 |
Claims  |
|
|
We claim:
1. Apparatus for determining systolic pressure of a living test subject
comprising means for applying a selectively changeable pressure to the
test subject adjacent a blood vessel; means for measuring a fluctuating
quantity proportional to a sum, said sum comprising a time-dependent
fluctuating component proportional to the amplitude of the pulsatile
pressure within the blood vessel plus the selectively changeable pressure
applied externally adjacent the blood vessel; means for converting said
quantity into a representation of a time derivative of said fluctuating
component thereof; means for obtaining the time integral of said time
derivative over an interval of predetermined limits in each of successive
blood pressure pulses, said interval being the time during which said time
derivative representation exceeds a predetermined reference level; means
for determining the maximum value attained by successive said integrals as
the applied pressure is changed; means for storing a representation of
said maximum integral value; means for determining when a said integral is
substantially equal to a predetermined fraction of said maximum integral
value for an applied pressure greater than the pressure applied when said
maximum integral value results; means for reading out said applied
pressure corresponding to said integral being substantially equal to said
predetermined fraction to said maximum value, said read-out pressure
corresponding to the systolic pressure of said subject; means for
establishing a threshold level signal representative of the magnitude of
substantially only the time derivative representing a systolic rise; means
for comparing said time derivative representation of said fluctuating
component of said quantity with said threshold level signal while said
time derivative representation exceeds said reference level to provide a
control signal indicative of whether or not said time derivative
representation exceeding said reference level is representative of a valid
systolic rise; and means responsive only to a validating indication that a
particular said time derivative representation exceeding said reference
level is a systolic rise for extending said integral of said particular
representation to said maximum integral value determining means and said
fraction of maximum integral value determining means.
2. The apparatus of claim 1 wherein said input signal is additionally in
the possible presence of high amplitude, low frequency interference and
wherein said time derivative means comprises a differentiating network for
converting said input signal into a representation of the first time
derivative thereof over a frequency band including said steeply rising
wavefront and into a representation of the second time derivative thereof
at the lower frequency of said possible high amplitude, low frequency
interference.
3. The apparatus of claim 1 wherein said threshold level signal
establishing means establishes said threshold level as a function of the
magnitude by which at least the immediately preceding time derivative
representation exceeding said reference level exceeds another reference
level.
4. The apparatus of claim 3 wherein said reference level and said other
reference level are the same.
5. The apparatus of claim 4 wherein said threshold level is proportional
only to the magnitude by which said immediately preceding time derivative
representation exceeds said reference level.
6. The apparatus of claim 5 wherein said threshold level is substantially
50% of said reference-exceeding magnitude of said immediately preceding
time derivative representation.
7. The apparatus of claim 6 wherein said means for obtaining the time
integral of said time derivative representation comprises accumulating
means, said time derivative representation being subdivided into
successive subinterval increments applied to and accumulated in said
accumulating means during each interval in which it exceeds said reference
level, the accumulated value of said subinterval increments for each said
reference-level-exceeding interval comprising a respective tentative said
integral, said systolic rise validating means being operative to extend
only said integrals attending respective valid systolic rises to said
maximum integral value determining means.
8. The apparatus of claim 6 wherein said means for obtaining the time
integral of said time derivative representation comprises accumulating
means, means for temporarily storing that portion of said time derivative
representation extending over an immediately preceding interval, said
immediately preceding interval being at least as long as the maximum
anticipated duration of the systolic rise portion of the blood pressure
pulses, means responsive to said time derivative representation going from
a level exceeding said reference level to a level not exceeding said
reference level and responsive to an indication of valid systolic rise
from said validating means for directing a last in-first out readout of
said portion of said time derivative representation stored in said storage
means, means for extending said readout from said storage means to said
accumulating means and for accumulating only those values of said readout
which exceed said reference level, the value of said time derivative
representation readout accumulated by said accumulating means being
extended to said maximum integral value determining means.
