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Description  |
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BACKGROUND OF THE INVENTION
This invention relates generally to medical devices for measuring blood
pressure and pertains more particularly to cuff-type systems employing a
pump and transducers.
Many types of blood pressure measurement devices are known in the prior
art. One example is U.S. Pat. No. 4,105,021 to Williams et al which shows
a system for applying an occluding inflatable cuff to a body member, with
the pressure applied thereby being cycled between a pressure greater than
systolic and a pressure less than diastolic. The pressurization cycle
described in this patent occurs as a controlled bleed of pressure from the
occluding cuff during the time that a plurality of blood pressure pulses
occur. The patent shows the use of a blood flow monitor and a pressure
monitor to measure pressure inside the cuff. A similar system is described
by Fletcher et al in U.S. Pat. No. 3,814,083, which describes a bleed-down
of the cuff pressure which was from 160-60 mm Hg in 30 seconds. Both
patents contemplate measuring both systolic and diastolic blood pressure
during a single cycle of the occluding cuff pressure.
Blood pressure measurement systems are also described by Lichowsky in U.S.
Pat. No. 3,905,354 which shows the measurement of pressure fluctuations in
a chamber attached to a patient's arm, wherein the fluctuations correspond
to heartbeats of the patient. Similarly, U.S. Pat. No. 4,074,711 to Link
et al discloses a transducer for measuring the fluctuating component of a
cuff pressure, which is representative of the pulsatile blood pressure.
A major problem with prior art methods of measuring blood pressure
non-invasively is that the process of obtaining a measurement requires an
undesirably long period of time. A problem faced with such prior art
devices is that the user is unable to tell what the blood pressure is
instantaneously and to tell how the blood pressure has changed over the
period of a few heartbeats. A further problem faced by such prior art
devices is that the accuracy of blood pressure measurements during a cuff
pressure cycle may be degraded if the blood pressure is not steady.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a blood pressure
measurement system.
This invention involves a blood pressure measurement system which includes
a pump and, attached to the pump, a cuff for encircling a body appendage
of a user. The pump provides an oscillating pressure to the cuff at a
frequency higher than the blood pressure frequency. The cuff pressure is
monitored and a detector is used to detect when the cuff pressure is equal
to the blood pressure. The detector may be any one of various types used
to sense blood flow, Korotkoff sounds, or discontinuities in the cuff
pressure. An electrical circuit is provided, either in analog or digital
form, to correlate the cuff pressure with the detected times and thus to
determine instantaneous values of blood pressure at the detected times. A
display may be provided to show a waveform envelope of the sequential
blood pressure measurements.
A significant feature of this invention is that it provides a non-invasive
blood pressure measurement system which performs blood pressure
measurements on a real time basis, i.e., the measurements substantially
reflect the instantaneous blood pressure.
Another advantage of the invention is that highly accurate blood pressure
measurements are achieved with a non-invasive device.
A further feature of the invention is that it provides a blood pressure
measurement system which generates a sequence of substantially continuous
blood pressure measurements.
Another feature of the invention is that it is compatible with major types
of automated sphygmomanometric systems.
These and other advantages of this invention will appear more fully from
the following description made in conjunction with the accompanying
drawings wherein like reference characters refer to the same or similar
parts throughout the several views.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a pictorial diagram of the blood pressure measurement system of
this invention showing electronic elements in block diagram form.
FIG. 2 is a block diagram of the analog electronic circuity for use in an
embodiment of this invention.
FIG. 3 is a block diagram of the digital electronic circuitry for use in an
embodiment of this invention.
FIG. 4 is a waveform diagram depicting the amplitude versus time behavior
of various quantities relevant to this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1, blood pressure measurement system 10 includes a
pump 12 having a piston 14 slidably mounted to cyclically reciprocate in
pump cylinder 16. Piston 14 is connected to crankshaft 18 by means of
crank 20, so that piston 14 reciprocates cyclically as shaft 18 is rotated
about shaft axis 22 in the direction of arrow 24. The stroke of piston 14
is adjustable by moving end 26 of crank 20 along the radius 28 of shaft
22. Shaft 22 is rotated by an external power source (not shown) such as an
electric motor.
Port 30 is a hole extending through the wall of cylinder 16 and is
positioned along the length of cylinder 16 so that port 30 is at the
lowermost portion of the strike of piston 14. Thus, port 30 is located so
as to allow pump cylinder 16 to vent through port 30 when piston 14
reciprocates to the lowest point of its strike during each cycle. Port 30
insures that the inside of cylinder 16 is equalized to atmospheric
pressure at least once during each cycle of piston 14. Note that port 30
is either covered by or behind piston 14 when piston 14 is not in its
lowermost stroke position.
