U.S. patent application number 11/334287 was filed with the patent office on 2006-06-01 for method and apparatus for calculating blood pressure of an artery.
Invention is credited to G. Kent Archibald, Timothy G. Curran, Orland H. Danielson, Marius O. Poliac, Roger C. Thede.
Application Number | 20060116588 11/334287 |
Document ID | / |
Family ID | 23535355 |
Filed Date | 2006-06-01 |
United States Patent
Application |
20060116588 |
Kind Code |
A1 |
Archibald; G. Kent ; et
al. |
June 1, 2006 |
Method and apparatus for calculating blood pressure of an
artery
Abstract
The present invention is a method for calculating blood pressure
of an artery having a pulse. The method includes applying a varying
pressure to the artery. Pressure waveforms are sensed to produce
pressure waveform data. Waveform parameters are derived from the
sensed pressure waveform data. Blood pressure is then determined
using the derived parameters.
Inventors: |
Archibald; G. Kent; (Vadnais
Heights, MN) ; Curran; Timothy G.; (Ramsey, MN)
; Danielson; Orland H.; (Roseville, MN) ; Poliac;
Marius O.; (St. Paul, MN) ; Thede; Roger C.;
(Afton, MN) |
Correspondence
Address: |
ALTERA LAW GROUP, LLC
6500 CITY WEST PARKWAY
SUITE 100
MINNEAPOLIS
MN
55344-7704
US
|
Family ID: |
23535355 |
Appl. No.: |
11/334287 |
Filed: |
January 18, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10439445 |
May 16, 2003 |
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11334287 |
Jan 18, 2006 |
|
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|
09594051 |
Jun 14, 2000 |
6589185 |
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10439445 |
May 16, 2003 |
|
|
|
09070311 |
Apr 30, 1998 |
6099477 |
|
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09594051 |
Jun 14, 2000 |
|
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|
08388751 |
Feb 16, 1995 |
5797850 |
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09070311 |
Apr 30, 1998 |
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08227506 |
Apr 14, 1994 |
5450852 |
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|
08388751 |
Feb 16, 1995 |
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08150382 |
Nov 9, 1993 |
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08227506 |
Apr 14, 1994 |
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Current U.S.
Class: |
600/494 ;
600/490 |
Current CPC
Class: |
A61B 2562/0247 20130101;
A61B 5/02233 20130101; A61B 5/021 20130101; A61B 5/02116 20130101;
A61B 5/02225 20130101; A61B 5/6843 20130101; A61B 5/02141
20130101 |
Class at
Publication: |
600/494 ;
600/490 |
International
Class: |
A61B 5/02 20060101
A61B005/02 |
Claims
1. An apparatus for non-invasively sensing a pressure pulse
waveform in an artery of a patient underlying body tissue,
comprising: a pressure transmission medium comprising an active
surface for placement upon the body tissue with a first dimension
generally parallel to the artery; a force application body
comprising a rigid section and a damping section, the rigid section
and the damping section having portions coupled to one another for
transferring force from the rigid section to the body tissue to
offset forces perpendicular to the artery, and the damping section
being disposed adjacent to the pressure transmission medium at
least along the first dimension for attenuating forces parallel to
the artery, the pressure transmission medium being disposed within
the force application body for being urged against the body tissue;
and a pressure transducer having a pressure-sensitive port isolated
from the force application body and coupled to the pressure
transmission medium.
2. The apparatus of claim 1 wherein the active surface has a second
dimension generally perpendicular to the artery that is greater
than a diameter of the artery.
3. The apparatus of claim 1 wherein the damping section comprises a
compressible foam material.
4. The apparatus of claim 1 wherein: the force application body
comprises an expansion cavity; and the pressure transmission medium
comprises a deformable portion for operationally deforming into the
expansion cavity upon an operational application of force to the
tissue by the force application body.
5. The apparatus of claim 1 wherein the force application body
comprises a deformable section disposed between the rigid section
and the damping section, for conforming to an anatomy of the
patient.
6. The apparatus of claim 1 wherein for conforming to an anatomy of
the patient: the force application body comprises a deformable
section disposed between the rigid section and the damping section
for operationally deforming upon an operational application of
force to the tissue by the force application body; and the pressure
transmission medium comprises a deformable portion for
operationally deforming upon an operational application of force to
the tissue by the force application body.
7. The apparatus of claim 6 wherein: the deformable section of the
force application body comprises a chamber having flexible solid
walls and containing a fluid; and the pressure transmission medium
comprises a chamber having flexible force-transmitting solid walls
and containing a fluid.
8. The apparatus of claim 1 wherein: the damping section comprises
a compressible foam body; and the pressure transmission medium is
encircled by the foam body.
9. The apparatus of claim 8 wherein: the force application body
further comprises a chamber partially filled with a fluid and
mounted to the rigid section; the foam body being mounted to the
chamber; and the pressure transmission medium being encircled by
the foam body and by the chamber.
10. The apparatus of claim 1 wherein: the force application body
further comprises an annular chamber partially filled with a fluid
and mounted to the rigid section; the damping section comprises an
annular compressible foam body mounted to the chamber; and the
pressure transmission medium comprises a fluid-filled chamber
encircled by the annular foam body and by the annular chamber.
11. The apparatus of claim 10 wherein: the fluid-filled chamber is
operationally deformable; and the force application body includes
an expansion cavity for operationally receiving a portion of the
fluid-filled chamber upon deformation thereof.
12. The apparatus of claim 11 wherein the fluid-filled chamber
comprises: a generally disk-like body comprising a first diaphragm
and a second diaphragm bonded along peripheral portions thereof to
form the chamber; the first diaphragm being operationally
deformable into the expansion cavity, and the active surface being
on the second diaphragm; the first diaphragm being bonded to the
annular foam body.
13. The apparatus of claim 12 wherein: the annular foam body has a
generally constant width; and the active surface of the second
diaphragm has a diameter that is greater than the width of the
annular foam body.
14. A sensor for sensing blood pressure pulses within an underlying
artery surrounded by tissue as the underlying artery is compressed,
the sensor comprising: a pressure transducer; an active area having
a dimension greater than a diameter of the artery for compressively
contacting tissue over the artery; a pressure transmission path for
transmitting pressure operationally present upon the active area to
the pressure transducer; and a compressible member for compressing
tissue over the artery, the compressible member being distinct from
the pressure transmission path and the pressure transducer, and at
least partially surrounding and in proximity to the pressure
transmission path for attenuating forces parallel to the
artery.
15. The sensor of claim 14 wherein: the compressible member is
annular; and the active area is circular and encircled by the
annular compressible member.
16. The sensor of claim 15 wherein the annular compressible member
comprises a foam material.
17. The sensor of claim 14 further comprising: an annular
deformable member; the compressible member being annular and bonded
to the annular deformable member; and the active area being
circular and encircled by the annular compressible member and the
annular deformable member.
18. The sensor of claim 14 wherein: the compressible member
comprises a foam material; and the active area is circular and
encircled by the compressible member.
19. A method for non-invasively sensing a pressure pulse waveform
in an artery of a patient at a measurement site through overlying
body tissue, comprising: applying pressure to the overlying body
tissue at least on both sides of the measurement site along the
artery with a compressible material, to neutralize forces exerted
by the overlying body tissue and to attenuate forces parallel to
the artery; urging a pressure transmission medium upon the
overlying body tissue at the measurement site, the pressure
transmission medium being distinct from the compressible material;
and detecting a pressure pulse waveform through the pressure
transmission medium.
20. The method of claim 19 wherein the pressure applying step
further comprises applying pressure to the compressible material
through a deformable material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent is a continuation of and claims the benefit of
prior U.S. patent application Ser. No. 10/439,445, filed May 16,
2003, entitled "METHOD AND APPARATUS FOR CALCULATING BLOOD PRESSURE
OF ANY ARTERY," which is a continuation of application Ser. No.
09/594,051, filed Jun. 14 2000, entitled "METHOD AND APPARATUS FOR
CALCULTING BLOOD PRESSURE OF AN ARTERY," which is a continuation of
application Ser. No. 09/070,311, filed Apr. 30, 1998, issued as
U.S. Pat. No. 6,099,477 and entitled "METHOD AND APPARATUS FOR
CALCULATING BLOOD PRESSURE OF AN ARTERY," which is a continuation
of application Ser. No. 08/388,751, filed Feb. 16, 1995, issues as
U.S. Pat. No. 5,797,850 and entitled "METHOD AND APPARATUS FOR
CALCULATING BLOOD PRESSURE OF AN ARTERY," which is a
continuation-in-part of application Ser. No. 08/227,506, filed Apr.
14, 1994, issued as U.S. Pat. No. 5,450,852 and entitled
"CONTINUOUS NON-INVASIVE BLOOD MONITORING SYSTEM," which is a
continuation-in-part of application Ser. No. 08/150,382, filed Nov.
9, 1993 entitled "CONTINUOUIS NON-INVASIVE BLOOD PRESSURE
MONITORING SYSTEM.revreaction., abandoned all of which are hereby
incorporated herein in their entirety by reference thereto.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to systems for measuring
arterial blood pressure. In particular, the invention relates to a
method and apparatus for measuring arterial blood pressure in
relatively continuous and non-invasive manner.
