Abstrict
A device for simulating the pressure readings obtained from the
arm of a living subject whose blood pressure levels are being determined
by an oscillometric blood pressure monitor, includes a pulse pressure
chamber for generating pressure pulses at a rate equivalent to a
preselected pulse rate. The input to the pressure chamber is attached
to the pressure cuff of the monitor and the output is connected
to the pressure transducer of the monitor. In order to balance the
pressure across the chamber and to apply the full cuff pressure
to the monitor transducer, a normally-open valve is connected across
the pressure chamber. This valve, however, is closed when the chamber
creates a pressure pulse so that the pulse is added to the applied
cuff pressure at the monitor transducer. A processor generates electrical
signals that control the amplitude of the pressure pulses created
by the chamber, depending on preselected values for simulated systolic,
mean arterial and diastolic pressures.
Claims
We claim:
1. A simulator device for simulating the pressure pulsations from
a subject whose blood pressure is being determined by an oscillometric
blood pressure monitor with an inflatable pressure cuff, comprising:
a. pressure chamber means with an input and an output for creating
a pressure pulse at its output related to an applied electrical
signal, the input being in pressure communication with the pressure
cuff, and the output being in pressure communication with the monitor;
b. means for generating the applied electrical signal in the form
of a series of pulses occurring at a preselected rate, the amplitudes
of said pulses being adjusted in response to the pressure in the
pressure cuff to simulate the pressure pulses of a test subject
with selected blood pressure values;
c. a controllable pressure valve connected between the input and
output of the pressure chamber, said valve normally being open;
and
d. means for closing said valve for a period substantially equal
to the period of said pressure pulse, so that the total pressure
at the output of the pressure chamber means is the sum of the pressure
pulse and the pressure applied from the pressure cuff;
e. wherein said pressure chamber means comprises
(i) a hollow housing with an input port coupled to the input of
said pressure chamber and an output port coupled to the output of
said pressure chamber,
(ii) a diaphragm dividing the interior of said housing between
said input port and said output port, and
(iii) means for causing movement of said diaphragm in response
to an electrical signal, the amplitude, direction and frequency
of the movement being related to the amplitude, polarity and frequency
of the electrical signal.
2. A simulator device as claimed in claim 1 wherein said means
for causing movement includes an electromagnet.
3. A simulator device as claimed in claims 2 or 1 wherein the means
for generating the applied electrical signal varies the peak amplitude
of the pulses in relation to selected values for simulated systolic,
mean arterial and diastolic pressures.
4. A simulator device as claimed in claim 3 wherein the means for
generating the applied electrical signal causes the peak amplitudes
of the pulses to vary such that:
when the applied cuff pressure is between the selected values for
diastolic and means arterial pressure, the peak amplitudes of the
pulses are linearly related to one another in the range between
a maximum pulse amplitude at means arterial pressure and a first
fraction of the maximum pulse amplitude at the diastolic pressure;
when the applied cuff pressure is between the selected values for
mean arterial pressure and systolic pressure, the peak amplitudes
of the pulses are linearly related to one another in the range between
a maximum pulse amplitude at mean arterial pressure and a second
fraction of the maximum pulse amplitude at the systolic pressure;
and
when the applied cuff pressure is between the selected value for
systolic pressure and a certain fixed pressure level significantly
exceeding systolic pressure, the peak amplitudes of the pulses are
linearly related to one another in the range between a pulse amplitude
equal to the second fraction of the maximum at the systolic pressure
and a pulse amplitude of zero at the fixed pressure level.
5. A simulator device as claimed in claim 4, wherein the means
for generating the applied electrical signal includes a microprocessor
programmed to calculate the linear relationships and to control
the pulse amplitudes in response to those calcalutions.
6. A simulator device as claimed in claim 1 wherein said means
for controlling the valve includes means for assuring that the valve
is open whenever there is a change in cuff pressure above a certain
value, even during the generation of a pulse.
