 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to an electronic sphygmomanometer and, in
particular, to a sphygmomanometer wherein accurate measurement of a
patient's systolic and diastolic pressures are obtained without requiring
the use of a sound transducer for detecting Korotkoff sounds.
It has long been known that an approximation of a patient's systolic blood
pressure and diastolic blood pressure can be obtained by detecting the
so-called Korotkoff sounds. Essentially, this measurement technique
utilizes an inflatable occluding cuff which usually is wrapped about a
patient's limb so as to close, or completely occlude, an artery.
Typically, the occluding cuff is wrapped about the arm in juxtaposition to
the brachial artery. When the cuff is inflated to a pressure which exceeds
the patient's systolic pressure, so as to close this artery, blood is no
longer capable of flowing therethrough. As the cuff is slowly deflated, a
point is reached whereat the patient's systolic pressure exceeds the cuff
pressure. Consequently, the artery opens for a short period during the
patient's cardiac cycle. Once the blood pressure during this cardiac cycle
falls below the cuff pressure, the artery once again is closed.
The pressure in the cuff which is equal to the maximum blood pressure
during a cardiac cycle is, of course, the systolic pressure. It is known
that when the blood pressure exceeds the actual cuff pressure, resulting
in the opening of the artery, turbulence in the blood stream is
accompanied by a sound which is the so-called Korotkoff sound. These
Korotkoff sounds occur each time that the artery is opened. Thus, as long
as the cuff pressure exceeds the lowest, or diastolic, pressure in the
cardiac cycle, the artery will be alternately opened and closed as the
cardiac cycle pressure traverses the cuff pressure. When the cuff pressure
falls below the lowest pressure point in the cardiac cycle, the artery
will remain opened, and the Korotkoff sounds no longer will be produced.
Consequently, by measuring the cuff pressure at the last Korotkoff sound,
a close approximation is made of the patient's diastolic pressure.
It is common practice to deflate the cuff at a rate which is much slower
than the cardiac cycle. For example, the cuff is deflated at a rate in the
range of 2mm Hg per second to 4mm Hg per second; so that it is expected
that a Korotkoff sound will be present for each millimeter of mercury
during the cuff deflation until the diastolic pressure is reached.
To detect the Korotkoff sounds, a suitable listening device has been
required. For manual measurements of blood pressure, a stethoscope is
applied to the patient's arm downstream of the occluding cuff. Because of
the relative insensitivity of a conventional stethoscope, and further in
view of ambient noises which can cause distraction or confusion in the
detection of an actual Korotkoff sound, it is necessary for a physician or
an otherwise skilled technician to take blood pressure measurements. This,
of course, is an inefficient and generally wasteful use of a physician's
time and skill.
Accordingly, there have been previous proposals for sphygmomanometers which
can be used to measure blood pressure without the assistance of a
physician or a highly skilled technician. In these earlier proposals, the
detection of the Korotkoff sounds are achieved automatically and the
associated cuff pressure readings are derived in conjunction with the
detected Korotkoff sounds by electronic apparatus. It is generally
believed that, prior to the instant invention, all of the earlier
proposals and systems proceeded upon the detection of the Korotkoff sounds
and, therefore, required the use of a microphone. Unfortunately, the
automatic detection of Korotkoff sounds is accompanied by various problems
and disadvantages. For example, the characteristic Korotkoff sounds of one
patient may be vastly different from those of another patient. In
particular, the amplitudes of these Korotkoff sounds may differ by many
orders of magnitude. As another example, in some patients, during cuff
deflation, but while the cuff pressure is between the patient's systolic
and diastolic pressures, the Korotkoff sounds will appear to cease but
then subsequently reappear. Since the finally detected Korotkoff sound is
assumed to correspond to the patient's diastolic pressure, this
interruption in the Korotkoff sounds will lead to erroneous measurements.
As another example, background noises, known as artifactual noise to
distinguish these noises from the Korotkoff sounds, can closely
approximate such Korotkoff sounds and thus will be falsely detected.
Attempts to overcome these, and other, problems are described in the
patent art. For example, in U.S. Pat. No. 3,405,707, it is proposed to
automatically simulate the selective process for discriminating Korotkoff
sounds which is used by a physician. Nevertheless, this proposal requires
the use of a microphone for sensing the Korotkoff sounds. Another
proposal, described in U.S. Pat. No. 3,771,515, also relies on the use of
a microphone.
Although it has been known that separate pressure signals are produced
generally in phase with the Korotkoff sounds during a cardiac cycle, as
mentioned in U.S. Pat. No. 3,349,763, nevertheless, there has been no
previous attempt to detect these pressure signals and to use them in
measuring blood pressure. In fact, even though pressure transducers have
been used to sense cuff pressure, as described, for example, in U.S. Pat.
