U.S. patent number 3,552,383 [Application Number 04/789,778] was granted by the patent office on 1971-01-05 for method and system for estimation of arterial pressure.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Dean T. E. Krueger, Louis C. Sheppard.
United States Patent |
3,552,383 |
Krueger , et al. |
January 5, 1971 |
METHOD AND SYSTEM FOR ESTIMATION OF ARTERIAL PRESSURE
Abstract
A method and apparatus for automatically following and
monitoring systolic and diastolic blood pressure in an animal
organism is disclosed. The pressure of an occluding cuff on the
organism is controlled by decisional logic to increase or decrease
in fixed increments the pressure in the cuff. The decision to
increase or decrease depends upon the existence and amplitude of
the Korotkoff sounds detected during a sample period which may
comprise a preset number of heartbeats. The Korotkoff signals are
analyzed for the purpose of indicating whether the cuff pressure is
just above or just below the systolic (or diastolic) blood pressure
and the actual cuff pressure is changed accordingly. Multiple
samples are made for systolic blood pressure and those samples for
which an acceptable Korotkoff pulse is detected are averaged. Then
multiple samples are made for diastolic blood pressure. The method
and apparatus is based on the typical Korotkoff sound envelope seen
when going from above systolic pressure to below diastolic
pressure.
Inventors: |
Krueger; Dean T. E. (Rochester,
MN), Sheppard; Louis C. (Birmingham, AL) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
25148655 |
Appl.
No.: |
04/789,778 |
Filed: |
January 8, 1969 |
Current U.S.
Class: |
600/495;
128/901 |
Current CPC
Class: |
A61B
5/02208 (20130101); Y10S 128/901 (20130101) |
Current International
Class: |
A61B
5/022 (20060101); A61b 005/02 () |
Field of
Search: |
;128/25AS,25M,25MS |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Claims
We claim:
1. A method for tracking a blood pressure of an animal organism,
comprising the steps of:
a. applying a first pressure to a member of said organism;
b. sensing a selected parameter of Korotkoff sounds resulting from
said first pressure;
c. placing each of said sounds into at least one of a plurality of
groups based upon specified ranges of said parameter;
d. generating, for a determinate time interval, a characteristic
measure for each of a plurality of said groups;
e. combining said plurality of measures with each other to form a
pressure-correction representation; and
f. applying a second pressure to said member in response to said
representation.
2. The method of claim 1 wherein said parameter is the intensity of
each of said sounds.
3. The method of claim 2 wherein each of said measures indicates
the number of said sounds placed within its associated group during
said interval.
4. The method of claim 3 wherein said pressure-correction
representation is further formed in response to the selection of
one of a plurality of blood pressure types to be tracked.
5. The method of claim 3 wherein said pressure-correction
representation is further formed in response to a
pressure-correction representation formed for a preceding time
interval.
6. The method of claim 3 wherein a first of said groups has an
intensity range extending from a first threshold to a second
threshold, and wherein a second of said groups has an intensity
range exceeding said second threshold.
7. The method of claim 6 wherein each of said measures assumes a
first value when said number is less than unity, a second value
when said number is unity, and a third value when said number is
greater than unity.
8. The method of claim 7 wherein said pressure-correction
representation has a first magnitude when the measure of said
second group assumes said first value, and when the measure of said
second group assumes said second value unless the measure of said
first group also assumes said second value.
9. The method of claim 7 wherein said representation has a second
magnitude when the measure of said second group assumes said third
value unless the measure of said first group also assumes said
third value.
10. The method of claim 7 wherein said representation has the same
magnitude as that of a pressure-correction representation formed
for a preceding interval when said first and second measures both
assume said second value, and when said first and second measures
both assume said third value.
11. The method of claim 8 wherein said first magnitude is positive
and said second magnitude is negative for a first blood pressure
type, and wherein said first magnitude is negative and said second
magnitude is positive for a second blood pressure type.