[0003] Blood pressure has been typically measured by one of four
basic methods: invasive, oscillometric, auscultatory and
tonometric. The invasive method, otherwise known as an arterial
line (A-Line), involves insertion of a needle into the artery. A
transducer connected by a fluid column is used to determine exact
arterial pressure. With proper instrumentation, systolic, mean and
diastolic pressure may be determined. This method is difficult to
set up, is expensive and involves medical risks. Set up of the
invasive or A-line method poses problems. Resonance often occurs
and causes significant errors. Also, if a blood clot forms on the
end of the catheter, or the end of the catheter is located against
the arterial wall, a large error may result. To eliminate or reduce
these errors, the set up must be adjusted frequently. A skilled
medical practitioner is required to insert the needle into the
artery. This contributes to the expense of this method. Medical
complications are also possible, such as infection or nerve
damage.
[0004] The other methods of measuring blood pressure are
non-invasive. The oscillometric method measures the amplitude of
pressure oscillations in an inflated cuff. The cuff is placed
against a cooperating artery of the patient and thereafter
pressurized or inflated to a predetermined amount. The cuff is then
deflated slowly and the pressure within the cuff is continually
monitored. As the cuff is deflated, the pressure within the cuff
exhibits a pressure versus time waveform. The waveform can be
separated into two components, a decaying component and an
oscillating component. The decaying component represents the mean
of the cuff pressure while the oscillating component represents the
cardiac cycle. The oscillating component is in the form of an
envelope starting at zero when the cuff is inflated to a level
beyond the patient's systolic blood pressure and then increasing to
a peak value where the mean pressure of the cuff is equal to the
patient's mean blood pressure. Once the envelope increases to a
peak value, the envelope then decays as the cuff pressure continues
to decrease.
[0005] Systolic blood pressure, mean blood pressure and diastolic
blood pressure values can be obtained from the data obtained by
monitoring the pressure within the cuff while the cuff is slowly
deflated. The mean blood pressure value is the pressure on the
decaying mean of the cuff pressure that corresponds in time to the
peak of the envelope. Systolic blood pressure is generally
estimated as the pressure on the decaying mean of the cuff prior to
the peak of the envelope that corresponds in time to where the
amplitude of the envelope is equal to a ratio of the peak
amplitude. Generally, systolic blood pressure is the pressure on
the decaying mean of the cuff prior to the peak of the envelope
where the amplitude of the envelope is 0.57 to 0.45 of the peak
amplitude. Similarly, diastolic blood pressure is the pressure on
the decaying mean of the cuff after the peak of the envelope that
corresponds in time to where the amplitude of the envelope is equal
to a ratio of the peak amplitude. Generally, diastolic blood
pressure is conventionally estimated as the pressure on the
decaying mean of the cuff after the peak where the amplitude of the
envelope is equal to 0.82 to 0.74 of the peak amplitude.
[0006] The auscultatory method also involves inflation of a cuff
placed around a cooperating artery of the patient. Upon inflation
of the cuff, the cuff is permitted to deflate. Systolic pressure is
indicated when Korotkoff sounds begin to occur as the cuff is
deflated. Diastolic pressure is indicated when the Korotkoff sounds
become muffled or disappear. The auscultatory method can only be
used to determine systolic and diastolic pressures.
[0007] Because both the oscillometric and the auscultatory methods
require inflation of a cuff, performing frequent measurements is
difficult. The frequency of measurement is limited by the time
required to comfortably inflate the cuff and the time required to
deflate the cuff as measurements are made. Because the cuff is
inflated around a relatively large area surrounding the artery,
inflation and deflation of the cuff is uncomfortable to the
patient. As a result, the oscillometric and the auscultatory
methods are not suitable for long periods of repetitive use.
[0008] Both the oscillometric and auscultatory methods lack
accuracy and consistency for determining systolic and diastolic
pressure values. The oscillometric method applies an arbitrary
ratio to determine systolic and diastolic pressure values. As a
result, the oscillometric method does not produce blood pressure
values that agree with the more direct and generally more accurate
blood pressure values obtained from the A-line method. Furthermore,
because the signal from the cuff is very low compared to the mean
pressure of the cuff, a small amount of noise can cause a large
change in results and result in inaccurate measured blood pressure
values. Similarly, the auscultatory method requires a judgment to
be made as to when the Korotkoff sounds start and when they stop.
This detection is made when the Korotkoff sound is at its very
lowest. As a result, the auscultatory method is subject to
inaccuracies due to low signal-to-noise ratio.
[0009] The fourth method used to determine arterial blood pressure
has been tonometry. The tonometric method typically involves a
transducer including an array of pressure sensitive elements
positioned over a superficial artery. Hold down forces are applied
to the transducer so as to flatten the wall of the underlying
artery without occluding the artery. The pressure sensitive
elements in the array typically have at least one dimension smaller
than the lumen of the underlying artery in which blood pressure is
measured. The transducer is positioned such that at least one of
the individual pressure sensitive elements is over at least a
portion of the underlying artery. The output from one of the
pressure sensitive elements is selected for monitoring blood
pressure. The pressure measured by the selected pressure sensitive
element is dependent upon the hold down pressure used to press the
transducer against the skin of the patient. These tonometric
systems measure a reference pressure directly from the wrist and
correlate this with arterial pressure. However, because the ratio
of pressure outside the artery to the pressure inside the artery,
known as gain, must be known and constant, tonometric systems are
not reliable. Furthermore, if a patient moves, recalibration of the
tonometric system is required because the system may experience a
change in gains. Because the accuracy of these tonometric systems
depends upon the accurate positioning of the individual pressure
sensitive element over the underlying artery, placement of the
transducer is critical. Consequently, placement of the transducer
with these tonometric systems is time-consuming and prone to
error.
[0010] The oscillometric, auscultatory and tonometric methods
measure and detect blood pressure by sensing force or displacement
caused by blood pressure pulses as the underlying artery is
compressed or flattened. The blood pressure is sensed by measuring
forces exerted by blood pressure pulses in a direction
perpendicular to the underlying artery. However, with these
methods, the blood pressure pulse also exerts forces parallel to
the underlying artery as the blood pressure pulses cross the edges
of the sensor which is pressed against the skin overlying the
underlying artery of the patient. In particular, with the
oscillometric and the auscultatory methods, parallel forces are
exerted on the edges or sides of the cuff. With the tonometric
method, parallel forces are exerted on the edges of the transducer.
These parallel forces exerted upon the sensor by the blood pressure
pulses create a pressure gradient across the pressure sensitive
elements. This uneven pressure gradient creates at least two
different pressures, one pressure at the edge of the pressure
sensitive element and a second pressure directly beneath the
pressure sensitive element. As a result, the oscillometric,
auscultatory and tonometric methods produce inaccurate and
inconsistent blood pressure measurements.
SUMMARY OF THE INVENTION
[0011] The present invention is an improved method for determining
blood pressure of an artery having a pulse. As a varying pressure
is applied to the artery, pressure waveforms are sensed to produce
sensed pressure waveform data. The sensed pressure waveform data
are then analyzed to derive waveform parameters. One or more blood
pressure values are derived based upon the waveform parameters.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a perspective view of a blood pressure monitoring
system having a sensor assembly mounted upon the wrist of a
patient.
[0013] FIG. 2 is a side view of the wrist assembly of the blood
pressure monitoring system of FIG. 1.
[0014] FIG. 3 is an end view of the wrist assembly.
[0015] FIG. 4 is a cross-sectional view of the wrist assembly.
[0016] FIG. 4A is an expanded cross-sectional view of the sensor
interface along section 4A-4A of FIG. 4.
[0017] FIG. 5 is a top view of the wrist assembly and cylinder of
the system of FIG. 1.
[0018] FIG. 6 is a bottom view of the wrist assembly and cylinder
with a portion removed.
[0019] FIG. 7 is an electrical block diagram of the blood pressure
monitoring system of FIG. 1.
[0020] FIG. 8 is a front elevational view of a monitor of the blood
pressure monitoring system of FIG. 1.
[0021] FIG. 9 is a graph illustrating blood pressure waveforms.
[0022] FIG. 10 is a graph illustrating a curve fit from points
taken from the waveforms of FIG. 9.
[0023] FIG. 11 is a graph illustrating a corrected and scaled
waveform taken from the waveforms of FIG. 9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
I. Overview
[0024] FIG. 1 illustrates blood pressure monitoring system 20 for
measuring and displaying blood pressure within an underlying artery
within wrist 22 of a patient. Monitoring system 20 includes wrist
assembly 24, monitor 26, cylinder 28, cable 30 and cable 32.
[0025] Wrist assembly 24 is mounted on wrist 22 for applying a
varying hold down pressure to an artery within wrist, and for
sensing blood pressure waveforms produced in the artery. Wrist
assembly 24 includes swivel mount 34, hold down assembly 36, sensor
interface assembly 38, waveform pressure transducer 40, hold down
pressure transducer 42, connection tube 44, wrist mount 46 and
wrist pad 48.
[0026] Cylinder 28, under the control of monitor 26, supplies fluid
pressure through cable 32 to wrist assembly 24 to produce the
varying hold down pressure. Cylinder 28 includes a movable piston
which is driven by stopper motor or linear actuator.
[0027] Electrical energization to wrist assembly 24 and pressure
waveform sensor signals to monitor 26 are supplied over electrical
conductors extending between monitor 26 and wrist assembly through
cable 30, cylinder 28 and cable 32. Drive signals to cylinder 28
are supplied from monitor 26 through electrical conductors within
cable 30.
[0028] Monitor 26 receives the pressure waveform sensor signals
from wrist assembly 24, digitizes the signals to produce pressure
waveform data for a plurality of beats, and performs waveform
analysis on the data. The waveform analysis extracts a plurality of
waveform parameters, which preferably include waveform shape,
relative amplitude and gain parameters. From the waveform
parameters, monitor 26 calculates or otherwise derives blood
pressure values, such as mean blood pressure, diastolic blood
pressure and systolic blood pressure. Monitor 26 then displays the
derived blood pressure values.