Description FIELD OF THE INVENTION
This invention relates to the automated measurement of blood pressure
by the oscillometric method and, more particularly, to devices for
calibrating oscillometric blood pressure monitors.
BACKGROUND AND PRIOR ART
The measurement of the blood pressure of human beings is usually
accomplished by applying a variable pressure through a pressure
cuff to the brachial artery in the upper arm of the test subject.
As the pressure is varied, the sound of the blood flow through the
artery or the pressure pulses generated by the artery are usually
measured and used as indications of various blood pressure values.
These two techniques, known as the ausculatory and oscillometric
methods, respectively, can both be automated.
With an automated ausculatory blood pressure monitor, a pump inflates
the cuff such that the brachial artery, is occluded. Then the pressure
is reduced in increments by the monitor and an audio transducer
or microphone located on the arm of the subject adjacent the distal
portion of the occluded artery converts the sound produced when
blood flow is reestablished into electrical signals. These sound
signals, known as "Korotkoff" or "K-sounds",
are first detected when the cuff pressure is at a level known as
the "systolic" pressure. The monitor continues to reduce
the cuff pressure until these K-sounds disappear. The cuff pressure
level where this occurs is known as the "diastolic" pressure.
Once the systolic and diastolic pressures have been determined,
the monitor deflates the cuff to zero pressure.
A typical automatic oscillometric blood pressure monitor also includes
a pump which first inflates a pressure cuff around the upper arm
of the subject to occlude the brachial cuff about the upper arm
of the subject to occlude the brachial artery and then decreases
the pressure in increments. However, instead of using an audio transducer
on the arm, the oscillometric monitor uses a pressure transducer
located in the monitor and connected to the cuff by a conduit. Even
when the cuff has occluded the artery, the pumping of the heart
of the subject causes the artery to flex or oscillate, resulting
in pressure pulses that are received in the cuff and the transducer.
As the pressure is decreased, the amplitude of the pulses increases
to a maximum and then decreases again. The cuff pressure when the
pulse amplitudes are at a maximum is known as "mean arterial
pressure" (MAP). Also, the cuff pressures above and below MAP
where the pulse amplitudes are predetermined fractions of the amplitude
at MAP, represent the systolic and diastolic pressures, respectively.
The testing of automatic blood pressure monitors is typically very
difficult because living subjects on which it may be tested may
have different pressure levels at different times, and different
subjects will have different pressure levels. Thus, the accuracy
of the monitor with respect to its intended purpose can only be
determined on a statistical basis by time consuming population studies.
Since this is not possible on a production basis, typically only
the electronics of the monitor are tested and a mercury manometer
is used to test the accuracy with which the pressure transducer
measures the cuff pressure. Then if the machine produces a correct
reading for a single test subject known to have a stable blood pressure,
it is assumed to be functioning properly. Obviously, the problem
with this method of testing monitors is that there is no assurance
that the monitor will operate to detect and measure pressure pulses
in addition to the cuff pressure, or to measure blood pressures
and pulse rates which are remote from those of the test subject.
Also, a properly functioning monitor may be rejected if the test
subject should have an unusual blood pressure level during the test.
It is known in the art to test automated ausculatory monitors by
creating an artifical arm in the form of an inner cylinder filled
with water in which sound waves approximating K-sounds are generated
by a transducer. An outer layer of water in a flexible bag transmits
the sound to the pressure cuff and microphone of the ausculatory
monitor, and also simulates the compliance of a real arm. At present,
however, there is no known method of simulating the arm of a test
subject so that the pressure pulses of the oscillometric method
can be generated for testing of a blood pressure monitor of that
type.
DISCLOSURE OF THE INVENTION
The present invention is directed to apparatus suitable for simulating
the arm of a living subject being tested for blood pressure readings
by the oscillometric method. This object is accomplished by automatically
inducing pressure pulses in the cuff of an oscillometric blood pressure
monitor, which pulses occur at a predetermined, selectable rate
and have amplitudes at various cuff pressures that indicate a selected
value for systolic, distolic and mean arterial pressure.
