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BACKGROUND OF THE INVENTION
Numerous means for obtaining blood pressure measurements are known
including both invasive and noninvasive means. A number of noninvasive
measuring means are disclosed in an article by C. S. Weaver, J. S.
Eckerly, P. M. Newgard, C. T. Warnke, J. B. Angell, S. C. Terry, and J.
Robinson, entitled "A Study of Non-Invasive Blood Pressure Measurement
Techniques" presented at a conference held at Stanford University on
September, 1978 and published by the Society of Photo-Optical
Instrumentation Engineers. Included in the article is a description of an
algorithm for processing cuff pressure measurements and associated R-wave
peak detector and Korotkov sound detector output signals for detecting
artifact information in the output from the Korotkov sound detector and
eliminating said artifacts from the blood pressure measurements. The
present invention involves an improved Korotkov sound processing
algorithm.
SUMMARY OF THE INVENTION AND OBJECTS
An object of this invention is the provision of an improved method of
identifying false signals in the output from a Korotkov sound detector
employed in apparatus for the measurement of blood pressure and/or
systolic slope of blood pressure waves in an artery of a subject.
An object of this invention is the provision of an automatic
computer-implemented technique for identifying and eliminating false
outputs from a Korotkov sound detector included in a blood pressure
measuring system, or the like, which technique is well adapted for use
during stress testing, or in an operating room and intensive care units of
hospitals where ECG signals normally are available.
The above and other objects and advantages of this invention are achieved
by means of a system which includes an inflatable cuff which is inflatable
to a pressure above systolic pressure and deflatable to a pressure below
diastolic pressure. A pressure transducer connected to the inflatable cuff
generates a signal which is a function of cuff pressure. A microphone
picks up Korotkov sounds, and artifacts, during deflation of the cuff, and
electrodes attached to the subject pick up electrocardiographic signals.
The peak of the ECG R-wave is detected by R-wave peak detection
techniques. Many, but not all artifacts, are removed from the microphone
output by use of K-sound detection techniques. The pressure transducer
output is converted to digital form for transfer to a digital computer and
storage in the computer memory. The time of arrival of the peak R-wave
signals and associated K-sound and artifact signals also is stored in the
computer memory. RK intervals comprising the time interval between the
time of arrival of an R-wave signal and the associated K-sound or artifact
signal are determined which, together with the associated cuff pressure,
establish a plurality of RK interval versus cuff pressure points, some of
which are true points and others of which are artifact points. These
points are stored in the computer memory. Certain RK interval values
outside normal ranges are eliminated as probably resulting from artifacts.
Remaining RK intervals are grouped in separate groups using a chaining
operation which involves estimating from one point at a high cuff
pressure, an RK interval value, RK, at a next lower cuff pressure at which
there is a point. If the difference between the RK interval value at said
lower cuff pressure and estimated interval value, RK, is within a selected
range, the actual value is added to the group. Now, using the newly added
point, an estimated RK interval value for the next lower cuff pressure at
which there is another point, is calculated and the difference between the
estimated value RK and actual RK interval value is determined. Again, the
point is added to the group of points if the difference is within a
predetermined range of RK interval values. If the difference is outside
the range, the point is not added to the group of points, and the RK
interval at the next lower cuff pressure containing a point is checked.