[0029] As shown in FIG. 1, monitor 26 includes control switches or
input keys 50a-50g, digital displays 52a-52c and display screen 54.
Input keys 50a-50c comprise hard keys for controlling monitor 32.
Input keys 50d-50g consist of software programmable keys which are
adaptable for various functions. Digital displays 52a-52c
continually display systolic, diastolic and mean blood pressure,
respectively. Display screen 54 displays the blood pressure pulse
waveforms and prompts to guide the operator.
[0030] In operation, sensor interface assembly 38 is positioned
over the radial artery. Wrist mount 46 maintains the position of
wrist assembly 24 including sensor interface assembly 38 on wrist
22. In response to fluid pressure supplied from cylinder 28 through
cable 32, hold down assembly 36 applies force and moves sensor
interface assembly 38 to vary the pressure applied to wrist 22
above the radial artery.
[0031] As this pressure is varied, distinct arterial pressure
waveforms are exhibited by the blood pressure pulse within the
underlying artery. Each waveform corresponds to a cardiac cycle.
Each arterial pressure waveform or shape is obtained by sensing and
measuring pressures exhibited by the pulse of the underlying artery
versus time during an individual cardiac cycle. Arterial pressure
applied to sensor interface assembly 38 and is transferred as a
fluid pressure from interface assembly 38 to waveform pressure
transducer 40 through tube 44. The electrical sensor signals from
transducer 40 are supplied to monitor 26 for digitization and
analysis.
[0032] The amplitude of each sensed waveform is a function of the
applied pressure applied to the artery by sensor interface assembly
38 and the amplitude of the arterial pulse. The shape
characteristics of at least one waveform and other parameters
derived from the sensed waveforms are used by digital signal
processing circuitry of monitor 26 to determine systolic, mean and
diastolic pressure. The calculated pressures are displayed by
displays 52a-52c and display screen 54.
II. Wrist Assembly 24
[0033] Wrist assembly 24 is shown in further detail in FIGS. 2-6.
Swivel mount 34 and hold down assembly 36 are side-by-side, and are
pivotally connected by swivel joint 60. Swivel mount 34 carries
transducers 40 and 42 and wrist pad 48. Sensor interface assembly
38 is pivotally connected to and is positioned below hold down
assembly 36. Wrist mount 46, which includes flexible wrist band 62
and wire loops 64 and 66, is connected between an outer end of
swivel mount 34 and teeter mount 68 at an opposite outer end of
hold down assembly 36.
[0034] FIG. 2 is a side elevational view illustrating wrist
assembly 24 in greater detail. Swivel mount 34 is a U-shaped body.
Swivel joint 60 is formed by a socket 70 of swivel mount 34 and
swivel ball 72 of hold down assembly 36. Socket 70 extends into a
channel within the U-shaped configuration of swivel mount 34 and is
sized for receiving swivel ball 72 which projects from an inner end
wall of hold down assembly 36. The ball socket swivel joint
provided by ball 72 and socket 70 permit swivel mount 34 and hold
down assembly 36 to rotate and pivot in virtually any direction so
as to better conform to wrist 22. To aid in pivoting swivel mount
34 with respect to hold down assembly 36, swivel mount 34 includes
an arcuate or beveled lower edge 74 along its inner end. Beveled
edge 74 permits hold down assembly 36 to pivot downward so as to
wrap around wrist 22 (or alternate anatomy) of a patient.
[0035] Swivel mount 34 further includes a tightening screw 76 which
extends across swivel mount 34 adjacent socket 70 and ball 72.
Tightening screw 76 permits socket 70 of swivel mount 34 to be
tightened about ball 72 so as to increase friction between socket
70 and ball 72 to adjust the level of force necessary to readjust
the positioning of swivel mount 34 and hold down assembly 36.
Untightening screw 76 permits ball 72 to be released from socket 70
such that hold down assembly 36 and sensor interface assembly 38
may be disassembled from swivel mount 34.
[0036] FIG. 3 is a end elevational view of blood pressure
monitoring system 20 of FIG. 1, illustrating teeter mount 68 in
greater detail. As shown by FIG. 3, teeter mount 68 includes
fulcrum 80 and tightening screw 82. Fulcrum 80 is generally a
triangular shaped member having two opposing slanted top surfaces.
Fulcrum 80 is coupled to loop 66 and thereby to wrist band 62.
Fulcrum 80 teeters about hold down assembly 36 and permits loop 66
and wrist band 62 to be adjustably positioned so as to better
conform to wrist 22. Tightening screw 82 extends through fulcrum 80
and threadably engages hold down assembly 36. Tightening screw 82
tightens fulcrum 80 against hold down assembly 36 so that the
position of fulcrum 80 may be frictionally set. In FIG. 3, fulcrum
80 is shown in a middle position, and can be rotated either a
clockwise or counterclockwise direction as needed.
[0037] Wrist assembly 24 stably and securely positions sensor
interface assembly 38 over the underlying artery of the patient.
Swivel mount 34 may be rotated and pivoted in practically all
directions about socket 70 and ball 72. Furthermore, teeter mount
68 permits wrist band 62 to be teetered or adjusted so as to better
conform with wrist 22 of the patient. Wrist band 62 wraps around
wrist 22 to secure sensor interface assembly 38 and wrist pad 48
adjacent wrist 22 of the patient. Because sensor interface assembly
38 is more securely and stably positioned above the underlying
artery of wrist 22, patient movement is less likely to reposition
sensor interface assembly 38. As a result, sensor interface
assembly 38 can be reliably located over the underlying artery so
that more accurate and consistent blood pressure measurements may
be taken.
[0038] As shown in FIG. 4, swivel mount 34 carries waveform
pressure transducer 40, hold down pressure transducer 42, and wrist
pad 48. Waveform pressure transducer 40 senses blood pressure
waveforms from the artery which is transmitted to transducer 40
from sensor interface assembly 38 through fluid tube 44 (FIG. 1).
Hold down pressure transducer 42 senses fluid pressure supplied by
cylinder 28 to hold down assembly 36, and is used as a safety
feature to detect an excess hold down pressure condition. Wrist pad
48 is preferably adhesively secured to plate 90 at a bottom surface
of swivel mount 34. Pad 48 is preferably made of a soft flexible
and compressible material so that swivel mount 34 better conforms
to the wrist of a patient. Plate 90 is preferably made of a metal
such as brass and is screwed to swivel mount 34 by screw 92.
Conductive plate 94 is secured within swivel mount 34 and is spaced
from plate 90 so that transducer 40 is positioned between plates 90
and 94. Transducer 40 preferably has a metallic conductive surface
such as brass which contacts conductive plate 94, which is
electrically grounded. As a result, brass plate 94 electrically
grounds transducer 40 so as to drain static charge from transducer
40.
[0039] As shown by FIG. 4, hold down pressure assembly 36 includes
swivel ball 72, housing 100, diaphragm 102, ring 104, piston 106,
piston rod 108, pin 110 and pin mount 112. Diaphragm 102 comprises
a generally circular sheet of flexible material such as reinforced
rubber. Diaphragm 102 is spaced from and cooperates with interior
cavity 114 formed within housing 100 to define pressure chamber
116. Pressure chamber 116 extends generally above and partially
around piston 106. Pressure chamber 116 receives pressurized fluid
from cylinder 28 through fluid passage 118 such that diaphragm 102
expands and contracts to drive piston 106 and piston rod 108 up and
down. As a result, a selected pressure may be applied to piston 106
and piston rod 108 so as to selectively apply a pressure to sensor
interface assembly 38, which is pivotally mounted to the lower end
of piston rod 108. By varying the volume of fluid within pressure
chamber 116, blood pressure monitoring system 20 applies a varying
hold down pressure to sensor interface assembly 38 and the
underlying artery.
[0040] Diaphragm 102 is supported in place by ring 104. Ring 104
encircles the outer perimeter of diaphragm 102 and captures an
outer perimeter or edge portion of diaphragm 102 between ring 104
and housing 100 so as to seal diaphragm 102 against housing 100.
Ring 104 is preferably adhesively secured to housing 100 and
diaphragm 102.
[0041] Piston 106 is preferably a disk or cylinder shaped member
which has its top surface preferably fixedly coupled (such as by an
adhesive) to diaphragm 102. Consequently, as fluid is supplied to
chamber 116, the volume of chamber 116 expands by moving piston 106
downward. Bore 120 extends from top to bottom of piston 106 and is
sized for receiving a portion of piston rod 108. Piston 106 mates
with piston rod 108 and exerts pressure upon piston rod 108 and
sensor interface assembly 38.
[0042] Piston rod 108 is coupled to piston 106 and sensor interface
assembly 38. Piston rod 108 includes plug 122, flange 124, stem
126, ball 128 and pin hole 130. Plug 122 is cylindrically shaped
and is press fit within bore 120 to secure piston rod 108 to piston
106. Flange 124 projects outwardly from plug 122 and fits within a
depression formed in the bottom surface of piston 106. As a result,
piston 106 presses against flange 124 of piston rod 108 to drive
piston rod 108. Alternatively, because piston rod 108 is secured to
piston 106 by plug 122, piston 106 lifts piston rod 108 as pressure
is decreased within pressure chamber 116. Stem 126 integrally
extends downward from flange 124 and has a length extending into
interface assembly 38. Ball 128 is integrally formed at the lower
end of stem 126 and is received within socket 132 of sensor
interface assembly 38. As a result, sensor interface assembly 38
pivots about ball 128 of piston rod 108.