In an illustrative embodiment of the invention, a device is provided
for simulating the pressure pulsations typically monitored when
blood pressure is measured by the oscillometric method. With this
device, the performance of an oscillometric blood pressure monitor
having an inflatable pressure cuff may be tested. The device has
a pressure pulse chamber that includes an electrically actuated
diaphragm adapted for generating pressure pulses related to an applied
electrical signal. On one side of the diaphragm there is an input
to the pressure chamber that is in pressure connection with the
pressure cuff. In order to eliminate the need to vary the pressure
pulses to compensate for applied cuff pressure levels, a controllable
pressure valve is connected across the pressure chamber to balance
the pressure across the diaphragm from the cuff. During a pressure
pulse, however, the valve is closed so that the selected pulse amplitude
is added to the cuff pressure, regardless of the cuff pressure.
As a result, the output of the chamber is a pressure level equal
to the sum of the pressure of the cuff and the pressure pulse created
by the actuation of the diaphragm.
The signal which drives the diaphragm has a repetition rate that
represents a selected pulse rate and an amplitude that varies with
the applied pressure in the cuff of the blood pressure monitor so
as to simulate the pressure pulsations of a subject having selected
diastolic, mean arterial and systolic blood pressure values.
In a preferred embodiment, the amplitudes of the pressure pulses
and rate are controlled by a microprocessor in response to selected
pulse rate and pressure values.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other features of the present invention will
be more readily apparent from the following detailed descriptioh
and drawings of an illustrative embodiment of the invention in which:
FIG. 1 illustrates the measurement of the blood pressure of a test
subject with an oscillometric blood pressure monitor;
FIG. 2 is a graph of the pressure detected by the transducer of
the blood pressure monitor of FIG. 1;
FIG. 3 illustrates apparatus according to the present invention
for testing the oscillometric blood pressure monitor of FIG. 1;
FIG. 4 is a graph of the equations for determining the pulse amplitudes
produced by the apparatus of FIG. 3;
FIG. 5 is a flow chart describing the operation of the apparatus
of FIG. 3;
FIG. 6 is an enlarged plan view of the pulse pressure chamber of
FIG. 3; and
FIG. 7 is a sectional view along line 7--7 of the pulse pressure
chamber of FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION
A conventional automatic oscillometric blood pressure monitor 10,
such as that sold by Critikon Corp. of Tampa, Fla. as the Dinamap.TM.
model 845, is shown in FIG. 1. In that figure, the monitor 10 is
being used to test the blood pressure of a test subject. This is
accomplished by inflating a pressure cuff 12 located about the upper
arm 14 of the subject and detecting pressure pulsations induced
in the cuff by the action of the subject's heart, while decreasing
the cuff pressure from an initial high level. For this purpose,
the monitor includes a pump 11 which is capable of automatically
inflating the cuff via a tube 13 to a pressure high enough to occlude
the brachial artery, e.g., about 170 mmHg, and a valve 15 capable
of reducing this pressure in steps as shown in FIG. 2. Pressure
pulses such as those shown at each step in FIG. 2, as well as the
applied pressure, are delivered to the monitor over a conduit 18
and are detected by a transducer 16 located in the monitor. The
transducer converts the pressure levels into an electrical signal
which will have the same form as the pressure signal in FIG. 2.
In the monitor, the amplitudes of the pulses are detected and stored
along with the corresponding values of the applied cuff pressure.
This data is then reviewed to determined the applied pressure level
at which the maximum pulse amplitudes occur. This pressure, which
is shown as 100 mmHg in FIG. 2, is designated MAP. The applied cuff
pressure above and below MAP at which the pulses are some fixed
ratio of the amplitude at MAP are designated the systolic and diastolic
pressures, respectively.