Nos. 3,450,131 and 3,508,537, there has been no attempt to use a pressure
transducer for sensing these pressure signals and for using same to
measure systolic and diastolic pressure. Rather, the classic technique
which relies upon the detection of the Korotkoff sounds, as described in
U.S. Pat. No. 3,371,661, has been maintained.
Unfortunately, all of these systems which require the detection of the
Korotkoff sounds are accompanied by many of the foregoing problems.
Attempts to avoid the disadvantages inherent in sensing the Korotkoff
sounds have not been entirely successful. Although the use of digital
techniques has improved the measuring sensitivities, the unreliability of
sound detection has tended to substantially nullify these improvements in
accuracy. Although still better results can be achieved by using highly
sensitive and discriminating microphones, the consequential increases in
manufacturing costs and maintenance of the sphygmomanometer do not
economically justify the use of such microphones.
OBJECTS OF THE INVENTION
Therefore, it is an object of the present invention to provide an improved
electronic sphygmomanometer wherein the aforenoted problems and
disadvantages attending prior art devices are avoided.
It is another object of the present invention wherein an electronic
sphygmomanometer is provided having no sound transducer for the detection
of Korotkoff sounds.
A further object of this invention is to provide a sphygmomanometer wherein
a single pressure transducer is used to detect the pressure in an
occluding cuff and to detect pressure pulsations during cardiac cycles of
a patient.
An additional object of this invention is to provide an improved
sphygmomanometer wherein systolic and diastolic pressures of a patient are
measured in accordance with digital techniques.
Yet another object of this invention is to provide a sphygmomanometer of
simple and low cost construction which is capable of achieving highly
accurate measurements of a patient's blood pressure.
A still further object of this invention is to provide a sphygmomanometer
which admits of simplified operation so that blood pressure measurements
can be taken without the assistance of a highly skilled technician.
Another object of the present invention is to provide a sphygmomanometer
which can be used by a patient for measuring his own blood pressure.
Various other objects and advantages of the present invention will become
apparent from the ensuing detailed description, and the novel features
will be pointed out in the appended claims.
SUMMARY OF THE INVENTION
In accordance with the present invention, a sphygmomanometer is provided
having an inflatable occluding cuff to be applied to a patient and
including a pressure transducer for producing an electrical analog signal
representative of the actual pressure in the cuff and for superposing
electrical pulses on the analog signal in response to pressure pulsations
during cardiac cycles of the patient; the analog signal is converted to a
digital representation of the cuff pressure, and the superposed pulses are
separately detected; the first detected pulse which is sensed while the
cuff pressure is reduced from a maximum value is used to produce a signal
representing the systolic pressure, and the final pulse which is sensed
when the cuff pressure is further reduced is used to produce a signal
representing the diastolic pressure; and the digital representation of
cuff pressure then obtaining when the systolic pressure representing
signal is produced as well as the digital representation of cuff pressure
then obtaining when the diastolic pressure representing signal is produced
are respectively displayed as systolic and diastolic pressures.
The digital representation of cuff pressure is produced by means of a
simplified, low cost analog-to-digital converting circuit. In one
embodiment thereof, this analog-to-digital converting circuit includes a
voltage controlled oscillator which generates a pulse train having a pulse
repetition rate which is linearly proportional to an analog signal applied
thereto; the pulse train being periodically sampled during predetermined
sampling intervals; and the number of pulses included in the pulse train
during the sampling interval being used as a direct digital representation
of the analog signal. For those oscillators wherein the pulse repetition
rate is not reduced to zero even though the input analog signal level is
substantially equal to zero, these zero-offset pulses are subtracted from
the sampled pulse train so as to produce a resultant pulse train wherein
the number of pulses is directly proportional to the analog signal level.
In accordance with a feature of the present invention, the pressure
measurements are displayed by conventional multi-digit seven-segment
light-emitting displays. To minimize costs, a single-digit display driver
is used to drive all of the display elements. A simple multiplexing
circuit is used to supply the display driver with each digit to be
displayed.
In a preferred embodiment of this invention, integrated circuit techniques
are used, so that the occluding cuff, control electronics and display
elements all can be incorporated directly into the cuff structure.
Furthermore, by sensing the pressure pulsations during a patient's cardiac
cycle, rather than by detecting the Korotkoff sounds, the occluding cuff
can be wrapped about any convenient limb. For example, the occluding cuff
may be wrapped about a patient's upper arm to surround the brachial artery
or about a patient's wrist to surround the radial artery.