[0043] Pin hole 130 axially extends through piston rod 108 and is
sized for receiving pin 110. Pin 110 is fixedly secured to housing
100 by pin mount 112 and extends through housing 100 into pin hole
130. Pin 110 has a diameter smaller than the diameter of pin hole
130 and extends into stem 126. Pin 110 guides the up and down
movement of piston 106 and piston rod 108 as pressure within
pressure chamber 116 is varied. Pin 110 prevents lateral movement
of piston 106 and piston rod 108 so that piston 106 and piston rod
108 apply only a perpendicular force to sensor interface assembly
38. As a result, pin 110 permits piston 106 and piston rod 108 to
move up and down while pin 110 remains fixedly supported by pin
mount cap 112 to housing 100. Pin 110 is preferably made from a
hard rigid material such as stainless steel.
[0044] As shown by FIG. 4, hold down pressure assembly 28 further
includes pressure supply passage 118, which extends from pressure
chamber 116 through swivel ball 72 where it connects with flexible
tubes 140 and 142 (shown in FIGS. 5 and 6). Flexible tube 140
extends through cable 32 from cylinder 28 to passage 118 in swivel
ball 72. Flexible tube 142 connects passage 118 to transducer 42 in
swivel mount 34. This allows transducer to monitor the fluid
pressure in chamber 116. Fluid supply tube 140 applies pressurized
fluid from cylinder 28 into pressure chamber 116 to vary the
pressure within chamber 116 so as to drive piston 106 and piston
rod 108.
[0045] FIGS. 4 and 4A illustrate sensor interface assembly 38 in
detail. FIG. 4 is a cross-sectional view of wrist assembly 24. FIG.
4A is an enlarged cross-sectional view of sensor interface assembly
38, taken along section 4A-4A of FIG. 4. Sensor interface assembly
38 includes top plate 150, upper V mount 152, lower V mount 154,
diaphragm lock 156, inner mounting ring 158, outer mounting ring
160, side wall diaphragm 162, damping ring 164, inner diaphragm 166
and outer diaphragm 168.
[0046] Top plate 150 is a generally flat annular platform having a
central bore 200, shoulder 202, shoulder 204, and side bore 206.
Central bore 200 receives and holds lower V mount 154. Upper V
mount 152 engages shoulder 202 and extends downward into bore 200
and into lower V mount 154. Rings 158 and 160 and the upper outer
end of side wall diaphragm 162 are mounted in shoulder 204.
[0047] Side bore 206 is defined within top 150 and extends through
top 150 so as to be in communication with fluid passage 208 defined
between upper and lower V mounts 152 and 154 and between upper V
mount 152 and diaphragm lock 156. Side bore 206 receives an end of
tube 44 so that tube 44 is in fluid communication with fluid
passage 208 and sensor interface chamber 210 (which is defined by
diaphragms 166 and 168). Fluid passage 208 and tube 44 provide
fluid communication between sensor interface chamber 210 and
transducer 40 eccentric to socket 132. As a result, piston rod 108
may be pivotally connected to sensor interface assembly 38 at a
lower pivot point.
[0048] Upper V mount 152 is a funnel-shaped socket which is sized
for receiving the lower or distal end of piston rod 108.
Preferably, upper V mount 152 extends through central bore 200 of
top plate 150 to a location near sensor interface chamber 210.
Upper V mount 152 is fixedly secured to an upper portion of top
plate at shoulder 202. Upper V mount 152 is supported by top plate
150 such that upper V mount 152 is spaced from lower V mount 154 to
define annular fluid passage 208. Fluid passage 208 is in fluid
communication with an sensor interface chamber 210. A fluid
coupling medium fills chamber 210, passage 208 and tube 44 all the
way to transducer 40. Upper V mount 152, which is made from a
material such as nylon and forms detent 220 and socket 132 for
pivotally receiving a ball member 128 of piston rod 108. As a
result, sensor interface assembly 38 may be pivoted about socket
132 so as to better conform to the anatomy of the patient.
Furthermore, because socket 132 is adjacent to sensor interface
chamber 210, sensor interface assembly 38 is pivotally coupled to
piston rod 108 about a low pivot point. This permits sensor
interface assembly 38 to be stably positioned above the underlying
artery. In addition, the low pivot point enables hold down assembly
36 to apply a more direct, uniform force on diaphragm 168. Thus,
the hold down pressure applied by hold down pressure assembly 36 is
more uniformly applied to the anatomy of the patient above the
underlying artery.
[0049] Lower V mount 154 is a generally cylindrical shaped member
including step or spar 230 and bore 232. An outer surface or
perimeter of lower V mount 154 projects outwardly to form spar 230.
Spar 230 engages the lower surface of top plate 150 to partially
support side wall diaphragm 162 which is partially captured between
top plate 150 and spar 230. In the preferred embodiment, adhesive
is used between the lower surface of top plate 150 and spar 230 to
fixedly secure the portion of side wall diaphragm 162 trapped
therebetween. Alternatively, spar 230 may be press fit against the
lower surface of top plate 150 to secure and support side wall
diaphragm 162. Spar 230 further divides the outer perimeter of
lower V mount 154 into two portions, an upper portion 234 and a
lower portion 236. Upper portion 234 fits within bore 200 of top
plate 150. Upper portion 234 is preferably adhesively secured to
top plate 150 within bore 200. Lower portion 236 extends below spar
230. Lower portion 236, spar 230 and side wall diaphragm 162 define
expansion cavity 240. Expansion cavity 240 enables upper diaphragm
166 to initially change shape while only experiencing a small
change in volume.
[0050] Diaphragm lock 156 is a thin, elongated, annular ring
including bore 250 and lower lip 252. Bore 250 extends through
diaphragm lock 156 and with upper V mount 152, defines a portion of
fluid passage 208. Lip 252 projects outwardly from a lower end of
diaphragm lock 156. Diaphragm lock 156 fits within bore 232 of
lower V mount 154 until an inner edge of diaphragm lock 156 is
captured between inserts, lip 252 and the lower end of lower V
mount 154. Diaphragm lock 156 is preferably adhesively affixed to
lower V mount 154. Alternatively, diaphragm lock 156 may be press
fit within lower V mount 154.
[0051] Side wall diaphragm 162, rings 158 and 160 and top plate 150
define an annular deformable chamber 260 coupled between top plate
150 and ring 164. Side wall diaphragm 162 is preferably formed from
a generally circular sheet of flexible material, such as vinyl, and
is partially filled with fluid. Diaphragm 162 has a hole sized to
fit around upper portion 234 of lower V mount 154. Diaphragm 162
includes outer edge portion 162a and inner edge portion 162b. Outer
edge portion 162a is trapped and held between outer ring 160 and
top plate 150. Inner edge portion 162b is trapped and supported
between top plate 150 and spar 230 of lower V mount 154. Diaphragm
162 is made from a flexible material and is bulged outward when
chamber 260 is partially filled with fluid. Chamber 260 is
compressible and expandable in the vertical direction so as to be
able to conform to the anatomy of the patient surrounding the
underlying artery. As a result, the distance between top plate 150
and the patient's anatomy can vary around the periphery of side
wall diaphragm 162 according to the contour of the patient's
anatomy. Furthermore, because fluid is permitted to flow through
and around chamber 260, pressure is equalized around the patient's
anatomy.
[0052] Damping ring 164 generally consists of an annular
compressible ring and is preferably formed from a foam rubber or
other pulse dampening material such as open celled foam or closed
cell foam. Ring 164 is centered about and positioned between side
wall diaphragm 162 and diaphragms 166 and 168. Damping ring 164 is
isolated from the fluid coupling medium within chamber 210. Because
ring 164 is formed from a compressible material, ring 164 absorbs
and dampens forces in a direction parallel to the underlying artery
which are exerted by the blood pressure pulses on sensor interface
assembly 38 as the blood pressure pulse crosses sensor interface
assembly 38. Because bottom ring 164 is isolated from the fluid
coupling medium, the forces absorbed or received by ring 164 cannot
be transmitted to the fluid coupling medium. Instead, these forces
are transmitted across ring 164 and side wall diaphragm 162 to top
plate 150. Because this path is distinct and separate from the
fluid coupling medium, chamber 210 and the fluid coupling medium
are isolated from these forces. In addition, ring 164 also presses
tissue surrounding the artery to neutralize or offset forces
exerted by the tissue.
[0053] Upper diaphragm 166 is an annular sheet of flexible material
having an inner portion 166a, an intermediate portion 166b, an
outer portion 166c and an inner diameter sized to fit around
diaphragm lock 156. Inner portion 166a is trapped or captured
between lip 252 of diaphragm lock 156 and the bottom rim of lower V
mount 154. Inner portion 166A is preferably adhesively affixed
between lip 252 and lower V mount 154.
[0054] Intermediate portion 166b lies between inner portion 166a
and outer portion 166c. Intermediate portion 166b is adjacent to
expansion cavity 240 and is isolated from ring 164 and chamber 260.
Because intermediate portion 166b is positioned adjacent to
expansion cavity 240, intermediate portion 166b is permitted to
initially move upward into expansion cavity 240 as chamber 260,
ring 164 and outer diaphragm 168 conform to the anatomy of the
patient surrounding the underlying artery while the experiences
only a small change in volume. As ring 164 is pressed against the
anatomy of the patient surrounding the artery to neutralize or
offset forces exerted by the tissue, diaphragm 168 is also
compressed. However, because intermediate portion 166b is permitted
to roll into expansion cavity 240, chamber 210 does not experience
a large volume decrease and a large corresponding pressure
increase. Thus, sensor interface assembly 38 permits greater force
to be applied to the anatomy of the patient through ring 164 to
neutralize tissue surrounding the artery without causing a
corresponding large change in pressure within chamber 210 as the
height of the side wall changes. As a result, sensor interface
assembly 38 achieves more consistent and accurate blood pressure
measurements.
