 |
Description  |
|
|
FIELD OF THE INVENTION
This invention relates to automated blood pressure measuring apparatus and,
more particularly, to stored program controlled monitors employing the
oscillometric method of detection characterized by data purification and
enhanced systolic, diastolic and mean blood pressure determination.
CROSS REFERENCE TO RELATED APPLICATIONS
Reference is hereby made to the following concurrently filed co-pending
commonly assigned patent applications: IMPROVED SPHYGMOMANOMETRIC CUFF
PRESSURIZING SYSTEM, Ramsey et al., Ser. No. 751,835; OSCILLOMETRIC BLOOD
PRESSURE MONITOR EMPLOYING NON-UNIFORM PRESSURE DECREMENTING STEPS, Ramsey
et al. Ser. No. 751,840; IMPROVED AUTOMATED MEAN ARTERIAL BLOOD PRESSURE
MONITOR WITH DATA ENHANCEMENT, Ramsey et al., Ser. No. 751,826; IMPROVED
AUTOMATED SYSTOLIC BLOOD PRESSURE MONITOR WITH DATA ENHANCEMENT, Ramsey et
al., Ser. No. 751,827.
BACKGROUND OF THE INVENTION
Automated blood pressure monitoring has rapidly become an accepted and, in
many cases, essential aspect of human and veterinary treatment. Such
monitors are now a conventional part of the patient environment in
emergency rooms, intensive and critical care units, and in the operating
theatre.
The so-called oscillometric method of measuring blood pressure is one of
the most popular methods in commercially available systems. This method
relies on measuring changes in arterial counterpressure, such as imposed
by an inflatable cuff, which is controllably relaxed or inflated. In some
cases the cuff pressure change is continuous, and in others it is
incremental. In substantially all, a transducer monitors arterial
counterpressure oscillations, and processing apparatus converts select
parameters of these oscillations into blood pressure data.
Of particular interest with respect to the principles of the present
invention are the concepts set forth in U.S. Pat. Nos. 4,360,029 and
4,394,034 to M. Ramsey, III, which are commonly assigned with the instant
invention. The Ramsey patents derive from common parentage, the former
including apparatus claims and the latter including method claims, their
division having been made in response to a restriction requirement during
the prosecution. Both patents, however, carry common disclosures of
apparatus and methods for artifact rejection in oscillometric systems,
which have been in practice in the commercially successful DINAMAP brand
monitors, which are manufactured and marketed by Critikon, Inc., of Tampa,
Fla., the assignee hereof. In accordance with the Ramsey patents, an
inflatable cuff is suitably located on the limb of a patient, and is
pumped up to a predetermined pressure. Thereupon, the cuff pressure is
reduced in predetermined fixed decrements, at each level of which pressure
fluctuations are monitored. These typically consist of a DC voltage with a
small superimposed variational component caused by arterial blood pressure
pulsations (referred to herein as "oscillatory complexes"). Therefore,
after suitable filtering to reject the DC component and to provide
amplification, pulse peak amplitudes above a given threshold are measured
and stored. As the decrementing continues, the peak amplitudes will
normally increase from a lower amount to a relative maximum, and
thereafter will decrease. The lowest cuff pressure at which the
oscillations have a maximum peak value is representative of mean arterial
pressure. The cuff pressures obtaining when stored oscillatory complex
pulse peak amplitudes bear predetermined fractional relationships with the
largest stored peak corresponding to the subject's systolic and diastolic
pressures.
The Ramsey patents devote considerable effort and disclosure to the
rejection of artifact data to derive accurate blood pressure data. Indeed,
as is apparent from FIG. 2 of the Ramsey patents, the most substantial
portion of the measurement cycle (denominated "T3") is devoted to the
execution of complex detection at the various pressure levels, measurement
of signal peaks of true complexes, and processing those peaks in
accordance with artifact rejection algorithms. Notwithstanding such
efforts, the signal peak data collected sometimes incorporates data
errors, i.e., a data pattern inconsistent with the above described typical
physiological response pattern of a subject as the artery occluding cuff
pressure monotonically decreases.