After the point at the lowest cuff pressure has been checked, the process
is repeated starting at the highest ungrouped point. When all of the
points have been grouped, the group with the greatest number of RK
interval vs cuff pressure points therein is selected as the group which
includes the greatest number of valid, or true, RK interval versus cuff
pressure points. A straight line using minimum mean-square fitting
techniques is fitted to these points and, with further processing, some
additional artifact points may be deleted from the group. With artifact
points deleted, the cuff pressure at the maximum and minimum RK interval
points provides a measure of the subject's systolic and diastolic blood
pressures, respectively. Using the techniques of this invention, accurate
blood pressure measurements may be obtained with ambulatory subjects in a
noisy environment.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood from the following description when
considered with the accompanying drawings. In the drawings:
FIG. 1 shows peak R-wave detector, K-sound detector, and cuff pressure
transducer output measurements obtained from a subject, plotted against
time;
FIG. 2 is a plot of RK interval versus cuff pressure with no artifacts
present;
FIG. 3 is a plot of RK interval versus cuff pressure which is similar to
that of FIG. 2 but which results when artifacts are included in the output
from the K-sound detector used in making the measurements;
FIG. 4 is a simplified block diagram of a system which may be employed in
the practice of this invention for identifying and deleting K-sound
artifacts fro the output of K-sound detector means;
FIGS. 5A and 5B, together, show a flow chart for use in explaining
operation of this invention;
FIG. 6 is a plot of RK interval versus cuff pressure showing points thereof
joined in separate groups;
FIG. 7 is a flow chart showing details of a chaining operation included in
steps of the flow chart of FIGS. 5A and 5B, and
FIG. 8 is a plot of RK interval versus cuff pressure for use in explaining
the chaining operation depicted in FIG. 7.
As is understood, electrocardiographic signals picked up by electrodes
attached to a subject's body include a large R-wave component which is
relatively easily detected. In FIG. 1, successive outputs from a peak
R-wave detector included in the present system are identified by reference
characters R1, R2-R20. At any given level of physical activity
substantially periodic peak R-wave signals are produced by the subject,
and, for simplicity, periodically occuring R-wave signals are shown in
FIG. 1. An inflatable cuff attached to the upper arm of the subject is
inflated and slowly deflated as shown by the plot of cuff pressure versus
time included in FIG. 1. The cuff is inflated to a pressure, P, above
systolic pressure, then is deflated through a pressure range at which
Korotkov sounds are produced. A microphone at the inflatable cuff senses
Korotkov sounds during deflation of the cuff. The microphone output is
coupled to a Korotkov sound detector, the output from which includes true
K-sounds identified by reference characters K8 through K20. In FIG. 1 the
numerical suffixes 1 through 20 for the R-wave detector output identify
the successive heartbeat periods, with corresponding suffixes being used
for the K-sound detector and pressure transducer outputs K and P,
respectively.
In FIG. 1, the Korotkov sound detector output is depicted on two lines, the
upper one of which includes only artifact-free K-sounds, and the lower one
of which includes only artifacts. The letter suffix A identifies those K
sound detector outputs which are artifacts, and where more than one
artifact output is produced during a given heartbeat period, a subsequent
numerical suffix is included to separately identify the same. Thus, in
FIG. 1, for heartbeat period 14, two K-sound artifact signals K14A1 and
K14A2 are shown. For heart beat periods 3, 7 and 9, only one artifact per
beat is shown, which are identified by reference characters K3A, K7A and
K9A, respectively. In signal processing for the removal of artifacts from
true K-sounds, use is made of the time interval between the occurrence of
the R-wave and the subsequent occurrence of the associated K-sound
detector output. Such RK intervals are identified by reference characters
RK together with a numerical suffix for the associated heart beat periods.
Where the K-sound detector output comprises an artifact, an additional
suffix A is included, and if more than one artifact per heart beat period
is produced, a subsequent numerical suffice distinguishes between the
same. Thus, in FIG. 1, RK intervals for true Korotkov sounds are
identified by reference characters RK8 through RK20 while RK intervals for
artifacts are identified by the reference characters RK3A, RK7A, RK9A,
RK14A1 and RK14A2.
