decreasing applanation sweep, the artery is overcompressed into a
"dog bone" shape, so that pressure pulses are not recorded. At the
end of the sweep, the artery is undercompressed, so that minimum
amplitude pressure pulses are recorded. Within the sweep, it is
assumed that an applanation occurs during which the arterial wall
tension is parallel to the tonometer surface. Here, the arterial
pressure is perpendicular to the surface and is the only stress
detected by the tonometer sensor. At this pressure, it is assumed
that the maximum peak-to-peak amplitude (the "maximum pulsatile")
pressure obtained corresponds to zero transmural pressure. Note
that other measures analogous to maximum pulsatile pressure,
including maximum rate of change in pressure (i.e., maximum dP/dT)
can also be implemented.
[0008] One prior art device for implementing the tonometry
technique includes a rigid array of miniature pressure transducers
that is applied against the tissue overlying a peripheral artery,
e.g., the radial artery. The transducers each directly sense the
mechanical forces in the underlying subject tissue, and each is
sized to cover only a fraction of the underlying artery. The array
is urged against the tissue, to applanate the underlying artery and
thereby cause beat-to-beat pressure variations within the artery to
be coupled through the tissue to at least some of the transducers.
An array of different transducers is used to ensure that at least
one transducer is always over the artery, regardless of array
position on the subject. This type of tonometer, however, is
subject to several drawbacks. First, the array of discrete
transducers generally is not anatomically compatible with the
continuous contours of the subject's tissue overlying the artery
being sensed. This can result in inaccuracies in the resulting
transducer signals. In addition, in some cases, this
incompatibility can cause tissue injury and nerve damage and can
restrict blood flow to distal tissue.
[0009] Other prior art techniques have sought to more accurately
place a single tonometric sensor laterally above the artery,
thereby more completely coupling the sensor to the pressure
variations within the artery. However, such systems may place the
sensor at a location where it is geometrically "centered" but not
optimally positioned for signal coupling, and further typically
require comparatively frequent re-calibration or repositioning due
to movement of the subject during measurement.
[0010] Tonometry systems are also commonly quite sensitive to the
orientation of the pressure transducer on the subject being
monitored. Specifically, such systems show a degradation in
accuracy when the angular relationship between the transducer and
the artery is varied from an "optimal" incidence angle. This is an
important consideration, since no two measurements are likely to
have the device placed or maintained at precisely the same angle
with respect to the artery. Many of the foregoing approaches
similarly suffer from not being able to maintain a constant angular
relationship with the artery regardless of lateral position, due in
many cases to positioning mechanisms which are not adapted to
account for the anatomic features of the subject, such as curvature
of the wrist surface.
[0011] Furthermore, compliance in various apparatus components
(e.g., the strap and actuator assembly) and the lack of soft
padding surrounding the sensor which minimizes edge effects may
adversely impact the accuracy of tonometric systems to a
significant extent.
[0012] One very significant limitation of prior art tonometry
approaches relates to the magnitude and location of the applied
applanation pressure during varying conditions of patient motion,
position, mean pressure changes, respiration, etc. Specifically,
even when the optimum level of arterial compression at the optimal
coupling location is initially achieved, there are commonly
real-world or clinical factors beyond reasonable control that can
introduce significant error into the measurement process,
especially over extended periods of time. For example, the subject
being monitored may voluntarily or involuntarily move, thereby
altering (for at least a period of time) the physical relationship
between the tonometric sensor and the subject's tissue/blood
vessel. Similarly, bumping or jarring of the subject or the
tonometric measurement apparatus can easily occur, thereby again
altering the physical relationship between the sensor and subject.
The simple effect of gravity can, under certain circumstances,
cause the relative positions of the sensor and subject blood vessel
to alter with time as well.
[0013] Furthermore, physiologic responses of the subject
(including, for example, relaxation of the walls of the blood
vessel due to anesthesia or pharmacological agents) can produce the
need for changes in the applanation level (and sometimes even the
lateral/proximal position of the sensor) in order to maintain
optimal sensor coupling. Additionally, due to the compliance of
surrounding tissue and possibly measurement system, the applanation
level often needs to adjust with changes in mean arterial
pressure.
[0014] Several approaches have heretofore been disclosed in
attempts to address the foregoing limitations. In one prior art
approach, an occlusive cuff is used to provide a basis for periodic
calibration; if the measured pressure changes a "significant"
amount or a determined time has elapsed, then the system performs a
cuff calibration to assist in resetting the applanation position.
Reliable pressure data is not displayed or otherwise available
during these calibration periods. See for example U.S. Pat. No.
5,261,414 to Aung, et al issued Nov. 16, 1993 and entitled
"Blood-Pressure Monitor Apparatus," assigned to Colin Corporation
(hereinafter "Aung"). See also U.S. Pat. No. 6,322,516 issued Nov.
27, 2001 and entitled "Blood-Pressure Monitor Apparatus," also
assigned to Colin Corporation, wherein an occlusive cuff is used as
the basis for calibration of a plurality of light sensors.
[0015] In another prior art approach, a pressure cuff or a pelotte
equipped with a plethysmographic gauge, such as an impedance or a
photo-electric device, is used to drive a servo control loop. See,
e.g., U.S. Pat. No. 4,869,261 to Penaz issued Sep. 26, 1989 and
entitled "Automatic noninvasive blood pressure monitor," assigned
to University J.E. Purkyne v Brne (hereinafter "Penaz"). In this
device, the sensor is connected through at least one amplifier and
a phase corrector to an electro-pressure transducer. All these
components constitute the closed loop of a servo control system
which (at least ostensibly) continuously changes the pressure in
the cuff and attempts to maintain the volume of the artery at a
value corresponding to zero tension across the arterial wall. The
servo control system loop further includes a pressure vibration
generator, the frequency of vibration being higher than that of the
highest harmonic component of blood pressure wave. A correction
circuit is also provided, the input of which is connected to the
plethysmographic sensor and output of which is provided to correct
the setpoint of the servo control system. The Penaz system
therefore in effect constantly "servos" (within a cardiac cycle) to
a fixed light signal level received from the sensor. Unlike the
Colin systems described above, the system continuously displays
pressure to the operator. However, the operation of the
plethysmographic sensor of Penaz limited the application of this
device to a peripheral section of a limb (preferably a finger)
where the peripheral pressure, especially under conditions of
compromised peripheral circulation, may not accurately reflect
aortic or brachial artery pressure. This presents a potentially
significant cause of error.
[0016] Yet another prior art approach uses a series of varying
pressure "sweeps" performed successively to attempt to identify the
actual intra-arterial blood pressure. The applanation pressure
applied during each of these sweeps is generally varied from a
level of arterial undercompression to overcompression (or
vice-versa), and the system analyzes the data obtained during each
sweep to identify, e.g., the largest pressure waveform amplitude.
See, e.g., U.S. Pat. No. 5,797,850 to Archibald, et al issued Aug.
25, 1998 and entitled "Method and apparatus for calculating blood
pressure of an artery," assigned to Medwave, Inc. (hereinafter
"Archibald"). The system of Archibald is not truly continuous,
however, since the sweeps each require a finite period of time to
complete and analyze. In practice the sweeps are repeated with
minimal delay, one after another, throughout the operation of the
device. During applanation mechanism resetting and subsequent sweep
operations, the system is effectively "dead" to new data as it
analyzes and displays the data obtained during a previous sweep
period. This is clearly disadvantageous from the standpoint that
significant portions of data are effectively lost, and the operator
receives what amounts to only periodic indications of the subject's
blood pressure (i.e., one new pressure beat display every 15-40
seconds).
[0017] Lastly, the techniques for non-invasive pressure measurement
disclosed by the Assignee of the present invention in U.S. Pat.
Nos. 6,228,034, 6,176,831, 5,964,711, and 5,848,970, each entitled
"Apparatus and method for non-invasively monitoring a subject's
arterial blood pressure" and incorporated herein by reference in
their entirety, include modulation of applanation level at, inter
alia, frequencies higher than the heart rate (e.g., sinusoidal
perturbation at 25 Hz). While the foregoing methods are effective,
Assignee has determined over time that it is desirable at certain
circumstances to control the applanation level according to other
modulation schemes and/or frequencies, and/or which are not regular
or deterministic in nature. Furthermore, certain modulation schemes
(e.g., intra-beat modulation) can place significant demands on the
applanation hardware, since more rapid (and often precise)
variations in applanation level must occur. Accordingly, the more
capable hardware required in such applications ultimately raises
the cost of the parent device in which it is used.
[0018] Based on the foregoing, there is needed an improved
methodology and apparatus for accurately and continuously
controlling the non-invasive measurement of parameters such as
pressure. Such improved methodology and apparatus would ideally
allow for, inter alia, continuous measurement (tonometrically or
otherwise) of one or more hemodynamic parameters, the measured
values of such parameters being reflective of true intra-arterial
parameters, while also providing robustness and repeatability under
varying environmental conditions including motion artifact and
other noise. Such method and apparatus would also be easily
utilized by both trained medical personnel and untrained
individuals, thereby allowing subjects to accurately and reliably
conduct self-monitoring if desired.
SUMMARY OF THE INVENTION
[0019] The present invention satisfies the aforementioned needs by
improved methods and apparatus for non-invasively and continuously
controlling the measurement of parameters in a fluidic system,
including arterial blood pressure within a living subject.
[0020] In a first aspect of the invention, an improved method of
continuously measuring pressure from a compressible vessel is
disclosed, the method generally comprising: disposing a pressure
sensor in proximity to the vessel; identifying a substantially
optimal level of compression for the vessel; achieving the
substantially optimal level of compression; measuring pressure data
from the vessel using the sensor; identifying non-optimal levels of
compression; and adjusting applanation to maintain or reacquire the
optimal level of compression. In one exemplary embodiment, the
sensor is applied tonometrically, and the method of maintaining
optimal level of compression comprises at least periodically
perturbing the level of compression of the vessel during the act of
measuring to produce an observable effect on the measured pressure
data; and adjusting the level of compression based at least in part
on the effect. In one exemplary embodiment, the vessel comprises a
blood vessel, and the applied perturbations comprises pseudo-random
binary sequences which are used to modulate the level of
compression applied to the vessel over time. Effects on the sensed
pressure are correlated to the modulation, and necessary
corrections applied thereto.
[0021] In a second aspect of the invention, an improved apparatus
for determining the blood pressure of a living subject is
disclosed, the apparatus generally comprising: a tonometric
pressure sensor device adapted for sensing the pressure at the skin
surface of a living subject and generating pressure data relating
thereto; and a processor adapted to run a computer program thereon,
the computer program defining a plurality of operating states, the
use of each of the operating states by the apparatus in determining
blood pressure being related at least in part to the pressure data.
In one exemplary embodiment, the computer program comprises three
(3) distinct sub-processes relating to transient detection and
compensation, servoing (or slower-rate changes to coupling), and
reacquisition after loss of coupling, respectively. The first
process in this exemplary embodiment employs four (4) distinct but
related states which govern applanation and lateral/proximal
positioning.
[0022] In a third aspect of the invention, a method of modulating
tonometric blood pressure measurements is disclosed. The method
generally comprises: providing a transducer adapted for determining
pressure, the transducer being disposed proximate a blood vessel;
and varying the level of compression applied to the blood vessel
over time, the act of varying comprising modulating the level of
compression. In an exemplary embodiment the modulation operates
according to a modulation sequence, wherein the modulation sequence
comprises a PRBS as previously described, although other random or
deterministic sequences may be used.
[0023] In a fourth aspect of the invention, a method of correcting
tonometric pressure measurements is disclosed, the method generally
comprising providing a transducer adapted for determining pressure,
the transducer disposed proximate to a blood vessel; varying the
level of compression applied to the blood vessel over time;
measuring pressure from the blood vessel using the transducer, and
correcting the measured pressure based at least in part on the
correlation between the acts of varying and measuring. In the
exemplary embodiment, the aforementioned modulation sequence is
used to generate effects within the sensed pressure waveform; these
effects are correlated to the modulation sequence and corrections
to the sensed pressure values generated based thereon.
[0024] In a fifth aspect of the invention, an improved apparatus
for measuring the blood pressure within a blood vessel is
disclosed. The apparatus generally comprises: a tonometric pressure
sensor adapted for sensing the pressure at the skin surface of a
living subject and generating an electrical, signal relating
thereto; a signal converter adapted to convert the electrical
signal to the digital domain; a digital processor in data
communication with the converter; a computer program adapted to run
on the digital processor, the program being further adapted to
modulate the position of the tonometric sensor over time according
to the aforementioned modulation sequence. In one exemplary
embodiment, an applanation apparatus controlled by the computer
program is coupled to the pressure sensor, and the level of
applanation varied according to the modulation sequence. In another
embodiment, the lateral and/or proximal position of the sensor is
controlled using a comparable modulation scheme.
[0025] In a sixth aspect of the invention, an improved method of
identifying changes in the coupling between a tonometric sensor and
the tissue of a subject is disclosed, the method generally
comprising: disposing the sensor in proximity to the tissue;
measuring data from the tissue using the sensor; and identifying at
least one change in a parameter associated with the coupling based
at least in part on the measured data. In one exemplary embodiment,
the measured parameters include pressure velocity and
acceleration.
[0026] In a seventh aspect of the invention, a transient-resistant
apparatus for determining the blood pressure of a living subject is
disclosed. The apparatus generally comprises a tonometric pressure
sensor adapted for sensing the pressure at the skin surface of a
living subject and generating a waveform relating thereto; motive
apparatus adapted to reposition the pressure sensor; a controller
operatively coupled to the motive apparatus; and a processor in
data communication with the sensor and operatively coupled with the
controller, the processor adapted to run a computer program
thereon, the computer program being configured to (i) establish an
initial substantially optimal position of the sensor for at least
one epoch; (ii) measure pressure data from the blood vessel using
the sensor; (iii) detect the occurrence of a transient event by
analyzing changes in the pressure data, the transient event
altering the coupling between the blood vessel and sensor; (iv)
initiate a sensor sweep via the controller and motive apparatus to
identify a second substantially optimal sensor position based on
the altered coupling; and (v) establish the second substantially
optimal sensor position for at least one epoch.
[0027] In an eighth aspect of the invention, an improved method of
identifying changes in the coupling between a tonometric pressure
sensor and the tissue of a subject during a blood pressure
measurement based on particular portions of the cardiac cycle is
disclosed. The method generally comprises: disposing the sensor in
proximity to the tissue; measuring pressure data from the tissue
using the sensor; determining at least one parameter from the data;
comparing the at least one parameter to a criterion, the criterion
being determined at least in part by the portion of the cardiac
cycle during which the pressure data was measured; and identifying
a change in coupling based at least in part on the act of
comparing. In one exemplary embodiment, the parameter comprises
both pressure velocity and acceleration, and the criterion
comprises threshold values for each relating to either the systolic
(upstroke) or diastolic (downstroke) portions of the cardiac
cycle.
