U.S. patent application number 10/754414 was filed with the patent office on 2005-03-03 for smart physiologic parameter sensor and method.
Invention is credited to Conero, Ronald S., Gallant, Stuart L..
Application Number | 20050049501 10/754414 |
Document ID | / |
Family ID | 29782235 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050049501 |
Kind Code |
A1 |
Conero, Ronald S. ; et
al. |
March 3, 2005 |
Smart physiologic parameter sensor and method
Abstract
A sensor assembly used for the measurement of one or more
physiologic parameters of a living subject which is capable of
storing both data obtained dynamically during use as well as that
programmed into the device. In one embodiment, the sensor assembly
comprises a disposable combined pressure and ultrasonic transducer
incorporating an electrically erasable programmable read-only
memory (EEPROM), the assembly being used for the non-invasive
measurement of arterial blood pressure. The sensor EEPROM has a
variety of information relating to the manufacture, run time,
calibration, and operation of the sensor, as well as application
specific data such as patient or health care facility
identification. Portions of the data are encrypted to prevent
tampering. In a second embodiment, one or more additional storage
devices (EEPROMs) are included within the host system to permit the
storage of data relating to the system and a variety of different
sensors used therewith. In a third embodiment, one or more of the
individual transducer elements within the assembly are made
separable and disposable, thereby allowing for the replacement of
certain selected components which may degrade or become
contaminated. Methods for calibrating and operating the disposable
sensor assembly in conjunction with its host system are also
disclosed.
Inventors: |
Conero, Ronald S.; (San
Diego, CA) ; Gallant, Stuart L.; (San Diego,
CA) |
Correspondence
Address: |
Robert F. Gazdzinski, Esq.
Gazdzinski & Associates
Suite 375
11440 West Bernardo Court
San Diego
CA
92127
US
|
Family ID: |
29782235 |
Appl. No.: |
10/754414 |
Filed: |
January 9, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10754414 |
Jan 9, 2004 |
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09652626 |
Aug 31, 2000 |
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6676600 |
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60152534 |
Sep 3, 1999 |
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Current U.S.
Class: |
600/449 |
Current CPC
Class: |
A61B 8/00 20130101; A61B
5/02158 20130101; A61B 8/06 20130101; A61B 2560/0252 20130101; A61B
5/1112 20130101; A61B 2562/08 20130101; A61B 2562/02 20130101; A61B
2018/00178 20130101; A61B 5/0002 20130101; A61B 2018/00988
20130101; A61B 5/6843 20130101; A61B 2560/0276 20130101; A61B 5/00
20130101 |
Class at
Publication: |
600/449 |
International
Class: |
A61B 008/02 |
Claims
1-36. cancel
37. A method of operating a device used for measuring at least one
parameter associated with a living subject, said device comprising
a host system and a detachable sensor assembly with associated
storage device having a first plurality of data stored therein, the
method comprising: placing said sensor assembly in data
communication with said host system, said host system having a
second plurality of data associated therewith; reading at least a
portion of said first plurality of data from said storage device;
evaluating said at least portion of said first plurality of data
and at least a portion of said second plurality of data; and
determining whether said sensor assembly is enabled for measuring
said at least one parameter based at least in part on said act of
evaluating.
38. The method of claim 37, further comprising reading said second
plurality of data from a storage device.
39. The method of claim 38, wherein the act of storing a first
plurality of data comprises storing said data within an
electrically erasable programmable read-only memory (EEPROM).
40. The method of claim 37, wherein said at least portion of said
first plurality of data comprises data relating to the date of
manufacture of said sensor assembly, and said at least portion of
said second data comprises another date which is later than that of
said date of manufacture.
41. The method of claim 37, wherein said at least portion of said
first plurality of data comprises data relating to the duration of
use of said sensor assembly, and said at least portion of said
second data comprises a parameter relating to a maximum allowed
duration.
42. The method of claim 37, wherein said at least portion of said
first plurality of data comprises data relating to the user of said
sensor assembly, and said at least portion of said second data
comprises a parameter relating to said user.
43. The method of claim 37, wherein said act of evaluating
comprises determining if said sensor assembly is compatible with
said host system.
44. The method of claim 43, wherein said act of determining if said
sensor assembly is compatible with said host system comprises
comparing a first multi-bit hexadecimal value in said at least
portion of said first data with a corresponding value of said
second data.
45. The method of claim 37, wherein said device comprises a device
adapted to measure a hemodynamic parameter, and said act of placing
comprises physically mating said detachable sensor assembly to a
substantially movable portion of said device.
46. The method of claim 37, further comprising calibrating said
sensor assembly based at least in part on said first data.
47. The method of claim 45, wherein said enabling for measurement
comprises a condition precedent to said act of calibrating said
sensor.
48. The method of claim 37, wherein said at least portion of said
first data comprises cryptographic data which is uniquely related
to complementary cryptographic data of said at least portion of
said second data.
49. A method of operating a device used for measuring at least one
parameter associated with a living subject, said device comprising
a host system and a detachable sensor assembly with associated
storage device having a first plurality of data stored therein, the
method comprising: placing said sensor assembly in data
communication with said host system; reading at least a portion of
said first plurality of data from said storage device; creating a
test condition within said sensor assembly; determining at least
one value relating to the operation of said sensor assembly
relating to said test condition; and determining whether said
sensor assembly is enabled for measuring said at least one
parameter based at least in part on said act of measuring.
50. The method of claim 49, wherein said sensor assembly comprises
a bridge circuit, and said portion of said first data comprises
data relating to the electrical output of said sensor assembly
during a shunted condition of said bridge circuit, and said act of
determining at least one value comprises: creating substantially
the same shunted condition within said sensor assembly; and
obtaining second data relating to the electrical output of said
sensor assembly.
51. A method of recording data, comprising: collecting first data
relating to at least one parameter associated with a living
subject; analyzing said first data to generate a first estimate of
said at least one parameter; collecting second data relating to
said at least one parameter; analyzing said second data to generate
a second estimate of said at least one parameter; comparing said
first estimate and said second estimate using at least one
acceptance criterion; and storing at least either said first data
or said second data in a storage device if said at least one
criterion is met.
52. The method of claim 51, wherein the acts of collecting said
first and second data each comprise collecting pressure data
derived from a tonometric pressure sensor, and said first and
second estimates comprise estimates of a blood pressure.
53. The method of claim 51, wherein the acts of analyzing said
first and second data comprise determining a time-frequency
representation for each.
54. The method of claim 51, wherein the acts of analyzing said
first and second data comprise: measuring a first hemodynamic
parameter from a blood vessel of said subject; measuring a second
parameter from said blood vessel; deriving a calibration function
based at least in part on said second parameter; and calibrating
the first hemodynamic parameter using said calibration
function.
55. The method of claim 54, wherein the act of measuring a first
hemodynamic parameter comprises measuring blood velocity.
56. The method of claim 51, further comprising: obtaining third and
fourth data relating to the identity of an individual during at
least a portion of said acts of collecting said first and second
data, respectively; and comparing said third and fourth data to
verify that said identity relating to each is the same.
57. A method of ensuring the adequacy of a sensor assembly used in
measuring at least parameter associated with a living subject,
comprising: encoding first data within said sensor assembly, said
first data being representative of a first time; placing said
sensor assembly in data communication with a host system; measuring
a second time using said host system; comparing said first and
second times using an acceptance criterion; and disabling said
sensor assembly from further use if said criterion is not
satisfied.
58. The method of claim 57, wherein the act of comparing comprises
calculating the difference between said first and second times, and
comparing said difference to said acceptance criterion.
59. A method of ensuring the condition of a degradable component
within a medical device, comprising: providing a degradable
component, said degradable component being adapted to measure at
least one physical parameter associated with a living subject;
measuring said at least one parameter of a living subject using
said degradable component to obtain first data; storing said first
data relating to said at least one parameter within a storage
device; measuring said at least one parameter a second time using
said degradable component to obtain second data; comparing said
first data to the second data using at least one predetermined
criterion; and disabling said medical device if said at least one
criterion is not satisfied.
60. The method of claim 59, wherein the act of measuring comprises
measuring pressure.
61. The method of claim 60, wherein said at least one predetermined
criterion comprises a difference in pressure.
62. The method of claim 59, further comprising deriving first and
second estimates of arterial blood pressure based at least in part
on said acts of measuring said at least one parameter at said first
and second times, respectively.
63. A method of operating a blood pressure measuring device
comprising a host system and a detachable pressure sensor assembly
with associated storage device having a first plurality of data
stored within, the method comprising: placing said sensor assembly
in data communication with said host system, said host system
having a second plurality of data associated therewith; reading at
least a portion of said first plurality of data from said storage
device; comparing said at least portion of said first plurality of
data to at least a portion of said second plurality of data; and
determining whether said sensor assembly is enabled for measuring
blood pressure based at least in part on said act of comparing.
64. A method of evaluating the performance of a plurality of
replaceable sensor assemblies each having a storage device
associated therewith, and first data adapted to differentiate each
sensor from at least a portion of said plurality, the method
comprising: operating each of said plurality of sensor assemblies;
generating second data resulting from said act of operating;
storing said second data; and analyzing said second data from at
least a portion of said plurality of sensor assemblies based at
least in part on said first data associated therewith.