Reference now is made to FIG. 2 of the drawings wherein a plot of RK
interval measurements as a function of cuff pressure is shown. In FIG. 2,
the plot includes RK interval measurements obtained using true K-sounds
only, and not artifacts. Such RK interval versus cuff pressure points are
identified as "true" points herein to distinguish the same from "artifact"
points obtained using K-sound detector outputs which are artifacts. For
the artifact-free plot shown in FIG. 2, a straight line 20 with a positive
slope can be fitted through the series of points using minimum
mean-squared fitting techniques. The slop, .DELTA.RK interval/.DELTA.
pressure, of the line 20, is inversely proportional to the systolic slope
of the blood pressure wave of the subject and, therefor, provides a
measure of the systolic slope of the blood pressure wave. The systolic
slope of the blood pressure wave, and hence the slope of line 20, varies
in accordance with exercise, with the systolic slope generally slowly
increasing with increasing exercise. At rest, before exercise, a slope of
approximately 1 (one) for line 20 is typical. In the operation of this
invention a "control slope" m.sub.c is computed using an estimated slope
of, say, 1 for the line 20 with the subject is at rest, before exercise.
Measurements of slope are described in detail below. Also, the maximum and
minimum cuff pressures at which true Korotkov sounds are obtained provide
a measure of the systolic and diastolic blood pressures, respectively, as
seen in FIG. 2.
For an ambulatory subject, or one undergoing stress exercise, the K-sound
detector output includes numerous artifacts. In FIG. 3, to which reference
now is made, a plot of RK interval measurements as a function of cuff
pressure is shown which includes not only true points but also includes
artifact points. A straight line 22 is shown fitted to the points using
minimum means-squared fitting techniques. Although the slope of the line
22 may approximate that of a curve fitting to only true points, artifact
points must be substantially eliminated from the plot if a relatively true
measure of systolic slope and/or of systolic or diastolic blood pressure
is to be obtained from the detected R-waves and K-sounds and associated
cuff pressure measurements. An algorithm using cuff pressure measurements
and times of occurrence of R-wave and K-sound outputs for identifying
artifacts in order that they may be deleted from the K-sound output is
described below. First, however, a simplified showing of a system which
may be used for implementing such an algorithm will be described, which
system includes means for obtaining necessary measurements of cuff
pressure along with measurements of the time of arrival of R-waves and
K-sounds.
Reference now is made to FIG. 4 wherein a system which is suitable for use
in the practice of this invention is shown comprising an inflatable cuff
30 for encircling a subject's limb, such as upper arm, and a pressure
source 32 connected to the cuff through a pressure controller 34. Cuff
pressure is sensed by a pressure transducer 36, the analog output from
which is connected through an amplifier 38 to the input of an analog to
digital converter 40 for conversion to digital signal form. The digitized
cuff pressure signal is connected through a digital multiplexer 42 to a
computer 44 which includes memory 44A where cuff pressure signals obtained
during a cuff deflation temporarily are stored for use in computer
systolic and diastolic blood pressure and/or the systolic slope of the
blood pressure waves during said cuff deflation.
With the cuff attached to the upper arm of the subject, the cuff is
inflated to a pressure above systolic pressure. Then, as the cuff pressure
is decreased, by R-wave-triggered decrementation, the first Korotkov sound
appears at the systolic pressure, and the last at the diastolic pressure.
A microphone 46 picks up the Korotkov sound (K-sound) at a plurality of
cuff pressures between systolic and diastolic pressures. The microphone
also picks up noise; the amount of noise produced depending upon the
physical activity of the subject. The microphone output signal is
amplified by amplifier 48, and the amplifier output is supplied both to a
signal converter 50 and a K-sound detector 52. The converter 50 simply may
include a one-shot for generation of a pulse output in response to an
amplified output signal from amplifier 48, which pulse output is connected
to the multiplexer 42. The K-sound detector 52 distinguishes between true
K-sounds and some artifacts, and produces an output in response to true
K-sounds and artifacts which are not eliminated by the detector. The
K-sound detector output is connected to an address input of the
multiplexer 42. In the presence of an output from the K-sound detector,
the output from converter 50 is connected through the multiplexer 42 to an
interrupt input of the computer 44 to produce a K-sound, or artifact,
timing signal which, together with an associated R-wave timing signal,
provides a measure of the RK interval.
