Abstrict
A blood pressure monitoring system for automatic unattended operation
uses curve fitting techniques determined during an initial inflation
period to determine cuff size. Based upon cuff size, the number
of important pre-determined operating parameters are determined
for use in controlling the remaining blood pressure reading operations.
The automatic blood pressure monitor according to the present invention
offers a simplified, cost effective construction utilizing a single
pump, a single valve and a single valve orifice.
Claims
What is claimed is:
1. An automatic, non-invasive blood pressure measuring device of
the type which detects blood pressure pulses in a patient's appendage,
comprising:
a cuff for constricting blood flow in the patient's appendage;
a pump connected to the cuff for inflation thereof in response
to a pump control signal;
valve means connected to the cuff for deflation thereof in response
to a valve control signal;
a pressure sensing means connected to the cuff to sense pressure
in the cuff and to send a pressure signal indicating a pressure-time
characteristic of said cuff in response thereto;
microprocessor means connected to said pump, said valve means and
said pressure sensing means, including means to observe the initial
pressure-time characteristic of said cuff during an observed inflation
period in which the pressure of the cuff is increased to a level
less than a target pressure needed to take a blood pressure reading;
and
said microprocessor means including means for determining the cuff
size by comparing the initial pressure-time characteristic of said
cuff with a set of stored pressure-time characteristics of cuffs
of known sizes, and means for determining, in response to said cuff
size determination, a cuff inflation rate, a cuff deflation rate
and an initial target pressure, said microprocessor means sending
control signals to said pump to inflate said cuff to said target
pressure according to said cuff inflation rate, and to deflate said
cuff at said deflation rate, using said at least one deflation pressure
drop step.
2. The device of claim 1 wherein said pressure sensing means is
employed to detect the blood pressure pulses in a patient's appendage.
3. The device of claim 1 wherein said microprocessor means sends
control signals to said pump to inflate said cuff to approximately
said target pressure at a substantially constant flow rate.
4. The device of claim 1 wherein said microprocessor means sends
control signals to said pump to deflate said cuff at a substantially
constant deflation rate.
5. The device of claim 1 wherein said microprocessor means sends
control signals to said pump to reduce the inflation rate immediately
before sending control signals to said valve to deflate said cuff.
6. The device of claim 1 wherein said set of stored pressure-time
characteristics of cuffs of known sizes comprises a table of a plurality
of discrete pressure and time values, said plurality corresponding
to the number of cuff sizes.
7. The device of claim 1 wherein, during said observed inflation
period, inflation is carried out for a preselected period of time.
8. The device of claim 7 wherein, during said observed inflation
period, inflation is carried out at a substantially constant flow
rate.
9. The device of claim 1 further comprising an acoustical detection
means which is employed to detect the blood pressure pulses in a
patient's appendage.
10. A method for the non-invasive automatic measuring of blood
pressure of a patient by detecting blood pressure pulses in a patient's
appendage, using a blood pressure cuff of unknown size to selectively
restrict blood flow in the patient's appendage, comprising the steps
of:
providing a pump connected to said cuff for inflation thereof in
response to a pump control signal;
providing a valve means connected to said cuff for deflation thereof
in response to a valve control signal;
providing pressure sensing means connected to said cuff to sense
pressure in said cuff and to send a pressure signal indicating a
pressure-time characteristic of said cuff in response thereto;
providing a microprocessor means coupled to said pump, said valve
means and said pressure sensing means;
sending a pump control signal to said pump to inflate said cuff
during an observation period to a pressure below that required to
take a blood pressure reading, and to observe the pressure-time
characteristic during said observation period;
comparing the pressure-time characteristic obtained during said
observation period to a set of pressure-time characteristics of
cuffs of known sizes so as to determine the cuff size employed;
determining, in response to the determination of the cuff size,
a target pressure and an inflation rate to inflate the cuff to approximately
the target pressure needed to take a blood pressure reading, the
deflation rate of the cuff during a blood pressure reading period;
inflating said cuff according to said inflation rate; and
deflating said cuff according to said deflation rate.
