Abstrict
An automatic blood pressure monitor having a blood pressure cuff,
an air pump applying pressurized air to the cuff, an air valve selectively
venting the cuff, and a pressure transducer generating an output
signal having a D.C. component indicative of the steady-state pressure
in the cuff, and an A.C. component indicative of oscillometric pulses.
The pressure transducer output is amplified and applied to an analog-to-digital
converter which generates a digital word indicative of the steady-state
cuff pressure. The amplified pressure transducer output is also
applied to one input of a differential amplifier having an output
that is applied to an analog-to-digital converter having a limited
operating range. The other input of the amplifier receives an offset
signal from a voltage source to offset the amplified pressure transducer
output so that the output of the differential amplifier will not
exceed the operating range of the analog-to-digital converter.
Claims
I claim:
1. An improved method for use in a blood pressure monitor of the
type having a blood pressure cuff, an air pump in fluid communication
with said cuff to direct pressurized air into said cuff, an air
valve in fluid communication with said cuff to selectively vent
said cuff to atmosphere, a pressure transducer in fluid communication
with said cuff generating a pressure signal indicative of the steady
state fluid pressure in said cuff corresponding to an occlusive
pressure exerted by said cuff on an artery and a transient fluid
pressure in said cuff indicative of the amplitude of an oscillometric
waveform, a processor in electrical communication with said air
pump and said air valve for selectively energizing said air pump
or valve to pressurize or depressurize said cuff, respectively,
said method allowing said oscillometric waveform to be sampled despite
relatively large changes in the steady state fluid pressure in said
cuff, said method comprising:
setting the steady state fluid pressure in said cuff to a predetermined
value;
sampling the pressure signal output by said pressure transducer
resulting from the steady state fluid pressure in said cuff, and
generating an offset signal corresponding thereto;
subtracting said offset signal from the pressure signal output
by said pressure transducer, thereby generating an offset pressure
signal;
sampling said offset pressure signal;
generating a plurality of digital numbers corresponding to a respective
plurality of samples of said offset pressure signal; and
analyzing said digital numbers to determine blood pressure.
2. The method of claim 1 further including the step of adjusting
magnitude of said offset signal by the steps of:
comparing each digital number to a predetermined range of said
digital numbers; and
adjusting said offset signal in the event that said digital number
is outside a predetermined range of said digital numbers so that
said offset pressure signal corresponds to a value that is closer
to the center of said predetermined range of said digital numbers.
3. An improved method for use in a blood pressure monitor of the
type having a blood pressure cuff, an air pump in fluid communication
with said cuff to direct pressurized air into said cuff, an air
valve in fluid communication with said cuff to selectively vent
said cuff to atmosphere, a pressure transducer in fluid communication
with said cuff generating a pressure signal indicative of the steady
state fluid pressure in said cuff corresponding to an occlusive
pressure exerted by said cuff on an artery and a transient fluid
pressure in said cuff indicative of the amplitude of an oscillometric
waveform, a processor in electrical communication with said air
pump and said air valve for selectively energizing said air pump
or valve to pressurize or depressurize said cuff, respectively,
said method allowing said oscillometric waveform to be sampled despite
relatively large changes in the steady state fluid pressure in said
cuff, said method comprising:
setting the steady state fluid pressure in said cuff to a predetermined
value;
sampling the pressure signal output by said pressure transducer
resulting from the steady state fluid pressure in said cuff, and
generating an initial offset signal corresponding thereto;
subtracting said initial offset signal from the pressure signal
output by said pressure transducer, thereby generating an initial
offset pressure signal;
sampling said initial offset pressure signal, and generating an
initial digital number corresponding thereto;
determining an initial digital offset value;
adding said initial digital offset value to said initial digital
number, thereby generating an initial oscillometric value;
continuing to subtract said initial offset signal from the pressure
signal output by said pressure transducer, thereby generating a
continuous offset pressure signal;
periodically sampling said continuous offset pressure signal, and
generating a respective digital numbers corresponding thereto;
adjusting the value of said initial offset signal if said offset
signal falls outside a predetermined range;
after each adjustment of said initial offset signal, calculating
a new digital offset value by summing the prior digital offset value
to the difference between two of said digital numbers taken at successive
times;
adding said new digital offset value to each of said digital numbers,
thereby generating a plurality of oscillometric values;
analyzing said oscillometric values to determine blood pressure.