9. The apparatus of claim 1 wherein said means for obtaining the time
integral of said time derivative representation comprises accumulating
means, said time derivative representation being applied to and
accumulated in said accumulating means during each interval in which it
exceeds said reference level, the accumulated value of each said
reference-level-exceeding intervals comprising a respective tentative said
integral, said systolic rise validating means being operative to extend
only said integrals attending respective valid systolic rises to said
maximum integral value determining means.
10. The apparatus of claim 9 including means for clearing said accumulating
means prior to each subsequent integrating accumulation.
11. The apparatus of claim 9 including means for converting at least said
reference-level-exceeding portions of said time derivative representation
to consecutively timed discrete increments thereof and wherein said
accumulating means sum said discrete increments which exceed said
reference level.
12. The apparatus of claim 1 wherein said means for obtaining the time
integral of said time derivative representation comprises accumulating
means, means for temporarily storing that portion of said time derivative
representation extending over an immediately preceding interval, said
immediately preceding interval being at least as long as the maximum
anticipated duration of the systolic rise portion of the blood pressure
pulses, means responsive to said time derivative representation going from
a level exceeding said reference level to a level not exceeding said
reference level and responsive to an indication of valid systolic rise
from said validating means for directing a last in-first out readout of
said portion of said time derivative representation stored in said storage
means, means for extending said readout from said storage means to said
accumulating means and for accumulating only those values of said readout
which exceed said reference level, the value of said time derivative
representation readout accumulated by said accumulating means being
extended to said maximum integral value determining means.
13. The apparatus of claim 12 including means for clearing said
accumulating means prior to each subsequent integrating accumulation.
14. A method for determining blood pressure of a living test subject
comprising:
applying a selectively changeable pressure to the test subject adjacent a
blood vessel;
measuring a quantity proportional to a sum, the sum comprising a
time-dependent fluctuating component proportional to the amplitude of the
pulsatile pressure within the blood vessel plus the selectively changeable
pressure applied externally adjacent the blood vessel;
converting said quantity into a representation of the time derivative of
the fluctuating component thereof, a portion of the time derivative in
each of successive blood pressure pulses being representative of the
systolic rise;
determining the time integral of said time derivative over an interval of
predetermined limits in each of successive blood pressure pulses as a
measure of this systolic rise, said interval being the time during which
said time derivative representation exceeds a predetermined reference
level;
establishing a threshold level signal representative of the magnitude of
substantially only the time derivative representation of a systolic rise;
comparing said time derivative representation of said fluctuating component
of said quantity with said threshold level signal while the time
derivative representation exceeds said reference level to provide a
control signal indicative of whether or not said time derivative
representation exceeding said reference level is representative of a valid
systolic rise;
processing selective ones of said time integral to provide an indication of
blood pressure; and
selecting for processing only the said time interval of said time
derivative representations indicated as being valid systolic rises. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
The present invention relates generally to waveform analysis, a particular
application of such waveform analysis being in the field of blood pressure
monitoring, particularly as relates to automatic monitoring of systolic
blood pressure.
The prior art is replete with devices for measuring systolic pressure of a
living subject. An old and simple device is a pressurizeable cuff used in
combination with a mercury manometer which reads pressure in the cuff and
a stethoscope which is used to listen to Korotkof sounds. In another
advanced method of measuring blood pressure, the distance from a blood
pressure cuff to the wall of an artery is accurately determined by
measuring Doppler shifts of sound waves reflected by the artery. In yet
other methods for measuring blood pressure intrusive devices are often
inserted directly into blood vessels.
Oscillometric methods of determining systolic pressure are also well known
in the art. In such methods, the operator observes the representation of
the strength of pulsations of pressure within an artery. This can be done
visually, as by watching the extent of bouncing at the top of a mercury
column in a mercury manometer which is in pressure communication with the
cuff or indirectly as by measuring the occlusion which occurs to a blood
vessel in the pinna of the ear as pressure is exerted thereon. These
oscillometric methods generally define systolic pressure to be the maximum
applied pressure with which threshold oscillations are observed to occur.