65. The method of claim 64, wherein said act of operating
comprises: attaching each of said replaceable sensors to a host
device; and performing at least one verification of each of said
sensor assemblies based at least in part on data stored in said
storage device.
66. The method of claim 64, wherein said act of generating second
data comprises generating at least one of a plurality of possible
error failure codes for said sensor assembly.
67. The method of claim 66, wherein said act of storing said second
data comprises storing said second data within said storage device
for that sensor assembly.
68. The method of claim 66, wherein said act of storing said second
data comprises storing said second data within a storage device
associated with a host device to which that sensor assembly is
connected.
69. The method of claim 68, further comprising aggregating second
data from a plurality of ones of said host devices in order to
perform said analyzing.
70. The method of claim 66, wherein said second data comprises
failure or error codes, and said act of analyzing comprises
identifying one or more commonalities in said codes across multiple
ones of said sensor assemblies.
71. Replaceable sensor apparatus adapted for measuring at least one
parameter associated with a living subject in concert with a host
device, said detachable sensor assembly further comprising a
storage device having a first plurality of data stored therein,
said sensor assembly further being adapted for qualification
according to the method comprising: placing said sensor assembly in
data communication with said host system; reading at least a
portion of said first plurality of data from said storage device;
creating a test condition within said sensor assembly; determining
at least one value relating to the operation of said sensor
assembly relating to said test condition; and determining whether
said sensor assembly is qualified for measuring said at least one
parameter based at least in part on said act of measuring.
72. Replaceable sensor apparatus adapted for use with a host device
and configured to prevent use thereof beyond a prescribed period of
time according to the method comprising: encoding first data within
said sensor apparatus, said first data being representative of a
first time; placing said sensor apparatus in data communication
with said host system; measuring a second time using said host
system; comparing said first and second times using an acceptance
criterion; and disabling said sensor apparatus from further use if
said criterion is not satisfied.
73. Replaceable tonometric sensor apparatus adapted to preserve
data integrity according to the method comprising: collecting first
data relating to at least one parameter associated with a living
subject using said sensor apparatus; analyzing said first data to
generate a first estimate of said at least one parameter;
collecting second data relating to said at least one parameter
using said sensor apparatus; analyzing said second data to generate
a second estimate of said at least one parameter; comparing said
first estimate and said second estimate using at least one
acceptance criterion; and storing at least one of said first data
and said second data in a storage device if said at least one
criterion is met.
74. Tonometric disposable medical sensor apparatus adapted for use
with a host device and having at least one pressure sensor and
storage device associated therewith, said storage device containing
both identifying and parametric information, said identifying and
parametric information being used in cooperation by said sensor
apparatus and host device to ensure that (i) said sensor apparatus
is compatible with said host device; (ii) said sensor apparatus is
functioning properly, and (iii) said sensor apparatus has not
expired.
75. The apparatus of claim 74, wherein said apparatus is further
adapted to verify use on only one patient.
Description
[0001] Pursuant to 35 U.S.C. 119(e), this application claims
priority benefit of U.S. provisional patent application Ser. No.
60/152,534 entitled "Smart Blood Pressure Sensor and Method" filed
Sep. 3, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the field of medical
instrumentation, specifically the use of electronic storage devices
for storing and retrieving data relating to, inter alia, particular
instruments or patients.
[0004] 2. Description of Related Technology
[0005] The ability to readily measure various physiologic
parameters associated with a living subject, such as arterial blood
pressure or ECG, is often critical to providing effective care to
such subjects. Typically, under the prior art, measurement of such
parameters is accomplished using a system comprising a host device
such as a portable or semi-portable monitoring station that is used
in conjunction with a replaceable/disposable probe or sensor
assembly, the latter being in direct contact with the subject and
measuring the physical parameter (or related parameters) of
interest. Such replaceable and disposable sensor assemblies are
highly desirable from the standpoint that the risk of transfer of
bacterial or other contamination from one patient to the next is
significantly mitigated; the portion of the sensor assembly (or for
that matter entire assembly) in contact with a given subject is
replaced before use on another subject.
[0006] However, despite the mitigated risk of contamination, the
use of such prior art disposable sensors also includes certain
risks. One such risk relates to the potential re-use of what are
meant to be single-use only components. Inherently, individuals or
health care providers may attempt to re-use such single use
components if there is no seeming degradation of the component or
perceived threat of contamination. However, in the case of certain
devices, the degradation of the component may be insidious and not
immediately perceptible to the user. For example, the offset (i.e.,
difference of voltage generated by the device at certain prescribed
conditions) associated with an elastomer-coated pressure transducer
used in a non-invasive blood pressure monitoring device may change
progressively in small increments over time due to swelling of the
elastomer coating resulting from exposure to certain chemical
substances. This variation in offset manifests itself as a change
in the ultimate blood pressure reading obtained using the device,
thereby reducing its accuracy. Hence, the readings obtained using
the instrument may appear to be reasonable or correct, but in fact
will incorporate increasing amounts of error from the true value of
the parameter, which may significantly impact the treatment
ultimately provided to the subject. Hence, what is needed is an
approach wherein any such degradable or single use components are
reliably replaced at the necessary interval such that performance
does not appreciably degrade.
[0007] A related issue concerns the re-use of such devices on
different patients. Specifically, if the "single use" components
are perceived by the user not to degrade rapidly, the user may be
tempted to use the device (including the single use transducer(s))
on several different patients. Aside from the aforementioned
performance issues, such repeated use may be hazardous from a
contamination standpoint, as previously discussed. Ideally,
portions of the device capable of transmitting bacterial, viral, or
other deleterious agents are disposed of and replaced prior to use
on another patient.
[0008] Another risk concerns the use of third party or
non-compliant sensors with the host device of the original
equipment manufacturer (OEM). While such third party sensors may
ostensibly be manufactured to the design specifications and
requirements of the OEM, in many cases they are not, which can
result in readings obtained using the system which are less than
accurate or even wholly non-representative of the parameter being
measured. Even OEM supplied disposable sensors may have defects.
Another troubling aspect is the fact that the caregiver or health
care professional who is provided with such disposable sensors may
have no means by which to verify the quality or acceptability of a
given replaceable sensor, and therefore the accuracy of any reading
they may obtain using that sensor may be called into question.
Hence, even if the majority of sensors within a given lot obtained
from the third party manufacturer are acceptable in terms of
performance, the caregiver often has no way of knowing whether the
next replacement sensor they use will perform as designed or
intended by the OEM and yield representative results. In the ideal
case, the quality of each individual replacement sensor would be
determined by the host system prior to use (such as when the new
replacement sensor is first installed on the host), and the
caregiver apprised of the results of this determination.
[0009] The calibration of the replaceable/disposable sensor,
whether OEM or otherwise, and the host system must also be
considered. Under the prior art approach, calibration is most often
performed on the system as a whole at a discrete point in time, and
is generally not performed before each use of the device after a
new sensor or probe has been installed. Hence, the calibration of
the host system and replaceable sensor as a whole is not specific
to each given sensor, but rather to a "nominal" sensor (i.e., the
one in place in the system when the calibration was performed). For
example, the system may be calibrated before first use, and then
periodically thereafter at predetermined intervals, or at the
occurrence of a given condition. Under this approach, changes in
the physical operating characteristics of the host system may
result in changes in the calibration over time. Due to any number
of intrinsic or external factors, the device may "drift" between
calibrations, such that a reading taken with the device immediately
following calibration may be substantially different from that
obtained using the same device and identical conditions immediately
before the next calibration.
[0010] Additionally, due to manufacturing tolerances and
variations, the performance of each individual replaceable sensor
may vary significantly from other similar devices, as previously
described. Such variations are generally accounted for by the OEM
by specifying a maximum allowable tolerances or variances for
certain critical parameters associated with the sensors; if these
tolerances/variances are met for a given replaceable sensor, then
the accuracy of the system as a whole will fall within a certain
(acceptable) tolerance as well. Ideally, however, the system would
be calibrated specifically to each individual replaceable sensor
immediately prior to use, a capability which is not present in
prior art disposable medical devices.
[0011] Another concern relates to the potential for surreptitious
alteration of data stored by an instrument prior to or during
operation. As with many other types of devices, the ability to make
a device "tamperproof" is of significant importance, in that this
provides the caregiver and subject with additional assurance that
the disposable sensor in use is the correct type of sensor for the
host system, that the sensor assembly and host system are properly
calibrated, and that the disposable sensor has not been used on
other subjects.
[0012] Lastly, it is recognized that prior art measurement systems
do not include the facility for evaluating the accuracy of a given
measurement or host/sensor combination after readings have been
taken. Many systems are capable of storing data relating to a
measurement obtained from a subject in terms of the estimated
value(s) derived by the system, yet none of which the Assignee
hereof is aware allow for the retrieval of data specific to a given
sensor or permit the system operator to evaluate the performance
(and accuracy) of the system historically. Such information is of
great potential utility in the medical field, especially with
relation to medical malpractice litigation, by enabling the
caregiver or OEM to reconstruct the operation of their equipment to
demonstrate that a given measurement obtained using a given sensor
and host unit was in fact accurate, that the disposable sensor had
been replaced prior to use on the patient, and the like. The
availability of this information may also produce the added benefit
of reduced medical malpractice insurance premiums for facilities
using such systems, since the potential for fraudulent claims
relating to the system is reduced.