4. An improved method for use in a blood pressure monitor of the
type having a blood pressure cuff, an air pump in fluid communication
with said cuff to direct pressurized air into said cuff, an air
valve in fluid communication with said cuff to selectively vent
said cuff to atmosphere, a pressure transducer in fluid communication
with said cuff generating a pressure signal indicative of the steady
state fluid pressure in said cuff corresponding to an occlusive
pressure exerted by said cuff on an artery and a transient fluid
pressure in said cuff indicative of the amplitude of an oscillometric
waveform, a processor in electrical communication with said air
pump and said air valve for selectively energizing said air pump
or valve to pressurize or depressurize said cuff, respectively,
said method allowing said oscillometric waveform to be sampled despite
relatively large changes in the steady state fluid pressure in said
cuff, said method comprising:
setting the steady state fluid pressure in said cuff to a predetermined
value;
sampling the pressure signal output by said pressure transducer
resulting from variations in the fluid pressure in said cuff caused
by oscillometric pressure pulses, and generating a pressure signal
corresponding thereto;
offsetting said pressure signal by an offset value, thereby generating
an offset pressure signal;
periodically sampling said offset pressure signal and generating
respective digital numbers corresponding thereto;
summing each of said digital numbers with an offset value, thereby
generating respective oscillometric values;
adjusting the magnitude of said offset pressure signal if said
offset value falls outside a predetermined range;
after each adjustment of said offset value, calculating a new offset
value by summing the prior offset value with the difference between
two of said digital numbers taken at successive times;
analyzing said oscillometric values to determine blood pressure.
5. A blood pressure monitor, comprising:
a blood pressure cuff;
a pneumatic controller in fluid communication with said cuff to
selectively vary a fluid pressure in said cuff responsive to a pneumatic
control signal;
a pressure transducer in fluid communication with said cuff, said
pressure transducer generating a pressure signal indicative of the
fluid pressure in said cuff corresponding to an occlusive pressure
exerted by said cuff on an artery and a transient fluid pressure
in said cuff indicative of the amplitude of an oscillometric waveform;
voltage generating means for generating an offset voltage corresponding
to a steady state pressure in said cuff;
offset means coupled to said voltage generating means and to said
pressure transducer for generating an offset pressure signal corresponding
to the difference between the pressure signal output by said pressure
transducer and said offset voltage;
analog-to-digital converter means having a predetermined operating
range coupled to said offset means, said analog-to-digital converter
means sampling said offset pressure signal, thereby generating a
plurality of digital numbers corresponding to a respective plurality
of samples of said offset pressure signal; and
processing means in electrical communication with said pneumatic
controller for generating said pneumatic control signal to cause
said pneumatic controller to vary the fluid pressure in said cuff,
said processing means being connected to an output of said analog-to-digital
converter means to receive said digital numbers and analyze said
samples to determine the blood pressure in an artery beneath said
cuff whereby oscillometric pressure pulses imparted to said cuff
can be sampled despite relatively large changes in the steady state
fluid pressure in said cuff.
6. The blood pressure monitor of claim 5 wherein said offset means
amplifies changes in the magnitude of said pressure signal, and
wherein said analog-to-digital converter means receives said pressure
signal as well as said offset pressure signal, said processing means
causing said voltage generating means to initially generate said
offset voltage from said pressure signal.
7. The blood pressure monitor of claim 5 wherein said processing
means further includes means for adjusting the magnitude of said
pressure offset signal by comparing each sample of said offset pressure
signal to the operating range of said analog-to-digital converter
means, and then adjusting said pressure offset signal in the event
that a sample of said offset pressure signal is outside a predetermined
portion of the operating range of said analog-to-digital converter
means so that said offset pressure signal is closer to the center
of the operating range of said analog-to-digital converter means.
8. The blood pressure monitor of claim 5 wherein said processing
means includes means for adjusting the magnitude of said offset
voltage generated by said voltage generating means if said offset
voltage falls outside a predetermined portion of the operating range
of said analog-to-digital converter means.
9. The blood pressure monitor of claim 8 wherein said processing
means includes means for calculating oscillometric signal samples
from said digital numbers, comprising:
means for summing each of said digital numbers with an offset value,
thereby generating respective oscillometric signal samples; and
after each adjustment of said offset value, calculating a new offset
value by summing the prior offset value with the difference between
two of said digital numbers taken at successive times.
Description TECHNICAL FIELD
This invention relates to automatic blood pressure monitors, and
more particularly, to an automatic blood pressure monitor that minimizes
the loss of oscillometric data caused by changes in the cuff pressure.
BACKGROUND OF THE INVENTION
Automatic blood pressure monitors are commonly used to periodically
measure the blood pressure of a patient. In most automatic blood
pressure monitors, a pressure cuff is attached to a patient's arm
over the brachial artery. The cuff is first pressurized with an
applied pressure that is high enough to substantially occlude the
brachial artery. The cuff pressure is then gradually reduced, either
continuously or in increments. As the pressure is reduced to systolic
pressure, the flow of blood through the brachial artery beneath
the cuff increases substantially.