Volume control piston 32 is slidably mounted inside volume control cylinder
34, which is coaxial and continuous with cylinder 16. Adjustment screw 36
is threadably mounted through cylinder 16 and has a knob 38 mounted on the
outside end thereof. Screw 36 is connected to piston 32 so that turning
screw 36 moves piston 32 along the length of cylinder 34. Thus, the
location of piston 32 may be changed to vary the volume inside cylinder
34.
Inflatable occlusion cuff 40 is preferably composed of a flexible material
for encircling a body appendage 42 (for example, an arm) of the user. Cuff
40 is of the usual construction adapted for occluding blood vessels in
appendage 42 when pressure is supplied to cuff 40.
Pressure supply tube 44 provides a fluidic connection between cuff 40 and
the insides of pump cylinder 16 and volume control cylinder 34 so that all
are fluidicly connected and filled with a fluid. Note that when piston 14
blocks the vent 30, a substantially closed volume is formed by cuff 40,
tube 44, cylinder 34 and cylinder 16. This fluid may be a gas such as air
or a liquid such as a low viscosity oil. The volume of such fluid required
varies with its compressability. For a fluid such as air, a volume of
between 250 and 300 cubic centimeters is suitable. For a fluid such as a
low viscosity oil, a volume of between 10 and 20 cubic centimeters is
suitable. When a fluid, such as low viscosity oil, having a relatively low
compressibility is used in this invention, it is preferable that cuff 40
be of the known, prior art type which is not substantially expansible in
outside circumference.
Pressure measurement tube 46 is threaded along the inside of tube 44 and
has an open distal end near cuff 40 and the opposite proximal end
connected to pressure transducer 48. Tube 46 is placed inside tube 44 to
minimize the number of connections which must be coupled to appendage 42,
to thereby minimize the inconvenience to the user of having tubes or
cables hanging from appendage 42. As described below, tube 46 and pressure
transducer 48 are used to measure the fluid pressure inside cuff 40.
Accordingly, the distal end of tube 46 is located near cuff 40 to avoid
inaccuracies due to pressure drops along the length of tube 44.
Pressure transducer 48 may be any of the usual types of transducers (such
as a diaphragm with a strain gauge) for converting fluid pressure to an
electrical signal. Transducer 48 serves to measure the fluid pressure
inside cuff 40 through tube 46. Transducer 48 is electrically connected to
electronic circuitry box 50 to transfer an electrical signal indicative of
the measured cuff fluid pressure.
Blood flow detector 52 is positioned on appendage 42 to detect the starting
and stopping of blood flow in appendage 42 due to occlusion by cuff 40.
Thus detector 52 detects the times at which the fluid pressure inside cuff
40 is equal to the blood pressure inside appendage 42. Detector 52 may be
of the photoplethysmographic type known in the prior art including a
photoelectric blood pulsation sensor positioned adjacent cuff 40. Such a
sensor can also be used to sense blood volume pulsations (the pulse) which
as described below is periodically interrupted by the pressures provided
by cuff 40. Detector 52 is electrically connected to box 50 to transfer an
electrical signal indicative of the blood flow in appendage 42.
In an alternative embodiment of this invention, microphone 54 is used as an
alternative blood flow detector to detector 52. As shown in FIG. 1,
microphone 54 mounts under cuff 40 on appendage 42 to detect Korotkoff
sounds characteristic of turbulent blood flow resulting from occluding
blood vessels, as is known in the prior art. Microphone 54 thus detects
the times at which the fluid pressure inside cuff 40 is equal to the blood
pressure inside appendage 42. Microphone 54, if substituted for detector
52 in an embodiment of this invention, would be electrically connected to
box 50 to transfer an electrical signal indicative of the blood flow in
appendage 42.
Electrical impedance plethysmograph 56 also shown in FIG. 1 is a further
alternative blood flow detector to detector 52 and is used as a part of an
alternative embodiment of this invention. Impedance plethysmograph 56
includes a spaced-apart pair of electrical conductors 58 and 60 encircling
the appendage 42, as is known in the prior art. The electrical impedance
between conductors 58 and 60 is known to vary as a function of the blood
flow, thus plethysmograph 56 also detects the times at which the fluid
pressure inside cuff 40 is equal to the blood pressure inside appendage
42. Impedance plethysmograph 56, if substituted for detector 52 in an
embodiment of this invention, would be electrically connected to box 50 to
transfer an electrical signal indicative of the blood flow in appendage
42.
As described in detail below, electrical circuit box 50 correlates the cuff
pressure measured by transducer 48 with the blood flow measurements of
detector 52. The measurements made by detector 52 are used by box 50 to
determine the times at which the fluid pressure inside cuff 40 is equal to
the blood pressure in appendage 42. Box 50 correlates the times obtained
by detector 52 with pressure measured by transducer 48 to generate an
electrical blood pressure signal indicative of the blood pressure inside
appendage 42.