With a typical mercury manometer and pressurized cuff, this pressure would
then be the highest pressure which the operator noted bouncing on the top
of the mercury column as the pressure in the cuff was slowly and
relatively uniformly reduced. However there are inaccuracies associated
with this method for determining threshold oscillations, since the inertia
of the mercury column does not allow it to noticeably respond to narrow
width pressure pulses.
Each of the aforementioned techniques or devices for measuring systolic
pressure exhibit some form of shortcoming such as inaccurate response to
narrow width pressure pulses or the requirement for sophisticated and/or
expensive measuring equipment.
There is described in U.S. Patent application Ser. No. 578,047, filed May
15, 1975 by Link et al for Apparatus and Process for Determining Systolic
Pressure, assigned to the present assignee and incorporated herein by
reference, a method and apparatus for automatically and relatively simply
obtaining accurate systolic blood pressure measurements, thereby
overcoming the shortcomings of the aforementioned devices. That device
determines systolic pressure by applying pressure to a living test subject
by changing pressure in a pressure cuff attached to the subject adjacent a
blood vessel; by measuring at the cuff a quantity proportional to a time
dependent fluctuating component representative of the pulsatile pressure
within the blood vessel, which quantity is proportional to the amplitude
of the pulsatile pressure; by determining the maximum value attained by
the quantity as the applied pressure is changed; by storing a
representation of the maximum value; by determining when the quantity is
substantially equal to about one half of the maximum value for an applied
pressure greater than the pressure applied when the maximum value occurs
or results; and by reading out the applied pressure corresponding to the
quantity being substantially equal to about one half of the maximum value,
the readout pressure corresponding to the systolic pressure of the
subject. The signal from the pressure cuff comprises a fluctuating
quantity proportional to a sum, that sum comprising a time dependent
fluctuating component proportional to the amplitude of the pulsatile
pressure within the blood vessel, which component has a steeply rising
wavefront between end diastole and systole, plus the selectively
changeable pressure applied externally adjacent the blood vessel by the
cuff.
In U.S. Patent application Ser. No. 754,201 by J. D. Haney and W. Jansen
for Systolic Pressure Determining Apparatus and Process Using Integration
to Determine Pulse Amplitude, filed Dec. 27, 1976, there is described a
systolic blood pressure monitor of the general type described in the
aforementioned U.S. patent application Ser. No. 578,047 and being improved
in a manner assuring increased accuracy in the determination of systolic
blood pressure. The improved monitor does not use a peak-to-peak detector
for determining the amplitude of the steeply rising wavefront of the
fluctuating component of the signal from the cuff, but instead,
differentiates the cuff signal to obtain the time derivative of the
fluctuating component, and then integrates a portion of the derivative.
The time derivative signal extends above a zero reference level from the
time of end diastole through systolic rise to the systolic peak. Thus, the
"above 0" area under the time derivative waveform is representative of the
peak-to-peak magnitude (diastolic-to-systolic) of a respective blood
pressure pulse. By integrating the "above 0" portion of the time
derivative waveform an integral value is obtained which is proportional to
the area under the waveform and, accordingly, is representative of the
peak-to-peak magnitude of the blood pressure pulse. This integral value is
then available for use on a beat-to-beat basis for determining the maximum
value attained by the fluctuating component from the cuff and later
determining one half of that maximum value in the determination of
systolic pressure.