[0013] Based on the foregoing, what is needed is an apparatus and
associated method useful for measuring one or more physiologic
parameters associated with a living subject wherein any degradable
or single use components associated with the apparatus may be
easily and reliably replaced so as to ensure that (i) the accuracy
of the system and any measurements resulting there from do not
degrade; (ii) cross-contamination between subjects does not occur;
and (iii) the operating history of the replaced components and
system as a whole may be subsequently retrieved for analysis.
SUMMARY OF THE INVENTION
[0014] The present invention satisfies the aforementioned needs by
an improved apparatus and method for monitoring the physiologic
parameters, such as for example arterial blood pressure, of a
living subject.
[0015] In a first aspect of the invention, an improved sensor
assembly incorporating an electronic storage element is disclosed.
In a first embodiment, the device comprises one or more ultrasonic
transducers and a removable (and disposable) pressure transducer,
the latter further including a storage device in the form of an
electrically erasable programmable read-only memory (EEPROM)
capable of storing data and information relating to the operation
of the sensor assembly, host system, and patient. The sensor EEPROM
includes a variety of information relating to the manufacture, run
time, calibration, and operation of the pressure transducer, as
well as application specific data such as patient or health care
facility identification. Portions of the data are encrypted to
prevent tampering. Furthermore, the host system is programmed such
that the sensor assembly will be rejected and rendered unusable by
the host if certain portions of the aforementioned data do not meet
specific criteria. In this fashion, system operational integrity,
maintainability, and patient safety are significantly enhanced. In
a second embodiment, one or more additional storage devices (e.g.,
EEPROMs) are included within the host system to permit the storage
of data relating to the system and a variety of different sensors
used therewith. In a third embodiment, a single storage device
(e.g., EEPROM) is associated with the entire sensor assembly,
including ultrasonic transducer(s) and pressure transducer, and
adapted to store and provide data relating thereto.
[0016] In a second aspect of the invention, an improved sensor
housing assembly is disclosed. In one exemplary embodiment, the
housing assembly comprises first and second housing elements which
are fabricated from a low cost polymer and which include recesses
containing the ultrasonic and pressure transducer elements,
respectively. The first housing element is adapted to removably
receive the second such that the active faces of the ultrasonic and
pressure transducer elements are substantially aligned when the
housing elements are assembled, and the second housing element (and
associated pressure transducer with EEPROM) can be readily disposed
of and replaced by the user when required without having to replace
or dislocate the first housing element. In a second embodiment, the
first housing element is also made optionally removable from the
sensor assembly such that the user may optionally replace just the
pressure transducer/EEPROM, the ultrasonic transducer(s), or both
as desired.
[0017] In a third aspect of the invention, an improved system for
measuring one or more physiologic parameters of a living subject is
disclosed. In one embodiment, the physiologic parameter measured
comprises arterial blood pressure in the radial artery of a human
being, and the system comprises the aforementioned sensor assembly
having at least one ultrasonic transducer capable of generating and
receiving ultrasonic signals, a pressure transducer capable of
measuring the pressure applied to its active surface, and a storage
device associated therewith; a local controller assembly in data
communication with the sensor assembly further including an
applanation/lateral device and controller, and a remote analysis
and display unit having a display, signal processor, and storage
device in data communication with the local controller assembly.
Calibration and other data pertinent to the sensor assembly which
is stored in the storage device (e.g., EEPROM) of the sensor
assembly is read out of the EEPROM and communicated to the analysis
and display unit, wherein the processor within the unit analyzes
the data according to one or more algorithms operating thereon.
Signal processing circuits present within the local controller
assembly are also used to analyze electrical signals and data
relating to the operation of the sensor assembly.
[0018] In a fourth aspect of the invention, an improved circuit
used in effectuating the calibration of the transducer element(s)
of the aforementioned sensor array are disclosed. In one exemplary
embodiment, the circuit comprises an analog circuit having a
transducer element, a span TC compensation resistor R.sub.a,
analog-to-digital converter (ADC), digital-to-analog converter
(DAC), and operational amplifiers. The voltage output of the
pressure transducer (bridge) is input to a first stage
instrumentation amplifier which amplifies the transducer output
signal. The amplified output is input to a second stage amplifier,
along with the output of the, DAC, which is subtracted from the
signal. The output of the second stage amplifier represents the
temperature compensated, zero offset output signal of the circuit.
The DAC converts a digital signal derived from the system processor
to compensate the output for the offset of the bridge, as well as
the temperature coefficient of the offset. The ADC is used to
measure the bridge voltage, which varies with temperature by virtue
of span compensation resistor Ra. Resistor Ra has a near zero TC,
while the bridge itself has a positive TC. Thus, the bridge voltage
varies with temperature, and can be correlated to the offset
variation with temperature. In a second embodiment, the DAC and
second amplifier are omitted, and replaced by a high resolution
ADC. The converter must have the dynamic range and signal to noise
ratio to measure the large output swing from the instrumentation
amplifier. This is true, because the output now contains both the
signal, and the offset error, and offset TC error. In this
embodiment, the bridge voltage still varies with temperature and is
digitized by the ADC after amplification, but all the compensation
is handled by digital signal processing in the system
processor.
[0019] In a fifth aspect of the invention, an improved method of
operating a disposable sensor in conjunction with its host system
is disclosed. In one embodiment, the method comprises storing at
least one data field within the aforementioned storage device of
the sensor, connecting the sensor to a host system, and determining
the compatibility of the sensor with the host device based at least
in part on the at least one data field. In a second embodiment of
the method, the sensor is operated in order to obtain data from at
least one living subject; this data is then stored within the
sensor and/or host system in order to provide a retrievable record
of the operation of the sensor and of the specific patient
tested.
[0020] In a sixth aspect of the invention, an improved method of
calibrating a transducer element used within a blood pressure
monitoring device is disclosed. The method comprises providing a
transducer element having a predetermined operating response and
associated storage device; determining the operating response for
the transducer; determining a plurality of calibration parameters
based on the determined operating response; storing data
representative of the calibration parameters within the storage
device; and calibrating the transducer during operation based at
least in part on the stored calibration parameters. In one
embodiment, the transducer element comprises a silicon strain beam
pressure transducer and the calibration parameters comprise
reference voltage and temperature values, linearity, sensitivity,
and shunted resistor values calculated using a series of
predetermined functional relationships. These calibration
parameters are stored in the EEPROM previously described at time of
manufacture. When used during normal operation, the output of the
pressure transducer is calibrated by the host system using the
pre-stored calibration parameters taken directly from the EEPROM
during each individual use.
[0021] In a seventh aspect of the invention, an improved method of
ensuring the condition of limited (e.g., single) use components
within a blood pressure monitoring device is disclosed. The method
generally comprises providing a blood pressure monitoring device
including a removable sensor assembly; measuring at least one
parameter of a living subject using the device and sensor assembly
to obtain first data; storing the first data relating to the at
least one parameter; measuring the at least one parameter at a
second time to obtain second data; comparing the stored first data
to the second data using a predetermined criterion; and disabling
the blood pressure measuring device if the criterion is not
satisfied. In one embodiment, the sensor assembly comprises a
pressure transducer and one or more ultrasonic transducers, which
are collectively used to gather parametric data relating to the
blood pressure within the radial artery of the subject. The
parametric data is stored within the EEPROM, and compared with
subsequent measurements taken with the same device using a
comparison algorithm. In this fashion, significant differences
between the parametric data obtained in successive readings is
detected, which indicates that the caregiver has used the device on
different patients. If certain acceptance criteria are exceeded,
the system generates a disable signal which prevents completion of
the analysis and display of the current measurement, as well as any
subsequent measurements, until the pressure transducer (and
optionally ultrasonic transducers) is/are replaced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1a is an exploded perspective view of a first
embodiment of the smart sensor assembly according to the present
invention.
[0023] FIG. 1b is top perspective view of the sensor assembly of
FIG. 1, shown assembled.
[0024] FIG. 1c is bottom perspective view of the sensor assembly of
FIG. 1, shown assembled.
[0025] FIG. 2a is a perspective assembly view of a second
embodiment of the smart sensor assembly of the invention.
[0026] FIG. 2b is a cross-sectional view of the sensor assembly
housing of FIG. 2, taken along lines 2-2.
[0027] FIG. 2c is perspective view of the sensor assembly of FIG.
2a, shown installed within a gimbal assembly.
[0028] FIG. 3 is a top plan view of a third embodiment of the
sensor assembly of the invention, having both removable/disposable
pressure and ultrasonic transducers.
[0029] FIG. 4 is a functional block diagram illustrating a first
embodiment of a physiologic parameter measurement apparatus
incorporating the smart sensor assembly of the invention.
[0030] FIG. 5 is a logical block diagram of a second embodiment of
the physiologic parameter measurement apparatus of the invention,
including a wireless communications link.
[0031] FIG. 6 is a logical flow diagram illustrating one exemplary
embodiment of the method of calibrating a disposable sensor element
according to the invention.