Further, in a contemporaneous invention (see M. Ramsey III, et al patent
application Ser. No. 751,840 for "OSCILLOMETRIC BLOOD PRESSURE MONITOR
EMPLOYING NON-UNIFORM PRESSURE DECREMENTING STEPS" filed on even date
herewith, the disclosure of which is incorporated herein by reference)
oscillometric blood pressure measurements are effected with non-uniform,
cuff pressure-dependent pressure decrements between successive oscillatory
complex peak measuring intervals. Such a method of effecting oscillometric
blood pressure measurements is facilitated by systolic, diastolic and mean
blood pressure determining algorithms not heretofore employed.
It is an object of the present invention to provide improved oscillometric
blood pressure determining apparatus and methodology.
More specifically, it is an object of the present invention to purify the
oscillatory complex peak amplitude data ensemble employed for blood
pressure determination.
Yet another object of the present invention is the provision of improved
algorithms, methodology and apparatus for determining diastolic blood
pressure.
SUMMARY OF THE INVENTION
A blood pressure cuff is applied about a subject's artery, and inflated
above the systolic level thus fully occluding the artery for a full heart
cycle. The cuff pressure is thereafter reduced to permit an increasing
flow through the progressively less occluded artery, and a measure of the
peak amplitudes of the successively encountered oscillatory complexes
stored in memory. Also retained is the cuff pressure obtaining for each
stored complex peak.
In accordance with varying aspects of the present invention, the stored
complex peak-representing data set is corrected for aberrations; and
improved data processing operates on the stored (and advantageously
corrected) pulse peak data and the corresponding cuff pressure information
to determine the subject's, diastolic pressure.
DESCRIPTION OF THE DRAWINGS
The above and other objects and features of the present invention will be
realized from the following detailed discussion of a specific,
illustrative embodiment thereof, presented hereinbelow in conjunction with
the accompanying drawing, in which:
FIG. 1 is a timing diagram illustrating data generation and correction
during an illustrative measurement cycle for oscillometric blood pressure
determination in accordance with the principles of the present invention;
FIG. 2 is a flow chart illustrating data purification for improved
oscillometric blood pressure determination;
FIG. 3 depicts oscillation amplitude processing for a systolic blood
pressure measurement in accordance with the present invention;
FIG. 4 is a program flow chart for the systolic blood pressure measurement
typified in FIG. 3;
FIG. 5 illustrates blood pressure interpolation for the processing mode of
FIGS. 3 and 4 (and by analogy for FIGS. 6-9 as well);
FIG. 6 depicts oscillatory complex measuring wave forms illustrating
diastolic blood pressure determination in accordance with the present
invention;
FIG. 7 is a program flow chart illustrating the diastolic blood pressure
measurement typified by FIG. 6;
FIG. 8 is a timing diagram depicting oscillatory complex peak amplitude
processing for mean arterial pressure measurements in accordance with the
present invention; and
FIG. 9 is a program flow chart illustrating the mean arterial pressure
determination typified by FIG. 8.
BEST MODE FOR CARRYING OUT THE INVENTION
U.S. Pat. Nos. 4,360,029 and 4,349,034, each to Maynard Ramsey, III, and
pending Maynard Ramsey, III et al application U.S. Pat. No. 4,543,962 for
"IMPROVED METHOD OF AUTOMATED BLOOD PRESSURE DETECTION" issued Oct. 1,
1985 are incorporated herein by reference, as is the aforementioned
co-filed Ramsey III et al application. These patents and patent
applications describe in detail the basic oscillometric method of
measuring blood pressure forming a background and a starting point for the
instant invention.
To review only briefly, an artery-occluding cuff is disposed on the
subject, e.g., about a subject's upper arm over the brachial artery. At
the inception of a measuring cycle, the cuff is inflated to a pressure
which fully occludes the brachial artery, i.e., prevents blood from
flowing therethrough at any point in the heart cycle. The cuff is then
progressively deflated, as in discrete steps. A pressure transducer is
coupled to the internal cuff pressure and provides an analog signal
characterizing the blood pressure oscillatory complexes when they begin to
occur (i.e., when the maximum heart pressure corresponding to contraction
of the heart's left ventricle exceeds the instantaneously obtaining
artery-occluding cuff pressure). The peak values of the complex signals
are determined in hardware or software.