When the blood flows through the brachial artery following each
contraction of the heart, it imparts a pulsatile movement to the
wall of the artery. This pulsatile movement is coupled to a blood
pressure cuff extending over the artery as minute changes in the
cuff pressure, which are known as oscillometric pulses. Automatic
blood pressure monitors employing the oscillometric method measure
and record the amplitude of the oscillometric pulses at a number
of cuff pressures. After the blood pressure measurement had been
completed, a table contains the oscillometric pulse amplitudes recorded
at each cuff pressure.
In theory, the systolic, diastolic, and mean arterial blood pressures
can then be determined from the values in the table using theoretical
and/or empirical definitions of these parameters as a function of
the amplitudes of these oscillometric pulses. However, blood pressure
measurements are often adversely affected by artifact, generally
produced by patient movement. Motion-induced artifact can substantially
alter the measured amplitude of oscillometric pulses, thus introducing
inaccuracies in the measurement of the patient's blood pressure.
Another problem associated with automatic blood pressure monitors
using the oscillometric method occurs when the cuff pressure is
changed from one pressure to another, particularly if the pressure
is incrementally changed at a relatively high rate. The cuff pressure
consists of two components, a relatively constant, or "DC",
component and a relatively variable, or "AC", component.
The relatively constant component defines the occlusive force of
the blood pressure cuff, while the relatively variable component
is produced by the oscillometric pressure pulses following each
contraction of the heart.
The cuff pressure and the oscillometric pulses are typically sensed
by a pressure transducer of conventional variety. The pressure transducer
outputs a signal having two components which, like the pressure
in the cuff itself, consist of a D.C. or relatively constant component
indicative of the occlusive force, and an A.C. or relatively variable
component corresponding to the oscillometric pulses imparted to
the cuff. The relatively constant D.C. component can be used as
an indication of occlusive force on the artery beneath the cuff,
while the relatively variable A.C. component represents the oscillometric
signal.
The magnitude of the relatively constant pressure is very much
greater than the magnitude of the pressure changes corresponding
to the oscillometric pulses. As a result, the amplitude of the D.C.
component in the transducer output signal, as well as the change
in the D.C. component when the cuff pressure is changed, is very
much greater than the amplitude of the A.C. component in the transducer
output signal. When the cuff pressure is changed, a transient A.C.
component is generated that is so much larger than the normal A.C.
component resulting from the oscillometric pulses that the circuits
which respond to the oscillometric pulses are overloaded or are
otherwise unable to process oscillometric pulses following a change
in cuff pressure. A similar phenomena can occur as a result of motion
artifact or from pneumatic leaks in the cuff or components in fluid
communication with the cuff. As a result, oscillometric data are
lost for a period of time following a cuff pressure change, thus
prolonging the time needed to obtain a blood pressure measurement.
SUMMARY OF THE INVENTION
The primary object of the invention it to provide an automatic
blood pressure monitor that is able to obtain oscillometric pulse
data immediately following a relatively large change in cuff pressure.
This and other objects of the invention are provided by a blood
pressure monitor having such conventional components as a blood
pressure cuff, an air pump applying pressurized air into the cuff,
an air valve selectively venting the cuff to atmosphere, and a pressure
transducer generating a pressure signal indicative of both the steady
state fluid pressure in the cuff corresponding to an occlusive pressure
exerted by the cuff, and a transient fluid pressure in the cuff
indicative of the amplitude of an oscillometric waveform. The pressure
transducer generates an output signal having a D.C. component corresponding
to the steady state fluid pressure in the cuff, and an A.C. component
indicative of the amplitude of an oscillometric waveform. The monitor
includes a voltage source generating an offset voltage corresponding
to the D.C. component of the pressure signal output by the pressure
transducer. An offset generator amplifies the output of the pressure
transducer and produces an offset pressure signal corresponding
to the difference between the offset voltage and the amplified pressure
signal. The offset pressure signal produced by the offset generator
is sampled by an analog-to-digital converter having a predetermined
operating range, thereby generating a plurality of digital words
corresponding to a respective plurality of samples of the amplified
offset pressure signal. These digital words are read by a processor
which analyzes the samples to determine the blood pressure in an
artery beneath the cuff. As a result, oscillometric pressure pulses
imparted to the cuff can be sampled despite relatively large changes
in the steady state fluid pressure in the cuff.
The processor preferably adjusts the magnitude of the offset signal
by first comparing each sample of the amplified offset pressure
signal to the operating range of the analog-to-digital converter.
