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Claims  |
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I claim:
1. A method for determining systolic pressure as the occluding pressure
existing in the cuff of a blood pressure measuring instrument bleeds down;
said instrument including means for allowing air in said cuff to bleed
out, and means for periodically sampling the instantaneous cuff pressure;
comprising the steps of:
(a) utilizing successive samples taken for detecting the presence of a
predetermined number of successive blood pressure pulses and the amplitude
of each,
(b) registering the occluding pressure in said cuff at the onset of each
blood pressure pulse,
(c) comparing the detected sequence of the relative amplitudes of said
predetermined number of blood pressure pulses with a plurality of known
valid sequences to determine if the detected sequence is valid, and
(d) if the detected sequence is valid, determining the systolic pressure to
be the registered occluding pressure at the onset of a predetermined one
of said blood pressure pulses.
2. A method in accordance with claim 1 wherein said predetermined number is
four.
3. A method in accordance with claim 1 wherein said predetermined blood
pressure pulse is the third in the sequence.
4. A method in accordance with claim 1 wherein step (a) includes the
sub-steps of:
(a1) maintaining an occluding cuff pressure value and continuously
up-dating it in accordance with a newly taken sample if the latter is
smaller,
(a2) determining the onset of a blood pressure pulse when a newly taken
sample exceeds said occluding cuff pressure value,
(a3) maintaining a maximum pressure rise value after the onset of a blood
pressure pulse, and continuously up-dating it in accordance with the
difference between a newly taken sample and said occluding cuff pressure
value if the difference is larger, and
(a4) determining that the blood pressure pulse has terminated when a newly
taken sample corresponds to the occluding cuff pressure value, with the
last up-date of the maximum pressure rise value representing the amplitude
of the blood pressure pulse.
5. A method in accordance with claim 1 further including the steps of:
(e) adding together the amplitudes of said predetermined number of blood
pressure pulses to derive a sum, and
(f) determining that the systolic pressure has not been validly determined
if said sum exceeds a predetermined threshold value.
6. A method in accordance with claim 5 further including the steps of:
(g) forming the difference between successive occluding pressures
registered in step (b), and
(h) starting over again if any difference formed in step (g) exceeds a
predetermined threshold level.
7. A method in accordance with claim 1 further including the steps of:
(e) forming the difference between successive occluding pressures
registered in step (b), and
(f) starting over again if any difference formed in step (e) exceeds a
predetermined threshold level.
8. A method in accordance with claim 1 wherein said predetermined number is
four, and in step (c) a valid sequence is one which (i) exhibits at least
three of its four successive amplitudes not decreasing in value, (ii) has
its third amplitude not less than both of its first and second, and (iii)
if its third amplitude is greater than its first but not its second, has
its fourth as the largest.
9. A method in accordance with claim 8 wherein said predetermined blood
pressure pulse is the third in the sequence.
10. A method for determining diastolic pressure as the occluding pressure
existing in the cuff of a blood pressure measuring instrument bleeds down;
said instrument including means for allowing air in said cuff to bleed
out, and means for periodically sampling the instantaneous cuff pressure;
comprising the steps of:
(a) utilizing successive samples taken for detecting the presence of
sequential blood pressure pulses, and determining the amplitude of each,
(b) registering the occluding pressure in said cuff at the onset of each
blood pressure pulse,
(c) deriving a threshold value in accordance with the amplitude information
determined in step (a),
(d) adding together the amplitudes of a predetermined number of the last
pulses detected to derive a value which is representative of the average
amplitude of said predetermined number of last pulses, and comparing said
representative value with said threshold value, and
(e) determining the diastolic pressure to be the registered occluding
pressure at the onset of a preselected one of said pulses in said
predetermined number of last pulses when said representative value is less
than said threshold value.
11. A method in accordance with claim 10 wherein said predetermined number
is four.
12. A method in accordance with claim 10 wherein said preselected one of
said pulses is the second.
13. A method in accordance with claim 10 wherein step (a) includes the
sub-steps of:
(a1) maintaining an occluding cuff pressure value and continuously
up-dating it in accordance with a newly taken sample if the latter is
smaller,
(a2) determining the onset of a blood pressure pulse when a newly taken
sample exceeds said occluding cuff pressure value,
(a3) maintaining a maximum pressure rise value after the onset of a blood
pressure pulse, and continuously up-dating it in accordance with the
difference between a newly taken sample and said occluding cuff pressure
value if the difference is larger, and
(a4) determining that the blood pressure pulse has terminated when a newly
taken sample corresponds to the occluding cuff pressure value, with the
last up-date of the maximum pressure rise value representing the amplitude
of the blood pressure pulse.