In the apparatus of the aforementioned application U.S. Ser. No. 754,201 by
Haney and Jansen, the integration of the time derivative waveform was
delimited by the positive-going crossing of the zero reference at the
beginning and the negative-going crossing of the zero reference at the
end. However, certain artifacts in the cuff signal during the diastolic
drop, such as due to patient movement, may result in the time derivative
waveform exceeding the zero reference for a brief time other than between
end diastole and the systolic peak. Although generally much smaller in
magnitude than the "above 0" passage of the signal derivative during
systolic rise, this otherwise superfluous "above 0" passage of the
artifact time derivative may be included in the determination of an
integral value for use in the beat-to-beat determination of that
peak-to-peak value representative of one half of the maximum peak-to-peak
value, and thereby impair the accuracy of the systolic pressure
determination.
SUMMARY OF THE INVENTION
In its broadest sense, the invention comprises an improved means and method
for identifying and quantizing an essentially periodic, steeply rising
wavefront of an input signal in the possible presence of low amplitude
interference, and possibly also high amplitude low frequency interference
signals. Such improved apparatus comprises means for obtaining a
representation of the time derivative of at least the steeply rising
wavefront portion of the input signal; means for obtaining the time
integral of the time derivative representation over an interval of
predetermined limits in each of successive repetitions of the steeply
rising wavefront, the interval being the time during which the time
derivative represntation exceeds a predetermined reference level; means
for establishing a threshold level signal representative of the magnitude
of substantially only the time derivative representing the steeply rising
wavefront; means for comparing the time derivative representation of the
input signal with the threshold level signal while the time derivative
representation exceeds the reference level to provide a control signal
indicative of whether or not the time derivative representation exceeding
the reference level is representative of a valid steeply rising wavefront;
and means responsive only to a validating indication that a particular
time derivative representation exceeding the reference level is a valid
steeply rising wavefront for recognizing the respective integral of the
particular time derivative representation as the quantized value of the
steeply rising wavefront.
The method of the invention comprises converting the input signal into a
representation of a time derivative of at least a steeply rising wavefront
portion of the input signal; determining the time integral of the time
derivative representation over an interval of predetermined limits in each
of successive representations of the steeply rising wavefront, the
interval being the time during which the time derivative representation
exceeds a predetermined reference level; establishing a threshold level
signal representative of the magnitude of substantially only the time
derivative representing the steeply rising wavefront; comparing the time
derivative representation of the input signal with the threshold level
signal while the time derivative representation exceeds the reference
level to provide a control signal indicative of whether or not the time
derivative representation exceeding the reference level is representative
of a valid steeply rising wavefront; and recognizing as the quantized
values of the respective steeply rising wavefront only the respective
integrals receiving a validating indication.
The method and apparatus of the invention additionally provide for
establishing the threshold level signal as a function of the magnitude by
which at least the immediately preceding time derivative representation
exceeding said reference level exceeds another reference level, that other
reference level normally being a zero reference and the same as said
reference level.
The method and apparatus of the invention additionally provide for doubly
differentiating the input signal in a low-frequency range to alternate
high amplitude, low-frequency components.
In one embodiment of the invention, the method and apparatus provide for
integrating the time derivative representation in accumulating means
whenever the representation exceeds the reference level, and to then
recognize as a valid integral value only that integral obtained during an
interval in which the time derivative representation at some time exceeded
the threshold level.
In another embodiment of the method and apparatus of the invention, an
immediately preceding portion of the time derivative representation having
a duration at least as long as the maximum anticipated duration of the
rise of the steeply rising wavefront is temporarily stored. When the time
derivative representation crosses the reference in the negative going
direction and a determination has been made that the representation had
exceeded the threshold level during the immediately preceding "above 0"
passage, the stored representation is then read out of storage in a last
in-first out sequence and the "above 0" portion thereof is integrated in
accumulating means to form the requisite integral value.
The apparatus of the invention finds particular utility in blood pressure
monitoring equipment wherein it is desired to know the peak-to-peak value
of each blood pressure pulse across the systolic rise (steeply rising
wavefront) from end diastole to systole. Such knowledge of the
peak-to-peak value of each blood pressure pulse it utilized in a preferred
embodiment for determining the systolic pressure of a living test subject.
The input signal may be obtained from a pressure cuff or other means.