The processor then adjusts the offset signal in the event that a
sample of the amplified offset pressure signal is outside a predetermined
portion of the operating range of the analog-to-digital converter
so that the amplified offset pressure signal is closer to the center
of the operating range of the analog-to-digital converter.
The processing means preferably generates samples of an oscillometric
waveform by summing the output of the analog-to-digital converter
with a digital offset value to obtain each sample. The digital offset
value is adjusted each time the value of the offset voltage is adjusted
by summing the previous digital offset value with the difference
in the output of the analog-to-digital converter before and after
the offset voltage adjustment .
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a presently preferred embodiment of
an automatic blood pressure monitor having means for minimizing
the loss of data during changes in cuff pressure.
FIG. 2 is a flow chart of a main computer program used to program
a microprocessor used in the automatic blood pressure monitor of
FIG. 1.
FIG. 3 is a flow chart of one embodiment of an initial offset subroutine
called by the main computer program shown in FIG. 2.
FIGS. 4A and 4B are diagrams showing waveforms obtained in the
presently preferred embodiment of an automatic blood pressure monitor
of FIG. 1.
FIG. 5 is a flow chart of one embodiment of a subroutine called
by the main computer program shown in FIG. 2 for adjusting the initial
offset set by the subroutine of FIG. 3 and for collecting data samples.
DETAILED DESCRIPTION OF THE INVENTION
One embodiment of an automatic blood pressure monitor 10 using
the inventive technique to minimizes the loss of oscillometric data
during changes in cuff pressure is illustrated in FIG. 1. The monitor
10 is composed of a number of hardware components, all of which
are conventional. The monitor 10 includes a conventional blood pressure
cuff 12 in fluid communication through tubes 14 with a conventional
pump 16, a conventional solenoid valve 18, and a conventional pressure
transducer 20. The pump 16 and solenoid valve 18 are electrically
connected to respective output ports of a conventional microprocessor
22 which controls the operation of the pump 16 and solenoid valve
18 in a conventional manner.
During the operation of the automatic blood pressure measuring
system, the pump 16 inflates the blood pressure cuff 12 to a pressure
that is greater than the expected systolic pressure, as indicated
by the pressure transducer 20. The solenoid valve 18 is then opened,
usually for a predetermined period, although it may be continuously
open to allow a slight leakage of air from the blood pressure cuff
12. However, the solenoid valve 18 normally allows air to escape
from the cuff 12 fairly rapidly in relatively small increments.
As the pressure in the cuff 12 is reduced, either gradually or incrementally,
the pressure in the cuff 12 is measured by the pressure transducer
20.
As mentioned above, the pressure in the blood pressure cuff 12
consists of two components, namely, a relatively constant, or "DC",
component and a relatively variable, or "AC", component.
The relatively constant component defines the occlusive force of
the blood pressure cuff 12. The relatively variable component is
produced by the minute change in the pressure of the cuff 12 following
each contraction of the heart. Thus, the relatively constant DC
component of the pressure in the cuff can be used as an indication
of cuff pressure, while the relatively variable AC component of
the pressure in the cuff 12 represents the oscillometric signal.
A signal from the pressure transducer 20 is applied to a conventional
analog-to-digital ["A/D"] converter 24 through a first
amplifier 23 and a differential amplifier 26. In the presently preferred
embodiment of the invention, the amplifier 23 generates a signal
that varies between 0-5 volts as the pressure in the cuff 12 varies
between 0-300 mm. of mercury. Thus, the signal at the output of
the amplifier 23 has a slope of 60 mm./volt. The output of the amplifier
23 is used as a "cuff pressure signal" to provide an indication
of the steady state pressure in the cuff 12. In the presently preferred
embodiment of the invention, the differential amplifier 26 boosts
the output of the pressure transducer 20 so that its output varies
between 0-5 volts as the pressure in the cuff 12 varies over a range
of 7 mm. of mercury. Thus, the signal at the output of the differential
amplifier 26 has a slope of 1.4 mm./volt. The output of the differential
amplifier 26 is used as the "oscillometric signal" from
which that patient's blood pressure is determined.
The A/D converter 24 has two input channels for alternately digitizing
either the output of the pressure transducer 20 or the output of
the differential amplifier 26, and alternately applying respective
digital words to the microprocessor 22. The A/D converter 24 used
in the presently preferred embodiment of the invention outputs a
10 bit digital word which can divide the analog input into 1024
values. The resolution of the signal at the output of the A/D converter
24 is thus about 293 .mu.m. (i.e. 300 mm./1024) for the cuff pressure
signal and 6.8 .mu.m (i.e. 7 mm./1024) for the oscillometric signal.