14. A method in accordance with claim 10 wherein in step (c) the derivation
of said threshold value includes the adding together of the amplitudes of
a preselected number of successive pulses.
15. A method in accordance with claim 14 wherein the successive pulses
whose amplitudes are used in the derivation of said threshold value are
those whose sum is the largest.
16. A method in accordance with claim 15 wherein said threshold value
includes a constant term which is independent of the amplitudes of said
successive pulses.
17. A method in accordance with claim 16 further including the step of:
(f) comparing said largest sum with a fixed value, and
(g) not performing step (e) if said largest sum does not exceed said fixed
value.
18. A method in accordance with claim 10 wherein in step (c) said threshold
value is derived by adding a constant term to a variable term which is a
function of said amplitude information.
19. A method in accordance with claim 18 further including the step of:
(f) comparing said variable term with a fixed value, and
(g) not performing step (e) if said variable term does not exceed said
fixed value.
20. A method in accordance with claim 10 further including the steps of:
(f) forming the sum of the amplitudes of a preselected number of the last
pulses detected,
(g) comparing the largest sum formed in step (f) with a fixed value, and
(h) not performing step (e) if said largest sum does not exceed said fixed
value.
21. A method for determining diastolic pressure as the occluding pressure
existing in the cuff of a blood pressure measuring instrument bleeds down;
said instrument including means for allowing air in said cuff to bleed
out, and means for periodically sampling the instantaneous cuff pressure;
comprising the steps of:
(a) utilizing successive samples taken for detecting the presence of
sequential blood pressure pulses, and determining the amplitude of each,
(b) registering the occluding pressure in said cuff at the onset of each
blood pressure pulse,
(c) deriving a threshold value in accordance with the amplitude information
determined in step (a),
(d) comparing amplitude data for a predetermined number of the last pulses
detected with said threshold value, said predetermined number being at
least one, and
(e) determining the diastolic pressure to be the registered occluding
pressure at the onset of a preselected one of said pulses in said
predetermined number of last pulses when said amplitude data corresponds
with said threshold value.
22. A method in accordance with claim 21 wherein said predetermined number
is four.
23. A method in accordance with claim 21 wherein said preselected one of
said pulses is the second.
24. A method in accordance with claim 21 wherein step (a) includes the
sub-steps of:
(a1) maintaining an occluding cuff pressure value and continuously
up-dating it in accordance with a newly taken sample if the latter is
smaller,
(a2) determining the onset of a blood pressure pulse when a newly taken
sample exceeds said occluding cuff pressure value,
(a3) maintaining a maximum pressure rise value after the onset of a blood
pressure pulse, and continuously up-dating it in accordance with the
difference between a newly taken sample and said occluding cuff pressure
value if the difference is larger, and
(a4) determining that the blood pressure pulse has terminated when a newly
taken sample corresponds to the occluding cuff pressure value, with the
last up-date of the maximum pressure rise value representing the amplitude
of the blood pressure pulse.
25. A method in accordance with claim 21 wherein in step (c) the derivation
of said threshold value includes the adding together of the amplitudes of
a preselected number of successive pulses.
26. A method in accordance with claim 25 wherein the successive pulses
whose amplitudes are used in the derivation of said threshold value are
those whose sum is the largest.
27. A method in accordance with claim 26 wherein said threshold value
includes a constant term which is independent of the amplitudes of said
successive pulses.
28. A method in accordance with claim 27 further including the step of:
(f) comparing said largest sum with a fixed value, and
(g) not performing step (e) if said largest sum does not exceed said fixed
value.
29. A method in accordance with claim 21 wherein in step (c) said threshold
value is derived by adding a constant term to a variable term which is a
function of said amplitude information.
30. A method in accordance with claim 29 further including the step of:
(f) comparing said variable term with a fixed value, and
(g) not performing step (e) if said variable term does not exceed said
fixed value.