BRIEF DESCRIPTION OF THE DRAWINGS
The following detailed description, given by way of example, will best be
understood in conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram illustrating the apparatus of the present
invention;
FIG. 2 is a partial block, partial logic diagram illustrating the control
electronics of this invention;
FIGS. 3A-3M are waveform diagrams which are helpful in explaining the
operation of the embodiment shown in FIG. 2;
FIG. 4 is a logic diagram illustrating the display apparatus which can be
used with the present invention; and
FIG. 5 is a logic diagram illustrating the analog-to-digital converter
which can be used with this invention.
DETAILED DESCRIPTION OF A CERTAIN PREFERRED EMBODIMENT
Referring now to the drawings, wherein like reference numerals are used
throughout, and in particular to FIG. 1, there is illustrated a block
diagram of the apparatus which can be used with the present invention. As
is conventional, the sphygmomanometer is comprised of an occluding cuff 10
which is adapted to be inflated by receiving a fluid, such as air, applied
thereto by a pressure hose 12. The occluding cuff 10 may be wrapped about
any convenient limb of a patient, such as the upper arm to surround the
brachial artery. Alternatively, the occluding cuff may be wrapped around a
patient's wrist. Suitable fastening members, not shown, are used to
maintain the cuff in stable position during inflation and during pressure
measurement.
In one embodiment, the occluding cuff 10 is manually inflated by the usual
technique of squeezing a bulb member 14. Fluid pulses applied from the
bulb 14 through the hose 12 to the cuff 10 can be smoothed by a suitable
regulator valve, an air chamber, or the like. For the present discussion,
it will be assumed that a regulating valve 16 is provided for this purpose
and the cuff 10 is subjected to a relatively rapid and smooth inflation.
As is also conventional, the cuff 10 is adapted to be deflated by the
suitable control of an escape valve 17.
A pressure transducer 18 is positioned in fluid communication with the cuff
10 and is adapted to produce an electrical analog signal which is
representative of the cuff pressure. As is apparent, the pressure
transducer 18 may be placed within the cuff or, as shown, the transducer
may be positioned in the pressure hose 12. Notwithstanding the specific
location of the pressure transducer 18, the transducer is further adapted
to detect the smoothed fluid pulses applied during cuff inflation and
pressure pulsations in the occluded artery of the patient, and to
superpose electric pulses on the analog signal in response to these
pressure pulsations. As will be described in greater detail hereinbelow
with respect to FIG. 3A, it is known that a pressure pulsation will occur
during a cardiac cycle when the patient's pressure, which varies from a
systolic peak to a diastolic peak, traverses the cuff pressure.
Accordingly, when the pressure fluctuation in a cardiac cycle exceeds the
pressure of the occluding cuff 10, the pressure transducer 18 serves to
superpose an electrical pulse on the cuff pressure-representing analog
signal. A typical pressure transducer which may be used with this
invention is produced by National Semiconductor Corporation of Santa
Clara, California and is identified as Model LX1600 or LX1700. Another
type of pressure transducer which can be used is manufactured by Stow
Laboratories, Inc. of Hudson, Massachusetts, and is identified as the
Pitran Pressure Transistor.
As illustrated, the output of the pressure transducer 18 is coupled in
common to an analog-to-digital converter 20 and to a pulse detector 22.
The analog-to-digital converter 20 is adapted to convert the analog signal
representing the cuff pressure to a digital representation. As the cuff
pressure changes, the digital representation produced by the
analog-to-digital converter 20 likewise changes.
The pulse detector 22 is adapted to sense the superposed pulses produced by
the pressure transducer 18 and to shape these pulses into desirable form.
The output of the pulse detector 22 is coupled in common to a first pulse
detector 24, designated the systolic pulse detector, and to a second pulse
detector 26, designated the diastolic pulse detector. As will be described
in greater detail below, the systolic pulse detector 24 is adapted to
sense the first pulse which is detected by the pulse detector 22. It is
appreciated that the first pulse which is detected by the pulse detector
22 is produced when the pressure in the occluding cuff 10 is equal to or
slightly less than the patient's systolic pressure. The diastolic pulse
detector 26 is adapted to sense the final pulse which is detected by the
pulse detector 22. It is appreciated that when the pressure in the cuff 10
is equal to or slightly greater than the patient's diastolic pressure, a
pulse will be produced by the pressure transducer 18. However, when the
cuff pressure falls below the patient's diastolic pressure, no further
pulses will be produced by the transducer. Thus, the last, or final, pulse
produced by the transducer 18 and detected by the pulse detector 22 is
sensed by the diastolic pulse detector 26.