In many modern blood pressure devices, the A/D converter 24 is
actually contained on the microprocessor chip. Whether the A/D converter
24 is on the microprocessor chip or whether it is located on a separate
chip, the microprocessor 22 has access to a digitized signals indicative
of the cuff pressure and the oscillometric signal. The microprocessor
22 thus extracts the two components of the pressure in the blood
pressure cuff namely (a) the pressure within the cuff and (b) the
minute change in the pressure of the cuff following each contraction
of the heart. Alternatively, separation of the cuff pressure into
these two components can be accomplished by external hardware filters,
as is quite common in older automatic blood pressure monitors. Whether
the separation is accomplished by external hardware filters or by
an algorithm internal to the microprocessor 22 is not important
for the current invention.
During a blood pressure measurement, the pressure in the cuff 12
(FIG. 1) is typically changed in pressure increments that are well
in excess of 10 mm. of mercury. It will be apparent that a change
in 10 mm should cause the output of the differential amplifier to
change by over 7 volts (i.e., [5 volts/7 mm.]* 10 mm.). Yet the
maximum output of the differential amplifier 26 is 5 volts. It is
thus apparent that the differential amplifier 26 and A/D converter
24 would be incapable of processing oscillometic pulses after the
pressure in the cuff was changed by pressure increments in common
use. The amplifier 26 and A/D converter 24 could be made able to
process oscillometric pulses after a 10 mm. change in cuff pressure
if the gain of the differential amplifier 26 was reduced. However,
reducing the gain of the differential amplifier would reduce the
resolution with which the A/D converter 24 could process oscillometric
pulses. The primary purpose of the invention is to be able to process
oscillometric pulses following relatively large changes in cuff
pressure without degrading the resolution with which the pulses
are processed.
With continued reference to FIG. 1, the blood pressure monitor
10 also includes a software controlled voltage source 32 which supplies
an offsetting voltage to the differential amplifier 26. As mentioned
above, the output of the pressure transducer 20 is applied to the
other input of the differential amplifier 26. The voltage output
by the voltage source 32 offsets the voltage output by the pressure
transducer 20 so that the voltage output by the differential amplifier
26 is preferably near the middle of its range (i.e., 2.5 volts,
in the example given) but in any case is between the end points
of its range (i.e., 0 volts and 5 volts, respectively, in the example
given). The voltage that is output of the voltage source 32 is thus
very close to the D.C. component of the pressure transducer 20.
A variety of conventional circuits may be used to implement the
software controlled voltage source 32. In the presently preferred
embodiment of the invention, the voltage source 32 is a digital-to-analog
converter generating a voltage in the 0-5 volt range responsive
to an 8 bit digital word. Thus, the voltage source 32 can generate
256 different analog voltages between 0 and 5 volts thereby providing
a resolution of about 19.5 mv. (i.e., 5 v./256). The presently preferred
embodiment of the invention would thus allow the cuff pressure to
be changed in about 20 mm. increments and still allow the voltage
source 32 to adjust the output of the amplifier 26 within the operating
range of the A/D converter 24.
As also mentioned above, the microprocessor 22 is of conventional
variety and, as is typical with such devices, is connected to a
random access memory 34 which is used for the storage of data, and
to either random access memory or read-only memory 36 which contains
the software for operating the microprocessor 22. Operator controls
38, such as a keyboard or buttons, are also connected to the microprocessor
22.
The microprocessor 22 is controlled by software that is stored
as a series of program instructions in the memory 36. A flow chart
from which object code can be easily and quickly written by one
skilled in the art is illustrated in FIGS. 2-4. With reference to
FIG. 2, a main program starts at 40 either through an operator command,
automatically at power-up, or when called by another program stored
in the memory 36. As is conventional with microprocessor-based systems,
the microprocessor 22 (FIG. 1) is initialized at 42 to set up the
software for subsequent processing, such as, for example, by establishing
tables that subsequently will contain data, by setting flags, and
by setting variables to known values. The program then checks at
44 to determine if enough oscillometric pulse amplitude data have
been collected in oscillometric data tables for evaluation.
The decision block 44 is first encountered prior to obtaining any
oscillometric pulse amplitude data. Thus, when the program initially
encounters decision block 44, the tables will not contain enough
data to be evaluated. As a result, the program will branch to 46
to calculate a target value for the pressure in the blood pressure
cuff 12 (FIG. 1). The target pressure for the cuff 12 will, of course,
be in excess of the cuff pressure before the measurement is started.