[0028] In a ninth aspect of the invention, an improved method of
maintaining an optimal or near-optimal level of compression (or
alternatively, position) for a blood pressure measuring apparatus
is disclosed. The method generally comprises using a correlation
process to analyze curves existing in relation to a "target", the
curves relating pressure as a function of a spatial position
parameter. In one exemplary embodiment, the pressure comprises
pulse pressure and/or diastolic pressure, and the spatial position
parameter comprises motor position (e.g., applanation, lateral, or
proximal drive motor). The method analyzes the slope of the curves
by, inter alia, obtaining a first derivative (e.g., d(Pulse
Pressure)/d(Motor Position) and/or d(Diastolic Pressure)/d(Motor
Position)), and correlates these slope values to identify the
optimal or near-optimal "target" position.
[0029] These and other features of the invention will become
apparent from the following description of the invention, taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a state diagram illustrating the relationship of
the four states associated with a first exemplary embodiment of the
first process of the present invention.
[0031] FIG. 1a is logical flow diagram illustrating the operation
of the exemplary embodiment of the first process of FIG. 1.
[0032] FIG. 2 is logical flow diagram illustrating the operation of
one exemplary embodiment of the second process (e.g., servoing or
maintaining optimal applanation level) according to the
invention.
[0033] FIG. 2a is a graph of pulse pressure versus diastolic
pressure for an exemplary patient.
[0034] FIG. 2b is a graph of tonometric pressure versus time for
the optimal arterial compression (applanation) of the patient of
FIG. 2a.
[0035] FIG. 2c is a graph of tonometric pressure versus time for
both optimal and non-optimal applanation level applied to the
patient of FIG. 2a.
[0036] FIG. 2d is a graph of an exemplary embodiment of the
modulation scheme according to the present invention, illustrating
the PRBS modulation value versus applanation motor step number.
[0037] FIG. 2e is a graph of the tonometric pressure obtained from
the patient of FIG. 2a with and without PRBS modulation applied to
non-optimal applanation, illustrating the effects of PRBS
modulation.
[0038] FIG. 2f is a graph of the corrected or restored tonometric
pressure waveform after application of PRBS modulation to the
non-optimal applanation profile.
[0039] FIG. 2g is a graph of an exemplary embodiment of the PRBS
modulation of the invention (PRBS length=7), illustrating the
correlation between modulation and corrected pulse pressure.
[0040] FIG. 2h is a graph of pressure versus beat number
illustrating the correlation between the weighted zero mean values
for exemplary pulse pressure and diastolic pressure, and PRBS
modulation.
[0041] FIG. 2i is a graph of pressure versus phase delay for pulse
pressure, diastolic pressure, and PRBS modulation according to one
embodiment of the invention.
[0042] FIG. 3 is a logical flow diagram illustrating one exemplary
embodiment of the method of determining the optimal initial
modulation according to the present invention.
[0043] FIGS. 3a and 3b are graphs illustrating various aspects of
the calculations supporting the methodology of FIG. 3.
[0044] FIG. 4 is logical flow diagram illustrating the operation of
one exemplary embodiment of the third process (e.g., reacquisition)
according to the invention.
[0045] FIG. 4a is a graphical illustration of an exemplary
embodiment of the fourth ("sweep") state entry criteria associated
with the third process of the invention.
[0046] FIG. 5 is a block diagram of one exemplary embodiment of the
apparatus for measuring hemodynamic parameters within the blood
vessel of a living subject according to the invention.
[0047] FIG. 6 is a logical flow diagram illustrating one exemplary
embodiment of the method of providing treatment to a subject using
the aforementioned methods.
DETAILED DESCRIPTION OF THE INVENTION
[0048] Reference is now made to the drawings wherein like numerals
refer to like parts throughout.
[0049] It is noted that while the invention is described herein
primarily in terms of a method and apparatus for the control of
non-invasive measurements of hemodynamic parameters such as blood
pressure obtained via the radial artery (i.e., wrist) of a human
subject, the invention may also be readily embodied or adapted to
monitor such parameters at other blood vessels and locations on the
human body, as well as monitoring these parameters on other
warm-blooded species. Similarly, the techniques of the present
invention can be applied to other similar fluidic systems which
have similar properties to those of the circulatory system of a
living being. All such adaptations and alternate embodiments are
readily implemented by those of ordinary skill in the relevant
arts, and are considered to fall within the scope of the claims
appended hereto.
[0050] As used herein, the term "hemodynamic parameter" is meant to
include parameters associated with the circulatory system of the
subject, including for example pressure (e.g., diastolic, systolic,
pulse, or mean pressure), derivatives or combinations thereof,
arterial flow, arterial wall diameter (and its derivatives), cross
sectional area of the artery, and arterial compliance.
[0051] Additionally, it is noted that the terms "tonometric,"
"tonometer," and "tonometery" as used herein are intended to
broadly refer to non-invasive surface measurement of one or more
hemodynamic parameters, such as by placing a sensor in
communication with the surface of the skin, although contact with
the skin need not be direct, and can be indirect (e.g., such as
through a coupling medium or other interface).
[0052] The terms "applanate" and "applanation" as used herein refer
to the compression (relative to a state of non-compression) of
tissue, blood vessel(s), and other structures such as tendon or
muscle of the subject's physiology. Similarly, an applanation
"sweep" refers to one or more periods of time during which the
applanation level is varied (either increasingly, decreasingly, or
any combination thereof). Although generally used in the context of
linear (constant velocity) position variations, the term
"applanation" as used herein may conceivably take on any variety of
other forms, including without limitation (i) a continuous
non-linear (e.g., logarithmic) increasing or decreasing compression
over time; (ii) a non-continuous or piece-wise continuous linear or
on-linear compression; (iii) alternating compression and
relaxation; (iv) sinusoidal or triangular waves functions; (v)
random motion (such as a "random walk"; or (vi) a deterministic
profile. All such forms are considered to be encompassed by the
term.
[0053] As used herein, the term "epoch" refers to any increment of
time, ranging in duration from the smallest measurable fraction of
a second to more than one second.
[0054] As used herein, the terms "spatial" and "position", although
described in terms of a substantially Cartesian coordinate system
having applanation (i.e., Z-axis), lateral (X-axis) and (Proximal
refers to closer to the heart) longitudinal or (proximal-distal)
(Y-axis) components, shall refer to any spatial coordinate system
including, without, limitation, cylindrical, spherical, and polar.
Such use of alternate coordinate systems may clearly be independent
of any particular hardware configuration or geometry (e.g., by
performing simple mathematical translations between a
Cartesian-based apparatus and the non-Cartesian coordinate system),
or alternatively make advantageous use of such geometries. The
present invention is therefore in no way limited to certain
coordinate systems of apparatus configurations. As one example, it
will be recognized that the methods and apparatus of the present
invention may be embodied using a cylindrical coordinate system
modeled around the radial artery, such that a particular point in
space for the tonometric sensor(s) can be specified by the Z, r,
and .theta. parameters. This approach may have advantages since the
forearm/wrist area of the human being very roughly comprises a
cylindrical form.
[0055] Lastly, the term "digital processor" is meant to include any
integrated circuit or other electronic device (or collection of
devices) capable of performing an operation on at least one
instruction including, without limitation, reduced instruction set
core (RISC) processors such as those manufactured by ARM Limited of
Cambridge, UK, CISC microprocessors, microcontroller units (MCUs),
CISC-based central processing units (CPUs), and digital signal
processors (DSPs). The hardware of such devices may be integrated
onto a single substrate (e.g., silicon "die"), or distributed among
two or more substrates. Furthermore, various functional aspects of
the processor may be implemented solely as software or firmware
associated with the processor.
Overview
[0056] In one fundamental aspect, the present invention comprises
methods and apparatus for controlling an applanation or other
positioning mechanism used in non-invasive hemodynamic parameter
measurements in order to, inter alia, maintain optimal coupling
between a parameter sensor and the blood vessel of interest.
Techniques for determining the optimal applanation level, position,
and coupling are described in detail in, e.g., co-pending U.S.
patent application Ser. No. 10/072,508 entitled "Method And
Apparatus For Non-Invasively Measuring Hemodynamic Parameters Using
Parametrics" filed Feb. 5, 2002, which is assigned to the Assignee
hereof and incorporated by reference herein in its entirety.
[0057] While the techniques described in the aforementioned
co-pending patent application have been determined by Assignee to
be highly effective, their robustness and utility in practical
(e.g., clinical) settings is enhanced through the addition of one
or more of the various aspects of the present invention. Such
additional robustness is highly desirable, since it effectively
removes many operational restrictions on a clinician, caregiver, or
user (hereinafter "operator") when measuring hemodynamic parameters
such as blood pressure. Specifically, the operator is substantially
relieved of having to monitor the signal derived from the
measurement apparatus to detect anomalies, motion artifact, and
under certain circumstances will even identify to the operator when
error conditions which cannot be corrected have in fact
occurred.
[0058] After the applanation and lateral (and proximal, if desired)
positions that provide the optimal mechanical coupling between the
system sensor and the underlying blood vessel have been determined,
the invention of the present disclosure is used to control and
adjust the level of applanation and/or the lateral/proximal
positions to maintain the optimal coupling under potentially
adverse environmental conditions such as might be encountered in
the average clinical setting. Due to the nature of the clinical
setting and all of its variables, not every environmental condition
or influence can always be compensated for, and hence the present
invention has as another function the ability to identify
conditions where changes in mechanical coupling have impacted the
accuracy or reliability of hemodynamic measurements in a meaningful
manner.
[0059] Three separate but substantially interactive processes are
used in the present invention to provide the aforementioned control
and identification functionalities: (i) a first process adapted to
identify sudden changes in the mechanical coupling, as indicated
for example by changes in the measured parameter (such as
tonometrically measured pressure or pressure velocity) that exceed
expected norms, and reacquire either/both the optimal applanation
level or lateral/proximal positions where appropriate; (ii) a
second process adapted to continuously identify time varying
changes in compression coupling, and controllably adjust the
applanation position accordingly ("servoing"); and (iii) a third
process adapted to operate interactively with the first state and
provide warning and protection against loss of optimal coupling in
one or more domains, as well as performing a new determination(s)
of optimal position in an optimized fashion.
[0060] The techniques and apparatus of the present invention may be
used with a single sensor (or array of sensors) as described in
detail herein and the aforementioned and incorporated co-pending
application, or in conjunction with literally any type of other
apparatus adapted for hemodynamic parameter measurement, including
for example the devices described in co-pending U.S. patent
application Ser. No. 09/815,982 entitled "Method and Apparatus for
the Noninvasive Assessment of Hemodynamic Parameters Including
Blood Vessel Location" filed Mar. 22, 2001, and Ser. No. 09/815,080
entitled "Method and Apparatus for Assessing Hemodynamic Parameters
within the Circulatory System of a Living Subject" also filed Mar.
22, 2001, both of which are assigned to the assignee hereof and
incorporated herein by reference in their entirety. For example,
ultrasound measurements of blood pressure via blood flow kinetic
energy or velocity can be used as a confirmatory technique for a
fundamentally tonometric pressure-based approach. As another
example, lateral positioning based on analysis of the acoustic
signals relating to vessel wall detection may be used in addition
to (or in place of) the pressure-based techniques described in the
originally cited co-pending patent application. Hence, the various
aspects of the present invention are advantageously compatible with
a number of different hemodynamic assessment techniques. It will
also be recognized that the techniques and apparatus described
herein are in no way limited to tonometric applications; rather,
these features may be implemented even in occlusive cuff or
pellot-based systems.
[0061] Since signals under measurement (e.g. pressure) are time
variant, iteration and optimization are substantially used by the
methodology of the present invention to account for this variation.
Specifically, the pressure signal associated with a blood vessel is
time variant over the short period of the cardiac cycle, over the
longer period of the respiratory cycle, and potentially over the
even longer or shorter period of hemodynamic changes resulting from
varying drug concentrations and volume changes. Accordingly, the
three processes referenced above utilize the aforementioned
applanation and lateral/proximal positioning mechanisms to
continually find and maintain the optimal position and level of
applanation, thereby maintaining an environment conducive for
accurate, continuous, and non-invasive parametric measurement. In
those very limited circumstances where such optimal position and
level cannot be reasonably or reliably maintained (such as an
abrupt and jarring dislocation of the apparatus from the subject's
anatomy), the present invention identifies such conditions
accordingly, and optionally alerts the operator or provides other
notification.
[0062] Table 1 below summarizes the functionality and features of
one exemplary embodiment of the invention incorporating the three
aforementioned processes.
TABLE-US-00001 TABLE 1 Feature First Process Second Process Third
Process Detection Time >5 Hz All Frequencies >20 Seconds
Frequency of Changes Recovery Method Applanation Sweep Continuous
Applanation Adjustment Sweep Time to Recover 10-20 seconds 20-120
seconds 10-20 seconds (Largely (Correlated to (Largely Uncorrelated
to Magnitude of Uncorrelated to Magnitude of error) error)
Magnitude of error) Lateral/Proximal Possible No No Search Pressure
Display No: Recovery Yes No: Recovery Continuous Sweeps Sweeps
Description of First Process
[0063] Referring now to FIG. 1-1a, a first exemplary embodiment of
the method of identifying sudden changes in the mechanical coupling
and reacquisition of either/both the optimal applanation level or
lateral/proximal positions according to the invention is described
in detail. A detailed discussion of the electronic and signal
processing apparatus used to support the operation of the processes
described herein is provided with respect to FIG. 5 below. It will
be appreciated that while portions of the following discussion are
cast in terms of applanation (and lateral/proximal positioning)
motors of the stepper-type, the techniques of the present invention
may be utilized in conjunction with other types of applanation and
positioning apparatus, and accordingly are in no way limited to the
specific embodiments of apparatus described herein.
[0064] It will also be recognized that while the first process is
described subsequently herein with respect to a tonometric pressure
transducer, the methodology of the invention can be applied more
generally to other signal domains. For example, sudden changes in
the mechanical coupling of an ultrasonic transducer to a subject's
tissue can be identified using, inter alia, parameters which exceed
physiological norms as indicia thereof or measurable distortion in
the measurement process. Sudden changes in mechanical coupling will
alter the measurement of many parameters, both physiological in
nature and otherwise.
[0065] Furthermore, it will be appreciated that while described in
the context of the aforementioned apparatus (i.e., a tonometric
pressure sensor which also acts to provide varying levels of
compression of the underlying tissue and blood vessel(s)), the
methodology of the present invention may be practiced using
apparatus having separate components which provide these functions.
For example, the control of the pressure sensor may be partly or
completely decoupled from the applanation control system, such that
the level of applanation can be varied independently from the
coupling of the active surface(s) of the sensor.