12. The method of claim 2 wherein the duration of said time
interval is established by counting a predetermined number of
heartbeats of said organism.
13. The method of claim 2 wherein said interval is delayed with
respect to the time of occurrence of said heartbeats.
14. The method of claim 2 wherein said pressure-correction
representation has a substantially constant absolute magnitude and
a variable sign.
15. The method of claim 2 wherein said second pressure is applied
after a predetermined time lag from the time at which said
representation is formed.
16. The method of claim 2 further comprising the step of generating
blood pressure indication from at least one of said applied
pressures.
17. The method of claim 16 wherein said indication substantially
equals said first applied pressure.
18. The method of claim 16 wherein said indication substantially
equals said second applied pressure.
19. The method of claim 16 further comprising the step of detecting
the existence of a preselected pattern of said measures, said blood
pressure indication being made substantially equal to said second
applied pressure when said pattern is detected during said
interval.
20. The method of claim 19 wherein said indication is made
substantially equal to said first applied pressure when said
pattern is not detected during said interval.
21. The method of claim 19 wherein said pattern represents the
presence of at least one of said sounds within at least one of said
groups during said interval.
22. A method of tracking a blood pressure in an animal organism
comprising the steps of:
a. applying constant pressure for a predetermined sample period to
said organism;
b. detecting the Korotkoff sounds during said period resulting from
said applied pressure;
c. analyzing said sounds on the basis of number and intensity to
determine a probable direction of deviation between said constant
pressure and the blood pressure being tracked; and
d. altering said applied pressure by a predetermined incremental
amount in a direction opposite to said probable direction of
deviation.
23. The method as claimed in claim 22 further comprising the steps
of:
a. repeating (a) through (d) of claim 22 for a plurality of sample
periods, said pressure being altered for each sample period;
and
b. averaging all said constant pressures which cause at least one
Korotkoff sound which meets a predetermined set of criteria.
24. The method as claimed in claim 23 wherein the step of analyzing
comprises:
a. classifying said sounds into classes of acceptable and loud
sounds according to their intensities relative to a predetermined
set of criteria;
b. counting the loud and acceptable sounds occurring during a given
sample period; and
c. generating an up or down pressure command signal dependent upon
the number of loud and acceptable sounds occurring during said
sample period and the particular blood pressure being tracked.
25. The method as claimed in claim 24 further comprising:
a. detecting the heartbeats of said organism; and
b. controlling the duration of said sample periods in accordance
with a preset number of detected heartbeats.
26. The method as claimed in claim 25 wherein the blood pressure
being tracked is the systolic blood pressure.
27. The method as claimed in claim 25 wherein the blood pressure
being tracked is the diastolic blood pressure.
28. Apparatus for automatically tracking the blood pressure of a
human organism comprising:
a. pressure applying means adapted to be connected to said organism
for applying pressure thereto;
b. detecting means for detecting Korotkoff sounds resulting from
certain pressure levels being applied to said organism; and
c. means for controlling incremental up and down variations in
pressure applied by said pressure applying means in accordance with
the number and a selected parameter of Korotkoff sounds detected
over a predetermined sample period.
29. Apparatus as claimed in claim 28 wherein said means for
controlling incremental up and down variation comprises:
a. voltage comparator means responsive to said detected Korotkoff
sounds for generating two sets of signals corresponding to
acceptable and loud sounds, respectively, whereby the
classification of the sounds into acceptable and loud is a function
of the sound intensity;
b. means for accumulating each of said two sets of signals for said
predetermined sample period;
c. decisional logic means responsive to said accumulated two sets
of signals for generating one of two pressure change command
signals; and
d. means responsive to said generated command signal for changing
by a predetermined incremental amount the pressure in said pressure
applying means.