As the measurement cycle progresses, the peak amplitude of the blood
pressure complexes generally become monotonically larger to a maximum and
then become monotonically smaller as the cuff pressure continues toward
deflation. The peak amplitude of the cuff pressure oscillation complexes,
and the corresponding occluding-cuff pressure values are retained in
computer memory. The aforementioned Ramsey patents and patent applications
illustrate previously employed algorithms for processing the stored blood
pressure complex peak values and concomitant pressure values to yield the
subject's mean arterial pressure. These patents and applications also
furnish detailed procedures for measuring oscillatory complex peaks;
procedures for testing complexes and rejecting bad data associated with
measurement-impeding artifacts (such as motion) during a measuring cycle,
and the like.
The oscillometric blood pressure measurements as typified by the
aforementioned Ramsey disclosures are effected under stored program
control, as via a microprocessor operative in conjunction with a program
containing read only memory (ROM or PROM), and a variable content random
access memory (RAM) which stores the cuff pressures, oscillatory complex
peak amplitudes, and other processing operand variables. The
microprocessor receives the cuff pressure readings generated by the
pressure transducer, for example as processed by a peak detector,
amplifier and analog-to-digital converter, and supplies all output control
signals required, e.g., to open and close one or more cuff deflating
valves.
The oscillometric method above described and more fully discussed in the
aforementioned Ramsey patents and applications may be conducted with
several variations. Thus, for example, the cuff may be inflated directly
by an air pump; and deflated in fixed, discrete steps under microprocessor
control. Alternatively, the cuff may be principally or entirely inflated
by the pressurized contents of an air reservoir; and/or deflation may
proceed in variable, cuff pressure-dependent steps via selected one or
ones of plural deflating valves. These latter alternatives achieve the
desideratum of condensing the time required for a composite measurement
cycle of operation.
Also, there are alternative procedures for measuring the oscillatory
complex peak amplitude at any prevailing cuff pressure. In one mode
heretofore employed, plural (e.g., two) complex peaks are measured at each
cuff pressure step during cuff deflation, and their average used as the
peak value. Since the peaks should be approximately equal, any marked
disparity (e.g., >20%) signals that some artifact error occurred and the
data is rejected. In a fast ("stat") mode, after several intervals of
qualifying (close or equal peak values) companion complexes are detected
to develop measurement confidence, only one pulse is required during
succeeding cuff deflation intervals thus speeding the composite
measurement period. Please see in this later regard the aforementioned
application filed July 9, 1984.
As alluded to above, it is sometimes the case when blood pressure complexes
are being examined for peak amplitude at any occluding pressure level that
improper data is developed. There are varying causes for such aberrations.
Perhaps the most common is spurious motion by the subject which generates
an inadvertent pressure impulse in the cuff which is sensed by the
pressure transducer which may be then incorrectly reflected in the blood
pressure measurement. Other causes include varying sources of interfering
electrical noise or internal cardiac or respiratory changes in the
subject. When a false complex peak amplitude value is generated, it is
discarded by the composite measuring apparatus and a discard-signalling
value (e.g., +1) retained in its place in memory.
A second form of spurious data occurs when the pattern of stored pulse peak
values departs from the physiologically mandated sequence of values which
progressively increase to a peak and then progressively decrease.
Attention will now be directed to data processing under stored program
control for purifying the data collected by the above-described blood
pressure measuring apparatus. Further, specific illustrative efficient
algorithms are discussed for in fact determining the subject's systolic,
diastolic and mean arterial blood pressures. Such data processing may be
effected on any computing equipment, preferably digital microprocessors
such as commercially available from a number of vendors. The program
instructions and sequences presented below are for illustrative purposes
only. Such instructions may in fact be implemented in any of diverse
program languages and sequences readily apparent to those skilled in the
art. In the signal processing below discussed, processing variables have
the following significance:
______________________________________
Variable Functional Quantity Represented
______________________________________
I. Variables Employed For All Data Processing Below Discussed
CP(I) The cuff pressure, measured by the
transducer pneumatically coupled to
the artery occluding cuff, obtain-
ing during the i-th deflation step.