[0032] FIG. 7 is a schematic diagram of a first embodiment of the
logic circuit of the invention.
[0033] FIG. 8 is a schematic diagram of a second embodiment of the
logic circuit of the invention.
[0034] FIG. 9 is a flow diagram illustrating one embodiment of the
method of evaluating the acceptability of the smart sensor assembly
of the invention in conjunction with the physiologic parameter
measurement apparatus of FIG. 4.
[0035] FIG. 10 is a flow diagram illustrating one embodiment of the
generalized method of encoding and storing data related to an
applanation measurement performed on a patient within the sensor
assembly of the invention.
[0036] FIG. 11 is a flow diagram illustrating one exemplary
embodiment of the method of encoding physiologic parameters within
the sensor of the present invention.
[0037] FIG. 12 is an exemplary plot of pressure and arterial blood
velocity data which is encoded using the method of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0038] Reference is now made to the drawings, in which like
numerals refer to like parts throughout. For purposes of clarity,
the following description of the subject invention is cast in the
context of arterial blood pressure measuring systems utilizing the
principle of arterial tonometry and ultrasonic wave analysis. Such
blood pressure monitoring systems are disclosed, for example, in
co-pending U.S. patent application Ser. No. 09/342,549, entitled
"Method and Apparatus for the Non-Invasive Determination of
Arterial Blood Pressure", filed Jun. 29, 1999, which is assigned to
the assignee hereof, and incorporated herein by reference in its
entirety. Alternatively, the methods and apparatus described in
co-pending U.S. patent application Ser. No. 09/534,900 entitled
"Method and Apparatus for Assessing Hemodynamic Parameters Within
the Circulatory System of a Living Subject" filed Mar. 23, 2000,
also incorporated herein by reference in its entirety, may be used
in conjunction with the present invention. Other methods and
apparatus, regardless of theory or principles of operation, may
also be substituted.
[0039] It is also noted that while the invention is described
herein in terms of a method and apparatus for assessing the
hemodynamic parameters of the circulatory system via the radial
artery (i.e., wrist) of a human subject, the invention may also be
embodied or adapted to monitor such parameters at other locations
on the human body, as well as monitoring these parameters on other
warm-blooded species. All such adaptations and alternate
embodiments are considered to fall within the scope of the claims
appended hereto.
[0040] The present invention generally comprises a "smart" blood
pressure sensor assembly which is used in conjunction with blood
pressure system and host device in order to provide the enhanced
functionality of the invention. This functionality includes, inter
alia, (i) the ability to pre-store data relating to the
manufacture, configuration, and calibration of the sensor assembly
prior to use; (ii) the ability to use the pre-stored data to
calibrate and enable/disable the sensor assembly during use, based
on certain parameters and analyses conducted when the sensor
assembly is connected to the host device; and (iii) the ability to
store data obtained by the sensors or designated by the subject or
caregiver within the sensor assembly and/or the host device during
use. Each of these aspects in one fashion or another enhances the
accuracy and reliability of blood pressure measurements taken with
the system, as described in greater detail in the following
paragraphs.
[0041] Sensor Assembly and Housing
[0042] Referring now to FIGS. 1a-1c, a first embodiment of the
smart sensor assembly 100 of the present invention is described. As
shown in FIGS. 1a-1c, the sensor assembly 100 of this embodiment
comprises a cover 102 and main sensor housing 104 which are mated
together to form the sensor body 106. The shape of the cover 102
and sensor housing 104 is generally that of an elongate rectangle,
although it will be recognized that other shapes may be used. A
mounting element 107 is formed in the cover 102 to permit, inter
alia, positional control of the assembly 100 by the local
controller assembly 444 (FIG. 4), as well as an electrical
penetration (not shown) for providing power for and data
communication with the assembly 100. In the illustrated embodiment,
the mounting element is generally spherical or ball-shaped to
permit the assembly to couple to the applanation mechanism 407 and
operate in a variety of orientations with respect to the local
controller 444, although other arrangements (such as universal
joints, Heim joints, etc.) well known in the mechanical arts may be
used.
[0043] The housing 104 and cover 102 of the illustrated embodiment
are fabricated from a high strength, low cost polymer such as
polycarbonate to provide the desired mechanical and electrical
properties while still making the disposal of the assembly 100
economically feasible. Other materials (both polymeric and not) may
be substituted depending on the properties and attributes
desired.
[0044] The main housing 104 and cover 102 enclose a number of
components, including a printed circuit board (PCB) 108, sensors in
the form of a pressure transducer chip 110 and ultrasonic
transducer (e.g., PZT) 112, bonding ring 114 for the pressure
transducer chip 110, and storage device 116. In the embodiment of
FIGS. 1a-1c, the storage device 116 and pressure and ultrasonic
transducers 110, 112 are mounted onto the PCB 108 in a manner well
understood to those of ordinary skill in the electrical arts,
although other arrangements may be used. The storage device 116 of
the embodiment of FIGS. 1a-1c is an electrically erasable
programmable read only memory (EEPROM), although it will be
appreciated that other types of storage devices such as an erasable
PROM (EPROM), ultraviolet EPROM (UVEPROM), SRAM, DRAM, SDRAM, flash
memory, or magnetic media may conceivably be used for a portion or
all of the desired functionality. The storage device 116 chosen for
use in the present embodiment is a 1K EEPROM device manufactured by
Microchip Corporation, which operates from a 5 volt supply and
utilizes a 2 wire serial link for data transfer. The 1K byte EEPROM
is small in size and packaged in a SOT-23 package. This device 116
allows for cost-effective non-volatile storage of up to 1024 bits
of information organized as 128 8-bit words. In the illustrated
configuration, the entire device may be written in a time period on
the order of one second, thereby providing rapid storage
capability. The device also nominally allows 1 million write
cycles, thereby allowing for extended use within a given sensor.
Data retention of the device is on the order of 100 years, thereby
greatly exceeding the anticipated shelf life of the assembly 100 as
a whole.
[0045] The aforementioned EEPROM is easily accommodated within the
sensor housing 104 on the PCB 108. Connections are made in the
present embodiment by wire bonding or alternate methods at the same
time the pressure transducer is connected 110, although other
assembly and bonding methods may be used. A gel cup (not shown) or
other means may optionally be used to protect the EEPROM 116 from
electrostatic discharge or other electrical or physical trauma.
Electrical signals are transferred in and out of the sensor
assembly 100 using a plurality of electrical conductors (not shown)
of the type well known in the art. Essentially any configuration of
electrical connector or coupling may be substituted depending on
the needs of the particular application.
[0046] Referring now to FIGS. 2a-2c, a second embodiment of the
sensor assembly according to the present invention is described. As
illustrated in FIG. 2a, the sensor assembly 200 of the second
embodiment comprises a two-lobed first housing element 202 and a
generally rectangular second housing element 204, the two elements
202, 204 fitting together to form a unitary assembly. The first
element 202 includes a pair of adapted recesses 206, 208 in which
at least a portion of respective ultrasonic transducer elements
210, 212 are received. The second housing element 204 similarly
includes a recess 214 in which the pressure transducer 416 with
associated storage device 218 is received. Two ultrasonic
transducer elements 210, 212 are used in the illustrated embodiment
to measure hemodynamic parameters, and blood velocity. These
factors allow correct positioning of the pressure transducer for
accurate measurements of blood pressure, as described in
Applicant's aforementioned co-pending applications. The pressure
transducer 216 comprises a silicon strain beam transducer element
which generates an electrical signal in functional relationship
(e.g., proportional) to the pressure applied to its sensing
surface. Similarly, the ultrasonic transducers 210, 212 comprises
piezoelectric (ceramic) devices which are capable of both
generating and receiving ultrasonic waves and/or pulses depending
on mode. In the illustrated embodiment, the ultrasonic transducers
210, 212 are tuned to generate ultrasonic frequencies centered at 8
MHz and 16 MHz respectively, although other center frequencies,
with varying bandwidths, may be used. The transducer elements 210,
212, 216 are frictionally received within the recesses of their
respective housing elements 202, 204 via an interference fit of the
type well known in the art, although other arrangements (such as
adhesives) may be used to retain the transducer elements in the
desired position(s).
[0047] The housing elements 202, 204 are formed from a low-cost
thermoplastic such as polycarbonate although it will be recognized
that other materials such as ethylene tetrafluoroethylene (i.e.,
Tefzel.RTM.), Teflon.RTM., PVC, ABS, or even non-polymers may be
substituted depending on the desired material and physical
properties (such as rigidity, tensile strength, compatibility with
certain chemical agents, ultrasonic transmission at certain
wavelengths, etc.).
[0048] As in the embodiment of FIGS. 1a-1c, the storage device 218
of FIG. 2a comprises an EEPROM, although other types of devices
including EPROM, UVEPROM, or even RAM may be substituted. The first
and second housing elements 202, 204 are adapted to fit together
such that the second element 204 is removable from the first
element 202. This aspect of the invention allows for the removal of
the pressure transducer element 216 and associated storage device
218 from the first housing element 202, thereby rendering the
former disposable if desired. Note however that the first housing
element 202 may also be made removable or separable from the local
controller 444 (FIG. 4), such that both components are separately
disposable (e.g., in the event that it is desired to operate one
type of transducer element for a period different than that for the
other type of transducer element).