[0055] Outer diaphragm 168 is a generally circular sheet of
flexible material capable of transmitting forces from an outer
surface to fluid within chamber 210. Outer diaphragm 168 is coupled
to inner diaphragm 166 and is configured for being positioned over
the anatomy of the patient above the underlying artery. Outer
diaphragm sheet 168 includes non-active portion or skirt 168a and
active portion 168b. Skirt 168a constitutes the area of diaphragm
168 where inner diaphragm 166, namely outer portion 166c, is bonded
to outer diaphragm 168. Skirt 168a and outer portion 166c are
generally two bonded sheets of flexible material, forces parallel
to the underlying artery are transmitted across skirt 168a and
outer portion 166c and are dampened by the compressible material of
ring 164.
[0056] Active portion 168b is constituted by the portion of outer
diaphragm sheet 168 which is not bonded to inner diaphragm 166.
Active portion 168b is positioned below and within the inner
diameter of ring 164. Active portion 168b is the active area of
sensor interface assembly 38 which receives and transmits pulse
pressure to transducer 40. Active portion 168b of diaphragm 168,
intermediate portion 166b of diaphragm 166 and diaphragm lock 156
define sensor interface chamber 210.
[0057] The coupling medium within chamber 210 may consist of any
fluid (gas or liquid) capable of transmitting pressure from
diaphragm 168 to transducer 40. The fluid coupling medium
interfaces between active portion 168b of diaphragm 168 and
transducer 40 to transmit blood pressure pulses to transducer 40.
Because the fluid coupling medium is contained within sensor
interface chamber 210, which is isolated from the side wall of
sensor interface assembly 38, the fluid coupling medium does not
transmit blood pressure pulses parallel to the underlying artery,
forces from the tissue surrounding the underlying artery and other
forces absorbed by the side wall to transducer 40. As a result,
sensor interface assembly 38 more accurately measures and detects
arterial blood pressure.
[0058] Sensor interface assembly 38 provides continuous external
measurements of blood pressure in an underlying artery. Because
sensor interface assembly 38 senses blood pressure non-invasively,
blood pressure is measured at a lower cost and without medical
risks. Because sensor interface assembly 38 is relatively small
compared to the larger cuffs used with oscillometric and
auscultatory methods, sensor interface assembly 38 applies a hold
down pressure to only a relatively small area above the underlying
artery of the patient. Consequently, blood pressure measurements
may be taken with less discomfort to the patient. Because sensor
interface assembly 38 does not require inflation or deflation,
continuous, more frequent measurements may be taken.
[0059] Furthermore, sensor interface assembly 38 better conforms to
the anatomy of the patient so as to be more comfortable to the
patient and so as to achieve more consistent and accurate blood
pressure measurements. Because chamber 260 is deformable and
partially filled with fluid, chamber 260 better conforms to the
anatomy of the patient and equalizes pressure applied to the
patient's anatomy. Because ring 164 is compressible and because
diaphragm 168 is flexible and is permitted to bow or deform
inwardly, ring 164 and diaphragm 168 also better conform to the
anatomy of the patient. At the same time, however, sensor interface
assembly 38 does not experience a large sudden increase in pressure
in sensor interface chamber 210 as ring 164 and diaphragm 168 are
pressed against the anatomy of the patient. Chamber 260 and ring
164 apply force to the anatomy of the patient to neutralize the
forces exerted by tissue surrounding the underlying artery. Because
chamber 260 and ring 164 are both compressible, the height of the
side wall decreases as side wall is pressed against the patient.
Diaphragms 166 and 168 are also conformable. However, because
intermediate portion 166b of inner diaphragm 166 is permitted to
move upward into expansion cavity 240, sensor interface chamber 210
does not experience a large volume decrease and a corresponding
large pressure increase. Thus, the side wall is able to apply a
greater force to the anatomy of the patient without causing a
corresponding large, error producing increase in pressure within
sensor interface chamber 210 due to the change in height of the
side wall and the change in shape of outer diaphragm 168.
[0060] At the same time, sensor interface assembly 38 permits
accurate and consistent calculation of blood pressure. Because of
the large sensing area through which blood pressure pulses may be
transmitted to transducer 40, sensor interface assembly 38 is not
as dependent upon accurate positioning of active portion 168b over
the underlying artery. Thus, sensor interface assembly 38 is more
tolerant to patient movement as measurements are being taken.
[0061] Moreover, sensor interface assembly 38 achieves a zero
pressure gradient across the active face or portion 168b of the
sensor, achieves a zero pressure gradient between the transducer
and the underlying artery, attenuates or dampens pressure pulses
that are parallel to the sensing surface of the sensor, and
neutralizes forces of the tissue surrounding the underlying artery.
Sensor interface assembly 38 contacts and applies force to the
anatomy of the patient across skirt 168a and active portion 168b.
However, the pressure within interface chamber 210 is substantially
equal to the pressure applied across active portion 168b. The
remaining force applied by sensor interface assembly 38 across
skirt 168a which neutralizes or offsets forces exerted by the
tissue surrounding the underlying artery is transferred through the
side wall (ring 164 and chamber 260) to top plate 150. As a result,
the geometry and construction of sensor interface assembly 38
provides the proper ratio of pressures between skirt 168a and
active portion 168b to neutralize tissue surrounding the underlying
artery and to accurately measure the blood pressure of the artery.
In addition, because the fluid coupling medium within sensor
interface chamber 210 is isolated from the side wall, pressure
pulses parallel to the underlying artery, forces from tissue
surrounding the underlying artery and other forces absorbed by the
side wall are not transmitted through the fluid coupling medium to
transducer 40. Consequently, sensor interface assembly 38 also
achieves a zero pressure gradient between transducer 40 and the
underlying artery.
[0062] FIG. 5 is a top view of wrist assembly 24. FIG. 5 further
illustrates portions of swivel mount 34 and cable 30 in greater
detail. Fluid tube 140 has one end connected to passage 118 in
swivel ball 72 and its other end connected to cylinder 28.
[0063] Fluid tube 142 extends between transducer 42 and passage 118
in ball 72. Fluid tube 142 fluidly connects pressure chamber 116
and transducer 42. As a result, transducer 42 senses the pressure
within pressure chamber 116. Transducer 42 produces electrical
signals representing the sensed hold down pressure within pressure
chamber 116. These electrical signals are transmitted by electrical
wires 280 which extend within cables 30 and 32 to monitor 26 (shown
in FIG. 1). As a result, monitor 26 may continuously verify that
the actual pressure within pressure chamber 116 is within a safe
range.
[0064] As further shown by FIG. 5, cable 32 additionally encloses
electrical wires 290 from transducer 40 (shown in FIG. 4).
Electrical wires 290 transmit electrical signals representing blood
pressure amplitudes sensed by transducer 40. Cable 32 also encloses
an electrical grounding wire 300 which is electrically connected
through resistor 302 (FIG. 6) to brass plate 94 (shown in FIG. 4)
and which electrically grounds transducers 40 and 42.
[0065] FIG. 6 is a bottom view of wrist assembly 24. FIG. 6
illustrates swivel mount 34 with pad 48 and plate 90 (FIG. 4)
removed. FIG. 6 illustrates the electrical connection between
transducers 40 and 42 and electrical wires 280 and 290,
respectively. As shown by FIG. 6, swivel mount 34 contains
electrical connector 304. Electrical connector 304 receives leads
306 of transducer 40. Leads 306 transmit the electrical signals
produced by transducer 40 representing the pressures and transmits
the electrical signals to electrical wires 290. Electrical
connector 304 further includes an electrical resistor 302
electrically coupled to brass plate 94. Resistor 302 is further
electrically coupled to grounded electrical wire 300. As a result,
static charge is drained through resistor 302 through electrical
connector 304 and through grounded wire 300. Electrical connector
304 permits transducer 40 to be removed and separated from swivel
mount 34.
[0066] Similarly, transducer 42 includes four electrical leads 310
which are electrically connected to electrical wires 280. In
contrast to transducer 40, however, transducer 42 is generally
fixed and mounted within swivel mount 34. As shown by FIG. 6,
swivel mount 34 electrically connects transducers 40 and 42 to
monitor 26 by electrical wires 280 and 290 carried within cables 30
and 32.
III. Monitor 26
[0067] FIG. 7 shows a block diagram of blood pressure monitoring
system 20. As best shown by FIG. 7, monitor 26 further includes
input signal processor 350, analog-to-digital converter 352,
microprocessor (and associated memory) 354, inputs 50a-50g,
cylinder drive 356, displays 52a-52c and 54, and power supply 358.
In operation, microprocessor 354 receives inputted signals from
inputs 50a-50g. Inputs 50a-50g may also consist of a keyboard or
other input mechanisms. Inputs 50a-50g permit microprocessor 354 to
perform a calibration.
[0068] Microprocessor 354 controls cylinder drive 356 to vary hold
down pressure applied by hold down pressure assembly 36 of wrist
assembly 24. Hold down pressure is applied to the anatomy of the
patient directly above the artery. The hold down pressure applied
by hold down pressure assembly 36 on sensor interface assembly 38
is increased over time. As the force or hold down pressure applied
by sensor interface assembly 38 increases, the amplitude or
relative pressure of the blood pressure pulse also increases until
a maximum amplitude results. Once the maximum amplitude or maximum
energy transfer results, the amplitude of the blood pressure pulse
begins to decrease as the artery begins to flatten out beyond the
point of maximum energy transfer.