CP(I) is an indexed array, i.e.,
there exists a plurality of
values for CP(I) characterizing
each of the -ideflation steps.
.phi.A(I) The peak amplitude of the
oscillometric oscillation (i.e.,
the complex peak amplitude) occur-
ring at the i-th step. Where
multiple complexes are measured
during each prevailing deflation
pressure, .phi.A(I) is the average of
two (or more) peak amplitudes
during the i-th step. .phi.A(I) is
an indexed array.
.phi.A(MAX) The peak value of the array of
averaged oscillatory blood
pressure complex amplitudes.
MAX The time interval when the peak
complex .phi.A(MAX) occurred.
II. Variables Specific To Systolic Pressure Measurement
LVL An intermediate processing
variable representing a predeter-
mined fraction of .phi.A(MAX).
SYS The subject's measured systolic
pressure.
III. Diastolic Pressure Variables
UDLVL and LDLVL
Intermediate processing variables
each representing a different frac-
tion of .phi.A(MAX).
DIAU, DIAL Intermediate processing variables
representing upper and lower inter-
polated diastolic pressure compu-
tational variables.
DIA The subject's measured diastolic
pressure.
IV. Mean Arterial Pressure Processing Variables
AMP The complex pulse peak for the
deflation interval following that
for which the pressure oscillation
amplitude was the maximum.
MAPL An intermediate processing variable
employed in the final mean arterial
pressure computation.
MAP The subject's mean arterial blood
pressure.
______________________________________
Turning now to FIG. 1, there is depicted wave forms with associated data
characterizing the generation of data for an oscillatory blood pressure
measurement--and purging (overcoming) bad data constituents. In accordance
with the above discussion, the cuff artery occluding pressure for a
measurement cycle, as measured by the cuff-associated transducer, is
characterized by a wave form 10. The cuff pressure rapidly increases to a
maximum above the subject's systolic pressure, and is then deflated in a
sequence of steps to a point below the diastolic pressure. The sequence of
cuff deflation steps is indicated by the time interval signalling digits
1, 2, . . . , (lowest row 18 in the data table portion of FIG. 1). The
internal pressure characterizing the cuff pressure at each step i is given
by the data array CP(1), CP(2), . . . (upper data table row 12).
Each step (time interval) is made sufficiently long to include at least two
heart beats. Accordingly, at least two cuff pressure complex pulses
21.sub.i and 22.sub.i are measured during each interval after such pulses
begin. Legends have been applied to pulses occurring during deflation
steps 6 and 9 to avoid clutter and loss of clarity in FIG. 1. No pulses
are measured during the first and second pressure steps (time intervals),
it being assumed that the cuff pressure [CP(1)=201 Torr., and CP(2)=194
Torr.] are sufficient during these periods to obviate blood flow through
the subject's artery for the full heart cycle. During the following
intervals 3, 4 . . . , two oscillometric complex pulses 21.sub.i and
22.sub.i are generated and measured, the two pulses having an average peak
amplitude 23.sub.i (the processor variable array value initially stored in
.PHI.A(I)). The measured oscillation amplitude array (.PHI.A(I)) is shown
in the second row 14 of the FIG. 1 data table for each time interval.
As above noted, assuming a perfect measurement, the oscillation pressure
amplitude .PHI.A(I) data row would not contain any +1 values which signify
an impeded measurement. Further, the data pattern in the second row of the
data table for the oscillation amplitudes would exhibit a pattern of
successively increasing numbers to a peak value, followed by progressively
decreasing values--all without adjacent equal .PHI.A(I) values. To the
extent that any .PHI.A(I)=1 values are stored, or to the extent that the
progressively increasing/decreasing pattern does not obtain, the data
processing in accordance with the instant invention functions to compute
appropriate corrected .PHI.A(I) values (the third data table row 15 in
FIG. 1) for the oscillation amplitude entries requiring correction.
In overview, where any .PHI.A(I)=1 values exist, they are replaced by the
average value of the oscillation amplitude in the two contiguous storage
cells, i.e.,
.PHI.A(I)=(.PHI.A(I-1)+.PHI.A(I+1))/2. Eq. 1.