[0066] The first process of the present embodiment continuously
checks for (sudden) changes in the mechanical coupling between a
tonometric pressure sensor and the underlying vessel/tissue. Sudden
changes in mechanical coupling can be identified by corresponding
sudden changes in tonometrically derived pressure P.sub.T (first or
second derivative of pressure) that exceed physiologic norms. A
velocity parameter, Vp[k], is calculated as in Eqn. 1:
P.sub.T[k]-P.sub.T[k-3] (Eqn. 1)
where k represents the current sample, k-3 represents three samples
in the past where the sample rate is 160 Hz. Tonometrically
measured acceleration, Ap[k], is calculated as in Eqn. 2:
Vp[k]-Vp[k-1]=P.sub.T[k]+P.sub.T[k-4]-(P.sub.T[k-1]+P.sub.T[k-3])
(Eqn. 2)
For each sample the pressure, velocity and acceleration are
compared with fixed (or deterministic) thresholds. If any one of
these parameters exceeds their respective thresholds, then a
process "event" is triggered. Note that the pressure, velocity, and
acceleration are typically greater during the systolic pressure
upstroke than during the diastolic pressure downstroke, hence the
thresholds of the present embodiment are set accordingly, providing
effectively a "buffer" between physiologic norms and process event
triggers. This buffer enhances system robustness, in that trigger
events occurring at physiologic norms are avoided.
[0067] For example, the ranges of velocity and acceleration of a
patient's blood pressure should fall within some limits around zero
(generally not symmetric). Changes in mechanical coupling could
also be observed as changes (velocity or acceleration) in
tonometrically observed pressure. The first process 100 focuses on
those changes in mechanical coupling that are both comparatively
large and rapid, thus producing velocity or acceleration values
that are not realizable from the patient's arterial pressure alone.
The buffer of the present embodiment comprises that "cushion"
between the range of measurable velocity and acceleration that is
naturally occurring (from patient's arterial pressure) and the
trigger threshold for the first process 100. It will be recognized,
however, that such buffer or cushion may be at least partly
obviated through use of one or more sensors that specifically
measure changes in mechanical coupling, such as for example a pad
sensor, which would allow the system to identify and possibly
respond to smaller changes and/or lower frequency changes in
mechanical coupling.
[0068] When sudden changes in mechanical coupling are detected,
tonometric pressure data are compared for periods occurring before
and after the process event. For instance, if the pulse pressure
(defined for the present discussion as the difference between
systolic and diastolic pressures) decreases from the maximum, or
the mean pressure changes significantly, then a limited scope
pressure sweep is implemented to achieve the optimal applanation.
If, despite this pressure sweep, a comparable pulse pressure is not
achieved, then a reacquisition state (described in greater detail
below) must be entered.
[0069] Note that in the present embodiment, the first process is
active except in conditions where the system initialization,
initial lateral search and applanation sweeps are being performed
or when operating in the aforementioned reacquisition state. The
second process is also active at the same time as the first
process, except in conditions where the first process is performing
the limited scope pressure sweep as previously described. It will
also be noted that during the limited scope pressure sweep, the
current value of P.sub.T is not displayed. Excessive periods, where
the current P.sub.T is not available limits the clinical utility of
the device, as previously described herein with respect to the
prior art. Thus, the present invention minimizes the need for the
limited scope pressure sweeps invoked by a process event, thereby
improving the overall performance and continuity of the technique
over prior art solutions.
[0070] The first process 100 of the present embodiment consists of
4 discrete but related states 102, 103, 104, 105 as illustrated in
FIG. 1; (i) first state ("normal operation") 102; (ii) second state
("event") 103; (iii) third state ("recovery") 104; and (iv) fourth
state ("sweep") 105. The impact of a process event on the servo
control system (described in greater detail below with respect to
FIGS. 2 and 5), depends largely on the then-exiting state. Each of
these four process states 102-105 are now described in detail.
[0071] (i) First (Normal Operation) State--The first state 102 is
the initial and default operating state. This state is entered when
a sweep completes, or following the detection of a sudden change in
mechanical coupling little change was observed between pre- and
post-pressure data. If a process event occurs, then the most recent
median filtered tonometric mean and pulse pressures are stored for
future comparisons. A temporal parameter (e.g., Time_of Last_Event)
is set to zero (units in seconds), and the process state is set to
the second state (event). If no process event is detected, the
first state (normal operation) is maintained. Note that in the
present embodiment, the second process 200 (i.e., servoing,
discussed below with respect to FIGS. 2-2i) is active during this
first normal operating state 102.
[0072] (ii) Second (Event) State--The second or event state 104
indicates that one or more process events 106 has recently
occurred, and the system is waiting for the perturbations relating
to the event(s) to subside before entering the next state. If a
process event occurs then, the temporal parameter (e.g.,
Time_of_Last_Event) is reset. However, if a sufficient time (i.e.,
2 seconds) has elapsed since the last event (as determined by the
existing value Time of_Last_Event), then the beat counter
(Beat_Counter) value is initialized for comparisons, and the
process is set to the third (recovery) state. The prescribed time
delay of the present embodiment advantageously minimizes the risk
of corrupted pressure data from being incorporated in the
post-process event pressure data. As in the first state described
above, the second process (servoing) is active during the second
state 103.
[0073] (iii) Third (Recovery) State--Entry into this third state
104 indicates that the recent process event has subsided, and the
system is collecting new tonometric beat pressure data to compare
with pre-perturbation data. Upon exit from the third state 104 and
before entering the next state, if a process event occurs, then the
Time_of_Last_Event parameter is reset, and the process state is set
to the second (event) state 103 per step 108. Otherwise, if a new
tonometric beat has been identified then, the system beat counter
parameter (e.g., Beat_Counter) is incremented for comparisons, and
new mean and pulse pressures are written to the storage device
(FIG. 5) for subsequent comparisons.
[0074] Note that if the beat counter has reached a predetermined
threshold value, then a comparison of the tonometric pulse and mean
pressures stored both before and after the triggering process event
is performed. If, upon performing this comparison, the mean
pressure has changed beyond a predetermined threshold, or the pulse
pressure has decreased substantially from pre- to post-event, then
the state of the process 100 is set to the fourth (i.e., sweep)
state 105, the sweep initialization parameter (e.g.,
Initialize_Sweep) is set to "true", and the second process
(servoing) is disabled. The motor position parameter (e.g.,
Motor_Position) is accordingly set to a target motor position
value. Target motor position in the illustrated exemplary
embodiment is set to either 0 (fully retracted) or -50000 (fully
extend out toward the radial artery) where units are motor steps.
Target motor position is set to 0 if the post-event mean pressure
is greater than the pre-event mean pressure. Target motor position
is set to -50000 if the post-event mean pressure is less than the
pre-event mean pressure. If the mean pressure has not increased
beyond the threshold (and the pulse pressure has not decreased
substantially between pre- and post-event), the state of the first
process 100 is set to the normal operating (first) state 102 per
step 110 of FIG. 1.
[0075] If the beat counter has not reached its predetermined
threshold, the first process 100 remains in the third (recovery)
state 104. As with the first and second states 102, 103 described
above, the second (servoing) process 200 remains, in the
illustrated embodiment, active during the recovery state 104.
[0076] (iv) Fourth (Sweep) State--Entry into this fourth state 105
indicates that the recent process event has caused a significant
change in the tonometrically measured pulse pressure and/or mean
pressure. In response, the system performs a limited scope pressure
sweep to reset the optimal applanation level. Specifically, if the
sweep initialization variable (Initialize_Sweep) is set true, the
initial search direction as determined for target motor position in
(iii) above, and the applanation motor(s) are moved in the proper
direction (ramp continuously in the present embodiment, although
other profiles may be used). Additionally, the sweep pressure
memory is initialized, and the "first pass" parameter flag (e.g.;
FirstPass_Flag) is set to "true."
[0077] If a new beat has been identified, then the process appends
the tonometric pressure data associated with the new beat to that
existing in the memory array and the Beat_Counter value is
incremented for comparisons. Specifically the data for each beat
includes average applanation position, mean tonometrically measured
pressure, systolic tonometric pressure, diastolic tonometric
pressure, and tonometric pulse pressure (i.e., systolic minus
diastolic), which are stored in parameter-specific one dimensional
arrays within memory.
[0078] If the measured mean pressure has reached its minimum goal
and current pulse pressure values (median filtered) are
significantly less than maximum pulse pressures (median filtered)
observed during the sweep, then additional analysis is performed.
Specifically, if the maximum pulse occurred close to beginning of
the applanation sweep, and the first pass flag (FirstPass_Flag)
equals "true" then the FirstPass_Flag is set to "false", and the
applanation motor(s) are moved, e.g., to ramp continuously, in the
direction opposite from the prior direction of travel. If the
maximum pulse pressure did not occur near the beginning of the
sweep, and if the maximum pulse pressure (median filtered) is a
large percentage (e.g., 80% or greater in the present embodiment)
of that occurring prior to the triggering event then the state of
the process 100 is set to the first state 102, and servoing at the
maximum pulse pressure is initialized. In the context of the
present embodiment, the measured mean pressure reaching its
"minimum goal" comprises the median filtered mean pressure at least
reaching and searching beyond the pre event trigger mean pressure.
Note that this requirement can be dependent on the direction of the
search (i.e., whether extending or retracting the sensor);
specifically, the median filtered mean pressure is greater than the
pre-event trigger mean pressure for the sensor extension case, or
less than the pre-event trigger mean pressure for the retraction
case.
[0079] However, if the maximum pulse pressure occurs not near the
beginning, and the maximum pressure value is not a large percentage
of the prior occurring value, then the reacquisition process (the
third process 400 discussed below with respect to FIG. 4) is
entered, and the first process 100 is disabled.
[0080] It is also noted that second process 200 (FIG. 2) is not
active during the fourth state 105 of the first process 100.
[0081] FIG. 1a provides a detailed flow chart representation of the
exemplary first process 100 of FIG. 1.
[0082] It will further be recognized that the first process 100 may
be applied to blood pressure measurements irrespective of the
mechanism used to originally attain optimal applanation position.
In this scenario, the first process operates effectively as if a
large transient event had occurred, and uses the foregoing method
(in conjunction with the third or reacquisition process 400
described below with respect to FIG. 4) to settle onto optimal
positions for these parameters.
[0083] It will be recognized that as referenced above, the first
process of the present invention need not operate using a
"physiologic" parameter. One exemplary alternative approach of the
present invention is to apply and accelerometer or force transducer
of the type well known in the art on or contiguous with the sensor
surface; i.e., not necessarily over the blood vessel of interest
itself. Similarly, such accelerometer or transducer may be located
on the apparatus coupling the sensor to the patient (e.g., wrist
brace or strap), or alternatively on the shaft (not shown) between
the actuating mechanism and the sensor/pad (or within the actuating
mechanism itself). Since the first process of the present invention
fundamentally detects rapid motion corresponding to potential
mechanical coupling disruptions, literally any physical
configuration and/or parameter which provides information relating
to such motion and disruptions may be used consistent with the
invention. As yet another alternative embodiment, an optical sensor
of the type well known in the electronic arts may be positioned
near the skin and accordingly used as the mechanism to detect
sudden changes in sensor/patient relative position.
[0084] It can be appreciated that the use of the tonometric
pressure sensor as the basis for measurement of the physical
parameter (as described in detail above with respect to the
exemplary embodiment) provides the benefits of both simplicity and
reduced cost by eliminating the need for an added sensor or added
complexity of the actuating mechanisms. However, certain benefits
relating to the decoupling of the parameter signal from the
arterial pressure signal (as compared to the use of the tonometric
pressure signal as described above) may be realized through use of
one of the alternate embodiments set forth above. For example, use
of a non-hemodynamic parameter allows for the separation of
mechanical coupling changes from the physiologic signal, since no
(or at least minimal) physiologic content exists in the
measurements obtained in this fashion. Furthermore, the use of
non-physiologic parameter (e.g., pad force or pressure as measured
by the force on the applanation motor shaft as described above)
allows the use of a much smaller buffer zone, since there is
effectively no overlap in the frequency and amplitude of the
pressure signal as measured by the pad relative to the pressure
changes induced by disruptions in mechanical coupling.
Description of Second Process
[0085] Referring now to FIGS. 2-2i, one exemplary embodiment of the
method of identifying changes in the compression coupling and
readjustment of the applanation level back to optimal (i.e.,
"second process") according to the invention is described in
detail. It will be appreciated that while the following discussion
of the exemplary embodiment is cast primarily in terms of the
adjustment of the tonometric applanation level (i.e., level of
compression), the techniques of the present aspect of the invention
may be equally applied to the other spatial domains associated with
the tonometric measurement environment; e.g., lateral position and
proximal position. Such applications may be coupled to that
associated with the applanation domain, or alternatively be
entirely independent.
[0086] It will also be appreciated that while the following
discussion is cast in terms of an exemplary embodiment utilizing
Pseudo Random Binary Sequences (PRBS) generally complying with a
structured sequence of the form (2.sup.n-1), other white noise,
random/pseudo-random, or pseudo-noise (PN) processes may be
substituted with success, and hence the following discussion is
merely illustrative of the broader principles of the invention. For
example, as one alternative, a pseudo-random generation algorithm
of the type well known in the communications arts (such as that
used for example in generating FHSS hop or CDMA pn "long code"
sequences) is seeded with a given initial seed value and generates
a pseudo-random sequence, the latter used to modulate the
applanation level in the present invention. Other perturbations or
sequences (any movement surrounding the optimal applanation
position including for example sinusoidal perturbations) may also
be substituted consistent with the present invention; however, the
methods described with respect to the exemplary embodiment above
have inherently good signal-to-noise ratio (SNR) across the
frequency band of interest.
[0087] FIG. 2 shows a logical flow diagram of the exemplary
embodiment of the second process 200. The process 200 generally
comprises first providing a transducer adapted for determining
pressure (step 202). The transducer is disposed proximate to the
blood vessel of interest (step 204), in order to provide coupling
of pressure signals from the blood vessel wall through the tissue
and to the active surface(s) of the sensor. Note that an
intermediary coupling agent (such as a gel) may be used if desired.
Next, an optimal or near-optimal state of vessel compression is
achieved per step 206. It will be recognized that such compression
may be applied via the pressure transducer itself, or alternatively
via another mechanism (such as a contact pad). The optimized level
of compression can be determined using, inter alia, the methods of
the aforementioned co-pending U.S. patent application Ser. No.
10/072,508 filed Feb. 5, 2002. The level of compression applied to
the blood vessel is next varied, over time (step 208). In the
illustrated embodiment, the act of varying the level of compression
per step 208 comprises modulating the level of compression in
comparatively small magnitude "perturbations" according to a
modulation sequence having particular desirable properties,
although other schemes (e.g., non-sequential) may be used. The
effects of the modulation on the observed pressure values (e.g.,
pulse pressure, diastolic, etc.) are then observed per step 210,
and corrections in the level of compression applied to the blood
vessel made per step 212 based on the observed effects of the
modulation sequence.
[0088] It will be appreciated that the second process 200 (and
associated apparatus) need not measure the applied pressure or
compression, such as via a force sensor or the pressure
transducer). Rather, the present embodiment is largely
effects-based in that applanation level (compression) can be
adjusted based simply on the observed effects of the modulation.
Hence, the applanation mechanism can advantageously be made "dumb",
thereby simplifying the mechanism as well as other aspects of the
system. However, if explicit monitoring of the applied force or
compression is desired, such intelligence can be utilized in
conjunction with the invention as well.