30. Apparatus as claimed in claim 29 wherein said two command
signals are up and down command signals and said means for changing
the pressure comprises;
a. a pressure servosystem connected to said pressure applying means
for controlling the pressure in said pressure applying means in
accordance with an analogue electrical quantity applied to a
control input thereof;
b. a digital storage means for storing a digital quantity;
c. means responsive to said command up signal for increasing by an
incremental amount the digital quantity stored in said digital
storage means;
d. means responsive to said command down signal for decreasing by
said incremental amount the digital quantity stored in said digital
storage means; and
e. digital to analogue converter connected between said digital
storage means and said pressure servo means for converting the
digital quantity stored in said storage means into an analogue
electrical quantity and applying said analogue electrical quantity
to said control input.
Description
BACKGROUND OF THE INVENTION
The invention is in the field of methods and apparatus for
measuring blood pressure in animal organisms.
It is known in the prior art to provide continuous monitoring of
systolic and diastolic blood pressure in animal organisms. This is
necessary in many cases, particularly where a patient has had
cardiovascular surgery. Only continuous monitoring can detect the
blood pressure variations at the time they occur. In general, the
prior art methods were based on the use of an occluding arm cuff
and a microphone or other detector and employ a ramp method to
identify the pressures at which Korotkoff sounds occur. These
methods are usually designated for isolated measurements on a
general patient population. Special problems are encountered when
indirect measurements of blood pressure must be made over many
hours or days in patients with cardiovascular disease.
In one known system the systolic pressure is followed and monitored
by raising the occluding cuff pressure above the systolic blood
pressure (or what is believed to be the systolic pressure) each
time an arterial pulse is detected by a transducer attached to the
organism. The cuff pressure is then allowed to leak below the
systolic pressure until the next arterial pulse is detected. For
following and monitoring the diastolic pressure, the Korotkoff
sounds are detected and the cuff pressure is controlled in a
reverse manner to that described above. That is, the detection of a
Korotkoff sound initiates a decrease in the cuff pressure to a
value believed to be below the diastolic pressure and the cuff is
then allowed to increase in pressure to above the diastolic
pressure until the next Korotkoff sound is detected.
The latter described system operates to only either decrease or
increase cuff pressure in response to detected indications, the
opposite form of pressure variation being left to a leakage
mechanism. As a result, the cuff pressure is never held at a
substantially constant rate and this magnifies the hunting effect
and degrades the accuracy of the output. Furthermore, in the above
system the operation is based on a sound or no-sound proposition
and no provision is made for taking the sound variations into
account.
SUMMARY OF THE INVENTION
In accordance with the present invention blood pressure is
monitored by controlling cuff pressure of an occluding cuff and
detecting and analyzing Korotkoff sounds over a sample period. The
Korotkoff sounds are analyzed to decide whether the cuff pressure
is above or below the systolic (or diastolic) blood pressure.
Following the decision, the cuff pressure is increased or decreased
accordingly by a predetermined increment. The cuff pressure,
analysis, and decision may be digitally accomplished in a
computerized mechanism, the digital pressure output being converted
to an analogue quantity for controlling a cuff pressure
servosystem.
During the sample period the cuff pressure is held constant and the
sample period is long enough to cover a plurality of heartbeats
thereby allowing for detection of multiple Korotkoff sounds. The
detected sounds are classified by an amplitude parameter into
"acceptable" and "loud" sound groups or classes. Acceptable sounds
are between first and second limits above the system noise level,
and loud sounds are above the second limit. The classification of
sounds is based on a typical Korotkoff sound envelope obtained when
cuff pressure is lowered from above systolic to below diastolic
level. The magnitude of the sounds increases at first as the cuff
pressure is decreased. It reaches a maximum when the cuff pressure
is between systolic and diastolic levels and decreases as the arm
cuff pressure approaches diastolic level. This phenomenon results
from the opening of the artery with each pulse when the arterial
pressure exceeds the cuff pressure. The method of the present
invention uses this variation in sound intensity or amplitude to
define systolic and diastolic pressures as those pressures at which
weak sounds at the ends of the sound envelope are detected.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1, 2, 3, and 4, taken together is a block diagram of a
preferred embodiment of the apparatus which is capable of carrying
out the method of the present invention.