31. A method in accordance with claim 21 further including the steps of:
(f) forming the sum of the amplitudes of a preselected number of the last
pulses detected,
(g) comparing the largest sum formed in step (f) with a fixed value, and
(h) not performing step (e) if said largest sum does not exceed said fixed
value.
32. A method for taking blood pressure measurements comprising the steps
of:
(a) occluding the artery of a patient with a pressurized occluding cuff and
then allowing the pressure in said cuff to bleed down, said cuff being in
communication with a pressure transducer which generates samples of the
instantaneous cuff pressure,
(b) controlling the generation of samples at a rate high enough to insure
that at least several samples are generated during each blood pressure
pulse of normal duration,
(c) registering the value of the sample which is generated at the start of
a blood pressure pulse, and maintaining it for the duration of at least
several but not all of the succeeding blood pressure pulses,
(d) deriving the height of a blood pressure pulse by subtracting the
respective value registered in step (c) from the largest sample value for
the respective blood pressure pulse, and maintaining said height for the
duration of at least several but not all of the succeeding blood pressure
pulses, and
(e) utilizing said at least several maintained values of samples and
heights for determining blood pressure measurements.
33. A method in accordance with claim 32 wherein the number of maintained
values for both samples are heights is substantially less than the number
of blood pressure pulses which normally occur during the course of a
measurement cycle.
34. A method in accordance with claim 32 wherein said maintained values are
utilized to determine systolic pressure.
35. A method in accordance with claim 32 wherein said maintained values are
utilized to determine diastolic pressure.
36. A method in accordance with claim 32 wherein in step (e) systolic
pressure is determined to be one of the sample values maintained in step
(c).
37. A method in accordance with claim 32 wherein in step (e) diastolic
pressure is determined to be one of the sample values maintained in step
(c).
38. A method in accordance with claim 32 further including the steps of
taking a sample while the occluding cuff is at atmospheric pressure to
derive a reference value, and causing the sample values processed in the
other steps to equal respective generated samples less said reference
value.
39. A method in accordance with claim 32 further including means for
determining when a sample value is less than a threshold value for
aborting a measurement cycle. |
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Claims |
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Description |
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This invention relates to methods and apparatus for the non-invasive
detection of arterial blood pressure and pulse rate, and more particularly
to instruments which perform the measurements automatically and in a
highly reliable manner.
The oldest and most widely used technique for measuring the blood pressure
of a patient is to completely occlude an artery by a pressurized cuff
whose pressure is then allowed to bleed down. A mercury manometer is used
to determine the pressure in the cuff, and a stethoscope is utilized to
listen for Korotkoff sounds. The cuff pressures when particular types of
sounds are heard are indications of systolic and diastolic pressures. The
various methods based on listening for Korotkoff sounds are inherently
inaccurate, especially when measuring diastolic pressure since what is
required is a determination of the disappearance of sound as it gradually
fades out. Even systolic pressure determinations are inaccurate because
what is often thought to be a first pressure pulse, and sometimes even a
second pressure pulse, are nothing more than artifacts which do not
represent a flow of blood through the still fully occluded artery. A mean
error of .+-.8 mm Hg can be expected in readings of systolic and diastolic
pressures based on Korotkoff sounds. (Best and Taylor, Physiological Basis
of Medical Practice, 9th Edition, Chapter 7, page 3-151.) Nor are
present-day automatic instruments based on Korotkoff sounds any more
reliable. Not only is it difficult to monitor electronically a fading
sound, but the methodologies employed do not provide consistent, reliable
results.
Other measurement approaches have met with equally little success. With
respect to oscillometric methods, it is very difficult to determine
diastolic pressure because one has to look for changes in oscillations of
a mercury column, and they are barely noticeable with narrow-width
pressure pulses. Hot-wire anemometer-type transducers offer somewhat
better accuracy, but they require the use of two cuffs (occluding and
sensing, in which the pressure in the sensing cuff is maintained
constant). One of the shortcomings of these and other prior art devices is
that two transducers and associated electronics are required.
Representative prior art, in addition to the Best and Taylor text referred
to above, are the following:
__________________________________________________________________________
Patent No.