[0049] In the illustrated embodiment, the second housing element
204 "snaps" into a channel 220 formed in the first housing element
202 such that the contact surfaces 222, 224 of each the first
housing element and that of the pressure transducer 225 are in
substantial planar alignment. In this fashion, the contact surfaces
222, 224, 225 each contact the skin (or interposed coupling medium)
of the subject concurrently, allowing for ready coupling of each of
the transducers to the subject. The snap functionality previously
described is accomplished using a series of transverse ridges 226
formed on exterior lateral surfaces 228a, 228b the first housing
element coupled with the extending inner edges 230a, 230b of the
removal tabs 232 formed on the corresponding sides of the second
housing element 204, although it will be appreciated that any other
types of arrangements for retaining the second housing element 204
in a given physical relationship with the first housing element 202
may be utilized, such arrangements being well understood by those
of ordinary skill in the mechanical arts. For example, other types
of snap arrangements (such as one or more raised pins or
protrusions, coupled with a complementary detent) may be used.
Alternatively, a frangible construction may be employed. As yet
another alternative, an adhesive such as a non-permanent
silicone-based adhesive, or a frictional interference construction
may be used to retain the second housing element 204 within the
first 202.
[0050] The removal tabs 232 of the second housing element 204 are
constructed such that when the tabs are grasped by the user (such
as between the thumb and forefinger) and compressed slightly, the
extending inner edges 230a, 230b of the tabs 232 disengage slightly
from the transverse ridges 226, thereby allowing the second housing
element 204 to be removed from the first 202 by pulling it
vertically there from. The sidewalls 240a, 240b of the second
housing element are designed to allow sufficient flexibility such
that when the tabs 232 are compressed, the sidewalls flex and
disengage the inner edges 230a, 230b from their respective ridges
226.
[0051] The first and second housing elements are also provided with
a groove 234 and vertical ridge 236 formed on corresponding mating
surfaces of the two components which act to align the second
housing element properly, and in one orientation only, within the
first housing element. Hence, it will be apparent that the second
housing element 204 may only be received within the first element
202 in one orientation, such that the transducer elements 210, 212,
216 are in proper alignment when the assembly 200 is properly
assembled.
[0052] The first housing element 202 is further equipped with a
pair of pivot pins 250a, 250b which are disposed linearly and
parallel to the longitudinal axis 252 of the first element 202. The
pivot pins 250 are received within respective bores (not shown)
formed in a first support element 254 of the gimbal assembly 256 as
shown in FIG. 2c, the latter being mounted to the local controller
device 444 (FIG. 4). This arrangement permits the sensor assembly
200 to rotate around at least the longitudinal axis 252, thereby
allowing the active surface of the transducer element 216 (as well
as the contact surfaces 222, 224 of the first housing element 202)
to orient themselves properly on the surface of the subject's skin.
A secondary pivot arrangement 260 transverse to the axis 252 is
also provided with respect to a second support element 258 as
shown, thereby allowing the sensor assembly 200 to rotate around
the transverse axis 264 in the direction 266. Hence, when coupled
to the gimbal 256, the sensor assembly 200 is advantageously
allowed three distinct degrees of freedom (two rotational, and one
in the vertical or normal direction), which permits the ultrasonic
transducer elements 210, 212 and the pressure transducer 216 to be
correctly oriented with respect to the skin of the subject at all
times during the measurement, even when the subject moves during
the measurement.
[0053] Additionally, it will be appreciated that while the
embodiment of the sensor assembly 200 of FIGS. 2a-2c includes a
generally two-lobed first housing element 202 and second housing
element with a single transducer element 216, even other
configurations may be used. For example, as illustrated in the
embodiment of FIG. 3, a single ultrasonic transducer 310 and single
pressure transducer 316 can be used, the two transducers being
aligned in a "side-by-side" configuration within the complementary
housing elements 302, 304. Hence, the embodiments of FIGS. 1a-3 are
merely illustrative of the broader concept of having either or both
the pressure transducer and/or ultrasonic transducer element(s)
(with any associated storage device) being separable from the local
controller and optionally disposable.
[0054] Apparatus for Physiologic Assessment
[0055] Referring now to FIG. 4, an apparatus for assessing the
physiologic parameters of a living subject and incorporating the
"smart" sensor assembly of the present invention. 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 physiologic
parameters, monitoring sites, and even types of living organism may
be utilized in conjunction with the invention in a broader
sense.
[0056] The apparatus 400 of FIG. 4 fundamentally comprises the
sensor assembly previously described used in conjunction with a
reusable "host" system 401 which controls and supports the
operation of the sensor assembly. Specifically, the sensor assembly
is contained within a local controller assembly 444, which is
coupled via electrical cable or other communications interface to a
remote analysis and display station 446. While the apparatus 400 of
FIG. 4 having the sensor assembly 200 of FIGS. 2a-2c is described
in detail in the following paragraphs, it will be apparent that the
assembly of FIGS. 1a-1c, or alternatively yet another
configuration, may be used with equal success.
[0057] Further included in the apparatus 400 are a pressure
transducer 416 for measuring blood pressure from the radial artery
tonometrically; an applanation device 407 coupled to the transducer
416 for varying the degree of applanation (compression) on the
artery; two ultrasonic transducers 410, 412 for generating
ultrasonic emissions and reflections thereof, these ultrasonic
emissions being used to derive blood velocity (and kinetic energy);
a signal processor 420 operatively connected to the pressure and
ultrasonic transducers 416, 410, 412 for analyzing the signals
generated by these transducers and generating a calibration
function based thereon; a signal generator/receiver 422 used to
generate ultrasonic signals for transmission into the artery, and
receive signals from the ultrasonic transducers 410, 412; and a
controller 426 operatively coupled to the applanation device 407
and the signal processor 420 for controlling the degree of
applanation pressure applied to the artery. The pressure and
ultrasonic transducers 416, 410, 412 are arranged within the sensor
assembly 200 previously described with respect to FIG. 2 herein.
The gimbal 256 is coupled to the sensor assembly 200 and the
applanation device 407 as shown in FIG. 2c in order to transfer the
applanation force from the device 407 to the sensor assembly 200
and ultimately to the skin of the subject. The applanation
mechanism 407 and sensor assembly 200 (along with apparatus
necessary to maintain the sensor assembly 200 in position on the
subject, such as a wrist brace or band) collectively comprise the
local controller assembly 444 which is mounted on the subject's
wrist, although it will be appreciated that other configurations
may be substituted. Furthermore, the various signal processing
and/or electronic components such as the processor 420 and
controller 426 may be located within the controller assembly 444 or
alternatively located remotely from the subject such as in the
monitoring and display station 446 if desired.
[0058] The local controller assembly 444 further includes portions
of the logic circuit (as described below with respect to FIGS. 7
and 8), which compensates or calibrates the pressure transducer
element 416 for offset, temperature effects, and non-linearities
characteristic of each individual pressure transducer element. This
calibration is advantageously conducted upon the initialization of
each new pressure transducer element 416 and associated EEPROM
device 418, thereby assuring the adequacy and proper calibration of
the apparatus 400 as a whole before use with that new transducer
element.
[0059] The analysis and display unit 446 comprises display, data
analysis, user control, and data storage functions for the
apparatus 400 including the display of raw data sensed by the
transducer elements, display of parameters calculated based on the
raw data by the processor and associated signal processing
algorithms, equipment status indications, display of information
stored within the storage device 218 of the sensor assembly (such
as pressure transducer manufacture date/location, calibration
parameters, etc.), name/SSN of the subject being monitored, etc. In
one embodiment, the analysis and display station comprises a
dedicated device having a CRT, TTT/LCD, LED, or plasma display
coupled with a variety of pre-specified control and data storage
functions. Alternatively, a laptop or handheld computer having
software adapted for performing each of the foregoing functions, or
those specifically chosen by the user, may be substituted. It will
be recognized that each of the foregoing display, storage, and user
control functions are well known to those of ordinary skill in the
electronic arts, and accordingly are not described further.
Analysis of the signals derived from the sensor assembly 200 is
described in the foregoing U.S. patent applications previously
incorporated herein.
[0060] The signal generator/receiver 422 generates electrical
signals or pulses which are provided to the ultrasonic transducers
410, 412 and converted into ultrasonic energy radiated into the
blood vessel. This ultrasonic energy is reflected by various
structures within the artery, including blood flowing therein, as
well as tissue and other bodily components in proximity to the
artery. These ultrasonic reflections (echoes) are received by the
ultrasonic transducers and converted into electrical signals which
are then converted by the signal generator/receiver 422 to a
digital form (using, e.g., an ADC) and sent to the signal processor
420 for analysis. In the present embodiment, the signal processor
comprises a microprocessor unit and a digital signal processor
(DSP) unit (not shown) in order to facilitate, inter alia, rapid
data processing and the control functionality previously described,
although it will be recognized that the processor 420 may
configured in other ways if desired. Depending on the type of
ultrasonic analysis technique and mode employed, the signal
processor 420 utilizes its program (either embedded or stored in an
external storage device) to analyze the received signals. For
example, if the system is used to measure the maximum blood
velocity, then the received echoes are analyzed for, inter alia,
Doppler frequency shift. Alternatively, if the arterial diameter
(area) is measured, then an analysis appropriate to the
aforementioned A-mode is employed. U.S. patent applications Ser.