11. The method of claim 10 wherein blood pressure readings are
taken in between said deflation pressure drops.
12. The method of claim 10 wherein at least two blood pressure
pulses are observed between consecutive deflation pressure drops.
13. The method of claim 10 wherein said cuff is inflated at a substantially
constant inflation rate.
14. The method of claim 10 wherein said cuff is deflated at a substantially
constant deflation rate.
15. The method of claim 10 wherein the inflation rate is reduced
immediately before deflating said cuff.
16. The method of claim 10 wherein said set of pressure-time characteristics
of cuffs of known sizes is stored in a table of a plurality of discrete
pressure and time values.
17. The method of claim 10 wherein said observed inflation period
is carried out for a preselected period of time.
18. The method of claim 10 wherein said observed inflation period
is carried out at a substantially constant flow rate.
19. The method of claim 10 wherein said pressure sensing means
is employed to detect the blood pressure pulses in a patient's appendage.
20. The method of claim 10 wherein, in said deflating step, said
cuff is deflated using a valve duty cycle determined in response
to said cuff size determination.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the real time monitoring of a
patient's blood pressure and in particular to the taking of continuous
automatic blood pressure readings.
2. Description of the Related Art
In working with a large number of different automatic blood pressure
reading systems, it has been recognized that the deployment of the
blood pressure cuff must be carefully considered in order to achieve
accuracy in the blood pressure readings taken. It has been observed,
for example, that the width of the blood pressure cuff (taken in
the direction along the length of the patient's arm) must be maintained
within certain ranges in order to prevent erroneous blood pressure
readings.
Most of the blood pressure cuffs in use today take the form of
a double-ended, elongated strip which is wrapped about as patient's
limb with ends of the blood pressure cuff partly overlapping. As
a minimal requirement, the amount of overlap must be sufficient
to allow proper self-attachment of the strip ends so as to free
an operator to perform other tasks, such as operating monitoring
equipment. Recently, attention has been paid to the amount of overlap
of the blood pressure cuff ends, with the appreciation that errors
in overwrap, either too large or too small, even if satisfactory
to allow blood pressure readings to be taken, result in an unwanted
shift of those readings.
In addition to variations encountered in applying a blood pressure
cuff to a patient's limb, a variation of blood pressure readings
also arises from the fact that, as a practical matter, there are
a relatively large number of different size cuffs by manufacturers
of blood pressure reading equipment. For example, systems having
nine or more differently sized blood pressure cuffs are not uncommon.
Cuff sizes typically include a smallest size blood pressure cuff
for neonatal patients and a largest blood pressure cuff size for
adult thigh readings. Some blood pressure reading equipment requires
the user to specify the cuff size by a special purpose input, such
as a special, identifying switch or some other pre-defined selection
means. Other systems require that special pneumatic fittings be
employed to provide a self-identification of the size of the blood
pressure cuff with which the fitting is associated. Accordingly,
some type of keying system between the blood pressure cuff and the
associated pneumatic circuitry is employed. The complexities in
taking blood pressure readings is growing at a time when increasing
demands are being made on care givers and other personnel charged
with the responsibility of taking blood pressure readings. The need
for an improved, automatic blood pressure reading system still exists.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system and
method for the automatic, continuous reading of blood pressure.
Another object of the present invention is to provide a system
and method of the above-described type which are suitable for use
with a plurality of differently sized blood pressure cuffs and which
automatically adapt for the accurate use of such cuffs.
A further object of the present invention is to provide a system
and method for the automatic reading of blood pressure in which
blood pressure cuff size is automatically determined at the initial
phase of a blood pressure reading, before the actual blood pressure
reading commences, allowing for the calculation of several parameters
important to the rapid, comfortable and safe reading of a patient's
blood pressure.