FIG. 5 is a table illustrating the decisional logic carried out by
the preferred embodiment shown in FIG. 2.
DETAILED DESCRIPTION OF THE DRAWINGS
That portion of the preferred embodiment shown in FIG. 1 operates
to analyze the Korotkoff sounds, classify these sounds into
acceptable and loud sounds, accumulate characteristic measures of
the acceptable and loud sounds over a sampling period, and control
the sampling period. A typical microphone, now shown, adapted for
use in detecting Korotkoff sounds may be placed on the organism
such as on the brachial artery of a human. As is well known, the
microphone converts the sounds into electrical impulses which are
applied through an amplifier 101 and a signal processor 102 to a
pair of voltage comparators 103 and 104. The specific form of the
signal processor 102 is not critical to the present invention but
it is designed for eliminating signals which could not result from
Korotkoff sounds. For example, the signal processor could be a
filter which filters out low frequency signals or it may be adapted
to count the zero crossovers between the maximum and minimum peaks
of any given signal. For example, in one specific embodiment
acceptable sounds were defined by the following criteria: (1 ) the
sound must be between 200 and 2,000 digital converter units above
the background noise; (2 ) the sound must be within plus or minus
42 milliseconds of a predetermined average lag time after the
occurence of a QRS complex in an EKG waveform; (3 ) the signal
resulting from the sound must have one to three zero crossings
between its maximal positive and maximal negative peaks. The zero
crossings could be detected by the signal processor 102 thereby
eliminating any signals not having the proper number of zero
crossings. As will be more fully understood below, the amplitude
limits and the time window mentioned above are controlled by the
apparatus of FIG. 1. For related sets of conditions, see
application Ser. No. 608,130, filed Jan. 9, 1967 now Pat. No.
3,450,131, by J. R. Vogt and assigned to the assignee herein.
Each of the voltage comparator units 103 and 104 has two inputs and
two outputs. To one input of each of the voltage comparator units
there is applied the output from the signal processor 102. A
reference voltage V.sub.1 is applied to the other input of
comparator 103 and a reference voltage V.sub.3 is applied to the
voltage comparator 104. The output terminals of the voltage
comparators are indicated by the designations 1 and .phi. and these
outputs are always complementary. In comparator 103, there is an
output at the 1 terminal whenever the input from the signal
processor is greater than the reference voltage V.sub.1. There is
an output at the 1 terminal of voltage comparator 104 if the signal
from signal processor 102 is greater than the reference voltage
V.sub.2, whereas there will be an output from the .phi. terminal of
comparator 104 when the signal from the signal processor 102 is
equal to or less than the reference voltage V.sub.2. The reference
voltages V.sub.1 and V.sub.3 are selected as the limits of
acceptable sounds. That is, a signal which is greater than V.sub.1
but less than V.sub.3 is an acceptable signal. When such a signal
occurs, the 1 output of comparator 103 and the .phi. output of
comparator 104 will be energized.
The latter two output terminals are connected to an AND gate 105.
The 1 output terminal from comparator 104, indicating a "loud"
sound is applied to AND gate 106. The remaining inputs to AND gates
105 and 106 are derived from EKG electrodes, amplifier 111, signal
processor 112, and a delay means 113. The latter means provides the
time lag referred to above. Specifically, the EKG electrodes are
connected to the organism in a manner well known in the art,
thereby creating electrical signals corresponding to heartbeats.
The heartbeat signals are amplified in amplifier 111, processed in
signal processor 112, and applied to the delay means 113. The
signal processor 112 may be a filter for filtering out undesirable
signals. For a pulse coming out of signal processor 102 to be
considered an acceptable Korotkoff pulse, it should occur within a
certain time window (lag time) after the QRS complex of the
electrocardiogram is detected by the EKG electrodes and associated
apparatus. (The "QRS complex" is a portion of an EKG waveform well
known in the art; see e.g., Dorlund's Illustrated Medical
Dictionary, 23rd Ed. (1957), page 433.) This time lag is provided
for by the delay means 113 which delays the "heartbeat" signal and
applies it to the AND gates 105 and 106. As pointed out above, in a
specific example, the time window may be plus or minus 42
milliseconds.