Date Inventor Title
__________________________________________________________________________
2,827,040
March 18, 1958
S.R. Gilford
Automatic
Sphygmomanometer
3,224,435
Dec. 21, 1965
M. Traite Method of Measuring
Blood Pressure
3,229,685
Jan. 18, 1966
D.L. Ringkamp
Blood Pressure
et al Measuring
3,480,005
Nov. 25, 1969
W.C. Edwards
Apparatus for Measuring
Blood Pressure With
Plural Brake Controlled
Indicators
3,581,734
June 1, 1971
M.E. Croslin et al
Sphygmomanometer
3,742,937
July 3, 1973
B. Manuel et al
Cardiac Monitor
3,742,938
July 3, 1973
T.J. Stern
Cardiac Pacer and
Heart Pulse Monitor
3,814,083
June 4, 1974
J.C. Fletcher
Apparatus and Method
et al FOr Processing
Korotkov Sounds
3,841,314
Oct. 15, 1974
R.E. Page Pulse Activity
Indicator
3,885,551
May 27, 1975
H.L. Massie
Artifact Rejection
For Blood Pressure
Monitoring
3,894,533
July 15, 1975
R.L. Cannon
Vital Sign Trend
Intuitive Display
System
3,903,872
Sept. 9, 1975
W.T. Link Apparatus and Pro-
cess For Producing
Sphygmometric Infor-
mation
3,978,848
Sept. 7, 1976
D.H. Yen et al
Monitoring Apparatus
And Method For Blood
Pressure and Heart
Rate
4,009,709
March 1, 1977
W.T. Link et al
Apparatus and Pro-
cess For Determining
Systolic Pressure
4,047,711
Feb. 21, 1978
W.T. Link et al
Apparatus And Pro-
cess For Determining
Systolic Pressure
__________________________________________________________________________
Other Publications:
1. L.A. Geddes et al "The Meaning of the Point of Maximum Oscillations i
Cuff Pressure in the Indirect Measurement of Blood Pressure, Part I",
Cardiovascular Research Center Bulletin, July-Sept., 1969, pages 15-25.
2. Physiological Basis of Medical Practice, Ninth Edition, John R.
Brobeck: Chapter 7, Section 3 "Measurement of Blood Pressure and Flow",
pages 148-163; Chapter 8, Section 3 "Control Mechanisms of the
Circulatory System", pages 164-188; Chapter 9, Section 3 "Regulation of
Systemic and Pulmonary Circulation", pages 189-210.
3. George E. Burch "Sphygmomanometric Cuff Size and Blood Pressure
Recordings", JAMA, 3 Sept. 1973, Vol. 225, No. 10, pages 1215-1218.
4. Electronic Design, Vol. 24, No. 19, September 13, 1976, page 28, "Semi
invade medical transducers; microprocessors monitor EKG and blood
pressure".
5. "Computer Automation of BloodPressure Measurements", Proceedings of th
IEEE, Vol. 63, No. 10, October 1975, pages 1399-1403.
The basic problem with most prior art automated blood pressure measuring
instruments is that they look for "gross" indications, e.g., the presence
of a pulse based upon a sound level or some other parameter reaching a
detectable level. From a theoretical standpoint, the most accurate
measurement determinations could be made were the pressure waveform in the
cuff actually traced out on paper during the course of a measurement
cycle, much as is done in the case of ECG waveform analysis. The pressure
waveform would show a decreasing occluding cuff pressure, on which blood
pressure pulses are superimposed. Such a paper trace would provide to the
physician the maximum amount of information from which systolic and
diastolic pressures could be determined. If a trace is not to be made and
an instrument is to perform the analysis, then ideally the processing
section of the instrument should be provided with the exact waveform of
the pressure in the cuff. It is possible to do this by sampling the cuff
pressure at a sufficiently high rate and to then process the samples. If
the sampling rate is so high that numerous samples are taken during the
occurrence of each pulse, then from the standpoint of information theory
the processing section of the instrument will have available sufficient
data from which the complete waveform may be reconstructed.
However, while this general principle may have been recognized by prior art
researchers, they have not employed effective methodologies in analyzing
the sampled data. One problem in this regard is that the analysis must be
done "on the fly". In the illustrative embodiment of the invention, a
sample is taken approximately every 2.5 milliseconds; thus 400 samples are
taken each second, and an 8 k memory would be required to store the data
for a measurement cycle of 20 seconds--if all of the data is to be stored
prior to the actual processing which determines the final measurement
values. A cost-effective instrument must therefore perform the processing
as samples are taken without storing a complete history of the pressure
waveform. The methodologies employed in the prior art for performing this
type of "on-the-fly" processing have not provided accurate or consistent
results.