[0069] Transducer 40 of wrist assembly 24 senses the amplitude and
shape of the blood pressure pulses within the underlying artery.
Transducer 40 creates electric sensor signals representing the
pressures exerted by the sensed blood pressure pulses. The sensor
signals are transmitted to input signal processor 350 of monitor
26. Input signal processor 350 processes the sensor signals and
filters any unwanted or undesirable noise and other effects. The
sensor signals are then transmitted from input signal processor 350
to analog-to-digital convertor 352. Analog-to-digital convertor 352
converts the sensor signal into digital form. A digital signal
representing the pressures of the sensed blood pressure pulses is
sent to microprocessor 354.
[0070] Based upon the digital sensor signals representing the
sensed pressures and shape of the blood pressure pulses,
microprocessor 354 determines wave shape information by measuring
amplitude and shape versus time of individual cardiac cycles. The
arterial wave shape information is determined by sampling the
arterial waves at a rate significantly above heart rate so that a
good definition of the arterial pressure wave is measured. From
wave shape information and other parameters derived therefrom,
microprocessor 354 calculates systolic, diastolic and mean blood
pressures.
IV. Method for Locating Sensor Interface Assembly Over Artery
[0071] FIG. 8 illustrates digital displays 52a-52c and display
screen 54 of monitor 26 in greater detail. As shown by FIG. 8,
display screen 54 further includes pressure scale 400, horizontal
guidelines 410 and digital readout 430. Monitor 26 also is used to
display blood pressure pulse waveforms so as to guide the operator
in positioning and locating sensor interface assembly 38 directly
over the underlying artery having a blood pressure pulse so that
more accurate blood pressure values may be determined.
[0072] To place sensor interface assembly 38 over an underlying
artery, sensor interface assembly 38 is located or positioned above
a known approximate location of the underlying artery. As sensor
interface assembly 38 is positioned over the underlying artery, a
constant hold down pressure is applied to sensor interface assembly
38 and to the underlying artery. Preferably, the pressure applied
to sensor interface assembly 38 should be as high as possible
without the diastolic portion 440 of blood pressure waveforms 450
distorting.
[0073] In response to the applied pressure, the underlying artery
exhibits a blood pressure pulse waveform for each cardiac cycle.
Sensor interface assembly 38 senses or receives the force exerted
by the blood pressure pulse as the pulse travels beneath the
sensing surface and transmits the pressures through the fluid
coupling medium to transducer 40. Transducer 40 in turn senses the
changes in pressure and converts the pressures into electrical
signals which represent the arterial pressure waveforms. The
signals are then transmitted through cables 30 and 32 to monitor
36. Monitor 36 samples the signals preferably at a rate of 128
samples per second. Monitor 36 then visually displays the sampled
signals received from transducer 40 and displays the signals
representing arterial pressure waveforms on display screen 54.
Display screen 54 is preferably indexed so as to provide a vertical
scale 400 with horizontal guidelines 410 for displaying pressure.
Guidelines 410 permit the maximum pressure amplitude of blood
pressure pulse waveforms at the particular location and at a
constant hold down pressure to be determined. A representative
series of blood pressure pulse waveforms 450 is illustrated on
screen 54 in FIG. 8.
[0074] To further aid the operator in determining the maximum
amplitude of blood pressure pulse waveforms, display screen 54
further includes a digital readout 430 which digitally displays the
maximum pressure amplitude exerted by the pulse in response to the
hold down pressure applied to the artery. As shown in FIG. 8, the
artery exhibits pressures which are in the form of blood pressure
pulse waveforms 450 when a constant hold down pressure of 80 mmHg
is applied to the underlying artery. Blood pressure pulse waveforms
450 exhibit a maximum amplitude of approximately 18 mmHg.
[0075] Once the maximum pressure amplitude exerted by the pulse at
a particular hold down pressure at the particular location is
determined and noted, sensor interface assembly 38 is repositioned
at a second location above the known approximate location of the
artery. The same constant hold down pressure is applied to sensor
interface assembly 38 and to the underlying artery of wrist 22. The
constant hold down pressure applied to the underlying artery is
preferably as close as possible to the constant hold down pressure
applied at the first location as indicated by display screen 54.
This can be done by applying a hold down pressure to sensor
interface assembly 38 at a constant force equal to one of
guidelines 410.
[0076] The maximum pressure amplitude exerted by the pulse in
response to the hold down pressure applied to the artery at the
second location can be determined from the analog display of the
blood pressure waveforms 450 on display screen 54 or the digital
readout 430 on display screen 54. The maximum pressure amplitude at
the second location is then noted or recorded for comparison with
maximum pressure amplitudes at other locations. Typically, sensor
interface assembly 38 will be repositioned at a plurality of
locations above a known approximate location of the artery while
applying a constant hold down pressure to the artery. At each
location, the maximum pressure amplitude exerted by the pulse in
response to the constant hold down pressure will be displayed on
display screen 54 and noted. At each location, the maximum pressure
amplitude indicated by display screen 54 is compared with maximum
pressure amplitudes exerted by the pulse in response to the
constant hold down pressure applied to the artery and indicated by
display screen 54 at the plurality of other locations. After the
maximum pressure amplitude corresponding to each of the plurality
of locations are compared, sensor interface assembly 38 and its
sensing surface are positioned at the particular location which
corresponds to the location at which the largest of the maximum
pressure amplitudes is exerted by the pulse in response to the
constant hold down pressure applied to the artery.
V. Method for Determining Blood Pressure Values
[0077] Once the sensor is properly positioned over the underlying
artery, blood pressure monitoring system 20 determines blood
pressure values from the sensed waveform pressure amplitudes sensed
by sensor interface assembly 38 and from other parameters derived
from the pressure amplitudes using a stored set of coefficients. A
pressure amplitude is determined at each sample point.
[0078] Blood pressure monitoring system 20 calculates a systolic
blood pressure valve (S), a mean blood pressure (M) and a diastolic
blood pressure (D) based upon the following formulas: M=F.sub.m
(P.sub.1.sup.m, . . . , P.sub.n.sup.m, . . . , C.sub.n.sup.m
S=F.sub.s (P.sub.1.sup.s, . . . , C.sub.1.sup.s, . . . ,
C.sub.n.sup.s) D=F.sub.d (P.sub.1.sup.d, . . . , P.sub.n.sup.d,
C.sub.1.sup.d, . . . , C.sub.n.sup.d) wherein F.sub.m, F.sub.s,
F.sub.d are linear or non-linear functions, P.sub.1.sup.m,
P.sub.1.sup.s, P.sub.1.sup.d, . . . , P.sub.n.sup.m, P.sub.n.sup.s,
P.sub.n.sup.d are parameters derived from waveform pressure
amplitudes and C.sub.1.sup.m, C.sub.1.sup.s, C.sub.1.sup.d, . . . ,
C.sub.n.sup.m, C.sub.n.sup.s, C.sub.n.sup.d are coefficients
obtained during training processes based upon clinical data.
[0079] In particular, blood pressure monitoring system 20
calculates a systolic blood pressure value (S), a mean blood
pressure value (M), a diastolic blood pressure value (D) based upon
the following formulas:
M=C.sub.1.sup.mP.sub.1.sup.m+C.sub.2.sup.mC.sub.2.sup.m+ . . .
+C.sub.n.sup.mP.sub.n.sup.m
S=C.sub.1.sup.sP.sub.1.sup.s+C.sub.2.sup.sP.sub.2.sup.s+ . . .
+C.sub.n.sup.sP.sub.n.sup.s
D=C.sub.1.sup.dP.sub.1.sup.d+C.sub.2.sup.dP.sub.2.sup.d+ . . .
+C.sub.n.sup.dP.sub.n.sup.d wherein P.sub.1.sup.m, P.sub.1.sup.s,
P.sub.1.sup.d . . . P.sup.n.sup.m, P.sub.n.sup.s, P.sub.n.sup.d are
parameters derived from waveform pressure amplitudes. Such
parameters may be calculated from shape characteristics of the
waveform or parameters calculated from functions such as curves
based upon relationships between particular points of several
waveforms. The parameters may be further based upon hold down
pressure values and time periods between particular points on the
waveforms. The value C.sub.1.sup.m, C.sub.1.sup.s, C.sub.1.sup.d .
. . C.sub.n.sup.m, C.sub.n.sup.s, C.sub.n.sup.d are coefficients
obtained during training processes based upon clinical data.
[0080] In addition, the pulse rate (PR) may also be determined
using the formula: PR = PR 1 + PR 2 + PR 3 + PR 4 4 ##EQU1##
[0081] To determine pulse rate, four individual waveforms or beats
are sensed and are time averaged to determine pulse rate.
Preferably, the waveforms used to determine pulse rates include the
waveform having largest maximum pressure amplitude, the two
waveforms prior to the waveform having the largest maximum pressure
and the waveform succeeding the waveform having the largest maximum
pressure. Once the four waveforms are identified, the pulse rate of
each waveform is determined. The sum of the pulse rate of the four
waveforms is then divided by four to calculate pulse rate PR. The
pulse rate (PR) for each waveform is based upon the following
formula: PR N .times. .times. beats .times. .times. per .times.
.times. minute .times. .times. ( N = 1 , 2 , 3 , 4 ) = 128 .times.