[0089] As previously discussed, one clinical objective of the
second process 200 is to maintain the tonometrically observed mean
pressure within a given value (e.g., +/-10 mmHg) of the optimum
tonometric pressure, which produces maximum pulse pressure. During
the second process 200, both the patient's arterial pressure and
the mechanical coupling between the tonometric transducer and the
underlying artery can change. Either type of change introduces a
variation in the tonometrically observed pressure. Hence, the
present invention seeks to differentiate between
physiologically-induced changes (e.g., those stemming from the
patient's physiology, such as for example due to the introduction
of pharmacological agents), and mechanical coupling changes in the
tonometrically observed pressure. It also seeks to constantly
correct for the second type of change (i.e., change in the
mechanical coupling).
[0090] Sudden changes in the mechanical coupling between the
tonometric pressure transducer and the artery (i.e. acceleration or
"bumping" of the transducer or the wrist) can be detected by
several techniques, as previously described herein with respect to
FIG. 1, as well as that of FIGS. 4-4a described below. Slower
changes in the mechanical coupling must be detected and corrected
by other means.
[0091] One method of detecting and correcting slower changes in
mechanical coupling involves perturbing the system by modulating
the compression of the artery and observing the resultant changes
in tonometrically measured pulse pressure. The method and degree of
perturbation should be optimized in accordance with the overall
clinical objectives.
[0092] Accordingly, the Assignee hereof has developed exemplary
clinical objectives for use in accordance with the exemplary
process 200 described herein. It will be recognized that these
objectives are merely illustrative, and may be adapted and modified
as needed to particular clinical environments or desired levels of
performance and accuracy.
[0093] (i) Display disruption--First, the disruption of the system
pressure display by the induced perturbation should be minimized.
Noticeable discontinuities in the pressure display and delays in
transfer of the pressure signal to the patient monitor (e.g., based
on a predetermined criterion such as delays of 0.1 seconds or
greater) are unacceptable.
[0094] (ii) Responsiveness--The tonometrically observed pressure
from 20 mmHg of optimum T-Line pressure to within 10 mmHg occurs in
accordance with a given period of time (e.g., 1 minute). From a
clinical perspective, excursions beyond roughly 10-15 mmHg in mean
tonometrically measured pressure from the actual intra-vascular
pressure (such as A-Line pressure) for extended periods, e.g.,
longer than 1-2 minutes, are often clinically undesirable. Although
measurement error can occur, as reflected by prevailing FDA
requirements for cuff accuracy (+/-5 mmHg mean error with a
standard deviation of 8 mmHg), more frequent and longer duration
divergences between tonometrically sensed pressure and true
intravascular pressure reduce the clinical desirability of a
device. Thus, a clinically useful system should operate such that
it responds with reasonable speed and accuracy to changes in
mechanical coupling.
[0095] (iii) Device Limitations--Limitations exist relating to the
motion of the applanation motor of the system. These limitations
include for example limits in the applied electrical power and
resulting output (mechanical) power and torque, the control of wear
over time (i.e., motor longevity), and limits in the motor velocity
and acceleration which preclude instantaneous (i.e. step) changes
in applanation. From bench data obtained by Assignee, diastolic
pressure in representative patients changes on average 7 mmHg per
1000 motor steps (within the range of 4-10 mmHg per 1000 motor
steps) at an applanation level near optimum. Furthermore, pulse
pressure changes for the same individuals an average of 8 mmHg per
1000 motor steps (ranging from 4-14 mmHg per 1000 motor steps). One
exemplary actuator and motor scheme utilized by the Assignee hereof
suggests a maximum rate of about 1000 motor steps per second.
Changes in actuator design to alleviate some of these limitations
are not considered. Hence, it can be inferred that maximum rates of
diastolic and pulse pressure change of about 7 mmHg/sec and 8
mmHg/sec, respectively, can be achieved with the aforementioned
exemplary apparatus.
[0096] (iv) Variations in Pulse Pressure--The patient's pulse
pressure is time variant. As is well documented in the literature,
arrhythmias can produce cyclical changes in pulse pressure (i.e.
pulsus alternans, wherein a succession of high and low pulses exist
in such a manner that a low pulse follows regularly a high pulse,
and this low pulse is separated from the following high pulse by a
shorter pause than that between it and the preceding high pulse.)
See, e.g., "Apparent Bigeminy and Pulsus Alterans in Intermittent
Left Bundle-Branch Block", Laszlo Littmann, M.D., and Jeffrey R.
Goldberg, M.D., Departments of Internal Medicine and Family
Practice, Carolinas Medical Center, Charlotte, N.C., USA, which is
incorporated by reference herein. It is well documented that
patient respiration can produce sizeable changes in pulse pressure
as well. Hence, a perturbation and servo-control system would
ideally be largely if not completely insensitive to cyclical and
random fluctuations in arterial pulse pressure.
[0097] In addition to the foregoing objectives and limitations, the
properties of the tonometric measuring and control system must be
determined. It is well known that the insertion of so-called "white
noise" into a system is a useful means of identifying properties
associated with that system. In the present context, the
introduction of such white noise generates a pattern which
effectively cannot be produced by the patient physiology. The
inputs to the system include applanation motor position, and the
"system" is the tonometrically obtained pulse pressure as a
function of applanation level. Cross-correlating the changes in
applanation position induced by the white noise with the resultant
observed pulse pressure produces a relationship between the
applanation motor position and pulse pressure. This relationship is
advantageously quite robust in the presence of random or periodic
fluctuations in pulse pressure, due largely to the insertion of the
white noise.
[0098] However, several considerations exist with respect to the
practical implementation of white noise modulation of applanation
motor position in the present invention. First, true "white noise"
assumes a normal or Gaussian distribution of motor position. Such
normal distributions can contain very large excursions from the
mean albeit with increasingly less frequency (theoretically not
bounded), whereas in contrast the motor position in the physical
implementation of the present invention is bounded.
[0099] Second, the time to travel from one position limit to the
other (if such travel is needed) is significant, as previously
discussed with respect to maximum motor rate. Instantaneous changes
in applanation mechanism position are therefore not possible.
[0100] Third, white noise identification theoretically requires an
infinite period time for convergence, even approximations of which
are not practical in the clinical setting. Ideally, a useful
clinical device would employ control systems which would converge
in a very short period of time, thereby enhancing the continuity of
the tonometric pressure measurement.
[0101] As is known in the mathematical arts, Pseudo Random Binary
Sequences (PRBS) are a defined sequence of inputs (+/-1) that
possess correlative properties similar to white noise, but converge
in within a give time period. In addition, the inputs can be
specified (and thereby optimized) to produce more effective
signal-to-noise ratio (SNR) within the constraints of the system.
One common type of PRBS sequence generator uses an n-bit shift
register with a feedback structure containing modulo-2 adders (i.e.
XOR gates) and connected to appropriate taps on the shift register.
The generator generates a maximal length binary sequence of length
(2.sup.n-1). The maximal length (or "m-sequence") has nearly random
properties that are particularly useful in the present invention,
and is classed as a pseudo-noise (PN) sequence. Properties of
m-sequences commonly include: [0102] (a) "Balance" Property--For
each period of the sequence, the number of `1`s and `0`s differ by
at most one. For example in a 63 bit sequence, there are 32 `1`s
and 31 `0`s. [0103] (b) "Run Proportionality" Property--In the
sequences of `1`s and of `0`s in each period, one half the runs of
each kind are of length one, one quarter are of length two, one
eighth are of length three, and so forth. [0104] (c) "Shift and
add" Property--The modulo-2 sum of an m-sequence and any cyclic
shift of the same sequence results in a third cyclic shift of the
same sequence. [0105] (d) "Correlation" Property--When a full
period of the sequence is compared in term-by-term fashion with any
cyclic shift of itself, the number differences is equal to the
number of similarities plus one (1). [0106] (e) "Spectral"
Properties--The m-sequence is periodic, and therefore the spectrum
consists of a sequence of equally-spaced harmonics where the
spacing is the reciprocal of the period. With the exception of the
de harmonic, the magnitude of the harmonics are equal. Aside from
the spectral lines, the frequency spectrum of a maximum length
sequence is similar to that of a random sequence.
[0107] Accordingly, detecting and correcting slower-rate changes in
mechanical coupling as previously described can be accomplished by
applying PRBS modulation of the applanation position, and observing
the resultant changes in tonometrically observed pulse pressure. In
one exemplary embodiment of the present invention, the physical
implementation of such a system contains three interactive
"components": (i) a modulator; (ii) a signal restoration entity;
and (iii) an identification/servo control entity. It will be
recognized by those of ordinary skill that the term "entity" as
used herein relates to any number of a wide variety of
implementations, ranging from a corporeal entity (e.g., electronics
and associated integrated circuits) to a completely virtual or
intangible one (e.g., one manifest in the form of algorithms,
routines, or software objects or components resident across the
various hardware environments of a system).
[0108] The following exemplary description illustrates the
operation of the aforementioned multi-component system according to
one embodiment of the invention.
[0109] Referring now to FIGS. 2a-2c, the characteristics and
response of an exemplary patient are described. As shown in FIG.
2a, the patient exhibits a given pulse pressure versus diastolic
pressure relationship 230. The maximum pulse pressure 232 (e.g., 42
mmHg in the illustrated example) occurs at a diastolic pressure of
about 75 mmHg 234.
[0110] Furthermore, it is assumed for purposes of illustration that
the applanation motor is held at a constant position (at the point
of optimal compression corresponding to maximal pulse pressure),
and that the patient has a time-invariant arterial pressure with a
heart rate of 60 bpm with the shape 236 shown in FIG. 2b. If the
patient's artery is not sufficiently compressed, a lower diastolic
pressure 237 (e.g., diastolic pressure=67 mmHg in this example)
will result, as indicated by the "sub-optimal" waveform 238 of FIG.
2c. Note that the pulse pressure (systolic minus diastolic) at a
tonometrically measured diastolic pressure of 67 mmHg is only
approximately 36 mmHg. Under this condition, the system must
identify the fact that the artery is under-compressed and adjust
the applanation level appropriately over time.
[0111] Referring now to FIGS. 2d-2e, the modulation entity of the
servo process 200 of the invention is described in the context of
the foregoing example. The modulator of the present embodiment
introduces changes in artery compression (applanation position)
over a limited range around the "optimal" operating point. These
changes are in the present embodiment synchronized with the
downward slope of the arterial pressure waveform, this downward
slope being associated with diastolic relaxation of the heart.
Other synchronizations (or even lack of synchronization) may be
used if desired, however. The modulations induced by the modulation
entity ramp the applanation mechanism position from one extreme to
an equal and opposite extreme (e.g., 400 motor steps in the present
embodiment) around the operating point over a brief period (e.g.,
0.5 seconds), although other profiles (symmetric or non-symmetric)
and durations may be substituted if desired. The decision to move
from one extreme to another is controlled in this embodiment by a
Pseudo Random Binary Sequence (PRBS) of the type previously
described. This modulation scheme produces changes in pressure
offset, and may produce highly correlated changes in pulse
pressure.
[0112] In the illustrated embodiment, a PRBS sequence of length=7
is implemented (i.e., 1, 1, 1, -1, -1, 1, -1) to modulate the
pressure waveform as shown in FIG. 2d. Note that for the clinical
application, the respiratory period of the patient, and its
corresponding cyclical fluctuations in pulse pressure, approximates
the repetition period of the PRBS of length 7. Hence, clinical
embodiments of the application incorporate a PRBS of appropriate
length such as length=15 (i.e., 1, 1, -1, 1, -1, 1, 1, 1, 1, -1,
-1, -1, 1, -1, -1) or length 31 (i.e., 1, 1, 1, 1, -1, 1, 1, -1,
-1, 1, 1, 1, -1, -1, -1, -1, 1, 1, -1, 1, -1, 1, -1, -1, 1, -1, -1,
-1, 1, -1, 1) Specifically, the PRBS sequence repeats every 7, 15,
31 beats if one does not allow for transition beats, and in the
exemplary case 11, 22, or 47 beats respectively allowing for
transition beats. Any noise source that repeats in the same time
base (sinusoidal noise frequency) will have a greater impact on
system performance than noise sources with other frequency content.
Respiration period occurs in the range of 5-7 seconds; hence,
during this period, anywhere from 4-14 heartbeats could be
observed. Thus, a PRBS sequence of length 7 with an effective
length 11 when transition beats are included falls directly within
the respiration period. The longer sequences do not have that
problem. Conversely, however, the noise rejection properties
require a complete cycle of data for proper function. Hence,
control using sequences that are excessively long are prone to
sluggish control, thereby detracting from system performance.
[0113] FIG. 2e depicts the practical implementation of PRBS changes
in applanation level. Practical mechanical considerations relating
to the applanation motor preclude step changes in applanation level
of sufficient magnitude to produce a significant change (e.g., 6
mmHg) in observed tonometric pressure. Thus, for the present
embodiment, the applanation position is ramped over a period of
time (e.g., 0.5 seconds), as shown in the PRBS portion 239 of FIG.
2e. Since no guarantee exists that the ramp will complete by the
end of the beat, a variable delay in the PRBS, which is a function
of heart rate and number of motor steps traveled, is included
within each transition period without loss of the correlative
properties. Typically this delay is 1 heart beat. but at high heart
rates could extend to two and possibly more beats. It is noted that
the PRBS portion 239 of FIG. 2e is dimensionless. In effect, two
classes of beats are created in the present implementation;
"measurement" beats and "transition" beats. When the motor is
moved, transition beats are added (e.g., for a length=15 sequence,
7 or 8 transition beats are added).
[0114] Referring now to FIGS. 2e and 2f, the signal restoration
entity of the invention is described in detail. As shown above, the
modulation entity will introduce changes in the measured pressure
waveform. These changes in the pressure waveform may be disruptive
to the clinician under certain circumstances. Note that the
PRBS-modulated pressure waveform 240 of FIG. 2e varies
significantly around the tonometric pressure 242 that would
otherwise be observed if the PRBS or other modulation was not
active. Hence, the signal restoration entity must anticipate the
changes in the observed tonometric pressure waveform introduced by
the modulation entity, and (mathematically or otherwise) restore
the modulated waveform to a shape that is clinically equivalent to
the un-modulated tonometric waveform.
[0115] Specifically, by implementing a linear ramp during the
period when the modulation is active, the original un-modulated
waveform can be restored. This process assumes the amount of change
that is observed by the modulation is not large (e.g., <roughly
6 mmHg in the illustrated embodiment) and is adaptively identified
(i.e., the cross-correlation of the PRBS modulation sequence and
diastolic pressure from which the average diastolic pressure has
been removed can be used to provide an estimate of the expected
change in pressure produced by the modulation).