Usually, a monitor such as that in FIG. 1 is tested for accuracy
by making sure the electronics and pressure transducer are functioning
properly and then using the monitor to test one or more people whose
blood pressure is known to be stable at a relatively fixed level.
However, if only a few people are used for this test, there is no
assurance that the monitor is capable of functioning over its complete
range. Also, repeated testing tends to fatigue and irritate the
test subjects, which may result in a change in their blood pressure
that may be interpreted as a malfunction in the monitor.
In order to overcome the problems inherent in testing automated
oscillometric blood pressure monitors, the simulator device shown
in FIG. 3 is used. This device effectively simulates the arm of
the test subject, but it can be set to give a wide variety of pulse
rates and pressure levels. More particularly, the simulator device
of FIG. 3 allows the pump of the monitor to inflate the cuff and,
at the proper rate and level, it induces pressure pulsations in
the cuff that duplicate those of a test subject's arm. This combination
of the pressure level from the pump and the pulsations of the simulator
are like that shown in FIG. 2 and are detected by transducer 16
of the monitor where they are processed to determined the preprogramed
blood pressure levels.
When using the simulator, the pump of monitor 10 is connected to
the pressure cuff 12 over conduit 13 in the same manner as when
a living subject is measured. However, the return line or conduit
18 for applying the cuff pressure to the transducer of the monitor
has the simulator device inserted in it for test purposes. A pulse
rate, a systolic pressure and a diastolic pressure for the simulator
are then selected by the user or operator. By means of a chart,
the operator also selects a MAP level that is compatible with the
selected values for systolic and diastolic pressure in a normal
human being. It should be noted that any value of MAP between the
selected values for systolic and diastolic pressure can be selected
for the simulator, but the monitor may reject a MAP setting that
is too far from normal levels. Of course, the selection of MAP may
be preprogrammed into the simulator so that for any systolic and
diastolic pressure, MAP is automatically selected. It is also possible
to construct the simulator so that it will automatically select
a variety of pulse rates and blood pressure levels so that the complete
range of the monitor is tested without human intervention.
Within the simulator, the selected pressure values and pulse rates
are directed to a processor, e.g., a microprocessor, that controls
the operation of the simulator. The blood pressure monitor is operated
in the usual manner while being tested by the simulator. It initially
inflates the cuff 12 to a pressure of about 170 mmHg as shown in
FIG. 2. This cuff pressure level is passed through the simulator
to the transducer 16 of the monitor via conduits 18', 18" and
a normally-open valve 21, which may be a Clippard EVO-3 valve. Since
valve 21 is located across a pressure chamber 30, the cuff pressure
is also applied to both sides of a diaphragm 36 (FIG. 6) located
within the chamber. The cuff pressure is further applied to a pressure
transducer 23 of the simulator over a conduit 24. As a result, an
electrical signal related to the cuff pressure is generated and
applied both to processor 20 and a control circuit 26 that operates
valve 21.
The processor 20 causes an electrical pulse generator 28 to produce
pulses at the selected pulse rate and with amplitudes that vary
with the cuff pressure created by the monitor in such a way as to
produce a signal simulating the selected blood pressure readings.
These pulses drive an electrical coil 34 (FIG. 7) in the pressure
chamber that causes diaphragm 36 to move and to produce a pressure
pulse with an amplitude related to that of the electrical pulse
applied to coil 34. At the same time the processor is activating
the pulse generator, it is supplying a pulse signal to valve control
circuit 26. This circuit in turn delivers a pulse to valve 21 so
as to close it during the pressure pulse produced by pressure chamber
30. Since the pressure level at the output 33 of pressure chamber
30 is at the cuff pressure level and the output has been separated
from the cuff by the closing of valve 21, the pressure chamber only
needs to drive the volume of air in conduits 29 and 18" leading
from the chamber to the monitor. Consequently, the pressure applied
to transducer 16 of the monitor will equal the pulse from the chamber
plus the cuff pressure, thus producing a signal such as that in
FIG. 2.