Means are provided for determining the maximum value attained by
successive recognized (i.e. valid) integral values, which values
correspond with the peak-to-peak value of the respective pulses. The
maximum determined integral value is stored and means are provided for
determining when a said integral value is substantially equal to about one
half of the maximum integral for an applied pressure greater than the
pressure applied by the cuff when the maximum integral value results, the
applied pressure at which that particular one half maximum integral value
occurs being read out as the systolic pressure.
It is a principal object of the present invention to provide an improved
apparatus and process/method for identifying and quantizing an essentially
periodic, steeply rising wavefront in the presence, or possible presence,
of low amplitude interference, and possibly also high amplitude, low
frequency interference.
It is another object of the present invention to provide an improved
apparatus and process/method for determining systolic pressure. Included
in this object is the provision of an improved apparatus and
process/method which determines systolic pressure in a manner which
reduces the possibility of false readings and thereby increases accuracy.
These and other objects and advantages of the present invention will be
apparent to those skilled in the art after referral to the detailed
description of the preferred embodiments in conjunction with the appended
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a prior art technique for the determination of the
peak-to-peak magnitude of the pulsatile pressure within a blood vessel;
FIG. 2 illustrates a technique for the determination of peak-to-peak
amplitude of the pulsatile pressure in accordance with the invention;
FIG. 3A illustrates a typical oscillometric envelope of the pulsatile
pressure of a blood vessel;
FIG. 3B illustrates the time derivative of the FIG. 3A waveform;
FIG. 3C illustrates a controlled mode timing diagram in accordance with the
basic method and apparatus of the invention;
FIG. 3D illustrates the time intervals obtained from the waveform of FIG.
3B in accordance with the basic apparatus and process of the invention;
FIG. 4 illustrates, in a block diagram, the apparatus and process of one
embodiment of the invention;
FIG. 5 illustrates a plot of the gain vs. frequency characteristics of a
differentiating network employed in a preferred embodiment of the
invention;
FIG. 6A represents an enlarged portion of the time derivative waveform
illustrated in FIG. 3B showing a validation threshold level and the timing
of various control states associated therewith in accordance with the
embodiment illustrated in FIG. 4;
FIG. 6B illustrates a control state diagram in accordance with FIG. 6A and
the embodiment of FIG. 4;
FIG. 7 illustrates a flow chart or decision tree of the control sequence
employed by the embodiment illustrated in FIG. 4 between successive heart
beats;
FIG. 8 illustrates, in an abbreviated block diagram supplemented by FIG. 4,
the apparatus and process of another embodiment of the invention; and
FIG. 9 illustrates a technique similar to that of FIG. 2 for the
determination of peak-to-peak amplitude of the pulsatile pressure and
further including threshold detection means for identifying particular
"above zero" passages of the waveform derivative as valid systolic rises.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1, there is illustrated a functional block diagram
of certain portions of the systolic pressure measuring apparatus described
in the aforementioned application U.S. Ser. No. 578,047. More
specifically, the functional blocks of FIG. 1 illustrate a filter network
100 having its output connected through amplifier 102 to the input of a
peak-to-peak detector 104. Filter network 100 receives an input signal
106a on input conductor 106. The input signal 106a comprises a
slowly-increasing ramp indicative of the applied cuff pressure and having
superimposed thereon the time-dependent fluctuating component
representative of pulsatile pressure within the blood vessel of the
subject, which component representative of pulsatile pressure has a
steeply rising wavefront relative to the remaining components during the
rise from end diastole to systole. Filter network 100 was typically
constructed such that its output waveform 100a had the linear effects of
the pressure ramp removed therefrom, however, any random and
uncontrollable deviation from the presumed linearity of the pressure ramp
would introduce errors in the signal 100a. For instance, if a large
perturbation in the cuff pressure ramp was encountered, filter 100
required a considerable time to recover and could allow some variation in
the base line 100b (dotted) from which the fluctuating signal proportional
to the amplitude of the pulsatile pressure within the blood vessel w as
measured. Accordingly, each time peak-to-peak detector 104 is operated in
response to sampling signals 108, the resulting output signal 110a
appearing on conductor 110 included those peak-to-peak errors introduced
by the variation in base line 100b.