[0116] Note that the foregoing process in essence adds or subtracts
a pressure correction offset to the measured pressure. When the
modulation entails extension of the sensor from the mechanism (in
the exemplary embodiment), the pressure offset correction is
subtracted from the measured pressure data, and vice-versa. The
value (units in mmHg) of the offset correction can not be directly
determined unless compared with a source of true intravascular
pressure (e.g., A-Line, thus defeating the purpose of the
tonometric sensor), but it can be estimated by evaluating the
change in diastolic, systolic, mean, pulse, or similar pressure
values correlated to the change in motor position. Thus, for
example, the cross-correlation between the PRBS and the diastolic
pressures (mean pressure removed) can be used to estimate the
offset correction. This estimate can be updated with each new beat
producing a continuous estimate of the offset correction. Note that
during applanation motor ramping, the offset correction of the
exemplary embodiment also ramps from one extreme to the other.
Additionally, it will be recognized that the amount of modulation
(e.g., number of motor steps in the illustrated embodiment) can be
adjusted to produce the desired amount of pressure change. In the
present embodiment, the modulation level is continuously adjusted
to achieve 5 mmHg peak-to-peak excursion subject to a limit; i.e.,
provided that the peak-to-peak excursion is limited to between 50
and 800 motor steps. Other modulation schemes and limits can be
used consistent with the invention, however.
[0117] Note that lead/lag relating to the assumed start of the
motor movement (as opposed to the actual start of movement), and
the introduced changes in pressure, can lead to small artifacts or
"bumps" in the pressure waveform display; however, these are often
imperceptible to the operator, and advantageously no points of
discontinuity exist in the display, unlike prior art systems.
[0118] Errors between the actual and predicted pressure change
(i.e., those predicted by the signal restoration entity relating to
the applied modulation) are exhibited as small jitter synchronized
with the PRBS in the diastolic pressure display. FIG. 2f depicts
the "restored" waveform 242; i.e., the waveform(s) of FIG. 2e after
correction by the restoration entity. Note that the error produced
by the linear ramp approximation is small compared to both (i) the
pulse pressure, and (ii) pixel resolution of the monitor. Thus, the
process of restoring a clinically equivalent waveform is readily
achieved using the techniques described herein.
[0119] Referring now to FIGS. 2g-2i, the identification/servo
control (ISC) entity of the present embodiment is described.
[0120] As shown in FIG. 2f, the corrected ("restored") pulse
pressure values associated with points on the restored waveform 242
fluctuate around corresponding ones of the nominal, non-modulated
sub-optimal applanation waveform 238. Further, it will be
recognized that these fluctuations, albeit comparatively small in
magnitude, generally correlate with the modulation in applanation
level, as illustrated by FIG. 2g.
[0121] The ISC entity of the present embodiment takes advantage of
the correlative properties of white noise. As shown in FIG. 2g, an
auto-correlation of the PRBS modulation is performed. The
auto-correlation of the PRBS signal has a gain equal to the PRBS
length (e.g., 7) for zero phase delay, and negative unity gain for
other phase delays until the PRBS repeats. The PRBS modulation,
time synchronized tonometrically measured pulse pressure, and
un-corrected diastolic pressures for the preceding example are
displayed in Table 2. Note that the PRBS values labeled "T"
indicate transition beats where the applanation motors are still in
the process of ramping from one position to the next. These beats
are removed from the subsequent cross-correlation without loss of
the correlative properties of the PRBS.
TABLE-US-00002 TABLE 2 Pulse Un-Corrected Beat PRBS Pressure
Diastolic Pressure 1 1 38 70 2 1 38 70 3 1 38 70 4 T 35 66 5 -1 34
64 6 -1 34 64 7 T 37 68 8 1 38 70 9 T 35 66 10 -1 34 64 11 T 37 68
12 1 38 70 13 1 38 70
FIG. 2h illustrates the weighted zero-mean values for pulse
pressure and diastolic pressure (after removing the "transition"
(T) beats) for the first 7 beats, and synchronized to the PRBS
modulation. It will be noted that the pulse pressure values 250 and
diastolic pressure values 252 are well correlated with the PRBS
modulation of applanation level 254.
[0122] Performing the cross-correlation between the PRBS modulation
of applanation and the pulse and diastolic pressures produces a
large signal at phase delay=0, as shown in FIG. 2i. For diastolic
pressure, the change induced by the modulation equals 21 mmHg
divided by the PRBS length=21/7=3 mmHg. This means that the
modulation process (extending the sensor out from operating point
"0" during the modulation) caused a 3 mmHg increase in diastolic
pressure. The total excursion (from PRBS="-1" to PRBS="1" thus
equals 6 mmHg (70 mmHg-64 mmHg) using the table above. Similarly,
the modulation-induced change in pulse pressure as shown in FIG. 2i
equals 14/7 or 2 mmHg. Thus, the system recognizes that increasing
compression (applanation) will increase the observed pulse
pressure. Subsequently, the control system can change the operating
point (applanation motor position around which PRBS modulation
operates) appropriately to maintain optimal coupling. Using this
approach, the control system can accurately track on a beat-by-beat
basis the motor position corresponding to the applanation level
that produces maximum pulse pressure.
[0123] A circular buffer arrangement is used in the exemplary
embodiment of the apparatus implementing the foregoing technique;
this advantageously allows the calculation to be updated once per
beat. It will be recognized, however, that other arrangements may
be used to implement the desired functionality.
[0124] It will also be recognized that the techniques described
above with respect to the second process may be equally applied to
the other domains of spatial variation; i.e., the lateral and/or
proximal search algorithms with proper selection of
random/pseudo-random sequence (e.g., PRBS) parameters, thereby
providing continuous tracking in the selected direction(s) as well
as in the application domain. Such application and selection are
readily implemented by those of ordinary skill given the present
disclosure, and accordingly are not described further herein.
[0125] Based on observations and testing performed by the Assignee
hereof, the performance of the present invention may be further
enhanced under certain circumstances by the inclusion of one or
more optional control and signal processing features; use of these
features can enable the system to respond more quickly to an event
by, inter alia, mitigating control overshoot and/or eliminating
unwanted noise and other artifacts from the processed signals(s).
These features include: (i) Hampel filtering of pulse and diastolic
pressures; (ii) the addition of a proportional component to the
control (servo) loop; (iii) the adjustment of integral control of
the servo loop through estimation of the SNR; (v) increasing the
precision of the diastolic cross-correlation; (vi) control of the
initial settings for the diastolic pressure cross-correlation
arrays; (vii) adjusting the integral gain based on the average
pulse pressure; and (viii) correcting for BMI or other scaling
artifact. Each of the foregoing features are now described in
detail.
[0126] (i) Hampel Filter for Pulse and Diastolic
Pressures--Improperly detected beats, noise, and cardiac
arrhythmias can introduce large one-time changes in pulse pressure
measurements that are not reflective of the applanation state of
the patient. In the context of the second process 200 described
above, these beats can potentially disrupt the feedback control.
One exemplary method of removing most of these beats comprises
independently applying a Hampel filter of the type well known in
the signal processing arts to each of the positive PRBS pulse
pressure and negative PRBS pulse pressure values in the respective
arrays. The Hampel filter is advantageously employed as opposed to
other filtering techniques including low pass filters or median
filters which increase the time lag in the servo control loop.
[0127] (ii) Addition of Proportional Component to Servo Loop
Integral Control--The PRBS-based algorithm described above operates
generally as a sophisticated block filter with a lag equal to 1/2
of the PRBS length. In the second process 200, transition beats
(PRBS length/2) are added to the computation, thereby creating a
lag (e.g., 11.25 beats in the above example) from a change in
coupling to its full impact to its identification through the
cross-correlation with the PRBS and subsequent servo control. This
lag can produce an overshoot in the integral servo control system
when recovering from a manually introduced step change in artery
compression (such as may be experienced when the NIBP measurement
apparatus is jarred), and the integral gain is set too large.
Adding a proportional component to the servo control algorithm and
retuning the integral control gain advantageously reduces the
magnitude of this overshoot. Since the servo control system
operates based on changes in the "target" applanation level, a
proportional control component may take the form of Eqn. 4
below:
M.sub.TP(t)=K.sub.p*(X.sub.corr[t]-X.sub.corr[t-k]) (Eqn. 4)
where M.sub.TP(t) is the new target applanation motor position,
X.sub.corr is the 0.sup.th delay of the cross-correlation of the
PRBS and zero mean pulse pressures, t is the current pulse, and k
is the number of beats past. In one exemplary embodiment, values of
(k=3) and K.sub.p=1.times.(integral gain) are utilized.
[0128] (iii) Integral Control of the Servo Loop by Estimating
SNR--The non-zero terms of the aforementioned cross-correlation
provide some indication of the noise potentially present in the
pulse pressure estimates. Adding a "governor" to the servo control
system which is triggered upon attaining one or more predetermined
criteria; e.g., when the non-zero terms (average absolute or
maximum absolute) are a percentage of the 0.sup.th term, can
decrease the sensitivity of the system to such noise. For example,
a manually introduced step change in artery compression can
introduce a large change in the operating state (see discussion of
the first process 100 above), which can drive the initial recovery
from the event in the wrong direction until the aforementioned
identification lag is overcome. Meanwhile, the non-zero elements of
the cross correlation also become large until the lag is also
overcome. The governor mechanism described herein mitigates the
effects of these non-zero elements during the lag period.
[0129] (iv) Improved Precision on Diastolic Cross-Correlation--As
described above with respect the "nominal" embodiment of the
present invention, cross-correlations are performed between the
diastolic pressure and the PRBS component. The accuracy of these
cross-correlation calculations may be increased by using the time
varying signed modulation signal as the basis of the
cross-correlation, rather than the PRBS as previously described.
When the signed modulation signal is implemented, the
cross-correlation value is divided by the average absolute
modulation signal for the period under consideration; otherwise,
the servo adjustment to subsequent modulation counts and operating
applanation position may be adversely impacted.
[0130] (v) Control of Initial Settings for Diastolic Pressure
Cross-Correlation Arrays--To provide an initially reactive system
and to speed initial convergence, the nominal system is initialized
to provide comparatively large modulations. On some patients,
however, the modulation (measured in the present context in terms
of "motor delta" which is defined as the estimated absolute change
in applanation motor position in steps required to change end
diastolic pressure by a pre-determined quantity such as 2.5 mmHg)
is initially excessive; e.g., up to 8 or 10 times the number of
motor steps otherwise required. If motor delta is set too large,
then the applanation motor will initially move much farther than
necessary/desired during PRBS, and the patient's diastolic pressure
will change much greater than anticipated. Waveform reconstruction
will not sufficiently compensate for changes in diastolic pressure,
thus shifts or oscillations in diastolic pressure will be
noticeable on the pressure display, which is undesirable.
Nonetheless, the large motor delta will aid in the rapid
convergence to the applanation position corresponding to maximum
pulse pressure. On the contrary, if the motor delta is set too
small, then applanation will not sufficiently excite the system,
thus slowing convergence to the applanation position corresponding
to maximum pulse pressure. Meanwhile, the restoration process will
overcompensate for the change in diastolic pressure with PRBS
modulation, and produce noticeable shifting in the displayed
pressure.
[0131] To address this issue, the initial modulation level can be
controlled, such that a predetermined maximum number of steps
(e.g., 150) are utilized, or alternatively by applying a more
sophisticated technique of determining the optimal initial
modulation as illustrated in FIGS. 3-3b. Specifically, the initial
applanation pressure sweep provides sufficient data to estimate the
necessary motor delta to change diastolic pressure by a
predetermined amount (e.g., 2.5 mmHg). The sweep data is first
obtained (step 302 of FIG. 3), and is used to generate the array of
diastolic pressure data values, aiDiastoleP[ ], and the array of
applanation position, alAppPos[ ], for all the beats in the sweep
(step 304). At the end of the applanation sweep process, the beat
which provided the maximum pulse pressure is identified as
iSysPointer (step 306).
[0132] In one exemplary embodiment of the method 300, outlying or
abhorrent values are first removed from alAppPos[ ] and
aiDiastoleP[ ] via a Hampel filter of the type well known in the
art, using for example a 3- or 4-standard deviations (a) outlier
test or comparable mechanism (step 308). Other filter types can
also be substituted, as will be appreciated by those of ordinary
skill.
[0133] Next, in step 310, those beats whose diastolic pressure
ranges from that associated with the optimum beat minus a
predetermined value (e.g., -10 mmHg) to that corresponding to
optimum beat plus the predetermined value (+10 mmHg) is
determined.
[0134] The slope of the diastolic pressure/applanation position
curve (in units of mmHg per motor step in the present embodiment)
over that region of interest is next determined in step 312. This
provides in effect a sensitivity of diastolic pressure to motor
position.
[0135] In step 314, the slope value(s) determined in step 312 are
used to calculate the number of applanation motor steps required to
change the diastolic pressure by a desired amount (e.g., motor
delta=2.5/slope in the illustrated embodiment). In the illustrated
embodiment, the PRBS process is simply a method of determining the
slope around the nominal.
[0136] Lastly, in step 316, the motor delta value is bounded within
acceptable limits which will reduce initial "overstepping" of the
modulation as previously described. For example, in one embodiment,
the allowed initial motor delta value is bounded on the low end by
40 motor steps, and on the high end by 400 motor steps.
[0137] It will also be recognized that a similar issue (i.e.,
"overstepping") may arise when initiating an applanation sweep
subsequent to the first process 100 described above with respect to
FIG. 1. Accordingly, the aforementioned methods of mitigating
excessive modulations can be employed in this context as well.
[0138] (vii) Gain Adjustment Based on Average Pulse
Pressure--Adjustments to integral gain (i.e., autocorrelation gain
with zero phase delay) is in the above-described embodiment
independent of the underlying average pulse pressure, as reflected
in the following relationship:
M.sub.TP(t)=(K.sub.i*K.sub.pp[t]*K.sub.n[t]*X.sub.corr[t])+M.sub.TP(t-1)
(Eqn. 5)
where M.sub.TP(t) is the new target applanation motor position,
M.sub.TP(t-1) is the previous target applanation motor position.
X.sub.corr is the 0.sup.th delay of the cross-correlation of the
PRBS and zero mean pulse pressures, t is the current pulse, K.sub.i
is the fixed integral gain, K.sub.pp[t] is the integral gain
modifier that is inversely related to pulse pressure, and
K.sub.n[t] is the integral gain modifier that is related to the
signal-to-noise ratio.
[0139] Thus, as an example, a pulse pressure cross-correlation of
magnitude 2 has the same control "impact" at an average pulse
pressure of 60 mmHg as it does at 20 mmHg. Making the value of this
gain quasi-inversely proportional to the underlying average pulse
pressure makes the control system more responsive both for
individuals with low pulse pressure, and for all individuals when
the system is not situated close optimum. It will be recognized
that the foregoing coupling between the integral gain and pressure
may take on other forms as well. For example, the gain adjustment
need not be proportional or quasi-proportional, but rather may be
based on a limited number of continuous or non-continuous discrete
pressure ranges if desired (e.g., 0-10 mmHg, >10-<25 mmHg,
etc.), or made deterministic upon other measured or observed
parameters. Furthermore, the gain adjustment may be coupled to
underlying criteria other than pulse pressure; e.g., diastolic or
systolic pressure, mean pressure, blood flow velocity or kinetic
energy, vessel diameter, body mass index, etc.