A systolic pressure display 28 and a diastolic pressure display 30 are
coupled in common to the analog-to-digital converter 20 and are adapted to
receive and display the digital representations produced by the converter.
In addition, the systolic pressure display 28 is coupled to the systolic
pulse detector 24; and the diastolic pressure display 30 is coupled to the
diastolic pulse detector 26. In a preferred embodiment of this invention,
the respective displays are optical displays formed, for example, by
multi-digit light-emitting elements. As is conventional in the art, the
pressure is measured and indicated in the form of millimeters of mercury.
Consistent with this convention, the displays 28 and 30 preferably provide
indications of systolic and diastolic pressure in millimeters of mercury.
In one embodiment thereof, the systolic pressure display 28 is responsive
to a signal applied thereto by the systolic pulse detector 24 when the
detector 24 senses the first pulse which is detected by the pulse detector
22, to thereby display the multi-digit number which is digitally
represented by the output of the analog-to-digital converter 20. That is,
as the digital representation produced by the converter 20 changes while
the cuff 10 is deflated from a maximum pressure, the pressure
representation produced by the converter is not displayed until the
systolic pressure display 28 is actuated by the signal produced by the
systolic pulse detector 24. At that time, the pressure then obtaining in
the cuff 10 closely approximates the patient's systolic pressure, and the
digital representation of this cuff pressure is stored and displayed in
the systolic pressure display 28. Further changes in the digital
representation produced by the analog-to-digital converter 20 will not
affect the data now stored in and displayed by the display 28.
In an alternative embodiment, the systolic pressure display 28 normally is
actuated so as to display the multi-digit representation of cuff pressure
which is produced by the analog-to-digital converter 20. Hence, as the
cuff pressure changes, for example, during cuff deflation, the systolic
pressure display 28 likewise changes. Now, when the systolic pulse
detector 24 senses the first pulse which is detected by the pulse detector
22, a signal is supplied to the systolic pressure display 28 to disable
that display from responding to further changes in the digital
representation supplied by the converter 20. Consequently, the last
indication of cuff pressure which is displayed by the systolic pressure
display 28 will be retained thereby once the initial pulse produced by the
pressure transducer 18 is detected.
The diastolic pressure display 30 is substantially similar to the systolic
pressure display 28 and operates in a similar manner. Hence, in one
embodiment, it is appreciated that the display 30 does not respond to the
digital representation of cuff pressure supplied thereto until the
diastolic pulse detector 26 senses the final pulse which is detected by
the pulse detector 22 and supplies an actuating signal to the diastolic
pressure display 30. At that time, the digitized representation of the
then obtaining cuff pressure will be supplied to and displayed by the
display 30. In the alternative embodiment, the pressure indication
displayed by the display 30 will be changed, or up-dated, as the output of
the analog-to-digital converter 20 is changed. The diastolic pressure
display 30 is inhibited from responding to further changes in the digital
representation supplied by the converter 20 once the diastolic pulse
detector 26 senses the final pulse which is detected by the pulse detector
22. Thus, it is seen that further deflation of the cuff 10 does not
disturb the pressure measurement indications displayed by the systolic and
diastolic pressure displays 28 and 30, respectively.
While the foregoing has assumed that the cuff 10 is inflated by squeezing
the bulb 14 in conventional manner, it is recognized that alternative
inflating devices may be used, if desired. For example, a suitable pump or
compressor can be used to supply fluid under pressure to the cuff 10. As a
further alternative, canisters of compressed gas may be used. Once the
cuff 10 has been inflated to a desirable pressure, i.e., to a pressure
well above the systolic pressure of the patient, the escape valve 17 may
be manually operated so as to slowly deflate, or bleed, the cuff. It
should be understood that, if desired, automatic deflating mechanisms may
be used. In that event, when the cuff pressure exceeds a desirable
threshold, as indicated by the pressure transducer 18 and/or the
analog-to-digital converter 20, the automatic deflating mechanism can be
actuated. This pressure threshold can be determined in accordance with the
physiological characteristics of a patient or, alternatively, may be set
at a level which is not expected to be exceeded by a patient's systolic
pressure. As one example, this threshold can be selected at 200mm Hg.
The block diagram of the electronic apparatus shown in FIG. 1 is depicted
in greater detail in the partial block, partial logic diagram of FIG. 2.
As shown therein, the pressure transducer 18 is coupled to the
analog-to-digital converter 20 which is comprised of a voltage controlled
oscillating circuit 204 coupled to a pulse train gating circuit 206. The
voltage controlled oscillating circuit 204 may be conventional, and as is
known, has an oscillating frequency which is proportional to an analog
signal applied thereto. Accordingly, an amplifying stage 202 may be used
to supply the analog signal produced by the transducer 18 to the voltage
controlled oscillating circuit 204.