No. 09/342,549 filed Jun. 29, 1999 and 09/534,900 filed Mar. 23,
2000, previously incorporated by reference herein, describe
adaptations of the apparatus 400 for time-frequency and hemodynamic
parameter blood pressure measurement, respectively.
[0061] FIG. 5 illustrates a second embodiment of the physiologic
parameter measurement apparatus of the present invention. In the
embodiment of FIG. 5, the system 500 further includes a radio
frequency (RF) transceiver chip 504 and associated processing of
the type well known in the art for transmitting the information
generated by the transducers elements 510, 512 and stored within
the storage device 518 to the host device via an associated antenna
506 located on the local control assembly 507 and receiver 508
located on the analysis and display device 523. The antenna 506 and
receiver 508 ideally comprise transponders, thereby enabling
two-way communication between the local control assembly 507 and
the analysis and display unit 523.
[0062] In the configuration of FIG. 5, the need for wiring or
conductors communicating the electrical signals between the local
control assembly 507 and the remote analysis and display unit 523
is advantageously obviated, thereby allowing the patient additional
mobility during blood pressure measurement, such as when being
transferred from one location in a hospital to another. It will
also be recognized that a number of different wireless transmission
methodologies (air interfaces) may be employed to transfer data
between these entities including, inter alia, point to point
transmission via the Infrared Data Association's ("IrDA") infrared
based wireless transmission standard; wireless radio frequency
("RF") based local area network ("LAN") connections based on the
IEEE 802.11 LAN access standard (including both frequency-hopping
and direct sequence spread spectrum variants); the "Bluetooth" 2.45
GHz frequency band based wireless communication specification, and
even the Home RF Shared Wireless Access Protocol. The construction
and operation of each of these air interfaces is well known in the
telecommunications arts, and accordingly is not described further
herein.
[0063] Referring now to FIG. 6, the method of calibrating a
disposable transducer element such as that of FIGS. 2a-2c using its
associated storage device is described. It will be recognized that
while the following discussion is cast in term of the calibration
of a silicon strain beam pressure transducer element, the general
principles described herein are equally applicable to wire strain
gage transducer elements or yet even other types of sensor
elements.
[0064] As shown in FIG. 6, the method 600 comprises first providing
a transducer element (such as the aforementioned pressure
transducer 216 of the embodiment of FIG. 2) having a predetermined
operating response per step 602. Next, the operating response of
the transducer element is determined in step 604. In the
illustrated embodiment of the method, the response of the
transducer element is determined by measuring, inter alia, the
bridge voltage (E.sub.b) and signal output voltage of the
transducer element (E.sub.s) under varying conditions of
temperature and applied pressure. Specifically, E.sub.b and E.sub.s
are measured for a series of increasing pressures at a first
temperature T.sub.o, and then again at a higher or "hot"
temperature T.sub.h. Table 1 below illustrates this principle
graphically. This step 604 is typically but not necessarily
performed by the transducer vendor or manufacturer.
1TABLE 1 Temp Press Eb Es Er Es-15K .degree. C. mmHg V mV V mV To
Po 0 Ebo Eso Eref Ecal To P1 50 Eb1 Es1 Eref To P2 100 Eb2 Es2 Eref
To P3 300 Eb3 Es3 Eref Th P4 0 Eb4 Es4 Eref Th P5 50 Eb5 Es5 Eref
Th P6 100 Eb6 Es6 Eref Th P7 300 Eb7 Es7 Eref
[0065] Next, in step 606, a series of conversion algorithms are
applied to the "raw" tranducer response data obtained in step 604
to convert the response data to calibration parameters useful for
calibrating the pressure transducer in-situ during operation.
Allowable ranges for the resulting calibration parameters are also
specified. Table 2 illustrates an exemplary set of calibration
parameters, allowable ranges, and conversion algorithms for a
typical silicon strain beam pressure transducer element.
2TABLE 2 Calculations for EEPROM Range Resolution Units Eref = Er @
Po 4.998 5.002 0.001 V Tref = To @ Po 20.0 24.0 0.1 oC Thigh = Th @
Po 38.0 42.0 0.1 oC Vos = Eso @ 0 -5.250 5.250 0.001 mV 1 Vos TC =
Es4 - Eso Th - To @ Po 30.0 30.0 0.1 uV/oC Eso = Ebo @ Po 1.0000
1.5000 0.0001 Volts 2 Ebo TC = Eb4 - Ebo Th - To @ Po 1.000 2.000
0.001 mV/oC 3 Sens = Es2 - Eso P2 - Po Rel to P2 35.0 80.0 0.1
uV/mmHg 4 Lin Error = 100 [ ( Es3 - Eso ) - 3 ( Es2 - Eso ) ] 3 (
Es2 - Eso ) @ To -1.50 1.50 0.01 % Ecal = Ecal @ Po 5.50 10.60 0.01
mV Assumptions = Vos TC and Ebo TC are independent of pressure
Sensitivity is independent of temperature
[0066] where:
[0067] E.sub.ref=Reference voltage used by vendor at measurement
(V)
[0068] T.sub.ref=Reference temperature used by vendor (degree
C)
[0069] T.sub.h="High" temperature used by vendor during raw data
measurement (degrees C)
[0070] V.sub.os=Offset voltage of transducer (bridge) at zero
applied pressure and Tref (mV)
[0071] V.sub.osTC=Temperature correction factor for offset voltage
of bridge (mV/degree C)
[0072] E.sub.b0=Bridge voltage at T.sub.ref (V)
[0073] E.sub.b0TC=Temperature correction factor for bridge voltage
(mV/degree C)
[0074] Sens=Sensitivity of bridge to pressure change (uV/mmHg)
[0075] Lin Error=Linearity error on non-linearity (%)
[0076] E.sub.cal=Shunted output voltage of bridge (mV)
[0077] The conversion algorithms of Table 2 are derived based on
the definition of the various calibration parameters (Table 3.
below), and the raw transducer response data. For example, in the
case of the temperature correction factor for the bridge voltage
(E.sub.b0TC), the bridge voltage taken at the reference temperature
T.sub.0 and zero pressure, or Eb.sub.0, is subtracted from the
bridge voltage taken at the "hot" temperature "T.sub.h" and zero
pressure (E.sub.b4), the resultant of which is divided by the
difference between the hot temperature and the reference
temperature (i.e., T.sub.h minus T.sub.0) to produce E.sub.b0TC.
The derivation of the other conversion algorithms is generally
analogous, and easily determined by those of ordinary skill in the
electronic arts.
[0078] Next, in step 608, the calculated calibrations parameters
(Table 3 below) are stored within the storage device (e.g., EEPROM)
of the transducer element for later recall during
calibration/operation (step 610). Appendix I illustrates exemplary
code useful for extracting the calibration parameters of Table 3
for a pressure transducer element from the EEPROM associated
therewith.
3TABLE 3 EEPROM DATA Er 5.002 V To 24.9 .degree. C. Th 40.0
.degree. C. Vos 3.4999 mV Vos-TC 29.9 ZuV/.degree. C. Ebo 1.4999 V
Ebo TC 1.960 mV/.degree. C. Sens 80.0 uV/mmHg Lin Error -1.99
Percent Ecal 10.60 mV
[0079] Referring now to FIG. 7, a first embodiment of the logic
circuit used for effectuating the calibration of the pressure
transducer element(s) as described with respect to FIG. 6 above is
disclosed.
[0080] In the embodiment of FIG. 7, the circuit 700 comprises an
analog circuit having a pressure transducer element 216 represented
in the form of an electrical bridge, an EEPROM 218, a span
compensation resistor R.sub.a, 720, reference voltage E.sub.ref
703, first and second analog-to-digital converters (ADC's) 702,
750, digital-to-analog converter (DAC) 704, three operational
amplifiers 706, 708, 710 of the type well known in the electronic
arts, a system processor 760, which can be any microprocessor,
microcontroller, or even a DSP device, electronic analog switch
770, and a precision shunt resistor R.sub.cal 780. The voltage
output of the pressure transducer (bridge) 716 is input to a first
stage operational amplifier 706 which amplifies the output of the
pressure transducer. The amplified output is input to a second
stage amplifier 708, along with the output of the DAC 704, which is
subtracted from the transducer signal. The output of the second
stage amplifier 708 represents the temperature compensated, zero
offset output signal of the circuit 700. In operation the system
processor 760 reads the data stored in the EEPROM 218, and sets the
DAC 704 to eliminate the offset. Since the offset is a function of
temperature, the system processor 760 also reads the output of the
first ADC 702, which is sensing the bridge voltage E.sub.b of the
pressure transducer 716. The bridge voltage varies with
temperature, and the correlation between its variation and the
offset variation with temperature is stored in the EEPROM. Since
this variation is small, it is amplified by the third stage
amplifier 710, before being read by the first ADC 702. As used
herein, term "offset" refers to the output voltage of the bridge
216 at zero applied pressure under specified conditions of
temperature. The second ADC 750 is used to convert the final analog
output of the second stage amplifier 708 to a digital
representation of the blood pressure. In this embodiment, the
system processor reads the EEPROM in the disposable transducer 216,
to obtain the transducer sensitivity (Sens). This factor is used to
scale the output of second stage amplifier 708 in the digital
domain so that a known relationship of the output of the amplifier
708 exists in mV per mmHg.