[0140] (viii) Correction for Scaling on Observed Pressure
Waveforms--Clinical observations made by the Assignee hereof
indicate that under some circumstances, limited changes in the
pressure displayed to the operator may be induced in part by the
modulation occurring during the second process 200 described above.
One cause of this behavior relates to the interaction of the
pressure waveform restoration and scaling (e.g., BMI) algorithms
with changing mean pressures. To address this behavior, an
alternate scaling implementation may be used. Specifically, the
high-pass filter (HPF) component of the pressure waveform (2.sup.nd
order 0.25 Hz cutoff frequency) is scaled, and combining the HPF
component multiplied by the scaling factor (e.g., BMI scale factor)
with the raw pressure waveform to produce the scaled pressure
waveform.
[0141] It will be recognized that the foregoing features (i)-(viii)
are purely optional in nature, and may be selected by the system
designer at time of apparatus design and manufacture based on the
anticipated applications. Alternatively, production devices may
incorporate the functionality for each enhancement (as well as
others), with the end-user having the ability to select which
features they wish to employ in particular applications (such as
via a GUI configuration menu, API, or similar mechanism).
[0142] As yet another alternative, the production device may be
configured to automatically or adaptively determine if particular
performance enhancements should be utilized. For example, during
start-up or monitoring, the device may be configured to institute
or "turn on" a given feature or group of features, monitor the
effects on the output data in light of prior data collected while
the enhancement feature(s) were inoperative, and then decide which
if any features should be utilized and under what conditions. As a
simple example, consider where the Hampel filter (Item (i) above)
is applied over time to the PBRS pulse pressure at times where
sudden change in values are expected (i.e., start or re-entrance
into the servo control system). The system may be programmed to
disable the Hampel filter during these periods of servo control or
during the period immediately following an ipsilateral
oscillometric cuff deflation.
[0143] Hence, the present invention contemplates the use of innate
"intelligence" within the device hardware and software adapted to
selectively control the application of one or more enhancement
features during device operation. Such innate control can be
readily implemented by those of ordinary skill given the present
disclosure, and accordingly are not described in greater detail
herein.
Interaction of First and Second Processes
[0144] The first process 100 and second process 200 described above
are in the exemplary embodiment adapted to operate in concert with
each other. As discussed, the first process 100 responds to sudden
changes in mechanical coupling between tonometric sensor and the
underlying artery while the second process is designed to, inter
alia, counteract lower frequency drifting in the mechanical
coupling. Generally speaking, the more quickly that the second
process 200 can respond to changes in mechanical coupling, the less
restrictive the constraints that are placed on the performance of
the first process 100. With the presence of the second process 200,
the first process 100 need not be reactive to small mechanical
coupling changes; the second process 200 can be used to provide
recovery without the need to disable current pressure display for
any period of time to perform the limited pressure search.
[0145] Accordingly, the following comprise exemplary values for
various parameters used by the first and/or second processes of the
invention, which are "tuned" so as to provide maximum efficiency
and efficacy of the two processes when they are both present in a
given system. It will be readily apparent that other values (and in
fact parameters) may be substituted depending on the particular
application(s) in which they are applied.
[0146] (i) Tonometric Pressure Velocity and Acceleration Triggers
used with first process 100:
POS_VEL_TRIGGER=45 mmHg:
[0147] (45 mmHg/3 samples)*(160 Sample/1 Second)=2400 mmHg/sec
NEG_VEL_TRIGGER=-20 mmHg:
[0148] (-20 mmHg/3 samples)*(160 Sample/1 Second)=-1067
mmHg/sec
POS_ACCL_TRIGGER=15 mmHg:
[0149] (15 mmHg/3 samples)*(160 Sample/1 Second.sup.2)=800
mmHg/sec.sup.2
NEG_ACCEL_TRIGGER=-12 mmHg:
[0150] (-12 mmHg/3 samples)*(160 Sample/1 Second.sup.2)=640
mmHg/sec.sup.2
MEAN_PRESSURE_CHANGE_TRIGGER=8 mmHg
[0151] (ii) Event trigger comparison of tonometric mean and pulse
pressures of first process 100:
[0152] PULSE_RANGE_PERCENT=10; a 10% decrease in decrease in
tonometric pulse pressure triggers a limited pressure sweep (fourth
state 105).
[0153] MEAN_RANGE_PERCENT=10; a 10% change in tonometric mean
pressure and +/-8 mmHg change in mean pressure triggers a limited
pressure sweep (fourth state 105).
Note that the second process 200 is in the exemplary embodiment
made active when first process 100 is active in either the first
state 102, second state 103, or third state 104 of the first
process 100. The Assignee hereof has also determined that under
certain circumstances, scrubbing or elimination of the beats
immediately surrounding a first process event from use in the
second process 200 may be helpful, since the measurement of mean
pressure and pulse pressure for beats surrounding the process event
are corrupted.
[0154] Additionally, the exemplary embodiment renders the second
process 200 inactive when the first process 100 is active in its
fourth state 105. The applanation motor position variable is set to
the target position upon entry into this fourth state 105, and the
second process 200 is reinitialized upon return of the first
process 100 from its fourth state 105 to its first state 102.
[0155] The second process 200 can also be called from within the
first process 100 using any one of a number of well known software
call routines in response to each new heart beat, and in concert
with the previously described first process states 102-105 and
initializations.
[0156] Additionally, the tonometric pressure velocity and
acceleration triggers (i.e., POS_VEL_TRIGGER, NEG_VEL_TRIGGER,
POS_ACCL_TRIGGER, and NEG_ACCEL_TRIGGER) associated with the first
process 100 can be increased to provide a larger buffer between
normal physiologic changes in pressure and trigger levels, as
follows: POS_VEL_TRIGGER=50 mmHg; NEG_VEL_TRIGGER=-25 mmHg;
POS_ACCL_TRIGGER=20 mmHg; and NEG_ACCEL_TRIGGER=-15 mmHg.
[0157] Furthermore, the checks of beat-to-beat changes in mean
pressure previously described with respect to the first process 100
may be eliminated when the two processes 100, 200 are used
concurrently. These mean pressure checks are designed primarily for
use as protection against slow changes in mechanical coupling (via
periodic sweep-calibration) when the first process 100 is used in a
stand-alone configuration (i.e., without the presence of the second
process 200). The presence of the second process 200 obviates the
need for this component, and thereby also possible false first
process events caused by arrhythmias (i.e. pulsus alternans) and
other physiologic events.
[0158] Concurrent use of the first and second processes 100, 200
also advantageously allows more frequent use of "tuning"
comparisons between pre- and post-event values of tonometrically
measured mean and pulse pressures associated with the first process
100. This feature reduces the frequency of periods where disabling
or freezing of the display of current pressure is required to
perform the limited applanation sweeps, by simply allowing the
second process 200 to recover from these smaller changes in
mechanical coupling. Exemplary values are as follows:
PULSE_RANGE_PERCENT=20; and MEAN_RANGE_PERCENT=20.
[0159] It will also be recognized that while the foregoing
exemplary embodiment of the second process 200 is interactive with
the first process 100, the second process may operate independently
of the first. For example, the second process may be used to adjust
and/or maintain the desired applanation level (or position in the
case of lateral and proximal cases) irrespective of the methodology
used to initially determine the optimal applanation/position. In
effect, the second process 200 of the invention used without the
first process 100 will hunt and eventually converge on the optimal
position itself. This approach, however, has been found by the
Assignee hereof to be less temporally efficient than the approach
previously described (i.e., determining optimal using the initial
sweep process), but may none-the-less be desirable in certain
circumstances where hardware/software simplicity can be traded for
longer acquisition and settling times. Hence, the present invention
should in no way be considered to be restricted to embodiments
wherein both first and second processes 100, 200 are employed.
Third Process
[0160] Referring now to FIGS. 4 and 4a, the third process of the
exemplary embodiment of the present invention is described.
[0161] During patient monitoring mode, the second process 200
previously described is capable of controlling the applanation of
the sensor/pad against the subject artery and overlying tissue,
thereby compensating for slow changes (drifts) in the mechanical
coupling between the sensor/pad and, the underlying tissue.
Furthermore, the second process 200 can be most effective over
applanation ranges where the pulse pressure is strong (higher
signal-to-noise ratio), which exist near the optimal applanation
position. However, for large shifts in the mechanical coupling
between sensor/pad and the tissue (i.e. flexing of the wrist), the
second process 200 may require several minutes to applanate to the
proper level to maximize tonometric pulse pressure. Thus, an
opportunity exists to improve the performance of the system as a
whole by detecting shifts in mechanical coupling that would incur
an extended recovery period, and implement a more direct recovery
process. The exemplary embodiment of the third process 400 shown in
FIG. 4 therefore takes a recovery "shortcut" as it were in those
limited circumstances where recovery via the second process 200
would require an undesirably long time.
[0162] Thus, an important goal of the third process 400 of the
present invention is to detect rapid shifts in mechanical coupling
that induce sizeable error in pulse pressure and/or diastolic
pressure, and implement an optimal recovery approach.
[0163] In a first exemplary embodiment, the third process 400 is
operated in conjunction with the first process 100 previously
described. Specifically, the third process 400 operates during the
first state 102 of the first process 100 (see FIG. 1), and triggers
the fourth state 105 when an appreciable shift in the mechanical
coupling is detected. Advantageously, the approach to detecting
rapid shifts in coupling described herein does not require any
significant mechanical or electrical changes to the system. The
approach is based on identifying changes in tonometric pressure
over a comparatively short period of time that jointly are of the
nature and degree to not likely occur physiologically. Such changes
also indicate that the second process 200 might require significant
time to properly recover. For example, when a patient's diastolic
pressure increases, pulse pressure typically remains constant (or
increases). Thus, detecting changes in pressure where diastolic
pressure increases and pulse pressure decreases significantly over
a short time period can be used to detect rapid shifts in
mechanical coupling. Furthermore, episodes where the pulse pressure
either remains constant or increases are not problematic regardless
of the change in diastolic pressure. Since the pulse pressure
remains very strong, the probability that the second process 200
can adjust the applanation level (if necessary) within a reasonable
period of time remains high.
[0164] In the exemplary embodiment of FIG. 4, the process for
detecting rapid shifts in mechanical coupling (third process 400)
employs one or more metrics for detecting joint shifts in
parameters. In the illustrated embodiment, diastolic pressure and
pulse pressure are used as the referenced parameters, although it
will be appreciated that other parameters (physiologic or
otherwise) may be substituted consistent with the invention.
[0165] On exemplary scheme for detecting rapid shifts in mechanical
coupling is depicted in FIG. 4a. The process 400 investigates
changes in the current block averaged pulse and diastolic pressures
from "qualified" block averaged pulse and diastolic pressures from
moving windows (e.g., both 12 beats and 24 beats in the past in the
illustrated embodiment). If the pulse pressure decreases and
diastolic pressure deviates from the previous diastolic pressures
(12 or 24 beats past), then fourth state 105 of the first process
100 is triggered.
[0166] Note that FIG. 4a depicts a percentage change in pulse
pressure (the selected parameter). Calculations may also be
performed based upon change in absolute blood pressure (mmHg),
where for example 40 mmHg is equivalent to 100% and should trigger
the fourth state 105 if either the percent change or absolute
change in pulse pressure in conjunction with the change in
diastolic pressure exceeds the prescribed thresholds. It will be
recognized, however, that other triggering criteria and schemes may
be utilized if desired. Such alternate criteria and schemes may
even be made specific to individual patient's or groups of
patients, based for example on historical or anecdotal data or
other indicia.
[0167] The operation of the exemplary embodiment of the rapid shift
detection algorithm according to the present invention is now
described in detail. As shown in FIG. 4, the algorithm of this
embodiment is based upon the waveform-restored but unsealed beat
pressure diastolic and pulse pressure algorithms. The pulse
pressure and diastolic beat pressure data are in this embodiment
processed through similar (yet not identical) parallel
sub-processes to calculate current and past pressure data for use
in the aforementioned threshold determinations of the third process
400. A primary difference between these two sub-processes is that
in the first sub-process 440, drops in pulse pressure are of most
concern, whereas in the second sub-process 442 changes in diastolic
pressure are considered. Exemplary embodiments of these
sub-processes 440, 442 are now described in greater detail,
although it will be appreciated that other parameters (e.g.,
besides pulse pressure and diastolic pressure) may be used as the
basis for rapid shift detection, and/or other specific
configurations of these sub-processes may be substituted.
[0168] Furthermore, while the exemplary algorithms and
functionality are described in terms of first-in-first-out (FIFO)
buffers, other buffering arrangements may be utilized depending on
the desired functionality for a given application. For example,
under certain circumstances, it may be desirable to replace
portions of data in a LIFO (last-in-first-out) manner.
Alternatively, "intelligent" (e.g., algorithmically driven) queuing
and de-queuing of data may be incorporated. All such alternate
approaches are readily implemented by those of ordinary skill in
the data processing arts, and accordingly not described further
herein.
i) Pulse Pressure Sub-process (Pre-filtering and Averaging)--The
following pre-filtering and averaging features are employed in the
exemplary embodiment of the first sub-process 440 used in analyzing
pulse pressure:
[0169] a. Hampel Filter--A Hampel filter (length 7) of the type
previously described is used to remove abhorrent pulse pressure
values from subsequent calculations, as shown in Eqn. 6 below. Note
that a by-product of the exemplary Hampel Filter is the calculation
of variance among the pulse pressures over the last 7 beats. This
information is used subsequently to determine if the current pulse
pressure should be included in the "acceptable" pulse pressure
circular buffer.
PP.sub.h(k)=Hampel Filter{PP(k),PP(k-1),PP(k-2), . . . PP(k-6)}
(Eqn. 6)
where k represents the current beat number, PP.sub.h(k) is the
Hampel filtered pulse pressure, and PP(k) is the current unfiltered
pulse pressure.
[0170] Furthermore, the Hampel filter of the present embodiment
also calculates the variance of the data. The variance is a measure
of distribution around the mean. It is computed as the average
squared deviation of each number from its mean, as illustrated in
Eqn. 7:
PP.sub.var(k)=((PP(k)-u).sup.2+(PP(k-1)-u).sup.2+ . . .
+((PP(k-6)-u).sup.2)/7, (Eqn. 7)
where k represents the current beat number, PP.sub.var(k) is the
variance of the pulse pressure over the last 7 beats, and u is the
average unfiltered pulse pressure for the last 7 beats.
[0171] b. Pulse Buffer--A pulse buffer (length=8 in the exemplary
embodiment) is a circular buffer containing the Hampel-filtered
pulse pressure values. With each beat, the oldest beat is replaced
with the most recent Hampel-filtered data.
[0172] c. Block Averager--A block averaging routine calculates the
mean for the Hampel-filtered pulse pressure data stored in the
aforementioned pulse buffer, as illustrated by Eqn. 8 below.