In accordance with the present invention as illustrated generally in FIG.
2, an input signal 206a having a waveform identical to that of waveform
106a in FIG. 1 is applied to the input conductor 206 to differentiating
network 200. Differentiating network 200 is constructed such that it
provides single differentiation of signal 206a over that range of
frequencies corresponding with the frequencies of the fluctuating signal
proportional to the amplitude of the pulsatile pressure within the blood
vessel, and doubly differentiates the input signal below that range of
frequencies in order to remove the offset effects of a linear pressure
ramp and any very low frequency perturbations which might have appeared in
the otherwise linear pressure ramp.
A filter or differentiating network having the properties required of
network 200 will possess the Gain v. Frequency characteristics illustrated
in FIG. 5 in which the Gain curve exhibits a -6db per octave slope in the
frequency range f.sub.1 -f.sub.2 and a -12db slope for frequencies below
f.sub.1. The frequency range f.sub.1 -f.sub.2 corresponds with the
bandwidth of the P.sub.ac signal comprising the fluctuating quantity
representative of the pulsatile pressure. Frequency f.sub.1 may be about
0.05-0.1 Hz and f.sub.2 may be about 10-20 Hz. Network 200 may
conveniently be provided by an electronic two-pole active filter providing
the -12 DB attenuation below f.sub.1 and an analog differentiator for
providing the -6 DB attenuation below f.sub.2. Above the f.sub.1 and
f.sub.2 corner frequencies the active filter and analog differentiator
respectively have substantially flat passbands. That portion of input
signal 206a representative of the pulsatile blood pressure is
differentiated and appears at the output of differentiating network 200 as
signal 200a, hereinafter designated P.
This P signal (200a) is applied through amplifier 202 to the input of an
integrator 204 which, by integrating the P signal over a predetermined
interval during each pulse, provides an output value corresponding with
the peak-to-peak pressure of each blood pressure pulse. Sample-and-hold
circuitry 205 associated with integrator 204 serves to sample the value
appearing at the output of integrator 204 at the end of each interval of
integration and to hold that value for an interim period until integration
of the next pressure pulse begins. Control of integrator 204 and
sample-and-hold circuit 205 is provided by the RESET/INTEGRATE/HOLD-signal
208 which controls the period of integration and serves to clear the
integrator prior to each new integration. The output from sample-and-hold
circuit 205 appears on line 210 as waveform 210a having a magnitude which
corresponds with the area under that portion of the waveform P being
integrated.
Referring to FIGS. 3A and 3D for an understanding of the theory underlying
the invention, it will be recalled from the aforementioned U.S. patent
application Ser. No. 578,047 that the systolic pressure is equal to
applied cuff pressure when the fluctuating quantity is about equal to one
half the maximum of value of the fluctuating quantity. The maximum value
of the fluctuating quantity is determined by measuring the diastole and
systole in successive blood pressure pulses. That pulse exhibiting a
maximum P-P amplitude is taken as the maximum value and the applied cuff
pressure is further increased such that the P-P amplitude decreases and
the systolic pressure is determined by noting the applied cuff pressure at
which the P-P pressure becomes one half of the P-P maximum.
FIG. 3A illustrates the time-dependent fluctuating component, P.sub.ac,
representative of pulsatile pressure within a blood vessel. The root ED of
each valley in the P.sub.ac waveform corresponds with the time of
diastole, or more specifically end diastole in a heart beat and the
waveform peak SP corresponds with the time of systole in the heart beat.
As earlier described, the signal from the cuff is differentiated to remove
the applied pressure ramp and low frequency random perturbations, and
results in the derivative P of waveform P.sub.ac, as represented in FIG.