.times. samples / sec No . .times. samples / beat N . ##EQU2##
[0082] FIGS. 9, 10 and 11 illustrate representative parameters
which may be used to calculate blood pressure values. FIG. 9
illustrates a sample series of waveforms exhibited by the
underlying artery as a varying pressure is applied over time. The
vertical scale indicates pressure in mmHg while the horizontal
scale indicates individual sample points at which the blood
pressure values exerted by the pulse are measured over time. In the
preferred embodiment, transducer 40 produces continuous electrical
signals representing waveform pressures which are sampled 128 times
per second.
[0083] In the preferred embodiment, the hold down pressure applied
by hold down pressure assembly 36 to sensor interface assembly 38
(shown in FIG. 1) is swept over a preselected range of increasing
hold down pressures. Preferably, the sweep range of hold down
pressures is begun at approximately 20 mmHg. The hold down pressure
applied by hold down pressure assembly 36 is then steadily
increased until two individual waveforms are sensed following the
sensed waveform having the largest pressure amplitude.
Alternatively, once the waveform having the largest maximum
pressure is sensed and identified, successive sweeps may
alternatively have a varying hold down pressure applied until a
preselected multiple of the mean hold down pressure of the waveform
having the largest maximum pressure amplitude is reached.
Preferably, each sweep range extends between the initial hold down
pressure of 20 mmHg and a final hold down pressure of approximately
150% of the mean hold down pressure of the waveform having the
largest maximum pressure amplitude during the previous sweep. In
addition, the sweep range may alternatively have an initial hold
down pressure of approximately 20 mmHg to a final hold down
pressure having a preselected absolute value. Alternatively, the
sweep could start at a high pressure and sweep low. As a safety
measure, the pressure within pressure chamber (sensed by transducer
42) and interface chamber 210 (sensed by transducer 40) are
continually monitored by monitor 26. If the ratio of the pressures
within pressure chamber 116 and chamber 210 fall outside of a
defined range of limits, an alarm is signaled.
[0084] After each hold down pressure sweep, blood pressure
monitoring system 20 begins a successive new sweep to calculate
new, successive blood pressure values. As a result, blood pressure
monitoring system 20 continually measures blood pressure within the
underlying artery without causing discomfort to the patient. As can
be appreciated, the sweep range of hold down pressure applied by
hold down pressure assembly 36 may have various initial and final
points. Furthermore, the hold down pressure applied by hold down
pressure assembly 36 may alternatively be intermittently varied.
For example, the hold down pressure may be increased or decreased
in a step-wise fashion.
[0085] Based upon sensed and sampled pressure waveform signals or
data produced by transducer 40 and sent to monitor 26 during each
sweep of hold down pressures, monitor 26 derives preselected
parameters for calculating blood pressure values from the derived
parameters and a stored set of coefficients. As indicated in FIG.
9, parameters maybe derived directly from the absolute waveform
pressures which vary as hold down pressure is varied over time.
Such parameters may be derived from the shape of the waveforms
including a particular waveform's slope, absolute pressure at a
selected sample point, a rise time to a selected sample point on a
waveform and the hold down pressures corresponding to a particular
sample point on a waveform. As can be appreciated, any of a variety
of parameters may be derived from the absolute waveform pressures
shown in FIG. 9. Parameters may further be based upon particular
points or functions of the sample points.
[0086] FIG. 10 illustrates an example of how values or parameters
of multiple waveforms 500 shown in FIG. 9 may be used to derive
additional parameters. FIG. 10 shows several data points 510. Each
data point 510 represents a selected waveform taken from the sweep
shown in FIG. 9. Curve 520 is derived by fitting points 510 to a
preselected function or relationship. Parameters such as the peak
530 are then derived from curve 520. As can be appreciated, various
other parameters such as slope may also be derived from curve 520.
Parameters derived from curve 520 are ultimately based upon
pressure waveforms 500 shown in FIG. 9 which are produced from
sensed pressure waveform data or signals from transducer 40.
However, because curve 520 is derived using a plurality of
waveforms 500, parameters derived from curve 520 represent the
overall relationship between the plurality of waveforms 500. In
other words, parameters derived from curve 520 represent the way in
which the plurality of waveforms 500 (shown in FIG. 9) are related
to one another. Data points 510 represent corrected, relative
waveform pressures. As can be appreciated, functions such as curves
may also be derived using absolute waveform pressure values which
are shown in FIG. 9.
[0087] A waveform is "corrected" by subtracting the hold down
pressure from the absolute pressure of the waveform to produce
relative waveform pressures (otherwise known as amplitudes).
Correcting a waveform eliminates characteristics of the waveform
which result from a continuously increasing hold down pressure
being applied to the artery during each waveform or cardiac
cycle.
[0088] FIG. 11 further illustrates other parameters which may be
derived from waveform pressure values as shown in FIG. 9. FIG. 11
illustrates waveform 600 selected from waveforms 500. Waveform 600
is preferably the waveform having the largest peak or maximum
pressure amplitude. Alternatively, waveform 600 may be any of the
waveforms 500 (shown in FIG. 9) such as waveforms immediately
preceding or succeeding the waveform having the largest maximum
pressure. As shown in FIG. 11, waveform 600 is corrected such that
the beginning point 602 and an ending point 604 have the same
absolute waveform pressure value. As further shown by FIG. 11,
waveform 600 is horizontally and vertically scaled to eliminate
gain from parameters derived from waveform 600. Preferably,
waveform 600 is scaled from zero to twenty-one beginning at
beginning point 602 and ending at ending point 604 of waveform 600
on the horizontal b axis. Preferably, waveform 600 is vertically
scaled from zero to one beginning at its base and ending at its
peak. Because waveform 600 is horizontally and vertically scaled,
parameters may be derived from waveform 600 for calculating blood
pressure values without the gain of the particular patient
affecting the calculated blood pressure value. Gains are caused by
the differences between the actual pressure exerted within the
artery and the pressures sensed at the surface of the wrist or
anatomy which is caused by varying characteristics of the
intermediate tissue. Scaling waveform 600 eliminates any gains
exhibited by individual patients. By using scaled values to locate
corresponding points or waveform pressure amplitudes on waveform
600, points on waveform 600 uniformly correspond to the same points
on waveforms exhibited by other patients.
[0089] As shown by FIG. 11, various parameters may be derived from
scaled, corrected waveform 600. As shown by FIG. 11, such
parameters include widths of waveform 600 at selected points along
the vertical y axis, ratios of individual waveform pressure
amplitudes at selected points along the horizontal b axis and the
amplitude of the waveform, the rise time or time elapsed from the
start of waveform 600 at point 602 to a selected point along the
vertical y axis. In addition, several other parameters may also be
derived from waveform 600, such as slope and other shape
characteristics.
[0090] Once the parameters to be used in calculating blood pressure
values are selected, coefficients corresponding to each parameter
must be determined. Coefficients represent the relationship between
a particular parameter set and the resulting blood pressure value
to be determined from a particular parameter set. Coefficients are
initially ascertained from clinical tests upon patients having
known blood pressure values. Typically, the known blood pressure
value is determined using the A-line method which is generally
accurate, although difficult to set up, expensive and medically
risky. As the blood pressure is determined using the A-line or
other methods, sensor interface assembly 38 is positioned over the
underlying artery of the patient. Hold down pressure assembly 36
applies a varying pressure to the artery of the patient having the
known blood pressure value. As discussed above, transducer 40
produces sensed pressure waveform signals or data representing
arterial pressure waveforms. Monitor 26 receives the produced
sensed pressure waveform data and derives preselected parameters
from the sensed pressure waveform data. Coefficients are then
determined using the derived values of the selected parameters and
the known blood pressure value. Each coefficient corresponding to
each selected parameter is a function of the known blood pressure
values and the derived parameters. Preferably, several patients are
clinically tested to ascertain the coefficients. Once obtained, the
coefficients are stored for use in non-invasively calculating blood
pressure values of other patients without the necessity of using
the more time consuming, expensive and risky A-line method and
without using the generally more inaccurate conventional blood
pressure measuring methods. Each particular coefficient is
preferably ascertained so as to be applicable for calculating blood
pressure values from the derived waveform parameters of all
patients. Alternatively, individualized coefficients may be used to
calculate blood pressure values from derived waveform parameters of
particular patients falling within a particular age group or other
specialized groups. The coefficients are preferably determined for
use with the same blood pressure monitoring system as will be used
to determine the particular blood pressure value of patients having
unknown blood pressure values. However, as can be appreciated, the
method of the present invention for ascertaining coefficients as
well as the method of the present invention for determining blood
pressure values may be used in conjunction with any one of a
variety of blood pressure monitoring systems including different
sensor assemblies and hold down pressure assemblies.
[0091] In addition to illustrating various methods by which
parameters may be derived from waveform pressure data, FIGS. 9, 10
and 11 illustrate particular parameters for use in calculating a
systolic, a mean and a diastolic blood pressure value of a
particular patient during an individual hold down pressure sweep.
According to the preferred method of the present invention, hold
down pressure assembly 36 applies a sweeping, continuously varying
hold down pressure to the underlying artery. Preferably, the hold
down pressure applied by hold down pressure assembly 36 during each
sweep begins at 20 mmHg and ramps upward over time until at least
two waveforms are detected by transducer 40 after the waveform
having the largest maximum pressure is identified. Based upon the
produced sensed pressure waveform data representing the waveforms
as representatively shown by FIG. 9, blood pressure monitoring
system 20 calculates systolic, mean and diastolic blood pressure
using a stored set of coefficients. Systolic blood pressure (S) is
calculated using the formula:
S=C.sub.1.sup.sP.sub.1.sup.s+C.sub.2.sup.sP.sub.2.sup.s+C.sub.3-
.sup.sP.sub.3.sup.s+C.sub.4.sup.sP.sub.4.sup.s+C.sub.5.sup.sP.sub.5.sup.s+-
C.sub.6.sup.sP.sub.6.sup.s+C.sub.7.sup.sP.sub.7.sup.s+C.sub.8.sup.sP.sub.s-
+C.sub.9.sup.s
[0092] Coefficients C.sub.1.sup.s-C.sub.1.sup.s are stored
coefficients ascertained according to the earlier described method
of the present invention. C.sub.9.sup.s is an offset value.