For example, consider the methodology for systolic pressure determination
disclosed in Link et al U.S. Pat. No. 4,009,709. Link et al theorize that
the DC pressure in the cuff (the value of the slowly changing occluding
pressure) when there is detected a blood pressure pulse whose amplitude is
one-half of a maximum amplitude value represents the systolic pressure,
where the maximum amplitude value is the maximum average amplitude over
four successive pulses. In the Link et al instrument, a "sliding average"
of the pulse amplitude over four successive pulses is taken, and a
threshold level is constantly up-dated to equal the maximum sliding
average. Link et al pump up the pressure continuously. As the occluding
pressure increases, the pulse amplitudes rise and then fall. By using an
increasing pressure during the measurement cycle, maximum pulse amplitudes
are detected before the occluding pressure reaches the relatively high
value which represents systolic pressure. It is in this way that the
threshold level is determined before a pulse is actually detected whose
amplitude is less than one-half of the threshold level. The Link et al
technique requires a smooth pump-up of the cuff pressure and thus does not
allow a cheap, conventional-type manually-operated bulb pump to be used.
On the other hand, it is possible to use a bulb to pump up the pressure to
a value which completely occludes the artery, and then to allow the
pressure to bleed down smoothly as in conventional instruments. But in
such a case, the systolic pressure is reached before the threshold level
can even be determined. This, in turn, requires that a considerable amount
of data be stored since "on-the-fly" processing is possible to only a
limited extent.
But quite apart from the difficulties in implementing such a technique, the
Link et al methodology has not proven to provide consistently correct
systolic pressure measurements. The basic premise of Link et al is that
the systolic pressure is the DC cuff pressure when a particular pulse is
detected, and that particular pulse is the first one in a decreasing
amplitude sequence whose amplitude corresponds to one-half of the maximum
amplitude (or, more accurately, the maximum average amplitude over four
successive pulses). This criterion has not been established, but even were
it valid the Link et al system does not take into account the existence of
artifacts. For example, if a patient moves his arm during the course of a
measurement cycle and in the process squeezes the cuff, there will be a
very large pressure rise which may control the maximum average pulse
amplitude which is used as the threshold value--the threshold value and
therefore the systolic pressure determination being completely erroneous
in such a case.
What is important in an automated blood pressure measurement instrument is
not only the selection of the proper criteria for determining systolic and
diastolic pressures, but also validation of the results. Throughout the
following detailed description of the invention, it will be noted that
considerable attention is paid to validating the measurement cycle. One
such example is the analysis of each individual pulse; a pulse is not
considered to be valid if its amplitude is too large. Another example
relates to the determination of systolic pressure. The sequence of pulse
amplitudes in the region of systolic pressure must be one of plurality of
predetermined valid sequences. It is this kind of constant concern for
validating the measurement results (both intermediate and final) which
contributes to reliable instrumentation.
In the illustrative embodiment of the invention a display is provided for
guiding the operator--physician or patient--through the measurement cycle.
As the bulb is used to pump up the cuff pressure, the operator is informed
not only of the instantaneous cuff pressure, but also of the particular
actions which are required. This, in and of itself, is an important
feature of the invention. Furthermore, for a measurement cycle to provide
accurate results, it is essential that the artery be completely occluded
before the cuff pressure is monitored for the presence of pulses. What the
system of the invention does is to check that no pulses have been detected
for about 2.5 seconds before it assumes that the artery has been
completely occluded and the cuff pressure should be allowed to continue
bleeding down. If full occlusion for 2.5 seconds is not ascertained, the
display informs the operator to pump up the cuff pressure.
The basic systolic pressure methodology involves an analysis of the
amplitudes of four successive pulses, when pulses first appear as the
occluding pressure bleeds down. (As will be described below, artifacts are
rejected and it is not necessarily the first four pulse amplitudes which
are operated upon.) Systolic pressure is taken to be the cuff pressure at
the onset of a particular one of the four pulses, but only if the pulse
amplitudes have a sequence which is one of a plurality of known valid
sequences, e.g., four successive pulses exhibit increasing amplitudes,
except for the third which may have the largest amplitude. There are quite
a few valid sequences, some of which will be described in detail below.