Correspondingly, where two contiguous oscillation amplitudes have the
proscribed equal values, the first of the contiguous equal pair is
replaced by the average of the amplitudes of the complex peaks measured at
the next lower and next higher occluding cuff pressures. See, for example,
Eq. 1 and, more particularly, the comparable relationship in functional
block 30 of FIG. 2.
Data flow effecting the data purification algorithm above-discussed is set
forth in the program flow chart of FIG. 2. FIG. 2 operates on the measured
average oscillation amplitudes (the second data table row 14 in FIG. 1)
and generates the corrected .PHI.A(I) values shown in the third row 15 of
FIG. 1. To this end, proceeding from a start block 10 (FIG. 2), step 15
reads the next value .PHI.A(I) (proceeding toward the right along the FIG.
1 data table row 14) and test 18 determines whether the value stored in
.PHI.A(I) equals the error-signalling value +1. If as is the usual case it
does not (indicating that the value measured was presumptively free of
artifacts and the like), control passes to equality test 27. However, if
the contents of .PHI.A(I) did equal +1 ("YES" branch of test 18),
functional block 23 implements Eq. 1, i.e., replaces the +1 former
contents of memory cell .PHI.A(I) corresponding to cuff pressure CP(I)
with the average value of the oscillation amplitude measured at the next
lower (.PHI.A(I-1)) and next higher non-plus one (.PHI.A(I+1)) deflation
steps. The processing steps 18 and 23 thus purge the measured pressure
peak amplitude storage contents (the second row of the FIG. 1 data table)
of all +1 values, replacing these by the average value of the measurements
made during immediately adjacent deflation steps (corrected .PHI.A(I)
contents being illustrated in row Test 27 next examines the current
operand .PHI.A(I) for the proscribed equality with the previous value
.PHI.A(I-1). If, as is normally the case, the contents of .PHI.A(I) and
.PHI.A(I-1) differ ("NO" branch from test 27), processing flows to test 32
to determine whether each of the N elements of .PHI.A(I) have been
processed. If they have not, control returns to block 15 to read in and
process the next .PHI.A(I) element of the array in the third row 15 of the
FIG. 1 data table. When all elements have been processed, control exits
from the FIG. 2 data purification routine to data processing point 33 to
proceed with the next (unrelated) task for the microprocessor.
If a data error has occurred ("YES" output of test 27 signalling that a
data value .PHI.A(I) equaled the previous value), control passes to step
30 which replaces the assumed erroneous element .PHI.A(I-1)--(the value
which should differ from .PHI.A(I) but did not) with the average of the
two immediately contiguous elements, as by
.PHI.A(I-1)=(.PHI.A(I)+.PHI.A(I-2))/2. Eq. 2.
Accordingly, the data purification routine depicted in FIG. 2 and
above-discussed replaces all error reading signifying .PHI.A(I)=1 values
with an interpolated estimated value; and purges the data table row 14
.PHI.A(I) array of data of any contiguous equal values. The corrected set
of .PHI.A(I) is shown in the third row 15 of the FIG. 1 data table. Thus,
for example, the oscillation amplitude value during the cuff pressure step
(time interval) "4" is corrected from the error-signalling +1 value to a
peak amplitude 14, representing the average of measurements 4 and 25 at
cuff pressures 187 Torr. and 153 Torr.during the immediately contiguous
time intervals 3 and 5. Similarly, the first (pressure step 6) of two
equal measured oscillation amplitude pulses of value 63 during periods 6
and 7, corresponding to occluding cuff pressures of 140 Torr. and 128
Torr., is corrected to a value of 44 representing the average of the
contiguous measured amplitudes of 63 and 25 units.
The corrected array .PHI.A(I) as represented by the third row 15 in FIG. 1
thus comprises values from which each of the systolic, diastolic and mean
arterial blood pressures may be determined either in accordance with the
improved algorithms below discussed or employing the algorithms of the
above referenced patents and patent applications. The data purification
above discussed provides more accurate measurements than was heretofore
the case; and also permits blood pressures to be determined more quickly,
obviating the need for repeated deflation steps when unacceptable artifact
or noise corrupted data is sensed.