Display 53 is preferably a video display electrically connected to box 50
for displaying a waveform 55 generated by box 50 indicative of the
measured blood pressure. Waveform 55 preferably consists of a sequence of
spaced-apart, parallel spikes; with the height of each of the spikes
indicating an instantaneous blood pressure value; and with the separation
of the spikes indicating time duration. Contrary to the impression which
may be conveyed by the waveform 55 of FIG. 1 which has been drawn in a
stylized fashion to accentuate the waveform 55, the spikes may be of quite
narrow width (short time duration) compared to the width (time) separating
the spikes. Also, the shapes of the tops of the spikes is not crucial to
operation of the embodiment of the invention described herein. Of course,
the exact characteristics of these spikes necessarily depends upon and
varies with the monitoring equipment used; however, monitoring equipment
may be used to produce an envelope of waveform 55 which resembles the
envelope of the waveform of the blood pressure in appendage 42.
Displays 57, 59 and 61 are preferably digital character displays
electrically connected to box 50 for displaying numerical digits
indicative of the measured systolic, diastolic, and mean blood pressure,
respectively. The contents of displays 57, 59 and 61 are generated by box
50 as a result of correlating the measurements of transducer 48 and
detector 52 to determine the measured blood pressure and to generate the
contents of displays 57, 59 and 61 based on the maximum, minimum, and
mean, respectively, of the measured blood pressure.
An analog electronic circuit implementation for the electronic box 50 is
shown in FIG. 2. Pressure signal conditioner 62 has an input 64 for
electrical connection to transducer 48 of FIG. 1. Conditioner 62
preferably comprises a power source for transducer 48 and an amplifier.
Detector signal conditioner 66 has an input 68 for electrical connection
to detector 52 of FIG. 1. Conditioner 66 preferably comprises a
differentiator and a signal level triggering switch.
The output of conditioner 62 is provided as an input to gate 70 which is
gated by the output of conditioner 66. Conditioner 66 gates the gate 70
when detector 52 (see FIG. 1) indicates that the pressure inside cuff 40
is the same as the blood pressure inside appendage 42, and allows the
output of conditioner 62 to propagate through gate 70. The magnitude of
the output of conditioner 62 is related to the pressure inside cuff 40.
Thus output 72 of gate 70 has a waveform consisting of a sequence of
spaced-apart spike pulses with a pulse corresponding to each time
conditioner 66 gates the gate 70, and with the height of each pulse
corresponding to the output of conditioner 62. Output 72 may be
electrically connected to display 52 of FIG. 1.
Display generator 74 preferably comprises analog and digital electronic
circuitry of the usual type for electrical connection to output 72 for
generating outputs 76, 78 and 80 to drive the numeric displays 57, 59 and
61, respectively, of FIG. 1. For example, generator 74 may comprise prior
art triggerable digital voltmeter circuits.
Pressure signal conditioner 62 may also include compensation circuitry,
such as an analog delay line, to compensate for a time delay between
detector 52 and transducer 48 due to detector 52 being positioned a
distance away from cuff 40.
Referring next to FIG. 3, a digital electronic circuit implementation is
shown for the electronic circuitry box 50 of FIG. 1. Pressure signal
sampler 82 has an input 84 for electrical connection to transducer 48 of
FIG. 1. Sampler 82 preferably comprises an analog to digital converter for
sampling the results of pressure measurements made by transducer 48.
Detector signal sampler 86 has an input 88 for electrical connection to
detector 52 of FIG. 1. Sampler 86 preferably comprises an analog to
digital converter for sampling the results of the blood flow measurements
made by detector 52.
The outputs of samplers 82 and 86 are provided as an input to computer 90.
Computer 90 correlates the outputs of samplers 82 and 86 by processing the
digital data acquired from both so that data acquired from sampler 82 is
selected which corresponds to data from sampler 86 indicating that the
pressure inside cuff 40 is equal to the blood pressure in appendage 42.
Essentially, computer 90 tests the data from sampler 86 to determine the
times at which detector 52 indicated that blood flow had started or
stopped, and then computer 90 selects data from sampler 82 corresponding
to the times determined. The data selected by computer 90 is a digital
electronic signal, composed of a plurality of binary bits inside computer
90, indicative of the blood pressure in appendage 42.
Outputs 96, 98 and 100 are digital output ports of computer 90 for driving
displays 57, 59 and 61 respectively. Computer 90 performs internal
mathematical calculations in a conventional, known manner with the data
which it acquires from samplers 82 and 86 in order to generate the
contents of displays 57, 59 and 61
A video output 92 is provided from display generator 94 to provide video
signals to display 53 to produce waveform 55 or other waveforms
representative of the measured blood pressure inside appendage 42. Display
generator 94 is of the usual, conventional type of computer graphics
system for connection to computer 90. Generator 94 operates to display the
data processed by computer 90.