The diastolic pressure methodology is actually similar to the Link et al
methodology for determining systolic pressure. (There is no apparent
reason why the same type of methodology should be effective to determine
both systolic and diastolic pressures; in fact, it is not effective for
systolic pressure determinations as taught by Link et al, but it is
effective for diastolic pressure determinations.) A threshold value is
determined based upon maximum pulse amplitude information; in the
illustrative embodiment of the invention, the threshold level partially
depends upon the maximum average pulse amplitude over four pulses. But the
threshold value is not based solely upon the maximum amplitude
information; it is also a function of a constant value. Moreover, instead
of comparing the amplitude of a single pulse with the threshold value in
order to determine diastolic pressure, the comparison involves the average
pulse amplitude over four pulses in the vicinity of diastolic pressure.
The methodology of the invention does not lend itself to a more detailed
general description. Suffice it to say that the method of the invention
allows "on-the-fly" analysis of samples taken at a sufficiently high rate
such that they allow the complete cuff pressure waveform to be reproduced.
The criteria for determining systolic and diastolic pressures have proved
to be accurate and reliable. Throughout the processing, validation checks
are performed. Any indication of erroneous measurements having been taken
results in an appropriate error message. Accurate measurements of pulse
rates are also provided. In connection with a pulse rate measurement,
while it is not particularly difficult to count detected pulses (as is
known in the prior art), it is the rejection of a measurement cycle due to
the presence of artifacts that gives rise to the high accuracy of my
method and apparatus.
Further objects, features and advantages of my invention will become
apparent upon consideration of the following detailed description in
conjunction with the drawing, in which:
FIG. 1 is a perspective view of the instrument of my invention;
FIG. 2 depicts a portion of the circuit board within the housing of the
instrument, and several of the components mounted on the board;
FIGS. 3-6 are a schematic of the circuit of the instrument, with the
figures being arranged as shown in FIG. 7;
FIG. 8 depicts two resistor networks utilized in the circuit of FIGS. 3-6;
FIG. 9 depicts the seven segments of each display element, together with
the fifteen characters which can be formed by energizing appropriate ones
of the segments (the 16th character is a blank, obtained by energizing
none of the segments);
FIGS. 10-13, 15-18, 20, 23 and 24 are flow charts depicting most of the
method of my invention, and should be read in conjunction with the
complete source listing which is reproduced below and which will be
described;
FIG. 14, which is not drawn to scale, depicts the cuff pressure throughout
a measurement cycle;
FIG. 19 depicts, in enlarged scale, the cuff pressure in the vicinity of a
single blood pressure pulse;
FIG. 21 depicts the envelope of the pulse amplitudes--not the cuff
pressure, but just the amplitudes of individual pulses such as that shown
in FIG. 19--throughout a measurement cycle; and
FIG. 22 depicts several illustrative pulse sequences which will be
discussed in conjunction with the systolic pressure measurement
methodology.
Hardware
FIG. 1 depicts the instrument of my invention. It includes a conventional
cuff 40, with tubing 42 connecting the cuff to pump-up bulb 44. As the
bulb is pumped, the pressure in the cuff rises. There is a bleed valve 45
in the bulb which allows air in the cuff to bleed out at a rate of several
mm Hg per second, the actual bleed rate depending upon the cuff pressure.
Tubing 47 connects the cuff to a manifold within the instrument housing.
The overall cuff arrangement is standard except that the take-off tubing
47 is extended to the instrument rather than to a mercury column as in
conventional blood pressure measuring instruments.
The instrument itself includes three switches and a twelve-character
display DP1 (under a red translucent strip 43). Switch S1 (on the top) is
the main on/off switch which, when operated, connects the internal
batteries to the circuit. (The unit also includes a jack 49 for insertion
of the plug of a charging circuit when it is necessary to recharge the
batteries.) Switch S2 is the reset/exhaust switch which is spring-loaded
to an open position. When it is momentarily closed, as will be described
below, the instrument resets and initiates a new cycle of operation.
Switch S3, another normally-open, spring-loaded push-button, is the
recall/cuff control. When it is operated, one of two different sequences
takes place depending upon the state of the instrument at the time the
button is operated. Toward the beginning of the overall cycle, operation
of switch S3 closes take-off tubing 47 as will be described shortly, so
that the pressure in the cuff can be pumped up by repeatedly squeezing
bulb 44. At the end of a measurement cycle, the final values are displayed
for only ten seconds, also as will be described below, and the display is
then blanked to conserve power. Operation of switch S3 causes the
previously determined values to be displayed once again, for another ten
seconds.