Attention will now be shifted to the particular method pursuant to which
the stored cuff pressure CP(I) and corrected blood pressure peak value
.PHI.A(I) information in the first and third data rows of FIG. 1 is
employed in accordance with other aspects of the present invention to
measure a subject's systolic, diastolic and mean arterial blood pressures.
Pulse complex wave form processing typifying systolic blood pressure
determination is illustrated in FIG. 3, and a flow chart for the
underlying data processing is set forth in FIG. 4. In overview, systolic
pressure is determined by:
(a) Finding the amplitude (.PHI.A(MAX)) of the largest blood pressure
oscillatory complex (which occurs at the time interval MAX):
(b) Finding an amplitude level (LVL) equal to a predetermined fraction of
the peak value .PHI.A(MAX). We have found a value of 0.5 to be
satisfactory for normal processing with something less (e.g., 0.45) for
stat (rapid deflation and/or single pulse) operation:
(c) Examining the corrected oscillation amplitude (.PHI.A(I)) values (third
row 15 in the FIG. 1 data table) starting at the MAX interval and
proceeding toward the higher cuff pressure direction (i.e., to the left in
FIGS. 1 and 3) to find the two contiguous oscillation amplitudes for which
.PHI.A(L)<.PHI.A(MAX)*0.5<.PHI.A(L+1); Eq. 3.
(d) Computing the interpolated cuff pressure (between CP(L) and CP(L+1))
assuming a linear variance in oscillation amplitude and cuff pressure
between the intervals L and L+1. This per se well known linear trapezoidal
interpolation is graphically depicted in FIG. 5. The interpolated cuff
pressure directly corresponds to the subject's systolic blood pressure
(SYS). Expanding upon the systolic pressure determining methodology set
forth above, the cuff pressure interval I=MAX when the largest oscillation
amplitude peak occurs is determined in any per se well known manner, (step
40 of the FIG. 4 flow chart corresponding to the interval MAX in FIG. 3).
Thus, for example, the following schematic BASIC sequence will suffice as
illustrative to find the interval MAX:
.PHI.AMAX=.PHI.A (1) Eq. 4.
MAX=1 Eq. 5.
F.PHI.R K=2 T.PHI.N Eq. 6.
IF .PHI.A(K)<.PHI.AMAX GOTO 70 Eq. 7.
.PHI.AMAX=.PHI.A(K) Eq. 8.
MAX=K Eq. 9.
70 NEXT K Eq. 10.
In brief, Equations 4 and 5 make an initial assumption that the peak value
occurred during the first interval and load a provisional peak value
storing variable .PHI.AMAX with the value .PHI.A(1). For an assumed N-time
interval measurement, the loop between Equations 6 and 10 sequentially
examines every element of the .PHI.A(I) array from 2 to N, updating
.PHI.AMAX only when the value .PHI.A(K)--(K being the loop index) exceeds
the previously assumed .PHI.AMAX value. When the processing exits from the
loop following instruction 70 in Equation 10 the variable MAX contains the
value of I such that .PHI.A(MAX) is the largest value in the array.
The next following step 42 sets a variable LVL equal to the predetermined
fraction of the peak amplitude .PHI.A(MAX) as by
LVL=.PHI.A(MAX)*0.5 Eq. 11.
The value LVL is shown by the dashed line 50 in FIG. 3.
The next following operation 45 finds the first time interval (L) preceding
MAX for which the oscillation amplitude peak is less than LVL, i.e., less
than one-half of the peak value .PHI.A(MAX), thereby finding the two
contiguous values (L, L+1) having peak amplitudes which bound the value in
LVL. Algorithms for conducting such a search are well known to those
skilled in the art, e.g.,
FOR J=1 TO MAX Eq. 12.
IF (.PHI.A(MAX-J)-LVL)<0 GOTO 140 Eq. 13.
NEXT J Eq. 14.
L=MAX-J Eq. 15.
Equations 12-15 simply comprise a DO or FOR-NEXT loop progressing from
MAX-1 toward L=1, exiting when the first sub-LVL value is obtained. The
appropriate interval identification (MAX-J) is stored in the variable
location L.