3B. Because waveform P.sub.ac exhibits zero slope at both end diastole
(ED) and the systolic peak (SP), the derivative waveform P will be of zero
magnitude at each of those times. Further, because P.sub.ac exhibits a
positive slope during the systolic rise between ED and SP, the P waveform
lies above the zero reference line during this interval. The zero-crossing
points ED and SP of the P waveform correspond with the points of maximum
amplitude between successive P.sub.ac pulses and thus the area under the P
waveform and above the zero reference between end diastole ED and the
systolic peak SP provides a value which corresponds with the P-P value of
the respective blood pressure pulse. This area is determined by
integrating the "above zero" section of the P waveform. It should be noted
that end diastole (ED) also corresponds essentially with the initiation of
the rise to systolic peak (SP).
FIG. 3C illustrates a control signal generally similar to that of signal
208 in FIG. 2 which clears or resets the integrator prior to the interval
of integration, then integrates the P signal over the interval of
integration, and finally samples and holds the value of the integration as
a representation of the P-P value of the respective blood pressure pulse.
This sequence of control events is repeated with the resetting operation
being indicated by R, the integrating operation being represented by
.intg., and the sample and hold operation being represented by S+H. In
fact, the sampled integral may be held longer than is suggested by the
brief duration of the S+H signal in FIG. 3C.
The results of integrating the P waveform between the limits of ED and SP
are illustrated in FIG. 3D. The magnitude of the integral at the time of
the systolic peak SP corresponds with the P-P value of the respective
blood pressure pulse.
In implementing the concept of integrating the P waveform over the interval
of systolic rise to obtain respective P-P values for the respective blood
pressure pulses or heart beats, standard circuitry may be used to detect
when the P waveform crosses the zero reference in the positive going
direction to begin the integration and to determine when it crosses the
zero reference in the negative going direction to terminate the
integration and/or perform the sample and hold function. The integrator
may be reset immediately after sample and hold and preferably continue
until the next positive going zero-crossing of P. The resulting integral
may then be considered as representing the P-P value of the respective
pulse. However, certain characteristics of the P.sub.ac waveform and/or
the presence of signal artifacts during the diastolic drop may result in P
appearing above the zero reference for a brief time other than between end
diastole and the systolic peak. For instance, as illustrated in FIGS. 3A
and 3B, if random muscular activity introduces a "high frequency" signal
artifact (ART) just prior to end diastole when the slope of the P.sub.ac
waveform is relatively flat, the derivative P waveform may present part of
the artifact as a "greater than zero" value and result in the tentative
values illustrated parenthetically in FIGS. 3C and 3D.
In accordance with an aspect of the invention, illustrated generally in
FIG. 9, a threshold level is established for discriminating between those
P values greater than zero which attend the systolic rise and those
signals, such as artifacts and the like, which do not attend the systolic
rise. The magnitude of P signal associated with the systolic rise is
normally significantly greater than the magnitude of any other (above
zero) portion of the signal (as from artifacts) and accordingly, this
allows discrimination between such signals. The determination that the P
waveform exceeds the threshold level during a particular "above zero"
passage serves to validate the integration of that "above zero" passage
between its respective ED and SP limits.