The microprocessor 22 then energizes the pump 16 (FIG. 1) at 48
while the output of the pressure transducer 20 is digitized by the
A/D converter 24. The frequency of the digitization is controlled
in a conventional manner by a conventional clock driven interrupt
routine (not shown). A conventional filtering algorithm may be applied
to the digitized signal in order to eliminate random noise from
the resulting cuff pressure. The microprocessor 22 continues to
energize the pump 16 at 48 until the cuff pressure is equal to the
target pressure. On subsequent passes through steps 46 and 48, the
target pressure calculated at 46 will be lower than the pressure
in the cuff 12, so that the microprocessor 22 will energize the
solenoid valve 18 at 48 to reduce the pressure in the cuff 12 to
the target pressure.
Upon reaching the desired pressure, the main program calls an initial
offset voltage subroutine at step 49. The initial offset voltage
subroutine software determines the initial offset voltage, i.e.,
the initial output from the software controlled voltage source 32.
The initial offset voltage subroutine is more fully described below
with reference to FIG. 3.
After return from the initial offset voltage subroutine at step
49, the main program calls an offset voltage adjust and data acquisition
subroutine at step 50 which is fully explained with reference to
FIG. 5. Briefly, the offset voltage adjust and data acquisition
subroutine provides further adjustments of the offset voltage applied
to the differential amplifier 26, and it determines the oscillometic
signal at the output of the A/D converter 24. The offset voltage
is adjusted from the initial offset voltage by sampling the oscillometric
signal at the output of the differential amplifier 26. If the oscillometric
signal is near one of the boundaries of the input range of the A/D
converter 24 as discussed above, an adjustment is made to the offset
voltage generated by the software controlled voltage source 32.
The offset voltage adjust and data acquisition subroutine also determines
the oscillometic signal at the output of the A/D converter 24 by
periodically sampling the output of the A/D converter 24 and compensating
for the effect of the offset voltage on the output of the A/D converter
24 so that the calculated value of the oscillometric signal will
be independent of the value of the offset voltage. As mentioned
above, these functions of the offset voltage adjust and data acquisition
subroutine are fully explained below with reference to FIG. 5.
When sufficient data are collected and averaged at a given cuff
pressure, control returns from the offset voltage adjust and data
acquisition subroutine at 50, and the main program then returns
to 44 where a decision is once again made as to whether or not enough
data have been collected to evaluate the tables and determine a
blood pressure. The answer once again will be negative on the second
pass though step 44. Therefore, the program will loop through 44,
46, 48, and 50 until sufficient data are collected. Each time the
program proceeds through 46 and 48, the pressure in the cuff is
decreased, usually in fixed steps, and the new offset voltage is
acquired.
The program continues to loop through 44, 46, 48, and 50 (FIG.
2) until at a determination is made at step 44 that there are sufficient
data in the oscillometric tables to permit evaluation of the table
so that a blood pressure value can be determined. The program then
branches to 52 where the tables are evaluated. At step 54, the table
evaluation is judged to be either complete or incomplete. If incomplete,
the program branches from step 54 to step 46 in order to collect
more data.
If the table evaluation at is judged to be complete at step 54,
then the program branches to step 56 where the results of the evaluation
are stored. The table evaluation results in either (a) a blood pressure,
or (b) an indication that there were too many artifacts to obtain
a blood pressure. Upon completion of a measurement attempt, the
program can also adjust parameters, such as screening and trigger
levels, which may aid in collection of the next blood pressure.
In any event, after the program displays and stores the results
at step 56, it exits to the calling procedure at 58.
FIG. 3 is a flow chart of a subroutine for the initial offset voltage
acquisition subroutine. At the end of every pump or bleed at step
48, the main program calls the initial offset acquisition subroutine
which it entered at step 70. The subroutine then waits at step 72
for a short time to allow pneumatic settling, i.e. to allow pressure
transients resulting from the relatively large change in cuff pressure
to decay. After the delay, the subroutine enters step 74 in which
the microprocessor 22 (FIG. 1) causes the A/D converter 24 to digitize
the cuff pressure signal at the output of the pressure transducer
20, thereby determining the D.C. level of the output of the pressure
transducer 20 after being boosted by the amplifier 23.
As mentioned above, in order to offset the oscillometric signal
at the output of the differential amplifier 26 to near the mid-range
of the A/D converter 24, the D.C. value of the pressure transducer
output must, in effect, be subtracted from the be pressure transducer
output. The digitized output of the differential amplifier 26 obtained
at step 74 is thus used in step 76 to set an approximate initial
value of the offset voltage to be generated by the software controlled
voltage source 32. Accordingly, at step 76 the microprocessor 22
(FIG. 1) outputs a digital word to the voltage source 32 to cause
the voltage source 32 to generate a voltage having a magnitude approximately
equal to the D.C. value of the cuff pressure signal at the output
of the pressure transducer 20, which was sampled at step 74. Since
the differential amplifier 26 offsets the output of the pressure
transducer 20 by the output of the voltage source 32, the resulting
output of the differential amplifier 26 should be within the operating
range of the A/D converter 24. After the initial offset voltage
has been determined and the software controlled voltage source adjusted
accordingly, the subroutine loops at 78 to allow the software controlled
voltage source 32 to stabilize.