An output from AND gate 105 means that an acceptable Korotkoff
sound has occurred, whereas an output from AND gate 106 means that
a loud Korotkoff sound has occurred. The acceptable and loud
Korotkoff sounds are counted in a pair of counters 107 and 108,
respectively. The counters accumulate the acceptable and loud
sounds during the sample period which may be a predetermined number
of seconds, or as in the case of the embodiment illustrated, may be
a predetermined number of heartbeats. A typical example would be
for the sample period to be five heartbeats in duration, thereby
allowing a maximum of five Korotkoff sounds during a sample period.
The sample period is controlled by a preset counter 114 and a
single shot 115. The preset counter 114 counts the heartbeats out
of the signal processor 112. After a predetermined number of
heartbeats has been counted, the single shot 115 is energized
thereby providing a pulse output which is applied to a pair of AND
gates 109 and 110. When the AND gates 109 and 110 are energized,
the contents of counters 107 and 108 are passed therethrough to the
analyzing and decisional logic system which is shown in FIG. 2.
The apparatus shown in FIG. 2 carries out the decisional logic
illustrated in FIG. 5. This decisional logic is designed to
increase or decrease the pressure in the occluding cuff 601 (FIG.
4) in predetermined small increments to cause the pressure in the
cuff to substantially follow the systolic or diastolic pressure,
depending upon which is being measured. The decision is based
entirely upon the number of acceptable sounds and loud sounds
detected during the sampling period. It will be noted that during
any sampling period the pressure in the occluding cuff 601 is held
constant.
A few examples will serve to illustrate the decisional logic of
FIG. 5. Referring to row 1 of the table, if the system is measuring
the systolic blood pressure and there are no acceptable sounds and
no loud sounds detected during the sample period, this probably
means that the cuff pressure is above the systolic blood pressure
and therefore a down signal is to be generated. If the diastolic
blood pressure is being measured then the absence of both
acceptable and loud signals indicates that the cuff pressure is
probably below the diastolic blood pressure and therefore an up
signal should be generated. Looking now at the third row in FIG. 5,
if there are no acceptable sounds detected during the sampling
period and there are more than one loud sounds detected during the
sample period, this probably means that the cuff pressure is in
between the systolic and diastolic blood pressures because that is
where the Korotkoff sounds are greatest. Consequently, if systolic
blood pressure is being measured, an "up" signal should be
generated whereas if diastolic pressure is being measured, a "down"
signal should be generated. The word "continue" in rows 5 and 10 of
the columns headed systolic and diastolic means that if the
conditions of acceptable and loud sounds are met, the command
signal should be the same as the command signal generated from the
prior sampling period. In other words, if the last command signal
was up, then the present command signal should also be up provided
the conditions indicated in row 5 or 10 of FIG. 5 are met.
The apparatus for performing the decisional logic of FIG. 5 is
shown in FIG. 2 and comprises a pair of digital comparators 201 and
202, latches 203--208, and 226, AND gates 209--217, 222--225, and
227--228, and OR gates 218--221, and 229--231. All of the elements
operate in the conventional manner well known in the art. Digital
comparator 201 receives the number of loud sounds from counter 108
in FIG. 1. The digital comparator 201 has three outputs indicating
respectively no loud sounds, one loud sound, and more than one loud
sound. These outputs are applied respectively to the set input
terminals of latches 203, 205, and 204. If any of the latches are
set, an output appears at its upper output terminal. The digital
comparator 202 is the same as digital comparator 201 and it
receives as its input the number of acceptable sounds from counter
107. The outputs from digital comparator 202 are applied to the set
input terminals of latches 206, 207, and 208. The only remaining
inputs to the apparatus of FIG. 2 are the diastolic and systolic
inputs to AND gates 222--225. The diastolic input is energized when
the diastolic blood pressure is being measured, whereas the
systolic input is energized when the systolic blood pressure is
being measured. The latter two logic inputs are derived from the
apparatus shown in FIG. 3 as will be explained more fully
hereafter. The total logic of FIG. 2 is self-explanatory but a
couple of examples will be described to demonstrate the
correspondence between the apparatus shown and the decisional logic
of FIG. 5.