PP.sub.hP(k)=[PP.sub.h(k)+PP.sub.h(k-1)+ . . . +PP.sub.h(k-7)]/8
(Eqn. 8)
where PP.sub.h(k) is the block averaged Hampel filtered pulse
pressure data. ii) Pulse Pressure Sub-process (Determining Current
Pulse Pressure)--The following features are utilized in the present
embodiment of the pulse pressure sub-process 440 for determining
the current pulse pressure:
[0173] a. Maximum--This feature of the algorithm determines the
maximum difference between the current pulse pressure and the block
averaged Hampel-filtered pulse pressure data, as shown below in
Eqn. 9. This maximum is used in subsequent analysis as the Current
Pulse Pressure variable.
If (PP.sub.h(k)>PP.sub.h(k))PP.sub.max[k]=PP.sub.h(k)
Else PP.sub.max[k]=PP.sub.h(k) (Eqn. 9)
where PP.sub.max[k] is used in subsequent comparisons to detect
shifts in mechanical coupling. Note that the trigger for the fourth
state 105 of the first process 100 is, in the illustrated
embodiment, dependant on a significant decrease in pulse pressure.
Thus, under conditions where the average pulse pressure is small,
the system should not trigger if the pulse pressure from the last
beat is large. iii) Pulse Pressure Sub-process (Determining Past
Qualified Pulse Pressures)--The following features are used in the
present embodiment for determining past (e.g., 12 & 24 beat)
qualified pulse pressure values.
[0174] a. Variance Buffer--In the exemplary embodiment, a variance
buffer (e.g., length=120) comprises a circular buffer containing
the variance in the pulse pressure for the last "x" (e.g. 7) beats,
as calculated within Hampel filter operation. With each beat, the
data of the oldest beat is replaced with the most recent
variance.
[0175] b. Block Averager and Standard Deviation--These features
calculate the mean pressure for the variance in the Hampel filtered
pulse pressure data stored in the buffer, as illustrated in Eqns.
10 and 11 below, respectively. With the buffer length set at a
comparatively large value (e.g., 120), these calculations provide a
statistical benchmark for typical average and range of variance
observed for the blocks of pulse pressure data. Output of these
algorithms is both block average and standard deviation (or
alternatively an equivalent measure that will enable detection of
pulse pressures that are not within normal limits of the average
mean pressure for the last number of beats n, where n=120 in the
present embodiment).
PP.sub.var(k)=[PP.sub.var(k)+PP.sub.var(k-1)+ . . .
+PP.sub.var(k-119)]/120 (Eqn. 10)
where PP.sub.var(k) is the block averaged Hampel filtered pulse
pressure data.
SD.sub.PPvar(k)=(((PP.sub.var)-PP.sub.var(k)).sup.2+(PP.sub.var(k-1)-PP.-
sub.var(k)).sup.2+ . . .
+((PP.sub.var(k-119)-PP.sub.var(k)).sup.2)/120).sup.1/2 (Eqn.
11)
where SD.sub.PPvar(k) is the standard deviation in the Pulse
Pressure Variance data.
[0176] c. Stationarity Limit--The stationary limit feature
calculates an upper limit of the pulse pressure variance or
standard deviation that permits the current block (e.g., 7 beats)
average pulse pressure to be included in the history of "qualified"
pulse pressure values for future comparisons. One exemplary
approach comprises comparing the variance of the current pulse
pressure block with the value (average pulse pressure+1 standard
deviation of the variances observed over the last 120 beats), which
constitutes the upper limit of acceptable pulse pressures variance,
as shown in Eqn. 12 below:
StationarityLimit.sub.PP(k)=PP.sub.var(k)+SD.sub.PPvar(k) (Eqn.
12)
Note, however, that other methods of determining an upper limit for
the observed variance may readily be substituted or used in
conjunction with the foregoing. For example, analysis of the
current variance (e.g., current variance <40.sup.th largest out
of 120 beats) may be utilized. It will be recognized that the
aforementioned median filter can be easily modified to recursively
determine this value. Other configurations may also be employed
consistent with the invention, such configurations being readily
determined by those of ordinary skill.
[0177] d. Identification of Pulse Pressure value to be included in
Pulse Pressure History Buffer--The exemplary embodiment of the
invention further includes functionality which determines whether
the current average pulse pressure value or most recent acceptable
pulse pressure value should be added to the circular buffer
containing a history of average pulse pressures, as shown in Eqn.
13 below. This is accomplished using the stationarity limit
calculated previously.
If(PP.sub.var(k)>PP.sub.var(k)+SD.sub.PPvar(k))PPhistory(k)=PPhistory-
(k-1)
Else PPhistory(k)=PP.sub.h(k) (Eqn. 13)
where PPhistory(k) is the history of "acceptable" Pulse
Pressures
[0178] e. Update Pulse Pressure History Buffer--In the exemplary
embodiment, a pulse pressure history FIFO buffer (e.g., length=24)
is employed. The history buffer comprises a circular buffer
containing the history of past "acceptable" average pulse pressure
values. With each beat, the data associated with oldest beat is
replaced with that of the most recent. Values which are a
prescribed number of beats in the past (e.g., 12 and 24 beats) from
this array are used in subsequent calculations to determine the
change in pulse pressure over this period.
iv) Diastolic Pressure Sub-process (Pre-filtering and
Averaging)--The following pre-filtering and averaging features are
employed in the exemplary embodiment of the first sub-process 440
used in analyzing pulse pressure:
[0179] a. Hampel Filter--The exemplary embodiment of the diastolic
sub-process 442 uses a Hampel filter (e.g., length 7) to remove
abhorrent diastolic pressure values from subsequent calculations,
similar to the pulse pressure sub-process 440 (see Eqn. 14 below).
A by-product of the Hampel Filter is the calculation of the
variance of diastolic pressure over the previous number (e.g., 7)
of beats. This information is used subsequently to determine if the
current diastolic pressure should be included in the "acceptable"
diastolic pressure circular buffer.
D.sub.h(k)=Hampel Filter{D(k),D(k-1),D(k-2), . . . ,D(k-6)} (Eqn.
14)
where k represents the current beat number, D.sub.h(k) is the
Hampel filtered diastolic pressure, and D(k) is the current
unfiltered diastolic pressure. Furthermore, the Hampel filter also
calculates the variance of the data, as shown in Eqn. 15:
D.sub.var(k)=((D(k)-u).sup.2+(D(k-1)-u).sup.2+ . . .
+((D(k-6)-u).sup.2)/7, (Eqn. 15)
where k represents the current beat number, D.sub.var(k) is the
variance of the diastolic pressure over the last 7 beats, and u is
the average diastolic pressure over the last 7 beats.
[0180] b. Pulse Buffer--A FIFO pulse buffer of a determinate length
(e.g., length=8) is used in the present embodiment; this buffer
comprises a circular buffer containing the Hampel filtered
diastolic pressure values. With each successive beat, the data for
the oldest beat is replaced with the most recent Hampel filtered
data.
[0181] c. Block Averager--A block averaging routine is used to
calculate the mean for the Hampel filtered diastolic pressure data
stored in the buffer, as shown in Eqn. 16 below:
D.sub.h(k)=[D.sub.h(k)+D.sub.h(k-1)+ . . . +D.sub.h(k-7)]/8 (Eqn.
16)
where D.sub.h(k) is the block averaged Hampel filtered Diastolic
pressure data. v) Diastolic Pressure Sub-process (Current Value
Determination)--The diastolic sub-process 442 determines the
current value of the diastolic pressure using a straightforward
methodology. Specifically, the current diastolic pressure is simply
the most recent block averaged Hamper-filtered diastolic pressure
value. Note that the trigger for the fourth state 105 of the first
process 100 is dependant on a significant change in diastolic
pressure. vi) Diastolic Pressure Sub-process (Determining Past
Qualified DiastolicPressures)--The sub-process 442 also contains
mechanisms for determining past qualified diastolic pressures
(e.g., those of 12 and 24 beats past), as follows:
[0182] a. Variance Buffer--A FIFO variance buffer of determinate
length (e.g., length=120) comprising a circular buffer containing
the variance in the diastolic pressure for the last 7 beats (as
calculated within Hampel filter operation) is used in the present
embodiment of the diastolic sub-process 442. With each beat, the
variance of the oldest beat is replaced with the most recent
variance.
[0183] b. Block Averager and Standard Deviation--These functions
calculate the mean for the variance in the Hampel-filtered
diastolic pressure data stored in the variance buffer. With the
buffer length set at a comparatively large value, these
calculations provide a statistical benchmark for the typical
average and the range of variance observed for the blocks of pulse
pressure. Output from, these processes is both block average and
standard deviation (or an equivalent measure) that will enable
detection of pulse pressures that are not within normal limits of
the average mean pressure for the last "n" beats), as shown in
Eqns. 17 and 18 below (for n=120):
D.sub.var(k)=[D.sub.var(k)+D.sub.var(k-1)+ . . .
+D.sub.var(k-119)]/120 (Eqn. 17)
SD.sub.Dvar(k)=(((D.sub.var(k)-D.sub.var(k)).sup.2+(D.sub.var(k-1)-D.sub-
.var(k).sup.2+ . . .
+((D.sub.var(k-119)-D.sub.var(k)).sup.2)/119).sup.1/2 (Eqn. 18)
where D.sub.var(k) is the block averaged Hampel filtered diastolic
pressure data.
[0184] c. Stationarity Limit--The stationary limit function of the
diastolic sub-process 442 calculates an upper limit of the
diastolic pressure variance (or standard deviation) that permits
the average diastolic pressure of the current block of data (e.g.,
7 beats-worth) to be included in the history of diastolic pressure
values for use in future comparisons, as shown in Eqn. 19:
StationarityLimit.sub.D(k)=D.sub.var(k)+/-SD.sub.var(k) (Eqn.
19)
[0185] d. Identify Diastolic Pressure value to be included in
Diastolic Pressure History Buffer--Using the stationarity limit
previously calculated, this feature of the diastolic sub-process
442 determines whether the current average diastolic pressure value
or most recent "acceptable" diastolic pressure value should be
added to the circular buffer containing a history of average pulse
pressures. If the new Diastolic Pressure is within limits of the
stationarity limit described above, then it is included in the
diastolic pressure history, else the most recent diastolic pressure
history value is duplicated.
[0186] e. Update Diastolic Pressure History Buffer--In the
exemplary embodiment, the diastolic subprocess 442 includes a
circular FIFO buffer of determinate length (e.g., length=24)
containing the history of past "acceptable" average diastolic
pressures. With each beat, the data associated with the oldest beat
is replaced with that of the most recent. Values derived from one
or more past beats (e.g., 12 and 24 beats in the past from the
current array) are used in subsequent calculations to determine the
change in pulse pressure over the period of interest, as shown in
Eqn. 20 below:
If(D.sub.var(k)>D.sub.var(k)+SD.sub.Dvar(k))Dhistory(k)=Dhistory(k-1)
Else Dhistory(k)=.sup.-D.sub.h(k) (Eqn. 20)
vii) Analysis for Detection of Shifts in Mechanical Coupling
[0187] a. Threshold detection--In order to detect rapid shifts in
mechanical coupling, the third process 400 of the invention
performs threshold detection over the prior first number (e.g., 12)
of beats in the exemplary embodiment as follows:
[0188] 1) Pulse Pressure Difference--The third process 400
calculates the difference between the current pulse pressure
(Current Pulse Pressure variable referenced with respect to Item
ii.a. of the pulse pressure sub-process 440 above) and the first
number (e.g., 12) of qualified pulse pressure beats in the past
(stored in the circular history buffer by the pulse pressure
sub-process 440 as previously described in iii.d, above). This
calculation is shown in Eqn. 21 below:
PulsePressureDifference12=PP.sub.max[k]-PPhistory(12) (Eqn. 21)
[0189] 2) Diastolic Difference--The third process 400 calculates
the difference between the current diastolic pressure (output from
the diastolic sub-process block averager as described above) and
the qualified diastolic pressure for, e.g., 12 beats in the past
(stored in the circular history buffer by the diastolic sub-process
as described in Item vi.d. above), as shown in Eqn. 22:
DiastolicPressureDifference12=D.sub.h(k)-Dhistory(12) Eqn. 22
[0190] 3) Detector--In accordance with the temporal threshold shown
in FIG. 4a, if the pulse pressure difference (Item vii.a.1) above)
is sufficiently negative, and the diastolic pressure difference
(Item vii.a.2) above) is sufficiently different from zero, then a
"Trigger 1" value 448 associated with the fourth state 105 of the
first process 100 is set to TRUE.
[0191] b. Additionally, the third process 400 of the invention
performs threshold detection over the prior second number (e.g.,
24) of beats in the exemplary embodiment as follows:
[0192] 1) Pulse Pressure Difference--The third process 400
calculates the difference between the current pulse pressure
(Current Pulse Pressure variable referenced above) and the
qualified second number (e.g., 24) of pulse pressure beats in the
past (stored in the circular history buffer by the pulse pressure
sub-process 440 as previously described), as shown in Eqn 23
below:
PulsePressureDifference24=PP.sub.max[k]-PPhistory(24) (Eqn. 23)
[0193] 2) Diastolic Difference: Calculates the difference between
the current diastolic pressure (as previously described) and the
qualified diastolic pressure 24 beats in the past, as illustrated
by Eqn. 24 below:
DiastolicPressureDifference24=D.sub.h(k)-Dhistory(24) (Eqn. 24)
[0194] 3) Detector--In accordance with the temporal (e.g., 24
second) threshold shown in FIG. 4a, i the pulse pressure difference
(Item vii.b.1) above) is sufficiently negative, and the diastolic
pressure difference (Item viii.b.2) above) is sufficiently
different from zero, then the "Trigger 2" value 450 for the fourth
state 105 of the second process is set TRUE.
[0195] c. Beat Evaluation over Most Recent Period--Additionally,
the third process 400 of the present invention is optionally
configured to evaluate beats detected within a prior interval
(e.g., prior five seconds), as follows:
[0196] 1) No beat detected during interval--If a beat of acceptable
quality has not been detected over the interval and "noise" on the
pressure signal has not caused the lack of a good beat, then the
"Trigger 3" value 452 for the fourth state 105 is set TRUE.
[0197] d. Fourth State Request Check--The third process 400
performs a logic check based on the presence of a Trigger 1,
Trigger 2, or Trigger 3 value 448, 450, 452 set to TRUE. If any of
the aforementioned Triggers are set TRUE, and the first process 100
is in the first state 102, then the first process 100 should enter
the fourth state 105 (i.e., accelerated recovery). All fourth state
Triggers 448, 450, 452 are then reset to FALSE.
[0198] Note that if the first process 100 is in either the second
state 103 or third state 104, the proper new state option is
subsequently determined. Alternatively, if the first process is
currently in the fourth state 105, then the aforementioned request
to enter the fourth state 105 is ignored.