Parameters P.sub.1.sup.s and P.sub.2.sup.s are derived from
relative waveform pressure amplitudes corresponding to scaled
values taken from a scaled and corrected beat as represented by
waveform 600 in FIG. 11. Preferably, parameter P.sub.1.sup.s is the
ratio defined by the waveform pressure amplitude on waveform 600
which corresponds to scale value b.sub.1 along the horizontal axis
divided by the maximum waveform pressure amplitude or peak (point
606) of waveform 600. Parameter P.sub.2.sup.s preferably is the
ratio defined by the waveform pressure amplitude of point 608 on
waveform 600 that corresponds to scale value b.sub.3 along the
horizontal b axis divided by the maximum waveform pressure
amplitude or peak (point 606) of waveform 600.
[0093] Parameter P.sub.3.sup.s is the rise time or the time elapsed
from the start of the waveform to a particular point along waveform
600 corresponding to a particular vertical scale value. Preferably,
parameter P.sub.3.sup.s is the elapsed time from the start of
waveform 600 to a point 610 on waveform 600 which has a vertical
height of approximately 0.18 that of a maximum pressure amplitude
or peak (point 606) of waveform 600. This rise time or elapsed time
is represented as 612 in FIG. 11.
[0094] Parameter P.sub.4.sup.s is the mean pressure of the
uncorrected waveform 500a (shown in FIG. 9) having the highest peak
or maximum pressure. Parameter P.sub.4.sup.s is indicated on FIG. 9
by point 700. Parameter P.sub.5.sup.s is the systolic point of the
uncorrected pressure waveform immediately following the uncorrected
pressure waveform having the largest maximum pressure. Parameter
P.sub.5.sup.s is represented by point 710 on FIG. 9.
[0095] Parameter P.sub.6.sup.s is a parameter taken from a function
such as a curve derived from values of a plurality of waveforms 500
(shown in FIG. 9). Preferably, parameter P.sub.6.sup.s is the peak
pressure of curve 520 shown in FIG. 10. The peak is represented by
point 530. Curve 520 is preferably generated by fitting the
relative waveform pressure amplitude of waveforms 500 (shown in
FIG. 9) to the function or mathematical expression of:
AMPLITUDE=exp (ax.sup.2+bx+c), wherein x=the mean pressure
amplitude of each pressure waveform.
[0096] Parameter P.sub.7.sup.s is a time value representing a width
of waveform 600 (represented by segment 614 between points 616 and
618) which corresponds to a selected percentage of the maximum
pressure amplitude or peak (point 606) of waveform 600. The time
elapsed between points 616 and 618 is determined by counting the
number of samples taken by monitor 26 which lie above points 616
and 618 on waveform 600. Preferably, parameter P.sub.7.sup.s is the
width of waveform 600 at a height of about 0.9 A, where A is the
maximum waveform pressure amplitude of waveform 600 (point
606).
[0097] Parameter P.sub.8.sup.s is the maximum slope of the
uncorrected waveform 500c immediately following the waveform 500a
having the largest maximum pressure or peak.
[0098] The mean blood pressure value (M) is calculated using the
formula:
M=C.sub.1.sup.mP.sub.1.sup.m+C.sub.2.sup.mP.sub.2.sup.m+C.sub.3.sup.mP.s-
ub.3.sup.m+C.sub.4.sup.mP.sub.4.sup.m+C.sub.5.sup.m
[0099] Coefficients C.sub.1.sup.m-C.sub.5.sup.m are stored
coefficients ascertained according to the earlier described method
of the present invention. Coefficient C.sub.5.sup.m is an offset.
Parameters P.sub.1.sup.m and P.sub.2.sup.m are derived from
relative waveform pressure amplitudes corresponding to scaled
values taken from the scaled and corrected beat as represented by
waveform 600 in FIG. 11. Preferably, parameter P.sub.1.sup.m is the
ratio defined by the waveform pressure (point 620) on wavefornm 600
which corresponds to the scale value b.sub.9 along the horizontal
axis divided by the maximum waveform pressure amplitude or peak
(point 606) of waveform 600. Similarly, parameter P.sub.2.sup.m is
the ratio defined by the waveform pressure on waveform 600 which
corresponds to scale value b.sub.13 along the horizontal axis
(point 622) divided by the maximum waveform pressure amplitude or
peak (point 606) of waveform 600.
[0100] Parameter P.sub.3.sup.m is identical to parameter
P.sub.4.sup.s used to calculate systolic blood pressure. Parameter
P.sub.4.sup.m is identical to parameter P.sub.6.sup.s used to
calculate systolic blood pressure.
[0101] Diastolic blood pressure values (D) are calculated using the
formula:
D=C.sub.1.sup.dP.sub.1.sup.d+C.sub.2.sup.dP.sub.2.sup.d+C.sub.3-
.sup.dP.sub.3.sup.d+C.sub.4.sup.dP.sub.4.sup.d+C.sub.5.sup.dP.sub.5.sup.d+-
C.sub.6.sup.dP.sub.6.sup.d+C.sub.7.sup.dP.sub.7.sup.d+C.sub.8.sup.d
[0102] Coefficients C.sub.1.sup.d-C.sub.8.sup.d are stored
coefficients ascertained according to the earlier described method
of the present invention. Coefficient C.sub.8.sup.d is an offset
value. Parameter P.sub.1.sup.d is derived from relative waveform
pressure corresponding to scaled values taken from a scaled and
corrected beat as represented by waveform 600 in FIG. 11.
Preferably, parameter P.sub.1.sup.d is a ratio defined by the
waveform pressure amplitude on waveform 600 which corresponds to
scale value b.sub.12 along the horizontal axis (point 624) divided
by the maximum waveform pressure amplitude or peak (point 606) of
waveform 600.
[0103] Parameter p.sub.2.sup.d is identical to parameter
P.sub.3.sup.d used to calculate the systolic blood pressure.
Preferably, parameter P.sub.3.sup.d is the width of segment 626
between points 628 and 630. Preferably points 626 and 628 are
points along waveform 600 that are located at a height of 0.875 A,
where A is the maximum pressure amplitude (point 606) of waveform
600. The width or time of parameter P.sub.3.sup.d is determined by
counting the number of individual waveform pressure amplitude
signals or samples generated by transducer 40 and transmitted to
monitor 26 which lie above points 626 and 628 on waveform 600. If
points 626 and 628 fall between individual waveform pressure
amplitude signals or samples, interpolation is used to determine
the time width of parameter P.sub.3.sup.d.
[0104] Parameter P.sub.4.sup.d is identical to parameter
P.sub.4.sup.s used to calculate systolic blood pressure. Parameters
P.sub.5.sup.d and P.sub.6.sup.d are calculated from absolute
waveform pressures as illustrated in FIG. 9. Preferably, parameter
P.sub.5.sup.d is the diastolic pressure value of the uncorrected
waveform having the largest maximum pressure value. This diastolic
value is represented by point 720. Parameter P.sub.6.sup.d is the
diastolic pressure value of the uncorrected waveform (waveform
500c) immediately following the waveform (waveform 500a) having the
largest maximum pressure amplitude or peak. Parameter P.sub.6.sup.d
is represented by point 730 on FIG. 9.
[0105] Parameter P.sub.7.sup.d is derived from absolute waveform
pressures illustrated in FIG. 9. To derive parameter P.sub.7.sup.d,
the slopes along the portions of each individual waveform 500 are
determined. Parameter P.sub.7.sup.d is the hold down pressure
applied to the underlying artery that corresponds to the point on
the particular waveform having the maximum slope corrected
amplitude. The slope corrected amplitude of a waveform is obtained
by multiplying its amplitude with the maximum slope over all
waveforms 500 and dividing the result with the slope corresponding
to the individual waveform. As can be appreciated, various
alternative parameters may also be used to calculate blood pressure
values under the method of the present invention.
VI. Conclusion
[0106] The present invention enables blood pressures of patients to
be continuously and non-invasively determined without the
complexity, cost, risks, and inaccuracies associated with the prior
methods and apparatuses for determining blood pressure. Wrist
assembly 24 securely mounts sensor interface assembly 38 upon wrist
22 of the patient so that patient movement does not alter the
optimal location of sensor interface assembly 38 found. The lower
pivot point of sensor interface assembly 38 causes pressure applied
by the sidewall of assembly 38 to the tissue above the underlying
artery to be uniform around the perimeter of the sidewall. As a
result, blood pressure monitoring system 20 samples more accurate
signals representing blood pressure pulse waveforms. By deriving
parameters from the waveform data and using stored coefficients,
blood pressure monitoring system consistently and accurately
determines blood pressure values.
[0107] Although the present invention has been described with
reference to preferred embodiments, workers skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention. For example,
although the determination of pressure values based upon waveform
parameters has been described using linear equations and stored
coefficients, other methods using non-linear equations, look-up
tables, fuzzy logic and neural networks also can be used in
accordance with the present invention.
* * * * *