For the first example, assume that during the sampling period there
are no acceptable sounds and there are no loud sounds. Digital
comparator 202 will provide an output which sets latch 206 and
digital comparator 201 will provide an output which sets latch 203.
The outputs from latches 203 and 206 are applied to the AND gate
209 whose output passes through OR gate 219 to AND gates 222 and
223. If the systolic logic signal is energized, the output from AND
gate 222 passes through OR gate 229, thereby causing a down command
signal to be generated. If the diastolic logic input is energized,
the output from AND gate 223 will pass through OR gate 230 thereby
causing an up command signal to be generated. These command signals
correspond to the ones listed in the first row of FIG. 5.
As a second example, consider row 5 of FIG. 5 wherein there is one
acceptable sound and one loud sound. The digital comparators 201
and 202 will set latches 205 and 208, respectively. The outputs
from latches 205 and 208 are applied to AND gate 216 whose output
is applied through OR gate 221 to the AND gates 227 and 228. The
other inputs to AND gates 227 and 228 are from the 1 and .phi.
outputs of latch 226. Latch 226 is set by a down command signal and
reset by an up command signal. Consequently, if the previous
command signal was a down command signal, the latch will be reset
and the AND gate 227 will be fully energized. The output from AND
gate 227 passes through OR gate 229 to generate another down
command signal. If the prior command signal had been an up command
signal, the AND gate 228 will be fully energized thereby providing
an output which passes through OR gate 230 to generate another up
command signal. Thus, the apparatus shown generates the proper
command signals as required by the decisional logic of table of
FIG. 5.
It will be noted that both the up and down command signals pass
through OR gate 231 and are applied to reset counters 107, 108, and
114 in FIG. 1. The resetting of the latter counters starts a new
sampling period.
The apparatus of FIG. 2 also provides an output from OR gate 218
which is applied to the apparatus of FIG. 3. The output from OR
gate 218, labeled 2E, occurs whenever at least one acceptable sound
is detected during the sample period. This is accomplished by
connecting the one and more than one outputs from digital
comparator 202 to the set input terminals of latches 208 and 207,
respectively, and connecting the outputs from latches 207 and 208
to the OR gate 218. Consequently, whenever a pulse appears at the
output of OR gate 218, it indicates that at least one acceptable
sound has been detected during that sample period.
The apparatus shown in FIG. 3 operates to generate a digital cuff
pressure signal, decrease or increase the cuff pressure signal in
response to the proper command, and average all of the cuff
pressure signals which cause at least one acceptable Korotkoff
sound to occur. The latch 401 is provided with the start signals
which are indicated respectively as a systolic and diastolic start
signals. The apparatus may be controlled by external means (not
shown) so that these signals are alternated periodically to thereby
cause the overall system to alternately monitor and track the
systolic blood pressure and the diastolic blood pressure. When the
latch 401 is set, indicating that systolic blood pressure is being
monitored, the output therefrom is applied in the form of a logic
signal to the AND gates 222 and 224 of FIG. 2. The output registers
503 and 507 contain the averages for the diastolic and systolic
blood pressures previously obtained. The systolic start signal
energizes the bank of AND gates 402 to pass the contents of
register 507 through the bank of OR gates 308 to the register 309
thereby inserting into register 309 a digital number corresponding
to the prior average systolic blood pressure. The bank of AND gates
403 is energized by the diastolic start signal to insert the prior
average diastolic blood pressure into register 309.