If there were a direct connection only between the cuff 12, transducer
16 and the output of pressure chamber 30, the pressure pulses would
have to be created against the pressure force and volume of the
entire cuff. Thus, the pulse amplitude would decrease at higher
cuff pressure levels and it might not be possible to create pulses
of reasonable amplitudes in the range of interest without using
an excessively large pressure chamber and consuming large amounts
of power. Also the amplitude of the signal from generator 28 would
have to be made to vary not only with the level needed to produce
the desired blood pressure levels, but also to compensate for the
volume and level of the cuff pressure that chamber 30 would have
to operate against. Further, the pressure differential across the
diaphragm would require it to be of a heavy construction. However,
by balancing the cuff pressure across the pressure chamber diaphragm
36 and using valve 21 to separate the output of the chamber from
the cuff during a pulse generation: (i) the resulting pressure pulse
is substantially independent of the applied cuff pressure, except
as determined by the applied electrical signal, (ii) the cuff pressure
at the input 32 of the chamber aids in pulse generation and (iii)
the pulses can be produced with a smaller and more energy efficient
pressure chamber.
As previously stated, the amplitudes of the pressure pulses from
a living test subject start at a low level and increase to a maximum
level at MAP while the cuff pressure is reduced from its high initial
level. In addition, the pulse amplitudes begin to decrease as the
pressure level is decreased below MAP. The locus of such pulse peaks
as shown in FIG. 4 is adequately represented by three straight line
segments I, II and III. These segments have the usual line equation
Y=mx+b, where Y is the amplitude, m is the slope, x is the cuff
pressure (PT) and b is the Y axis intercept for each line, i.e.
B.sub.1, B.sub.2 and B.sub.3. The processor 20 has an input from
pressure transducer 23 and in this way the processor obtains information
as to the current cuff pressure (PT). Using this information along
with the preset values for systolic pressure, diastolic pressure
and MAP, the processor computes the required pulse amplitude according
to the equations for the lines shown in FIG. 4. With this information
an amplitude control section of the processor generates a series
of digital signals that specify the shape and amplitude of the pulse.
Thus, pulse generator 28 can be a digital-to-analog converter whose
output is used to drive the electromagnetic coil 34 which operates
diaphragm 36 to generate the pressure pulses. The larger the output
of generator 28, the greater is the deflection of diaphragm 36 and
the resulting pressure pulse.
The rate at which the pulses are generated is determined by the
heart pulse rate selected by the user. This input is directed to
a clock control portion of the processor which selects a division
ratio of the basic processor clock from a heart pulse counter and
uses it to time the operation of the amplitude control section.
This timing signal from the heart pulse counter is also delivered
to the valve control circuit 26 along with a signal from pressure
transducer 23. In the valve control circuit, the timing signal is
not only amplified to a level sufficient to drive the valve closed,
it is logically combined with the signal from pressure transducer
23 so that valve 21 is automatically opened when there is a change
in pressure of, e.g., 5 mmHg, indicating a controlled change in
cuff pressure caused by the monitor. This is done to insure that
the pressure across the diaphragm is balanced and that the pressure
applied to the monitor is always at the level of the cuff pressure,
even if a pulse is being produced. The valve control circuit, however,
is set so that a change in pressure of less than 3 mmHg, for example,
representing the output pulse from the pressure chamber, will not
cause the valve to open.
A flow chart of the operation of processor 20 is shown in FIG.
5. In the situation in which processor 20 is a stored program microprocessor,
as opposed to separate sequential logic blocks as generally indicated
in FIG. 3, the flow chart of FIG. 5 would represent the program
for that microprocessor stored in a memory circuit 22.