ECG electrodes 60 attached to the subject's body pick up ECG signals which
are amplified by amplifier 62 and then supplied to a converter 64 and to
an R-peak detector 66. As with the converter 50, the converter 64 also may
include a one-shot for generation of a pulse output in response to the
R-wave component of the amplified ECG signal. The pulse output from the
converter 64 is connected to the multiplexer 42 for connection as an
interrupt input to the computer 44. The R-peak detector detects the R-wave
of the ECG signal while discriminating against noise and other ECG signal
components, such as the P and T wave components. The R-peak detector
output is supplied as an address input to the multiplexer 42 for
connection of the output from the converter 64 to an interrupt input of
the computer 44 when an R wave is detected. The difference in time between
the arrival of an R wave input and associated true and/or artifact K-sound
signal(s) at the interrupt inputs to the computer provides a measure of
the RK and/or RKA interval, which interval value is temporarily stored in
the computer memory 44A for use with other such true and artifact K-sound
interval values obtained at different cuff pressures. A clock 44B is
included in the system for use in making the above-described time interval
measurements.
Another address input for the multiplexer 42 is obtained from the computer
44 through a control unit 70. Under control of unit 70, the multiplexer 42
is switched for connection of cuff pressure signals from the A/D converter
40 to the computer 44. Also, multiplexer address information is supplied
to the computer 44 through the control unit 70 for use by the computer in
controlling operation of the multiplexer. A keyboard 72 may be included
for manual supply of information to the computer, such as the name of the
subject being tested, etc. Data display and recording unit 74 may be used
a display and/or record information output from the computer, such as
systolic slope, systolic and diastolic blood pressure, or the like.
The computer 44 implements a novel process for identifying, and
eliminating, artifacts included with true K-sound signals in order that
accurate blood pressure and/or systolic slope measurements may be obtained
using only true K-sounds. The process, in general, will be best understood
with reference to the flow chart of FIGS. 5A and 5B. It will here be noted
that one or more programming steps may be involved in the actual
implementation of the indicated operation. Since the programming of such
steps for the indicated operations is well within the skill of the average
programmer, a complete program listing is not required and is not included
herein.
With the cuff 30 and transducers 36 and 60 properly secured to the subject,
the test is started as indicated by START step 100, at which time system
power is turned on or a reset operation is performed, by means not shown.
Initialization step 102 includes initial settings of counters, registers
and the like, in the computer 44. A control slope m.sub.c is used by the
algorithm for identifying, and discarding, artifacts. For the first
activity, R-peak, K-sound and cuff pressure measurements are obtained
while the subject is at rest. Measurements obtained during this resting
activity are used for computing an initial control slope m.sub.c. As noted
above, the K-sound detector output is relatively free of artifacts while
the subject is at rest, thereby ensuring that a relatively accurate value
of control slope m.sub.c is computed.
With the subject at rest, before exercise, cuff inflation step 108 is
entered wherein the cuff 30 is inflated under control of the computer to a
pressure above systolic blood pressure through operation of the cuff
pressure controller 34 to occlude blood flow in the brachial artery. Next,
at step 110, the cuff pressure is reduced to a pressure at which true
Korotkov, or artifact, sounds are first detected which, for true Korotkov
sounds, is the systolic blood pressure. At this point, the cuff pressure
is entered into the computer memory 44A through use of the transducer 36,
amplifier 38, A/D converter 40 and digital multiplexer 42, as indicated by
step 112.
Next, at step 114, an R-peak wave is detected and its time of arrival is
entered into the computer memory. The time of arrival of an associated
Korotkov sound also is entered into the computer memory, as is the time of
arrival of artifacts, if any, in the K-sound detector output. As seen in
FIG. 1 and described above, for any given R-peak wave, the time of arrival
of the true K-sound and that of one or more artifacts may be stored.
At step 116 the RK interval is calculated, and the RK interval value, or
values, are stored (step 118) with the associated cuff pressure. The cuff
pressure, at step 120, is then reduced an incremental amount of, say, 4
mmHg. The decision step 122 next is performed to determine whether or not
cuff pressure remains above diastolic pressure. If the decision is
affirmative, step 112 is again entered, whereupon the new reduced cuff
pressure value is stored, together with new associated RK interval values.