It is preferred that the voltage controlled oscillating circuit 204 produce
a pulse train whose pulse repetition rate is directly proportional to the
pressure-representing analog signal applied thereto. Thus, desirably, if
the pressure is equal to 0mm Hg, the pulse repetition rate likewise should
be zero pulses per second (pps). Similarly, when the pressure is equal to
200mm Hg, the pulse repetition rate should be K times 200 pps, where K is
an integer. However, it has been found that, with many commercially
available voltage controlled oscillating circuits, the pulse repetition
rate is not reduced to zero even though the input analog signal applied
thereto is substantially equal to zero. This characteristic feature of
such a voltage controlled oscillating circuit is graphically represented
by the curve 205. It is recognized that if the pressure P is reduced to
zero, zero offset pulses are produced. However, as the pressure P is
increased, the pulse repetition rate is directly proportional to, and thus
linearly increases with, the pressure. The pulse train gating circuit 206,
coupled to the output of the voltage controlled oscillating circuit 204,
is adapted to remove the zero offset pulses, whereby the curve 205 is
effectively shifted so as to intersect the origin, as shown in the curve
207.
As will be described in greater detail with reference to FIG. 5, the pulse
train gating circuit 206 is adapted to periodically sample a portion of
the pulse train produced by the voltage controlled oscillating circuit
204, and to transmit this sampled portion to further apparatus. The number
of pulses included in this sampled portion of the pulse train is directly
proportional to the cuff pressure, as sensed by the pressure transducer
18. The zero offset pulses attending the operation of the voltage
controlled oscillating circuit 204 are removed from the sampled portion of
the pulse train. A clock circuit 210 is coupled to the pulse train gating
circuit 206 and is adapted to produce a periodic sampling pulse having a
predetermined sampling interval. Therefore, it should be readily
appreciated that if the number of pulses included in the sampled portion
of the pulse train corresponds to the cuff pressure in millimeters of
mercury, an indication of the cuff pressure can be readily attained merely
by counting the number of pulses transmitted during a sampling interval.
The pulse detecting circuit 22 is coupled to the pressure transducer 18 and
comprises a pulse detecting and shaping circuit 222, a bistable
multivibrator 226 (hereinafter a flip-flop circuit) and an OR circuit 228.
The pulse detecting and shaping circuit 222 is adapted to detect the
superposed pulses which are produced by the transducer 18. Accordingly,
this circuit may comprise a conventional amplifying stage, together with
filter and differentiating circuits which are well known to those of
ordinary skill in the art. Hence, the pulse detecting and shaping circuit
222 is adapted to produce an output pulse that is relatively free of noise
and which exhibits a desirable amplitude and duration.
The flip-flop circuit 226 is conventional and admits of two stable states.
In the illustrated embodiment, this flip-flop circuit may comprise a R-S
flip-flop having a set input terminal coupled to the pulse detecting and
shaping circuit 222 and a reset input terminal coupled through an
inverting circuit 230 to the OR circuit 228. For the purpose of the
present discussion, it will be assumed that the flip-flop circuit is
adapted to be set, or actuated, to its first, or "set" state when a binary
"1" is applied to the set input terminal. Conversely, the flip-flop
circuit is adapted to be reset, or actuated, to its second, or "reset"
state when a binary "1" is applied to the reset input terminal. Consistent
with this assumption, it will be further assumed that a binary "1" is
represented by a positive DC level, whereas a binary "0" is represented by
a lower DC level. More particularly, the binary "0" may be represented by
a negative DC level or by ground potential. As is understood, when the
flip-flop circuit 226 is in its "set" state, a binary "1" is produced at
the 1 output terminal and a binary "0" is produced at the 0 output
terminal. Conversely, when the flip-flop circuit is in its "reset" state,
a binary "0" is produced at the 1 output terminal and a binary "1" is
produced at the 0 output terminal.
Although the foregoing convention of designating binary signals will be
used throughout this discussion, it should be readily understood that
various other representations and compatible devices can be used, if
desired. Thus, a binary "1" can be represented by a ground potential and a
binary "0" can be represented by a higher, positive potential. Also,
signals of negative polarity can be used.