Referring to FIG. 9, in which those components functionally identical to
corresponding components in FIG. 2 are identically numbered, the input
signal 206a is differentiated by differentiating network 200 to provide
the P waveform which is passed through amplifier 200 to the respective
inputs of integrator 204, a threshold detector 912, and a zero-crossing
detector 907. The zero-crossing detector 907 may correspond with means,
not shown in FIG. 2, which established the interval of integration and
resulted in the control signal 208 therein. The threshold detector 912
establishes a signal magnitude threshold value above which the P waveform
is presumed to be indicative of a valid systolic rise. When the incoming P
waveform exceeds the threshold level of detector 912, a signal is provided
to the input of validating logic 914 indicative of such threshold level
having been exceeded. Similarly, the validating logic 914 receives an
input from the output of the zero-crossing detector 907 to define when the
P waveform crosses a zero reference in the positive going direction and
also in the negative going direction. The output 908" from validating
logic 914 is applied to the RESET input of integrator 204 for resetting
the integrator at least substantially at the beginning of each desired
period of integration beginning with the P waveform crossing the zero
reference in the positive going direction. The output 908 from validating
logic 914 is applied to the "sample" input of the optional sample
-and-hold circuit 205 and serves to store the integral value accumulated
by integrator 204 between the positive going and negative going zero
crossings of the P waveform only if threshold detector 912 has provided an
indication that the P waveform during that interval was in fact a valid
systolic rise. The output 910 of sample-and-hold circuit 205 varies from
the output of 210 of FIG. 2 only where the latter might have included an
invalid output value representative of a systolic rise when in fact only
an artifact was present.
While a threshold of fixed magnitude above the zero reference might be
utilized if an "above zero" portion of the P waveform did not vary in
magnitude in successive pulses, such is not the case, particularly when
using the present oscillometric blood pressure monitoring techniques in
which the ac pressure signal P.sub.ac increases from a small amplitude at
a low applied pressure to a large amplitude at a larger applied pressure
and then to a smaller amplitude at a still larger applied pressure.
Therefore, the threshold level, indicated as TRLD in FIG. 3B, is selected
to be a function of the magnitude of the systolic rise portion of the P
signal over one or more of the immediately preceding blood pressure
pulsations. The increase (and subsequently decrease) in magnitude of
successive systolic rises in the P waveform is sufficiently gradual, and
the relative amplitude of any "above zero" non-systolic rise components of
the P waveform are sufficiently small, that a dynamic threshold which
corresponds with 50% of the maximum "above zero" amplitude of the systolic
rise of the P waveform during the preceding pulse is herein considered
sufficient for recognizing only those "above zero" portions of the P
waveform which, in fact, attend the systolic rise.
It will be appreciated that the dynamic threshold level might be
established by summing and weighting several prior systolic rise portions
of the P waveform in which case threshold TRLD might be at a preselected
level greater or less than 50% of the magnitude of the immediately
preceding systolic rise. *An analog example of a dynamic threshold
detector of the type suitable for application herein is described in
greater detail in U.S. Pat. No. 3,590,811 to Harris for
Electrocardiographic R-wave Detector. Digital means for establishing a
dynamic threshold level will be described hereinafter in greater detail.
Reference is now made to FIGS. 4,6, and 7 for a more detailed description
of the apparatus and process of one aspect and embodiment of the
invention. The apparatus is described with reference to the functional
block diagram of FIG. 4 which provides for the digital processing of the
analog signal received from transducer 23. However, it will be appreciated
that analog implementation is similarly possible. More specifically,
discrete electronic components, discrete digital chips, microprocessor
technology and structure, or digital computer can be employed. FIGS. 6 and
7, respectively, comprise a state diagram and a flow chart, or decision
tree, associated with the processing of the P signal between successive
blood pressure pulses corresponding with successive beats of the heart.
Generally speaking, the signal processing steps of the improved blood
pressure monitoring apparatus and technique of the invention, illustrated
in FIGS. 6 and 7, correspond with that portion of the FIG. 4 apparatus
which integrates the P signal between the limits ED.fwdarw.SP.
The arm 11 of a test subject with artery 13 therein is surrounded by a
typical blood pressure cuff 15. Typically, the brachial artery located in
the upper arm is employed for this type of blood pressure measurement.
Attached to the cuff via conduits 17 and 21 are pump 19 and pressure
transducer 23, respectively. Transducer 23 has a transfer function such
that its electrical output is substantially representative of its pressure
input up to the limit of information contained in the pulse pressure. The
pressure transduce | | |