When the cuff pressure is reduced below diastolic pressure, decision step
122 is negative, and step 124 is entered and the process of eliminating
artifacts from the K-sound detector output begins. At step 124, RK
intervals calculated at step 116 are checked for the presence of an
interval which is more than a predetermined number, A, of heartbeats from
an adjacent RK interval. Any RK interval which is more than, say, 3
heartbeats from an adjacent interval is deleted from the list of RK
intervals stored at step 118. In FIG. 1, RK interval RK3A (occurring at
heartbeat period 3) is more than three heartbeats from the nearest RK
interval (here, RK7A at heartbeat period R7) and, in accordance with step
124 is deleted from the store of RK intervals obtained during the
illustrated cuff deflation. Generally, such isolated RK intervals result
from artifacts and should not be included in subsequent computations.
For any subject under any exercise condition, RK intervals are practically
never less than 100 ms and rarely are greater than 400 ms. At step 126,
the store of RK intervals is checked and those intervals equal to or less
than B, say, 100 ms or equal to or greater than C, say, 400 ms, are
deleted from the list. Minimum values employed in this test may range, for
example, from 25 to 150 ms, and maximum values from 350 to 600 ms.
Under certain conditions during a cuff deflation, the cuff pressure may
rise, due, for example, to physical pressure exerted thereon during
exercise. At step 128, the cuff pressure is checked for any rise therein
which may have occured during a cuff deflation. If there has been a
pressure rise during a heartbeat period, only the first-occuring RK
interval is accepted, and any subsequent RK intervals which may occur
during the period are deleted. K-sound detector outputs which occur during
the pressure increase often are produced by sounds generated by such
increase and are assumed to be artifacts.
At decision step 130, the total number of RK intervals remaining following
steps 124 through 128 is checked. If the number is less than, D, the RK
intervals for the deflation are not processed, and operation returns to
step 108 for the start of another cycle of operation. With 4 mmHg steps
during cuff deflation, a plurality of true K-sounds normally are produced.
For this test, a value of D in the range of between 4 and 10 typically is
employed. It will be apparent that steps 124 through 128 may be performed
in any desired order.
If a sufficient number of RK interval points remain after step 130, another
decision step 132 is entered at which a determination is made whether or
not a control cycle is being processed. As noted above, once a control
slope m.sub.c has been determined, such a control slope, or some function
thereof, may be used in subsequent chaining operations. Chaining involves
grouping together RK interval points, in a manner described below, to
eliminate those points which fail to fit the group which includes the most
points. If a control slope has been determined during a prior cycle, or
cycles, operation, such control slope is used in the chaining operation
which follows, as indicated at step 134. If no such control slope has been
established, an estimated slope is used for the chaining operation, as
indicated at step 136. In FIG. 6, a plot of RK interval versus cuff
pressure is shown wherein three groups, or chains, of RK interval points
have been established using the chaining process of this invention, the
groups being identified by reference characters C1 through C3. Details
involved in the chaining steps 134 and 136 are included in the flow chart
of FIG. 7, described below following completion of the description of the
flow chart of FIGS. 5A and 5B.
The chain which includes the greatest number of RK interval points
established at step 134, or at step 136, is selected for the next step 138
where a straight line is fitted to the chain using a minimum mean-squared
algorithm. In FIG. 6, straight line 140 is shown fitted to chain C2 in
accordance with step 138 of the flow chart. At step 142, the distance of
the RK interval points in the chain from the straight line 140 is
determined, and all points more than a specified distance therefrom are
deleted from the chain. For example, RK interval points more than, say, 20
ms from the line 140 may be deleted. In FIG. 6, RK interval point
identified by reference character 144 is eliminated by operation of step
142.