The subroutine then digitizes the output of the differential amplifier
at 80, i.e. after the amplified output of the pressure transducer
20 has been offset by the initial offset voltage from the voltage
source 32. The digitized value is then compared to a threshold at
step 82. The threshold used at step 82 is preferably near the upper
limit of the operating range of the A/D converter 24. In the presently
preferred embodiment of the invention, this upper threshold is set
at 3/4 of full scale value, i.e., a digital output value of 768
which is three quarters of 1024 (1024=2.sup.10) corresponding to
an input of 3.75 volts. If the output of the differential amplifier
26 is above this threshold, the subroutine branches to 84. The subroutine
then causes the microprocessor 22 to adjust the initial offset voltage
generated by the software controlled voltage source 32 so that the
output of the voltage source 32 is closer to the amplified output
of the pressure transducer 20. The offset voltage generated by the
voltage source 32 will therefore bring the output of the differential
amplifier 26 closer toward the center of the operating range of
the A/D converter 24. The subroutine then loops back to step 78.
If the output of the differential amplifier 26 was found to be
below the upper threshold at step 82, the subroutine proceeds to
86 where the digitized output of the differential amplifier 26 is
compared to a threshold that is near the lower limit of the operating
range of the A/D converter 24. In the presently preferred embodiment
of the invention, this lower threshold is set at 1/4 of full scale
value, i.e., a digital output value of 256 which is one quarter
of 1026 corresponding to an input of 1.25 volts. If the output of
the differential amplifier 26 is below this lower threshold, the
subroutine branches to 88. The subroutine then causes the microprocessor
22 to adjust the offset voltage generated by the software controlled
voltage source 32 so that the output of the voltage source 32 is
closer to the amplified output of the pressure transducer 20. The
adjusted offset voltage from the voltage source 32 will therefore
bring the output of the differential amplifier 26 closer toward
the center of the operating range of the A/D converter 24. The subroutine
then loops back to step 78.
Either initially or after the output of the voltage source 32 has
been adjusted as described above, the program will determine at
steps 82 and 86 that the output of the differential amplifier 26
is below the upper threshold and above the lower threshold, respectively.
The subroutine will then proceed to step 90 to calculate a digital
offset value that compensates for the offset voltage so that the
value of the oscillometric signal used to determine blood pressure
will be insensitive to the value of the offset voltage. The rational
for the digital offset value is best understood with references
to FIGS. 4A and 4B.
FIG. 4A shows the output of the differential amplifier 26 as a
function of time. At point A, an adjustment is made to the offset
voltage, thereby causing a step change in the output of the differential
amplifier 26 that is applied to the A/D converter 24 from point
A to point B. If the microprocessor 22 read the output of the A/D
converter 24 as being truly indicative of the oscillometric signal,
then the microprocessor 22 would erroneously determine that the
oscillometric signal changed suddenly at point A. Thus, the microprocessor
22 must be able to compensate for or cancel out the changes in the
output of the A/D converter 24 that are due to changes in the offset
voltage in order to determine the true oscillometric signal. As
a result, the oscillometric signal reconstructed by the microprocessor,
which is shown in FIG. 4B, has the same value at A (before the offset
voltage adjustment) as it has at point B (after the offset voltage
adjustment).
The above-described compensation is accomplished by the microprocessor
22 initially setting a digital offset value to an arbitrary value,
such as 4000 hex which corresponds to 16,384 decimal. If, for example,
the output of the A/D converter 24 is 810 at point A, an oscillometric
value calculated by adding 810 to the initial digital offset of
16,384 is 17,194. However, the 810 output of the A/D converter 24
is above the 768 upper threshold used in the above example. As a
result, the offset voltage output by the software controlled voltage
source 32 would be increased to reduce the input to the A/D converter
24. If the offset voltage is adjusted down to point B, the output
of the A/D converter 24 might now be, for example, 600. The difference
between the outputs of the A/D converter at points A and B would
thus be 168 (i.e., 768 less 600). The microprocessor 22 then adjusts
the digital offset by this amount so that the new digital offset
is now 16,594 decimal (i.e., 16,384+810-600). However, the new value
of the A/D converter output at point B is, as mentioned above, 600
which, when added to the new digital offset of 16,594, is 17,194.
This new oscillometric value of 17,194 at point B is the same as
the old oscillometric value of 17,194 obtained by summing the initial
digital offset of 16,384 with the 810 output of the A/D converter
24 at point A. Thus, the microprocessor 22 is able to calculate
the same oscillometric value at points A and B despite the abrupt
change in the signal applied to the A/D converter 24.