The display itself consists of 12 character positions, each of which has
seven light-emitting diode segments as shown at the left of FIG. 9.
Depending upon which of the segments are energized, any one of 15
characters can be displayed at each position, the 15 characters also being
shown in FIG. 9 and it being obvious which of the seven segments are used
to form each of the fifteen characters. A blank may be displayed simply by
energizing no segments. The display elements are also used to form
numerals as is well known in the art.
The instrument also includes a light-emitting diode LD1 (under a red
translucent area 51 on the case) which, when illuminated, represents one
of two things. First, the light is on whenever the system is in the
process of detecting a blood pressure pulse (a rise in the occluding cuff
pressure). Second, after the final display has been blanked in order to
conserve power, the light is turned on to indicate to the operator that
the display can be recalled if switch S3 is momentarily operated. Lastly,
the positions on the display of the final measurement values are printed
on the case.
FIG. 2 depicts just one part of the circuit board 18 on which the circuit
components are mounted within the housing. Switch S3 can be seen in the
drawing. In addition, a manifold 20 is mounted on the board, and spaced
from it by spacer 22. The manifold provides open communication between
input pipe 24 (on which take-off tubing 47 of FIG. 1 is placed), a pipe
segment 26, and a valve V1. The valve is normally open, but when its two
leads (not shown) have a potential applied across them, the valve closes.
A pressure transducer T1 is mounted on the other side of the board--the
side on which all of the chips used in the circuit are mounted--and the
input port of the transducer is connected to pipe segment 26. It is
apparent that since pipe 24 is connected via take-off tubing 47 to the
cuff, transducer T1 has as its "input" the cuff pressure. Valve V1 is used
to open the cuff to the atmosphere, within and through the housing, so
that the cuff pressure can rapidly decrease at the end of a measurement
cycle. The valve is closed automatically by the circuit after switch S3 is
operated so that the pump-up procedure can commence. It is important to
note that transducer T1 is located within the instrument housing and is
not positioned in the cuff (although it could be). Thus there are no
circuit elements which are in contact with the patient.
The schematic of the circuit is shown in FIGS. 3-6. Many of the chips are
identified on the schematic, and the omitted chip identifications, as well
as the component values, are as follows (many of the resistors are
contained in four resistor networks, identified by the symbols RA1-RA4,
which will be discussed below):
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C1 22uf R18* 15K RA(1)
C2 .01uf R19 10K
C3 6.8uf R20 2K
C4 6.8uf R21* 20K (RA1)
C5 .01uf R22 1K
C6 .47uf R23* 5K (RA2)
C7 .01uf R24 100
C8 .01uf R25* 10K (RA1)
C9 22uf R26 100
C10 .01uf R27* 7.5K (RA1)
C11 .47uf R28* 402K (RA1)
C12 .01uf R29* 10K (RA1)
C13 .47uf R30* 330 (RA4)
C14 .01uf R31* 10K (RA1)
C15 10f R32* 330 (RA4)
C16 22uf R33* 330 (RA4)
C17 .47uf R34* 330 (RA4)
C19 1uf R35* 162K (RA1)
C20 1uf R36* 40.2K (RA1)
C21 .01uf R37* 4.7K (RA3)
C22 270pf R38* 4.7K (RA3)
C23 68pf R39* 4.7K (RA3)
C24 20pf R40* 4.7K (RA3)
C25 20pf R41 47 (1/2W)
C26 .1uf RA1 Custom 16-Pin Dip, 1%
C27 .1uf RA2 Custom 16-Pin Dip, 1%
R1* 18.7K (RA2) RA3 Bourns 4310R-102-472
R2* 19.6K (RA2) (10-Pin Sip), 1%
R3* 1M (RA2) RA4 Bourns 4310R-102-331
R4* 10K (RA2) (10-Pin Sip), 1%
R5* 18.7K (RA2) D1 IN4001
R6* 1M (RA2) D2 IN4001
R7* 10K (RA2) D6 1N4001
R8* 21.5K (RA2) Z1 1N5523
R9 1K IC3 LM324
R1 | | |