The digital contents of register 309 is converted into an analogue
quantity by the digital-to-analog converter 310 and the output
therefrom is applied to the occluding cuff servo (FIG. 6) for
controlling the pressure of the occluding cuff. Thus, the pressure
in the cuff corresponds to the digital number contained in register
309. The pressure is increased or decreased by small incremental
amounts in response to the up and down command signals
respectively. This is accomplished by digital subtractor 304,
digital adder 305, preset register 303, and AND gates 306 and 307.
When an up command signal occurs, it is applied to a delay signal
shot 301 which energizes the bank of AND gate 306. The other inputs
to AND gate bank 306 is from digital subtractor 304 which operates
to subtract the contents of the preset register 303 from the
contents of the pressure register 309. The value held in preset
register 303 may be equivalent to an incremental pressure having a
constant magnitude of 4 millimeters of mercury as a specific
example. Thus, the new digital value inserted into register 309
amounts to a decrease corresponding to 4 millimeters of mercury.
When an up command signal is received, the delay single shot 302 is
energized thereby energizing the bank of AND gates 307 to pass the
output from adder 305 into register 309. The digital adder 305
operates to add the incremental amount from register 303 to the
amount held in register 309 thereby increasing the pressure by the
incremental value of 4 millimeters of mercury.
Each digital quantity held in register 309 is a measure of the
occluding cuff pressure for the instant sample period. An accurate
reading of the systolic or diastolic blood pressure is obtained by
averaging the cuff pressures over a plurality of sample periods.
However, only those cuff pressures are averages which result in at
least one acceptable Korotkoff sound. The averaging is controlled
by a counter 502, adders 506 and 510, registers 505 and 509,
dividers 504 and 508, and output registers 503 and 507. The
apparatus for obtaining the average systolic pressure is the same
as that for obtaining the average diastolic pressure so only the
apparatus for obtaining the systolic pressure will be explained.
Assuming that systolic pressure is being monitored and followed by
the system, latch 401 will provide one input to the bank of AND
gates 404. If the present cuff pressure, which corresponds to the
digital number held in register 309, results in at least one
acceptable Kortkoff sound an input pulse 2E, from OR gate 218 in
FIG. 2, will fully energize the bank of AND gates 404 thereby
passing the contents of register 309 into the adder 510 of the
systolic averaging circuitry. The pulse 2E also is applied to the
counter 502 which contains a number corresponding to the number of
sample periods during which at least one acceptable Korotkoff sound
is detected. A second input to the adder 510 is from the register
509, and the output from the adder 510 is entered into register
509. As a result of the adder-register combination, the sum of the
cuff pressures is placed in register 509 and is updated for each
new cuff pressure that results in a sound pattern containing at
least one acceptable Korotkoff sound. The sum in register 509 is
divided in divider 508 by the number of sample periods during which
acceptable Korotkoff sound was detected. This is accomplished by
applying the contents of counter 502 to the divider 508. The output
from divider 508 is applied to output register 507 and represents,
in digital form, the systolic pressure of the patient on whom the
occluding cuff is placed.
The occluding cuff and the analogue control mechanism for inflating
and deflating the occluding cuff is illustrated generally in FIG.
4. The occluding cuff is illustrated as an arm cuff 601 having air
duct connections to an inflate valve 602 and a deflate valve 603.
The valves 602 and 603 may be solenoid controlled valves which
operate in response to signals from a conventional analogue
servodevice 605. The actual pressure in arm cuff 601 is measured by
a conventional strain gauge 604 whose output, representing in
electrical form the cuff pressure, is applied to the pressure servo
605. The other input to the pressure servo 605 comes from the
digital to analogue converter 310 of FIG. 3. The servo operates in
a conventional manner to maintain a zero difference between its two
input signals. Consequently, if there is an incremental increase in
the cuff pressure signal 3A from the converter 310, the pressure
servo will energize solenoid 602 long enough to bring the strain
gauge output up to the level of the cuff pressure input signal.
* * * * *