[0081] In the illustrated embodiment, the span compensating
resistor R.sub.a 720, value is chosen to be 1000 ohms based on the
bridge impedance, span TC, and reference voltage E.sub.ref 703,
which is chosen to be 5 V. It will be recognized, however, that a
range of values for R.sub.a are possible from a few hundred ohms to
several thousand ohms. Reference voltages other than 5 V may also
be used. The main functions of resistor R.sub.a 720, are to
temperature compensate the span sensitivity of the transducer to
temperature, and to provide signal E.sub.b, which provides an
output which is a function of the temperature of the bridge. The
calibration and functionality of the transducer and the system can
be checked by periodically turning on the analog switch 770, which
shunts one side of the bridge with precision resistor R.sub.cal
780. In one embodiment, this operation is initially performed at
the time of manufacture, and the result is stored in the EEPROM as
E.sub.cal. The resistor 780 is selected to have a near zero TC, and
a precise value that gives a response approximately equal to the
equivalent of 100 mmHg. When the disposable transducer element is
first connected, the system processor turns on the switch 770 and
reads the output of the circuit 700. It then compares the value
obtained with E.sub.cal, which is stored in the EEPROM. If the
results match within specified limits, then system accuracy is
ensured. Note that this calibration verification generally is
performed when the sensor is off the wrist of the subject. Since
the system can control and sense the applanation of the sensor,
this condition is ensured, and calibration will typically be
checked prior to a blood pressure measurement interval.
[0082] During operation, the bridge voltage E.sub.b of the circuit
700 is sampled at an interval of once per second, and the input
value to the DAC recalculated in order to continually update the
DAC output provided to the second stage amplifier 708. In this
fashion, the output of the bridge (transducer element) is
continually compensated for temperature.
[0083] In a second embodiment of the logic circuit 800 shown in
FIG. 8, portions of the functionality of the circuit of FIG. 7 are
alternatively accomplished using higher resolution first and second
A/D converters 802, 850 to read the output of the bridge amplifier
806, and the bridge voltage E.sub.b. Rather than subtracting out
the offset voltage, and the offset variation with temperature in
analog circuitry, the embodiment of FIG. 8 digitizes both the
signal and the error terms, and does the subtraction in software
running on the system processor 860, as is well known in the art.
Temperature variations of the bridge 216 are sensed by reading
E.sub.b with the first ADC 802. Corrections to the output signal
are accomplished in the digital domain by the system processor 860.
Note that in yet another alternate embodiment (not shown), one A/D
converter and an analog multiplexer are used in place of the first
A/D converters 802, 850 of the embodiment of FIG. 8.
[0084] Appendix II hereto illustrates various exemplary
applications of the smart sensor assembly of the invention,
including storing data that enhances sensor and system performance
and reliability. It is noted that the applications described in
Appendix II are not exhaustive, but rather merely illustrative of
the broader concept of the invention disclosed herein.
[0085] Referring to Appendix II, Items 1 through 17 therein
represent exemplary data that is encoded on the EEPROM during
sensor manufacturing according to the present embodiment. Each of
these items is described in greater detail below with reference to
Appendix I. Note that each of these Items can be considered
optional, and furthermore that other configurations (such as
different coding schemes, ranges/bits assigned to each parameter,
etc.) may be used consistent with the invention. he blood pressure
readings obtained from a subject.
4 Item Description 1. Data is encrypted on the EEPROM 116 using a
32-bit encryption key of the type well known in the cryptographic
arts. This encryption frustrates unauthorized access to the data
present on the EEPROM after encoding. In one embodiment, data is
encrypted on the EEPROM using a 32-bit key, and a 32-bit vendor
verification code (Appendix II, Item 2). Unless the verification
code matches, the system will reject the sensor as a non-matching
or non-acceptable sensor. The encryption key is the public part of
the key. It is combined with the private part known only to the
sensor vendor and customer, and is used to encrypt the data stored
on the EEPROM. Sensors which have data which is not encrypted or
encrypted incorrectly are rejected by the system upon startup. 2. A
32-bit hexadecimal vendor verification code is provided within the
EEPROM 116. Unless the verification code of the sensor assembly
matches that stored in the host system 401 (FIG. 4), the host
system will reject the sensor as being non- compatible. In this
fashion, use of the sensor assembly with a non-compatible host, or
vice versa, is prevented, thereby removing a potential source of
error in the blood pressure readings obtained from a subject. 3.
The manufacturing location is encoded on the sensor. Up to 255
different locations may be coded in the illustrated embodiment.
This information may be used for inventory purposes, to track
defective lots of items, etc. 4. The date of manufacture is
encoded. This date along with the real date derived from a real
time clock chip (not shown) in the sensor/blood pressure measuring
instrument allows rejection of any sensor assembly that is beyond
its shelf life specification (for example, 2 years) when the sensor
is connected to instrument. Hence, an "out of date" sensor assembly
cannot be used with any host device, even if compatible. 5. The
vendor lot code is encoded in the sensor. Since the manufacturing
date and location is also encoded, multiple lots per day can be
tracked using the present invention. 6. A unique serial number for
each senor assembly (and/or each specified transducer element) may
also be encoded. The 32-bit field of the illustrated embodiment
allows up to 4.295 billion combinations. 7.-15. Items 7-15
characterize the performance of the transducer and allow
calibration, linearization, and temperature compensation of the
transducer when it is connected to the system. As illustrated in
Appendix II, Items 7-15 represent the values of V.sub.ref, T.sub.o,
T.sub.h, and V.sub.os, V.sub.os TC, E.sub.b0, E.sub.b0 TC, Sens,
E.sub.cal, and Lin Error, respectively, as previously described
with respect to FIG. 6. Each sensor will have unique values for
these items due to manufacturing tolerances. Note that E.sub.b0 and
E.sub.b0 TC advantageously allow the system the ability to measure
the temperature of the sensor directly, and perform calibration of
the system based thereon. When used on the radial artery, this
temperature will be approximately equal to the patient skin
temperature at the wrist when the sensor is in contact with the
wrist. 16. A common validation test for pressure transducers is to
shunt one side of the bridge with a precision resistor, which
causes the bridge to output a full-scale reading. In the present
embodiment, this procedure is performed at the time of manufacture,
and the actual resulting bridge output for each sensor assembly (as
well as any critical test parameters, if desired) encoded into its
corresponding EEPROM 116. This same test is then performed in the
blood pressure measuring system when the sensor is connected
thereto, which validates the accuracy of the sensor electronically.
The result of this validation is also optionally stored in the
sensor storage device. 17. The ultrasound signals generated by the
ultrasonic transducer elements are processed by the system and play
a fundamental role in both the placement and positioning of the
transducer element(s) and the accuracy of the pressure signal
derived from the pressure transducer. A "quality" factor for these
processed signals is stored in the EEPROM. This quality factor is
derived by the signal processing algorithms and represents the S/N
(signal to noise ratio) of the ultrasound signals, as is well
known. The higher the S/N, the better accuracy the overall system
can achieve.
[0086] Referring again to Appendix II, Items 18 through 35 therein
represent exemplary data that the exemplary ultrasonic blood
pressure measuring system used in conjunction with the sensor of
the present invention writes to the EEPROM 116 during the time that
the sensor assembly is connected to the host system. These items
are described in greater detail below.
5 Item Description 18. (including Items 19-22) When a sensor is
first connected to the blood pressure measuring system, the host
system reads the first "n" (e.g., 10) items from the sensor's
storage device 116, and first validates that the transducer is a
compatible sensor as previously described. The host system then
utilizes the sensor calibration information to adjust the system
electronics accordingly to optimize the performance of the system
for that sensor. The date and time of first connection is also
written to the sensor. Once this occurs, the pressure calibration
validation test of the present embodiment is performed which mimics
the test performed at the time of manufacturing by shunting the
bridge with the same value used on the sensor manufacturing line.
The result is stored in the sensor. Additionally, the measuring
system also optionally includes separate EEPROMs or other storage
devices in, inter alia, the sensor positioning head and the system
enclosure, which allows these devices to retain unique information
such as their own serial numbers or calibration parameters. Such
serial numbers may also be stored in the sensor, thereby allowing
for tracing the specific measuring system/positioning head the
sensor was being used with at a given time. 23. If any one or more
of the tests previously described fail, a code is written back to
the sensor indicating what the error(s) were. Table 4 illustrates
exemplary error codes, although other codes and arrangements may be
used. TABLE 4 Code Error 0 Sensor OK 1 Non VW Sensor 2 EEPROM
read/write failure 3 US sensor failure 4 Cal Test Failure 5 Railed
High 6 Railed Low 7 (Reserved) 8 (Reserved) 9 (Reserved) 24. The
total time that the sensor is in an electrically powered state is
also stored in the sensor. In the present embodiment, this data
field is updated within the storage device at periodic intervals
(such as every minute), although other schemes for triggering
updates, and update frequencies, may be used if desired. 25. The
total time that the sensor is performing measurements while powered
(as opposed to merely being in an electrically powered state), may
also be stored in the sensor storage device. As above, this field
is updated on at a periodic interval. 26. The remaining allocated
power on time may also be written to the sensor storage device. In
an exemplary embodiment, this time is the measurement time (such as
24 hours) plus an additional time (3 hours, for example) to support
device setup, although other arrangements may be used. 27. The
remaining run time may also be stored in the sensor storage device.