At step 146, a check of RK interval points adjacent the diastolic end of
the chain is made to determine whether or not the RK interval of the end
point is less than that of its neighboring point. An end point (such as
point 145 shown in FIG. 6) having an RK interval greater than the
neighboring point is deleted from the chain. When an end point is deleted,
the above test is repeated until an end point having an RK interval which
is less than the neighboring point is located. In this manner, a point in
the chain adjacent diastolic pressure at which there is an upward
deflection of the chain is selected as an end point.
Next, at step 148, another check of points adjacent the diastolic end of
the chain is made for any point, or group of, say, two points which is
more than E heartbeats from an adjacent point, where E is, say, 3. Such
point, or group of points, is deleted from the chain as likely being
artifacts.
At the following step 150, a straight line is fitted to the remaining
points of the selected chain using the minimum mean-squared algorithm in
the manner of step 138. Steps 152, 154 and 156 which follow are similar to
the above-described steps 142, 146 and 148 respectively, except the
operations now are performed on the chain of RK interval points and
straight line fitted thereto at step 150. At step 158, another straight
line is fitted to the points which remain. The cuff pressures at the
shortest and longest RK intervals included in the chain are a measure of
the respective diastolic and systolic blood pressure of the subject, which
pressure measurements may be displayed, stored, recorded or the like, as
indicated at step 160. The slope of the straight line fitted at step 158
is determined at step 162, which slope provides a measure of the systolic
slope of the blood pressure waves during the cuff deflation. This value
may be stored, recorded, displayed, or the like, as desired. At decision
step 164, step 108 is reentered if the test is to be continued. If not,
the test is ended at step 166.
Reference now is made to the flow chart of FIG. 7 which includes details of
the chaining operation shown at step 134 of FIG. 6A. As noted above,
chaining involves the dividing of RK interval versus cuff pressure points
obtained during a cuff deflation into groups. At step 170, a check is made
for any ungrouped points. Of any remaining points, that point at the
highest cuff pressure is stored in a first group of points at step 172.
Referring to the RK interval versus cuff pressure plot of FIG. 8, at the
beginning of the chaining operation, point (RK.sub.n-1, C.sub.n-1), which
is the highest upgrouped cuff pressure point, is stored in group, or
chain, 1 (one) of points at this step. Next, at step 174, another check is
made for any ungrouped points and, if any points remain, an estimate of
the RK interval for the next lower cuff pressure point is made at step
176. In FIG. 8, the point at the next lower cuff pressure is point
(RK.sub.n, C.sub.n). An estimated RK interval RK.sub.n for cuff pressure
C.sub.n is computed by projecting a line with a control slope m.sub.c from
point (RK.sub.n-1, C.sub.n-1) to C.sub.n. The difference between the
estimated and actual RK values is determined at step 178. In FIG. 8, this
difference is identified as .DELTA.RK.sub.n =RK.sub.n -RK.sub.n. If an
artifact is present, the absolute difference .DELTA.RK.sub.n almost always
falls outside the 40 ms, range, and at step 180.DELTA.RK.sub.n is compared
to, say, 40 ms to determine whether or not it is within the 40 ms range.
If it is within this range, the point (RK.sub.n, C.sub.n) is stored in a
group with point (RK.sub.n-1, C.sub.n-1) at step 182, and decision step
184 is entered. If not within range, decision step 184 is entered without
storing, or grouping the point. For the plot of FIG. 8, point (RK.sub.n,
C.sub.n) is outside the range whereby point (RK.sub.n, C.sub.n) is not
stored with the group 1 points.
At step 184, a check for additional points at lower cuff pressures is made.
If it is determined that one or more such points remain, step 176 is
reentered, wherein an estimated RK interval for the next lower cuff
pressure point is made. In FIG. 8, the next lower cuff pressure point is
(RK.sub.n+1, C.sub.n+1), and the estimated point is (RK.sub.n+1,
C.sub.n+1). Again, the difference in RK interval between the actual and
estimated points exceeds 40 ms, whereby this point also is not included
with the group 1 points.