[0199] It will be recognized that while the foregoing embodiment of
the third process methodology addresses the problem of identifying
rapid shifts in mechanical coupling based on a substantially
probabilistic approach (which is tailored using an understanding of
common changes in a patient's arterial pressure during the course
of various physiologic events), this approach does not measure
directly (or even indirectly) changes in the mechanical coupling
between the tonometric pressure sensor and its associated contact
pad and the underlying tissue. Accordingly, the exemplary
implementation of the third process 400 is not immune to error. The
second process 200 of the present invention, however,
advantageously insulates the system against failure of the third
process 400 to detect rapid changes in coupling, since the second
process will converge on the optimal level of applanation
irrespective of the third process (albeit over a period of several
minutes) as previously described. Furthermore, false triggering by
the third process 400 (i.e., indication that a rapid coupling
change has been experienced when in fact it has not) will induce an
applanation sweep, and possibly a lateral/proximal position sweep,
which enables the system to recover as well. Hence, any errors
associated with the probabilistic implementation of the third
process 400 do not adversely affect the accuracy of the system, but
rather merely the speed with which it converges on the proper
applanation level and/or lateral or proximal position. The
exemplary embodiment of the present invention will therefore not
generate "bad" data, but rather simply not update data until
optimal applanation/position is achieved.
[0200] It will also be noted that examination of a patient's data
history as described above with respect to the pulse pressure and
diastolic sub-processes 440, 442 may encompass examinations of
selected segments of the data history for that patient as well as
the examination and comparison of segments of data for that patient
against comparable data for other patients. Furthermore, the
analysis described above may be applied in both historical and/or
predictive fashion; for instance, one or more historical data
segments may be analyzed via an algorithm which predicts future
ranges or values for one or more parameters. If the subsequent
measurement of the parameter(s) is not within the prediction,
instigation of the applanation/position sweep(s) can then conducted
and the optimal position reacquired. For example, wherein an
analysis of the historical data for a patient relating to diastolic
pressure indicates that a future measurement within a given time
epoch .tau. outside the range of 50-80 mmHg would correspond to an
unphysical situation or event, any diastolic pressure reading
outside that range occurring within .tau. could trigger
reacquisition.
[0201] It will further be recognized that numerous combinations of
analyzed parameters (e.g., systolic, diastolic, pulse, or mean
pressure, and combinations or derivations thereof), time periods
(historical, historical/predictive, or purely predictive), and
acceptance/rejection criteria (e.g., parameter range at a discrete
epoch, continuity or variation over time, statistics, etc.) may be
utilized either alone or in combination consistent with the present
invention to effectuate the goal of maintaining optimal position of
the sensor under all operating environments and conditions. All
such methods and approaches are readily implemented within the
framework of the present invention by those of ordinary skill in
the programming and mathematical arts, and accordingly are not
described further herein.
System Apparatus for Hemodynamic Assessment
[0202] Referring now to FIG. 5, an apparatus for measuring
hemodynamic properties within the blood vessel of a living subject
is now described. In the illustrated embodiment, the apparatus is
adapted for the measurement of blood pressure within the radial
artery of a human being, although it will be recognized that other
hemodynamic parameters, monitoring sites, and even types of living
organism may be utilized in conjunction with the invention in its
broadest sense.
[0203] The exemplary apparatus 500 of FIG. 5 fundamentally
comprises an applanation assembly (including one or more pressure
transducers 522) for measuring blood pressure from the radial
artery tonometrically; a digital processor 508 operatively
connected to the pressure transducer(s) 522 (and a number of
intermediary components) for (i) analyzing the signals generated by
the transducer(s); (ii) generating control signals for the stepper
motor 506 (via a microcontroller 511a operatively coupled to the
stepper motor control circuits); and (iii) storing measured and
analyzed data. The motor controllers 511, processor 508, auxiliary
board 523, and other components may be housed either locally to the
applanator 502, or alternatively in a separate stand-alone housing
configuration if desired. The pressure transducer 522 and its
associated storage device 552 are optionally made removable from
the applanator 502.
[0204] The pressure transducer 522 is, in the present embodiment, a
strain beam transducer element which generates an electrical signal
in functional relationship (e.g., proportional) to the pressure
applied to its sensing surface 521, although other technologies may
be used. The analog pressure signals generated by the pressure
transducer 522 are converted into a digital form (using, e.g., an
ADC 509) after being optionally low-pass filtered 513 and sent to
the signal processor 508 for analysis. Depending on the type of
analysis employed, the signal processor 508 utilizes its program
either embedded or stored in an external storage device to analyze
the pressure signals and other related data (e.g., stepper motor
position as determined by the position encoder 577, scaling data
contained in the transducer's EEPROM 552 via I2C1 signal, need for
reacquisition per FIG. 4, etc.).
[0205] As shown in FIG. 5, the apparatus 500 is also optionally
equipped with a second stepper motor 545 and associated controller
511b, the second motor 545 being adapted to move the applanator
assembly 502 laterally across the blood vessel (e.g., radial
artery) of the subject as described above. A third stepper motor
(not shown) and associated controls may also be implemented if
desired to control the proximal positioning of the applanation
element 502. Operation of the lateral positioning motor 545 and its
controller 511b is substantially analogous to that of the
applanation motor 506, consistent with the methodologies previously
described herein.
[0206] As previously discussed, continuous accurate non-invasive
measurements of hemodynamic parameters (e.g., blood pressure) are
highly desirable. To this end, the apparatus 500 is designed to (i)
identify the proper level of applanation of the subject blood
vessel and associated tissue; (ii) continuously "servo" on this
condition to maintain the blood vessel/tissue properly biased for
the best possible tonometric measurement; optionally (iii) scale
the tonometric measurement as needed to provide an accurate
representation of intravascular pressure to the user/operator; and
(iv) identify conditions where transient or "unphysical" events
have occurred, and correct the system accordingly to regain the
optimal applanation level and lateral/proximal positions.
[0207] During an applantion "sweep", the controller 511a controls
the applanation motor 506 to applanate the artery (and interposed
tissue) according to a predetermined profile. Similarly, the
extension and retraction of the applanation element 502 during the
later states of the algorithm (i.e., when the applanation motor 506
is disposed at the optimal applanation position, and subsequent
servoing around this point) are controlled using the controller
511a and processor 508. Such "servo" control schemes may also be
employed with respect to the lateral and proximal motor drive
assemblies if desired, or alternatively a more static approach
(i.e., position to an optimal initial position, and then reposition
only upon the occurrence of an event causing significant
misalignment). In this regard, it will be recognized that the
control schemes for the applanation motor and the lateral/proximal
positioning motor(s) may be coupled to any degree desired
consistent with the invention.
[0208] The apparatus 500 is also configured to apply the
methodologies of the first, second, and third processes 100, 200,
400 previously discussed with respect to FIGS. 1-4, as well as the
initial sweep and scaling methodologies described in the
aforementioned co-pending patent application Ser. No. 10/072,508
previously incorporated by reference herein. Details of the
implementation of these latter methodologies are provided in the
co-pending application, and accordingly are not described further
herein.
[0209] The physical apparatus 500 of FIG. 5 comprises, in the
illustrated embodiment, a substantially self-contained unit having,
inter alia, a combined pressure transducer 522 and applanation
device 500, motor controllers 511, RISC digital processor 508 with
associated synchronous DRAM (SDRAM) memory 517 and instruction set
(including scaling lookup tables), display LEDs 519, front panel
input device 521, and power supply 523. In this embodiment, the
controllers 511 is used to control the operation of the combined
pressure transducer/applanation device, with the control and
scaling algorithms are implemented on a continuing basis, based on
initial operator/user inputs.
[0210] For example, in one embodiment, the user input interface
comprises a plurality (e.g., two) buttons disposed on the face of
the apparatus housing (not shown) and coupled to the LCD display
579. The processor programming and LCD driver are configured to
display interactive prompts via the display 579 to the user upon
depression of each of the two buttons.
[0211] Furthermore, a patient monitor (PM) interface circuit 591
shown in FIG. 5 may be used to interface the apparatus 500 to an
external or third-party patient monitoring system. Exemplary
configurations for such interfaces 591 are described in detail in
co-pending U.S. patent application Ser. No. 10/060,646 entitled
"Apparatus and Method for Interfacing Time-Variant Signals" filed
Jan. 30, 2002, and assigned to the Assignee hereof, which is
incorporated by reference herein in its entirety, although other
approaches and circuits may be used. The referenced interface
circuit has the distinct advantage of automatically interfacing
with literally any type of patient monitor system regardless of its
configuration. In this fashion, the apparatus 500 of the present
invention coupled to the aforementioned interface circuit allows
clinicians and other health care professionals to plug the
apparatus into in situ monitoring equipment already on hand at
their facility, thereby obviating the need (and cost) associated
with a dedicated monitoring system just for blood pressure
measurement.
[0212] Additionally, an EEPROM 552 is physically coupled to the
pressure transducer 522 as shown in FIG. 5 so as to form a unitary
unit which is removable from the host apparatus 500. The details of
the construction and operation of exemplary embodiments of such
coupled assemblies are described in detail in co-pending U.S.
application Ser. No. 09/652,626, entitled "Smart Physiologic
Parameter Sensor and Method", filed Aug. 31, 2000, assigned to the
Assignee hereof, and incorporated by reference herein in its
entirety, although other configurations clearly may be substituted.
By using such a coupled and removable arrangement, both the
transducer 522 and EEPROM 552 may be readily removed and replaced
within the system 500 by the operator.
[0213] It is also noted that the apparatus 500 described herein may
be constructed in a variety of different configurations, and using
a variety of different components other than those specifically
described herein. For example, it will be recognized that while
many of the foregoing components such as the processor 508, ADC
509, controller 511, and memory are described effectively as
discrete integrated circuit components, these components and their
functionality may be combined into one or more devices of higher
integration level (e.g., so-called "system-on-chip" (SoC) devices).
The construction and operation of such different apparatus
configurations (given the disclosure provided herein) are readily
within the possession of those of ordinary skill in the medical
instrumentation and electronics field, and accordingly not
described further herein.
[0214] The computer program(s) for implementing the aforementioned
first, second, and third processes (as well as scaling) are also
included in the apparatus 500. In one exemplary embodiment, the
computer program comprises an object ("machine") code
representation of a C.sup.++ source code listing implementing the
methodology of FIGS. 1-4, either individually or in combination
thereof. While C language is used for the present embodiment, it
will be appreciated that other programming languages may be used,
including for example VisualBasic.TM., Fortran, and C.sup.+. The
object code representation of the source code listing is, compiled
and may be disposed on a media storage device of the type well
known in the computer arts. Such media storage devices can include,
without limitation, optical discs, CD ROMs, magnetic floppy disks
or "hard" drives, tape drives, or even magnetic bubble memory.
These programs may also be embedded within the program memory of an
embedded device if desired. The computer program may further
comprise a graphical user interface (GUI) of the type well known in
the programming arts, which is operatively coupled to the display
and input device of the host computer or apparatus on which the
program is run.
[0215] In terms of general structure, the program is comprised of a
series of subroutines or algorithms for implementing the
applanation and scaling methodologies described herein based on
measured parametric data provided to the host apparatus 500.
Specifically, the computer program comprises an assembly
language/micro-coded instruction set disposed within the embedded
storage device, i.e. program memory, of the digital processor or
microprocessor associated with the hemodynamic measurement
apparatus 500. This latter embodiment provides the advantage of
compactness in that it obviates the need for a stand-alone PC or
similar hardware to implement the program's functionality. Such
compactness is highly desirable in the clinical and home settings,
where space (and ease of operation) are at a premium.
Method of Providing Treatment
[0216] Referring now to FIG. 6, a method of providing treatment to
a subject using the aforementioned methods is disclosed. As
illustrated in FIG. 6, the first step 602 of the method 600
comprises selecting the blood vessel and location to be monitored.
For most human subjects, this will comprise the radial artery (as
monitored on the inner portion of the wrist), although other
locations may be used in cases where the radial artery is
compromised or otherwise not available.
[0217] Next, in step 604, the applanation mechanism 502 is placed
in the proper location with respect to the subject's blood vessel.
Such placement may be accomplished manually, i.e., by the caregiver
or subject by visually aligning the transducer and device over the
interior portion of the wrist, by the pressure/electronic/acoustic
methods of positioning previously referenced, or by other means.
Next, the first applanation element 502 is operated per step 606 so
as to applanate the tissue surrounding the blood vessel to a
desired level so as to identify an optimal position where the
effects of transfer loss and other errors associated with the
tonometric measurement are mitigated. Co-pending U.S. patent
application Ser. No. 10/072,508 previously incorporated herein
illustrates one exemplary method of finding this optimum
applanation level.
[0218] Once the optimal level of applanation for the applanator
element 502 is set, the pressure waveform is measured per step 608,
and the relevant data processed and stored as required (step 610).
Such processing may include, for example, calculation of the pulse
pressure (systolic minus diastolic), calculation of mean pressures
or mean values over finite time intervals, and optional scaling of
the measured pressure waveform(s). One or more resulting outputs
(e.g., systolic and diastolic pressures, pulse pressure, mean
pressure, etc.) are then generated in step 612 based on the
analyses performed in step 610. The relevant portions of the first,
second, and third processes 100, 200, 400 of the present invention
are then implemented as required to maintain the subject blood
vessel and overlying tissue in a continuing state of optimal or
near-optimal compression (as well as maintaining optimal
lateral/proximal position if desired) per step 614 so as to provide
continuous monitoring and evaluation of the subject's blood
pressure. This is to be distinguished from the prior art techniques
and apparatus, wherein only periodic representations and
measurement of intra-arterial pressure are provided.
[0219] Lastly, in step 616, the "corrected" continuous measurement
of the hemodynamic parameter (e.g., systolic and/or diastolic blood
pressure) is used as the basis for providing treatment to the
subject. For example, the corrected systolic and diastolic blood
pressure values are continuously generated and displayed or
otherwise provided to the health care provider in real time, such
as during surgery. Alternatively, such measurements may be
collected over an extended period of time and analyzed for long
term trends in the condition or response of the circulatory system
of the subject. Pharmacological agents or other courses of
treatment may be prescribed based on the resulting blood pressure
measurements, as is well known in the medical arts. Similarly, in
that the present invention provides for continuous blood pressure
measurement, the effects of such pharmacological agents on the
subject's physiology can be monitored in real time.
[0220] It is noted that many variations of the methods described
above may be utilized consistent with the present invention.
Specifically, certain steps are optional and may be performed or
deleted as desired. Similarly, other steps (such as additional data
sampling, processing, filtration, calibration, or mathematical
analysis for example) may be added to the foregoing embodiments.
Additionally, the order of performance of certain steps may be
permuted, or performed in parallel (or series) if desired. Hence,
the foregoing embodiments are merely illustrative of the broader
methods of the invention disclosed herein.
[0221] While the above detailed description has shown, described,
and pointed out novel features of the invention as applied to
various embodiments, it will be understood that various omissions,
substitutions, and changes in the form and details of the device or
process illustrated may be made by those skilled in the art without
departing from the spirit of the invention. The foregoing
description is of the best mode presently contemplated of carrying
out the invention. This description is in no way meant to be
limiting, but rather should be taken as illustrative of the general
principles of the invention. The scope of the invention should be
determined with reference to the claims.
* * * * *