In one embodiment, this field is the difference between 24 hours
and the actual run time. The field is optionally updated on a
periodic basis as previously described. 28. The total number of
measurement events is also stored in the sensor storage device. One
measurement event is defined in the present embodiment as the start
of a measurement cycle to the end of a measurement cycle. The cycle
can be any length of time. 29. The total number of "power on"
cycles that a sensor sees is stored in the sensor storage device.
In one embodiment, the power on/off switch on the instrument is
electronic in nature, thereby facilitating counting the number of
such events and storing them within the EEPROM. In another
embodiment, the voltage at a certain node within the circuitry is
sensed; when the voltage level exceeds a predetermined value
(corresponding to the fully powered-up state of the sensor),
another event is recorded. Many other arrangements are possible,
all being well understood by those of ordinary skill in the
electronic arts. 30. The sensor replacement code is also stored in
the sensor. Examples of sensor replacement codes are shown in the
Table 5 herein. By utilizing the run time and power on time data as
well as the initial power on date and time, the duration of sensor
use may be accurately monitored and controlled. In one exemplary
embodiment, a utilization window of a predetermined interval (e.g.,
30 hours) from the initial power up time) may be defined. Even if
the sensor were not used during the interval, it would expire
concurrent with the expiration of the interval. Within the
interval, shorter periods corresponding to power-on time and
mesurement time may be defined. For example, up to 24 hours of
actual measurement time, and 27 hours of actual power up time,
would be allowed during the aforementioned 30 hour interval. TABLE
5 Code Error 0 Sensor OK 1 Non VW Sensor 2 24 Hr measurement time
expired 3 27 Hr power on time expired 4 30 Hr utilization time
expired 5 Shelf life expired 6 Wrong mechanism 7 Wrong instrument 8
Wrong Patient 9 Wrong Unit 10 Wrong Hospital 11 (Reserved)
[0087] It is noted that the sensor replacement and error codes
previously described (Items 23 and 30) are useful for service
issues. In one possible scenario, failure analysis may be performed
on one or more subsets of a given sensor population (such as those
sensors which failed during use over a given period of time at a
given health care facility). The contents of the sensor's EEPROM
may be downloaded to allow analysis of the individual failures. For
example, if a large percentage of the aforementioned sensor
failures occurred when the sensors were connected to a particular
mechanism, the likely source of the failures, i.e., the common
mechanism, could be divined as uniquely identified by its EEPROM
serial number. Other scenarios are possible, all considered to be
within the scope of the present invention.
6 31.-35. During use, the blood pressure measurement system
periodically checks the pressure sensor calibration (ideally, not
during measurements) and write the result of one or more such tests
to the sensor storage device. Also, during measurements, the system
periodically writes the blood pressure, applanation value, and time
and date to the sensor storage device. This data is subsequently
retrievable and useful in evaluating the actual performance of the
sensor and the measurement system, which might prove useful in a
variety of circumstances (such as, inter alia, medical malpractice
or products liability litigation).
[0088] Items 36 through 39 of Appendix II illustrate additional
data which may optionally be recorded within the storage device
according to the present invention. These items can be configured
by the health care provider/physician if desired. For example, the
hospital name and care unit as well as the patient name and
attending may be entered into the storage device(s) of the sensor
and/or measurement system. If the hospital staff wanted to be sure
that a sensor was only used on one patient, or only on a particular
individual, or only within a specific ward or department (such as
the Operating Room or Intensive Care Unit), the system may be
configured to facilitate this. If use outside of the allowed
parameters was detected, the sensor would be rejected, and the
appropriate replacement code stored in the sensor's EEPROM.
[0089] It is also noted that since the present embodiment stores
data in the sensor itself, the utilization of the sensor can be
controlled across multiple mechanisms, systems, and power down
events. Additional protection may also be gained by downloading a
write authorization code to the sensor. For example, if attempts
were made to write to the sensor to reset its run time, and the
correct write authorization code was not utilized, the system would
reject the sensor. The 32-bit encryption algorithm previously
described further thwarts such attempts. Other security or
cryptographic techniques well known in the art may be used to
implement this protective functionality as well.
[0090] Method of Operation
[0091] Referring now to FIG. 9, a first embodiment of the general
method of evaluating the acceptability of a disposable sensor
assembly according to the present invention is described. It is
noted that the embodiment of FIG. 9 is merely illustrative of the
broader concept of the invention, and is not meant to be
restrictive in any way. As shown in FIG. 9, the method 900
comprises a first step 902 of storing data within the storage
device (e.g., EEPROM) of the sensor assembly. As discussed with
reference to Appendix II above, the data may take on any number of
forms including the serial number or other identifying data of the
probe, its date of manufacture, powered-on time to date, etc. Such
data may be loaded upon manufacture, during operation/testing, or
both. In step 904, the smart sensor assembly is connected to the
"host" ultrasonic measurement system 401 (FIG. 4) previously
described. Next, in step 906, the data stored within the sensor
storage device is read by the host and evaluated for compatibility
between sensor and host (e.g., is the sensor of the type designated
for use with a particular host), remaining lifetime, suitability of
application (e.g., is the sensor suitable for use with the given
patient or at the location indicated by the host), etc. If the
sensor assembly (or component thereof) is somehow incompatible or
otherwise restricted from use with the host in step 908, the host
generates an indication of rejection (step 910) and optionally
encodes data within the host and/or sensor assembly storage device
to this effect per step 912. The user is then prompted to change
the sensor assembly or incompatible component in steps 914 and
916.
[0092] Referring now to FIG. 10, one embodiment of the data
encoding and storage method of the present invention is described.
While the following description is cast in terms of a tonometric
non-invasive blood pressure measurement, it will be apparent to
those of ordinary skill that the methodology of FIG. 10 may be
adapted to other types of parametric measurements and devices.
[0093] As shown in FIG. 10, the method 1000 comprises a first step
1002 of performing an applanation "sweep" using the ultrasonic
blood pressure measurement system previously described. When data
from the sweep is obtained, the pressure, velocity, and wall
(diameter) data is analyzed per step 1004. Based on this analysis,
the "best fit" curves for the patient are determined per step 1006.
These best fit curves are determined using any one of a number of
different methods, one exemplary method being described herein with
reference to FIG. 11. Next, in step 1008, the patient is again
applanated using the measurement system to determine the adequacy
of the curves generated in step 1006. If the curves are adequate
(as based on predetermined criteria such as those described below
with reference to FIG. 11, or other selected criteria) in step
1010, the curves are then stored per step 1012 in the storage
device of the sensor assembly. Stored data may include the measured
pressure, blood velocity, and arterial diameter, or any other
parameters related to the applanation as desired.
[0094] Referring now to FIG. 11, a specific exemplary method of
encoding physiologic parameters is disclosed. As shown in FIG. 11,
the method 1100 comprises collecting pressure and velocity data
from a given patient using the sensor 1100 to perform applanation
sweeps per step 1102. Next, in step 1104, the data obtained during
step 1102 is analyzed using the aforementioned time-frequency or
hemodynamic parameter measurement methods as set forth in
Applicant's aforementioned copending U.S. patent applications
previously incorporated herein, although it will be recognized that
other methods of analysis may be substituted. The result of this
analysis is an estimate of mean blood pressure within the measured
patient. Next, in step 1106, steps 1102 and 1104 are repeated in
order to derive a second estimate of the mean blood pressure. In
step 1108, the two preceding estimates of mean blood pressure are
evaluated to determine if they fall within predetermined acceptance
criteria. In the illustrated embodiment, the allowance band of 5 mm
Hg (i.e., the last two estimates must be within 5 mm Hg of each
other) is used, although other values and in fact other criteria
may be substituted. This step 1108 provides reasonable assurance
that the sensor is placed correctly on the subject, is being used
on the same patient (i.e., the disposable sensor assembly is not
being reused on another subject), and the data is not
corrupted.
[0095] Lastly, in step 1110, the collected data (including for
example mean pressure, heart rate, peak blood flow at mean
pressure, kinetic energy, arterial diameter at mean pressure, and
transform peak values) are stored in the EEPROM or other storage
device previously described. This data collectively (or subsets
thereof) comprises a characteristic signature for a given sensor
assembly and patient.
[0096] Referring now to FIG. 12, an exemplary plot of measured
pressure and blood flow/velocity data obtained during a system
calibration cycle and which is subsequently encoded using the
present invention is described. The plot 1200 illustrates the
applanation (pressure) sweep 1202 as a function of the number of
samples; note that per the illustrated embodiment, a sampling rate
of 200 samples per second is chosen, although other rates or even
variable rates may be used. Also illustrated is the blood
flow/velocity profile 1204 as a function of the number of
samples.
[0097] 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, display, 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.
[0098] 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 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.
* * * * *