It will soon become apparent that the flip-flop circuit 226 is adapted to
be set to its "set" state in response to each pressure pulse which is
detected by the pulse detecting and shaping circuit 222. The flip-flop
circuit is adapted to be reset to its "reset" state during those clock
intervals when a pulse train produced by the voltage controlled oscillator
204 is not sampled. Accordingly, the reset circuit of the flip-flop
circuit 226 is formed of the OR circuit 228 which includes a first input
terminal coupled to the zero output terminal of the flip-flop circuit and
a second input terminal adapted to be supplied with the clock pulses
produced by the clock circuit 210. As is known, an OR circuit is a logic
element which is adapted to produce a binary "1" at its output terminal
when a binary "1" is supplied to any input terminal thereof. A binary "0"
is produced by an OR circuit only if each input terminal is supplied with
a binary "0". The output terminal of the OR circuit 228 is coupled to the
reset input terminal of the flip-flop circuit 226 by the inverting circuit
230. The inverting circuit is adapted to invert the logical sense of the
binary signal produced by the OR circuit. Hence, a binary "1" is inverted
to a binary "0" and, conversely, a binary "0" is inverted by the inverting
circuit to a binary "1".
The inverting circuit 230 is further coupled in common to the systolic
pulse detecting circuit 24 and to the diastolic pulse detecting circuit
26. The systolic pulse detecting circuit is comprised of a flip-flop
circuit 242, an AND-gate 244 and an AND-gate 246. The flip-flop circuit
242 is similar to the aforedescribed flip-flop circuit 226 and, therefore,
may comprises a R-S flip-flop. As shown, the reset input terminal of the
flip-flop circuit 242 is coupled to the inverting circuit 230. The set
input terminal of the flip-flop circuit 242 is coupled to a predetermined
stage of a counter circuit 302 included in the diastolic pressure display
30 for a purpose to become apparent. At the present time, it merely may be
noted that the flip-flop circuit 242 is adapted to be set to its "set"
state when the counter circuit 302 is incremented to a predetermined
count, and is adapted to be reset to its "reset" state immediately
following the sensing by the pulse detecting and shaping circuit 222 of
the first pressure pulse.
The 1 output terminal of the flip-flop circuit 242 is connected in common
to respective input terminals of the AND-gates 244 and 246. As is known,
an AND-gate is a conventional coincidence circuit of the type wherein a
binary "1" is produced at its output only when a binary "1" is supplied to
each input terminal. The AND-gate produces a binary "0" when a binary "0"
is applied to any input terminal.
The output terminal of the AND-gate 244 is coupled to the counter circuit
282 and is adapted to supply a reset signal thereto. To this effect, the
AND-gate 244 includes a second input terminal which is connected to the
clock circuit 210 by a monostable multivibrator circuit 216. The
monostable multivibrator circuit 216 is a conventional one-shot circuit
capable of producing a pulse of predetermined duration in response to a
positive transition applied thereto. Thus, the one-shot circuit 216
supplies relative narrow pulses to the AND-gate 244 at each leading edge
of a clock pulse.
The AND-gate 246 is adapted to transmit the sampled pulse train produced by
the pulse train gating circuit 206 to the counter circuit 282.
Accordingly, the other input terminal of the AND-gate 246 is connected to
the pulse train gating circuit 206 and the output terminal of the AND-gate
is connected to the counter circuit 282. It may be appreciated that when
the flip-flop circuit 242 is in its "set" state, the AND-gate 246 is
enabled to transmit the sampled pulse trains to the counter circuit.
However, once the flip-flop circuit 242 is reset to its "reset" state, the
AND-gate 246 is disabled from transmitting further pulses to the counter
circuit. As will soon become apparent, the resetting of the flip-flop
circuit 242 is partially controlled by the clock circuit 210 so as to
avoid a premature disabling of the AND-gate 246, and thereby avoiding the
possibility of transmitting only an incomplete portion of the sampled
pulse train.
The diastolic pulse detecting circuit 26 is comprised of a flip-flop
circuit 262 and first and second AND-gates 264 and 266, respectively. The
flip-flop circuit 262 is similar to the aforedescribed flip-flop circuit
242 and includes a set input terminal coupled to the inverting circuit 230
and a reset input terminal coupled to the clock circuit 210 by a
monostable multivibrator circuit 214. This multivibrator circuit comprises
a conventional one-shot circuit for producing a positive pulse of
predetermined duration in response to a negative transition applied
thereto. Thus, the one-shot circuit 214 is adapted to produce output
pulses upon detecting the trailing edges of the clock pulses supplied by
the clock circuit 210. As an alternative embodiment thereof, the one-shot
circuit 214 may be substantially identical to the one-shot circuit 216,
i.e., it may produce an output pulse in response to a positive transition
applied thereto, and an inverting circuit, not shown, may be used to
connect the clock circuit 210 to the one-shot circuit 214.