FIG. 5 is a flow chart of a subroutine called by the main program
at step 50 (FIG. 2) to adjust the offset voltage from the initial
value, to collect data samples of the oscillometric signal at the
output of differential amplifier 26, and to calculate oscillometric
values in order to reconstruct the true oscillometric signal. At
step 100, the microprocessor 22 causes the digital-to-analog converter
24 to generate a digitized sample of the output of the differential
amplifier 26. An oscillometric value corresponding to the true oscillometric
signal when the sample is taken is then calculated at 102 by adding
the digital word at the output of the A/D converter 24 to the initial
digital offset value calculated by the subroutine explained above
with reference to FIG. 3. The result of this addition is stored
in the random access memory 34 for use by the analysis portion of
the program in step 50.
The subroutine then proceeds to step 104 where the output of the
A/D converter 24 obtained at 100 is compared to an upper threshold
that is near the upper limit of the operating range of the A/D converter
24. If the output of the analog-to-digital converter 24 is above
this threshold, the subroutine branches to 106 where an adjustment
is made to the software controlled voltage source 32 so that the
output of the voltage source 32 is closer to the output of the pressure
transducer 20. The adjusted output from the voltage source 32 will
therefore bring the output of the differential amplifier 26 beneath
the upper threshold and closer toward the center of the operating
range of the analog to digital converter 24. The subroutine then
proceeds to step 108 which is discussed below.
If the output of the differential amplifier 26 was found to be
below the upper threshold at step 102, the subroutine proceeds to
110 where the digitized output of the differential amplifier 26
is compared to a lower threshold that is near the lower limit (i.e.
negative voltage limit) of the operating range of the A/D converter
24. If the output of the differential amplifier 26 is below the
lower threshold, the subroutine branches to 112. The subroutine
then causes the microprocessor 22 to adjust the output of the software
controlled voltage source 32 so that the output of the voltage source
32 is above the lower threshold and closer to the output of the
pressure transducer 20. The adjusted output from the voltage source
32 will therefore bring the output of the differential amplifier
26 closer toward the center of the operating range of the analog
to digital converter 24. The subroutine then proceeds to step 108
which is discussed below. If the subroutine determined at step 110
that no adjustment of the software controlled voltage source 32
was required, the subroutine returns to the main program of FIG.
2 via step 118.
If the subroutine determined at either step 104 or step 110 that
adjustment of the software controlled voltage source 32 was required,
then the subroutine waits at 108 to allow the software controlled
voltage source 32 to stabilize. After the delay, the subroutine
proceeds to 114 where the microprocessor 22 causes the A/D convertor
24 to generate a digitized sample of the output of the differential
amplifier 26.
It will be recalled that the subroutine had previously generated
a digitized sample of the output of the differential amplifier 26
at step 100. Thus, the subroutine obtains two digitized samples
each time the offset voltage applied to the differential amplifier
is to be adjusted. However, the time between obtaining the digitized
sample at 100 and obtaining the digitized sample at 114 is sufficiently
short in relation to the frequency spectrum of the output of the
pressure transducer 20 so that the two digitized samples should
be substantially the same. Therefore, any significant difference
between the digitized sample obtained at 100 and the digitized sample
obtained at 114 must be due to the change in the value of the offset
voltage. Using the example explained above with reference to FIGS.
4A and 4B, the sample obtained at step 100 would be 810, and the
sample obtained at 114 would be 600. The subroutine of FIG. 5 calculates
a new digital offset value as explained above by summing the 210
difference with the current digital offset value (e.g., 16,384)
to obtain the new offset value (e.g., 16,594). The subroutine then
returns to the main program through 118.
The offset voltage generated by the voltage controlled voltage
source 32 is thus set by the subroutine of FIG. 3 to an initial
offset value each time the pressure in the cuff 12 changes, and
it is then adjusted as needed by the subroutine of FIG. 5 each time
an oscillometric sample is taken at that cuff pressure. Each time
that the subroutine of FIG. 5 is executed, the digital offset value
calculated during the previous pass through the subroutine of FIG.
5 will be used at step 102 to calculate a new oscillometric value,
and, if the offset voltage was adjusted, a new digital offset value
is calculated at step 116.
It is thus seen that the inventive blood pressure monitor is capable
of accurately measuring oscillometric pulses despite relatively
large changes in sampled cuff pressure caused by cuff pressure changes,
motion artifact, and pneumatic leaks. Furthermore, the inventive
monitor is able to do so without requiring the use of an A/D converter
having an extraordinary large dynamic range.
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