When operating, the processor first initializes its circuits as
shown by function block 40 in FIG. 5. Then the processor checks
in decision block 41 to see if the cuff pressure signal PT derived
from transducer 23 is above 10 mmHg. If it is not, the processor
continues to loop until this condition changes. As soon as the cuff
pressure increases above 10 mmHg, indicating that the pressure monitor
has begun to take a reading by inflating the cuff, the processor
moves on to decision block 42 where it checks to see if a heart
rate signal, HPCNT, has been set. If no heart rate level has been
set, the processor returns to its initial state. If it has been
set, the processor moves to decision block or state 43 where it
determines if the cuff pressure is above systolic levels. Suitable
programming can be included to assure that this decision is not
made until the cuff pressure has reached its maximum level and has
begun to decrease, i.e. a delay may be added to the decision level
to assure that the decision in block 43 is not made during inflation
of the cuff, shown as section A of the curve in FIG. 2.
When the cuff pressure is above systolic, the processor generates
pulse information for pulses whose amplitudes are related to the
line equation identified by I in FIG. 4. Thus, as the pressure decreases
in the range above systolic pressure, the pulse amplitudes increase
in value. The curve I in FIG. 4 is selected to be zero at a pulse
level of 255, but this level was picked arbitrarily based on the
capacity of storage elements used in the processor. The amplitude
of the curve at systolic pressure is some preselected fraction F.sub.1
of the maximum at MAP. Thus, in the range of pressures above systolic,
the value for the slope of the line, i.e. m, as well as the zero
axis crossing b are set in logic block 44 and these values are substituted
in the line equation in decision block 48. The processor then converts
the line equation into the generation of pulses having an amplitude
according to the line equation in logic block 49, after which it
loops through the blocks 41-44, 48 and 49 again until the applied
cuff pressure is below systolic.
When the cuff pressure is below systolic, the processor moves from
decision block 43 to decision block 45, which is operative to direct
the processor to logic block 46 when the applied pressure is between
systolic pressure and MAP. The pulse amplitudes in this region are
represented by the line equation II in FIG. 4. The values for the
equation of this line, i.e., the slope m and zero crossing B.sub.1,
are generated in block 46 and are applied to equation block 48 which
in turn causes pulse generator block 49 to generate another series
of pulses which increase a larger amount for each pressure decrement,
than the pulses for pressures above systolic. This process continues
until the cuff pressure is equivalent to MAP. At this point, the
pulse amplitudes begin to decrease according to equation III in
FIG. 4. The values for this equation are derived from logic block
47 in the flow chart. Thus, by knowing the selected systolic, MAP
and diastolic pressures, the processor is able to duplicate the
variation in pulse amplitudes of a living test subject.
The details of the pressure chamber itself are shown in FIGS. 6
and 7. In these figures, it can be seen that the pressure chamber
is a generally cylindrical housing about three inches in diameter
and made of any suitable material, for example clear plastic. Located
in the middle of the housing is a diaphragm 36 which typically can
be a conventional audio speaker with a driving coil 34. For operation
in the pressure chamber, an audio speaker that has a large magnet
and a small cone support with a round shape, should be selected.
The speaker should have its frame removed and all of the backing,
except for the spider cloth net. Then, a latex coating should be
applied to the spider.
The output of generator 28 is connected to the driving coil 34
of the speaker via leads 38, thereby causing the diaphragm 36 to
flex in response to signals from the generator. This flexing of
the diaphragm produces pressure pulses at the output 33 of the pressure
chamber. As previously explained, the input 32 to the pressure chamber
has the cuff pressure applied to it. Thus, the chamber, under the
control of processor 20 and with the assistance of valve 21, is
capable of producing pressure pulsations similar to those produced
by the arm of a subject under going a test for blood pressure readings
according to the oscillometric method.
While the present invention has been particularly shown and described
with reference to a preferred embodiment thereof, it will be understood
by those skilled in the art that various changes in form and details
may be made therein without departing from the spirit and scope
of the invention. |