Finally, the value of the systolic pressure is estimated by assuming a
linear variation in cuff pressure between the values CP(L) and CP(L+1),
and a linear variation between the corresponding oscillation amplitude
.PHI.A(L) and .PHI.A(L+1). Thus, in accordance with the per se well known
trapezoidal interpolation equation, the systolic pressure SYS may be
determined (step 47 of FIG. 4) by
##EQU1##
To illustrate employing the data of FIG. 1, 50% of the peak amplitude (70)
is 35, and thus the pulse complex measurements of time intervals 5 and 6
are selected for systolic pressure computation. The Eq. 16 software
interpolation implementation yields:
SYS=153+((140-153).times.(35-25)/(44-25)); Eq. 17.
=146 Torr. Eq. 18.
assuming three significant figures.
Pulse complex wave form processing characterizing diastolic blood pressure
determination is illustrated in FIG. 6; and a flow chart for the
underlying diastolic data processing algorithm is depicted in FIG. 7. In
overview, diastolic pressure is determined by:
(a) the amplitude (.PHI.A(MAX)) of the complex (which occurs at the time
interval MAX);
(b) Finding an amplitude level (UDLVL) equal to a first predetermined
fraction of the peak value .PHI.A(MAX). We have found a value of 0.69 to
be satisfactory for normal processing and 0.72 for rapid ("stat")
processing;
(c) Examining the corrected oscillation amplitude (.PHI.A(I)) buffer 15
(FIG. 1) starting at the MAX interval and proceeding toward the lower cuff
pressure direction (i.e., to the right in FIGS. 1 and 6) to find the two
contiguous oscillation amplitudes for which
.PHI.A(UD).ltoreq..PHI.A(MAX)*0.69.ltoreq..PHI.A(UD-1); Eq. 19.
(d) Finding the interpolated cuff pressure (between CP(UD-1) and CP(UD))
assuming a linear variation in oscillation amplitude and cuff pressure
between the intervals UD-1 and UD (processing variable DIAU in FIG. 7);
(e) Examining the stored .PHI.A(I) oscillation amplitude values at
pressures starting at the lowest CP measured for a contiguous pair
bounding the peak amplitude .PHI.A(MAX) multiplied by a second factor
lower than the first factor (e.g., 0.55), i.e., where
.PHI.A(LD).ltoreq..PHI.A(MAX)*0.55.ltoreq..PHI.A(LD-1); Eq. 20
(f) Computing the interpolated cuff pressure between CP(LD) and CP(LD-1)
corresponding to MAX times the 0.55 factor. This lower interpolated cuff
pressure is associated with the variable designation DIAL; and
(g) Determining the subject's diastolic pressure (DIA) as the average of
the upper and lower interpolated values DIAU and DIAL, i.e.,
DIA=(DIAU+DIAL)/2. Eq. 21.
The above-described procedure is illustrated in the blood pressure complex
depiction of FIG. 6 and the FIG. 7 flow chart. The peak .PHI.A(max) is
first located as by the processing of Equations 4-10. The upper and lower
peak amplitude fractions DIAU and DIAL are next determined (steps 64 and
65 of FIG. 7 corresponding to the labeled horizontal dash lines in FIG.
6). Step 69 then finds the first time interval (UD) following MAX at which
the peak amplitude .PHI.A(UD) is lower than the value stored in DIAU (as
by processing analogous to that of Equations 12 through 15 replacing
"MAX-J" with "MAX+J"). Thereafter, step 72 performs the trapezoidal
interpolation analogous to that of FIG. 5, determining the cuff pressure
(DIAU) corresponding to the UDLVL complex amplitude value. It is observed
that the time interval UD-1 coincides with the interval MAX when the peak
complex value occurred since, for the data case illustrated, the first
pulse complex following MAX less than 0.69.times..PHI.A(MAX) occurred in
the next time interval MAX+1.
The functional steps 73 and 74 of FIG. 7 perform in a manner directly
analogous to operations 69 and 72. locating the cuff pressure DIAL by
interpolation for the intervals when the peak complex amplitudes bound the
LDLVL value equal .PHI.A(MAX) times 0.55. This latter search is conducted
from .PHI.A(i) at the lowest CP, then working toward higher CP's. Finally,
the subject's diastolic pressure (DI | | |