The next lower cuff pressure point (RK.sub.n+2, C.sub.n+2) is checked and
the .DELTA.RK.sub.n is determined to be within the 40 ms range. Now, the
result of decision step 180 is negative, whereupon step 182 is entered for
storage of point (RK.sub.n+2, C.sub.n+2) with the group 1 points.
In accordance with the present invention, when a new RK interval vs cuff
pressure point is added to the group, a line with the control slope
m.sub.c is projected from the newly added point to the next lower cuff
pressure which includes an ungrouped point. As seen in FIG. 8, a new line
with slope m.sub.c is shown projected from point (RK.sub.n+2, C.sub.n+2)
newly added to the list. This method of chaining provides for much more
accurate grouping of points than is provided in the arrangement disclosed
in the above mentioned article, "A Study of Noninvasive Blood Pressure
Measurement Techniques" wherein the line is projected from the highest
cuff pressure points to the lowest, and not from newly added points.
In FIG. 8, it will be seen that (RK.sub.n+5, C.sub.n-5) is the last point
to be added to chain 1. At this point in the operation, decision step 184
is negative, whereupon the group number is incremented at step 186, and
decision step 170 is reentered. If more ungrouped points exist, the
chaining operation is repeated starting with the point at the highest cuff
pressure not yet included in a chain, or group. In FIG. 8, point
(RK.sub.n, C.sub.n) is selected at step 172 for storage with points of a
second group. After point (RK.sub.n+1, C.sub.n+1) has been identified as a
group 2 point, at step 180, and stored at step 182, no more ungrouped
points at a lower cuff pressure remain, and the operation loops back
through step 186 to step 170. Now, since there are no remaining ungrouped
points, decision step 170 is negative, and step 138 (FIG. 5A) is entered
for fitting of a straight line to the group which includes the greatest
number of points in the manner described above.
Step 136 involves essentially the same operations as above-described step
134 except that an estimated slope rather than a control slope is used in
projecting a line from one point to a next lower cuff pressure which
includes a point. As noted above, the estimated slope is used during a
control cycle during which the subject is stationary, and artifacts are at
a minimum. The slope of the straight line fitted to points at step 158 and
calculated at step 162 during a control cycle is employed in step 134 on
subsequent cuff deflations during which the subject is active. If desired,
a plurality of control cycles may be performed, and the average slope
obtained therefrom may be used as the control slope.
The slope of the line fitted at step 158 normally changes during exercise.
For any given cuff deflation, the slope obtained during one or more
preceeding cuff deflations, or some function thereof, may be used in step
134 as the control slope. The cuff deflation cycles may be repeated often
enough such that the change in slope between cycles is minimal. Thus, the
slope obtained during one cuff deflation may provide an accurate control
slope for use in a next cycle. In an alternative arrangement, an estimated
stope of, say, 1 may be employed in all chaining operations, without
establishment of a control slope.
The invention having been described in detail in accordance with
requirements of the Patent Statutes various other changes and
modifications will suggest themselves to those skilled in this art. For
example, the novel method is not limited to use with apparatus illustrated
in FIG. 4. Instead of using R-wave and K-sound detectors, the ECG signal
and/or Korotkov sound waveforms may be digitized, and the digital signals
supplied to the computer for software R-wave and/or K-sound detection,
with the time of arrival of the software R-wave and/or K-sounds being
stored in the computer memory. Also, a recording of the necessary inputs
may be made, and the recorded signals/played back to provide the computer
inputs. Such recording of signals for processing is particularly useful
for long term monitoring of blood pressure of ambulatory subjects.
Portable equipment for automatic cuff inflation and deflation, and
K-sound, ECG and cuff pressure recording is well known. Obviously, high
speed playback of the recorded signals is possible, so long as
compensation is made for any time differences, if any, which may result
therefrom. It is intended that the above and other such changes and
modifications shall fall within the spirit and scope of the invention
defined in the appended claims.
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