The operation of the invention is best explained using the waveforms 102
through 112 plotted in FIG. 4 on a common, horizontal time axis extending
from 0 to 13. Waveform 102 represents the arterial blood pressure as it
actually exists inside appendage 42 during a single heart beat. Waveform
104 represents the fluid pressure inside cuff 40 as measured by transducer
48. The vertical axis for the plot of waveforms 102 and 104 extends from
pressure P1 through pressure P4.
Pressure P2 is the diastolic blood pressure and pressure P3 is the systolic
blood pressure. Pump 12 periodically varies the pressure in the cuff 40 to
produce the periodic waveform 104 which extends from P1 to P4 and passes
through both the systolic and diastolic blood pressure levels. It is
preferable that pressure P1 be equal to the atmospheric pressure so as to
minimize the amount of time that venous blood flow in appendage 42 is
occluded by cuff 40. Port 30 in pump cylinder 16 insures that pressure P1
will remain constant at the atmospheric pressure. The venting of cylinder
16 by port 30 occurs at the times indicated by pressure waveform 104
decreasing to pressure P1.
The period of blood pressure waveform 102 extends from 0 to t13. As
illustrated, approximately six periods of cuff pressure waveform 140 occur
during the period of waveform 102. The period and frequency of the blood
pressure waveforms of human beings vary from person to person and are
dependent on many factors such as physical condition and exertion. It is
expected that the blood pressure waveform frequency (heart beat rate) may
range from 50 to 150 beats per minute. It is preferable that the frequency
of cuff pressure waveform 104 be relatively higher, on the order of 10
compressions per second. Thus, the period of waveform 104 is preferably
substantially shorter than the period of waveform 102. Waveform 104
crosses over and intersects waveform 102 at the times indicated as t1
through t12. At those crossover points occurring at times t1 through t12,
the pressure in cuff 40 is equal to the blood pressure in appendage 42.
Detector waveform 106 is a stylized drawing displaying, in general, a
waveform typical of the waveforms expected from the blood flow
measurements of detector 52 or impedance plethysmograph 56. As described
below, waveform 106 displays electrical "crossover signals" based upon the
discontinuities detected by the invention when the blood flow starts or
stops. During the time period from t1 to t2, the cuff pressure waveform
104 is less than the blood pressure waveform 102, therefore arterial blood
is flowing, as indicated by the relatively high level of waveform 106.
During the time period from t2 to t3, the cuff pressure waveform 104 is
more than the blood pressure waveform 102, therefore arterial blood flow
is prevented by cuff 40, as indicated by the relatively low level of
waveform 106. At time t3, arterial blood flow is started, or allowed to
flow by cuff 40, thus causing a discontinuity in waveform 106. At time t4,
arterial blood flow is stopped from flowing by cuff 40, thus causing a
discontinuity in waveform 106. Discontinuities occur in waveform 106 at
each of the times t1 through t12. Such discontinuities are detected by
electronic circuitry box 50 as indicating that the fluid pressure inside
cuff 40 is equal to the arterial blood pressure inside appendage 42.
Waveform 108 is a representation of the Korotkoff noise signal blips picked
up by microphone 54 at each of the times t1 through t12 when the pressure
inside cuff 40 is equal to the arterial blood pressure inside appendage
42. Such blips are detected by electronic circuitry box 50.
Waveform 110 is identical to waveform 104 except that the small
oscillometric variations occurring at each time t1 through t12 have been
magnified. Waveform 112 is identical to waveform 102. Discontinuities are
produced in the cuff pressure waveform 110 at each of the times t1 through
t12 due to the fact that cuff 40 is part of a relatively closed fluidic
system and thus its internal pressure is sensitive to changes in the
volume of appendage 42. The expansion of arteries in appendage 42 when
blood is allowed to start flowing causes the pressure in cuff 40 to
increase slightly. The contraction of arteries in appendage 42 when blood
is caused to stop flowing causes the pressure in cuff 40 to decrease
slightly. The expansion and contraction of arteries cause volume changes
in appendage 42 and result in the above mentioned discontinuities in
waveform 110.
The slight increases and decreases in the pressure in cuff 40, as shown in
waveform 112, produce the above-mentioned discontinuities in waveform 110
which may be used by electronic circuitry box 50 in the same way as
signals coming from detector 52. A high pass filter and amplifier of
usual, conventional construction (not shown) may be used to condition the
output of transducer 48 to provide a detection signal to replace the
function of detector 52. Thus the need for a separate detector 52,
microphone 54, or impedance plethysmograph 56 may be eliminated.
Transducer 48 may be used both to measure the fluid pressure inside cuff
40, and to detect the times when the pressure inside cuff 40 is equal to
the arterial blood pressure inside appendage 42.
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