The 1 output terminal of the flip-flop circuit 262 is connected in common
to respective input terminals of the AND-gates 264 and 266. The AND-gate
264 is adapted to supply reset pulses to a counter 302 included in the
diastolic pressure display 30. To this effect, the other input terminal of
the AND-gate 264 is connected to the aforementioned one-shot circuit 216,
and the output terminal of the AND-gate 264 is connected to a reset
terminal of the counter 302.
The AND-gate 206 is adapted to transmit the sampled pulse trains produced
by the pulse train gating circuit 206 to the counter circuit 302.
Accordingly, the other input terminal of the AND-gate 266 is connected to
the pulse train gating circuit 206, and the output terminal of this
AND-gate is connected to a counter input terminal of the counter circuit
302. It is appreciated that the AND gate 266 is enabled to transmit the
sampled pulse trains only when the flip-flop circuit 262 is in its "set"
state. It will be recognized that the flip-flop circuit 262 is set to its
"set" state when a pressure pulse is sensed by the pulse detecting and
shaping circuit 222 and is reset to its "reset" state at the conclusion of
a sampling interval. In this manner, a complete sampled portion of the
pulse train is transmitted to the counter circuit 302 so long as the
pressure pulses are sensed.
The systolic pressure display 28 is comprised of the counter circuit 282,
designated the systolic counter, and display elements 286, designated the
systolic display. The systolic counter 282 may comprise any conventional
counting circuit such as a binary counter, a BCD counter, a decade
counter, or the like. In the preferred embodiment, the systolic counter
282 is incremented in response to each pulse transmitted thereto by the
AND-gate 246. As is known, the contents of a decade counter remain
unchanged until the counter is subsequently incremented or until the
counter is cleared, or reset.
Preferably, the systolic counter 282 is comprised of three cascaded decade
counters which are adapted to count in decimal form the pulses applied
thereto. The decimal count attained by the systolic counter 282 is
supplied to and displayed by the systolic display 286. If the systolic
display 286 is comprised of three seven-segment display elements, it is
appreciated that a three-digit number corresponding to cuff pressure will
be stored in the systolic counter 282 and directly displayed by the
systolic display 286. Of course, depending upon the construction of the
systolic counter 282, suitable gating and converting circuits may be used
to produce a numerical display corresponding to the number of pulses which
are supplied to the counter.
The diastolic pressure display 30 is comprised of the counter 302,
designated a diastolic counter, and a display 304, designated a diastolic
display. As described above, a predetermined stage 303 of the diastolic
counter 302 is connected to the set input of the flip-flop circuit 242.
Thus, when the diastolic counter is incremented to a particular count, a
binary "1" is produced by the stage 303 and is supplied to the flip-flop
circuit 242. The diastolic counter and display are substantially identical
to the aforedescribed systolic counter and display and, in the interest of
brevity, further description thereof need not be provided.
The operation of the apparatus illustrated in FIG. 2 now will be described
in conjunction with the waveform diagrams shown in FIGS. 3A-3M. Let it be
assumed that the cuff 10 is inflated to a maximum pressure, as indicated
in FIG. 3A. During inflation, fluid pulses to the cuff are detected by the
pulse detecting and shaping circuit 222 to set the flip-flop circuit 226
to its "set" state. As will be described, this sets the flip-flop circuit
262 to its "set" state so as to enable the AND-gate 266 to transmit pulse
train pulses to the diastolic counter 302. Now, when maximum cuff pressure
is attained, the artery surrounded by the occluding cuff 10 is closed.
Also, it may be recognized that as the analog signal produced by the
pressure transducer 18 increases in accordance with the cuff inflation,
the oscillating frequency of the voltage controlled oscillating circuit
204 likewise increases. Hence, the number of pulses included in the
sampled portion of the pulse train, as produced by the pulse train gating
circuit 206, is relatively large. These pulses are transmitted by the
AND-gate 266 to the diastolic counter 303 so that, when the maximum
pressure level is reached, the stage 303 supplies a binary "1" to the set
input terminal of the flip-flop circuit 242, thereby setting this
flip-flop circuit to its "set" state. Hence, the AND-gates 244 and 246 are
enabled to transmit reset pulses as well as pulse train pulses to the
systolic counter 282.
After the maximum pressure in the occluding cuff 10 is reached, the cuff
then is slowly deflated, as indicated by the pressure curve 302a of FIG.
3A. As this pressure decreases, the pulse repetition rate of the voltage
controlled oscillating circuit 204 likewise decreases, as shown in FIG.
3C. Of course, the clock circuit 210 produces a constant, periodic clock
pulse of the type shown in FIG. 3D. Each clock pulse interval is assumed
to have a width T which, it is recognized, is equal to the pulse train
sampling interval. Hence, the output of the pulse train gating circuit 206
is shown in FIG. 3E. The cross-hatched area in FIG. 3E represents the zero
offset pulses which are subtracted from the sampled pulse train, as
described above. Therefore, the total number of pulses included in the
sampled pulse train, shown in FIG. 3E, is directly proportional to the
pressure then obtaining in the occluding cuff.
As shown in FIG. 3A, cardiac cycles 304a are superimposed over the pressure
curve 302a. These cardiac cycles represent pressure pulsations in a
patient and range from a maximum systolic pressure 306 to a minimum
diastolic pressure 308. Let it be assumed that the cardiac cycle first
exceeds the pressure curve 302a at the point 304'. Consequently, a
pressure pulse will be sensed by the transducer 18 and an electrical pulse
will be superposed on the analog signal level. After detection and shaping
by the pulse detecting and shaping circuit 222, this initially sensed
pulse 310 will appear as shown in FIG. 3B. From this point on, it is seen
that the cardiac cycles 304a traverse the pressure curve 302a during both
the rising and falling portions of the cardiac cycle. However, pressure
pulses will be produced only at positive transitions and the pulse
detecting and shaping circuit 222 will sense these pressure pulses as
shown in FIG. 3B. The last intersection between the pressure curve 302a
and the cardiac cycles 304a occurs at 304". At this point, the pulse
detecting and shaping circuit 222 produces the final pulse 312. Since
subsequent cardiac cycles exceed the cuff pressure, no further pressure
pulses will be sensed and the pressure transducer 18 will not superpose
any further pulses on the analog signal level. This is depicted by the
absence of pulses following the pulse 312 in FIG. 3B.
The first pulse 310 which is sensed by the pulse detecting and shaping
circuit 222 serves to set the flip-flop circuit 226 to its "set" state, as
shown in FIG. 3F. At this time, a postive clock pulse is present,
resulting in a binary "1" supplied through the OR circuit 228 to the
inverting circuit 230, as shown in FIG. 3G. Hence, the combination of the
OR circuit and the inverting circuit is not capable of resetting the
flip-flop circuit 226. Also, since the output produced by the inverting
circuit 230 is a binary "0", it is appreciated that the flip-flop circuit
242 remains in its "set" state and the flip-flop circuit 262 remains in
its "reset" state.
Now, upon the termination of the positive clock pulse, a binary "0" is
applied to each input terminal of the OR circuit 228, resulting in a
binary "0" supplied to the inverting circuit 230, whereat the binary "0"
is inverted. At this time, the flip-flop circuit 226 is reset, as is the
flip-flop circuit 242. This is shown in FIG. 3H. Prior to the resetting of
the flip-flop circuit 242, it is appreciated that the sampled pulse trains
shown in FIG. 3E are transmitted through the AND-gate 246 to the systolic
counter 282, as shown in FIG. 3J. Also, immediately prior to the
transmission of the sampled pulse train, the AND-gate 244 is enabled to
transmit the clock pulse leading edge pulses produced by the one-shot
circuit 216 to the reset terminal of the systolic counter 282. Thus,
immediately before receiving each succeeding sampled pulse train, the
systolic counter 282 is reset to an initial, or zero, count. Therefore, it
is seen that, while the flip-flop circuit 242 remains in its "set" state,
the systolic counter 282 periodically receives representations of the
actual pressure then obtaining in the occluding cuff 10. However, once the
flip-flop circuit 242 has been reset, the AND-gates 244 and 246 now are
disabled. Consequently, the periodic reset pulses no longer are applied to
the systolic counter 282, as shown in FIG. 3K. Also, the periodic sampled
pulse trains no longer are supplied to the systolic counter, as shown in
FIG. 3J. Hence, the last-received sampled pulse train shown in FIG. 3J is
stored in the systolic counter. Therefore, it is fully appreciated that
the count of the systolic counter 282 is periodically updated so as to
represent the deflating pressure in the occluding cuff until the first
pulse 310, which is produced by the pressure transducer 18 and is sensed
by the pulse detecting and shaping circuit 222, occurs. Then, the
last-received count, or pressure indication, is retained by the systolic
counter 282 until a subsequent pressure-measuring operation is performed.
As the count stored by the systolic counter 282 is periodically updated in
response to the deflating occluding cuff, the pressure measurement
represented by this count is displayed by the systolic display 286. In one
embodiment, the display 286 provides indications of each updated count
stored by the c | | |