These and other objects of the present invention are provided in
an automatic, non-invasive blood pressure measuring device of the
type which detects blood pressure pulses in a patient's appendage,
comprising:
a cuff for constricting blood flow in the patient's appendage;
a pump connected to the cuff for inflation thereof in response
to a pump control signal;
valve means connected to the cuff for deflation thereof in response
to a valve control signal;
a pressure sensing means connected to the cuff to sense pressure
in the cuff and to send a pressure signal in response thereto;
microprocessor means connected to said pump, said valve means and
said pressure sensing means, including means to observe the initial
pressure-time characteristics of said cuff during an observed inflation
period in which the pressure of the cuff is increased to a level
less than a target pressure needed to take a blood pressure reading;
and
said microprocessor means including means for determining the cuff
size by comparing the initial pressure-time characteristic of said
cuff with stored pressure-time characteristics of cuffs of known
sizes, and means for determining, in response to said cuff size
determination, a cuff inflation rate, a cuff deflation rate and
at least one deflation pressure drop step size, said microprocessor
means sending control signals to said pump to inflate said cuff
to said target pressure according to said cuff inflation rate, and
to deflate said cuff at said deflation rate, using said at least
one deflation pressure drop step.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an automatic blood pressure monitor
according to the principles of the present invention;
FIG. 2 shows an inflation profile with operation according to principles
of the present invention;
FIG. 3 is a graph showing the pump flow rate associated with the
operating curve of FIG. 2; and
FIG. 4 shows an initial portion of the operating curve, taken on
an enlarged scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to the drawings, and initially to FIG. 1, the present
invention is directed to an automatic blood pressure monitor 10
with automatic cuff size determination and cuff pressure control.
A cuff 12 of conventional construction is coupled through a hose,
piping or other conduit means to a pneumatic control system, including
a pump 16, a valve 18 and a pressure transducer 20. The cuff 12
is wrapped about the patient's arm and operated so as to apply varying
amounts of pressure sufficient to selectively occlude and release
blood flow through the patient's brachial artery.
Pump 16 is preferably of the positive displacement type operated
under control of an electronic system 24. Most preferably, the pump
is controlled by duty-cycling the driver circuitry of the pump.
The driver portion of system 24 is coupled by control circuit wiring
26 to a microprocessor 30. Microprocessor 30 is of a conventional
type issuing control instructions, e.g., in the form of a pulse
train, to the pump driver of system 24. The presence or absence
of pulses in the pulse train control the duty cycle of the pump,
which in turn directly controls the pump output, i.e., the inflation
rate and inflation volume of cuff 12.
The valve 18 is pneumatically coupled to cuff 12 and provides selective
venting or deflation of the cuff in a controlled manner, preferably
by duty-cycling the driver circuitry 34 of the valve. Valve driver
34 is coupled by control circuitry 36 to microprocessor 30. The
valve 18 is preferably of the on/off control type (as opposed to
more costly proportional valves). Depending upon the duty cycle
control signal transmitted through conductor 36 to valve control
circuitry 34, the valve is held closed or open with a number of
different deflation rates.
A pressure transducer 20 monitors the pressure of cuff 12 and sends
an electrical output signal indicating the pressure, via conductor
40 which couples the pressure transducer 20 to an analog/digital
converter 33. The analog/digital converter 44 is in turn coupled
to microprocessor 30 by conductor 46. Preferably, the driver circuitry
24 of pump 16 operates under closed loop control implemented by
microprocessor 30. Similarly, the valve driver circuitry 34 of valve
18 undergoes closed loop operation under control of microprocessor
30. Although principles of the present invention may be readily
employed with pneumatic control systems having multiple pumps, multiple
valves and/or multiple orifices, present invention is particularly
advantageous in providing heretofore unattainable control with simple,
low-cost components, including a single pump, a single valve and
a single valve orifice arrangement.
In the preferred embodiment, the monitoring system uses an oscillometric
method of determining blood pressure, sensing pulses with pressure
transducer 20. If desired, as an alternative, an acoustic, ultrasonic
or strain gage transducer 52 could be located in the vicinity of
cuff 12 for audibly monitoring blood flow in the brachial artery.
The alternative transducer is shown coupled to conventional pulse
discriminator circuitry 54 which could also be implemented, for
example, in microprocessor 30. Together, the transducer and pulse
discriminator circuitry, either standing alone or incorporated in
microprocessor 30, detect the presence of blood flow in the patient's
appendage, monitor the number of heart-induced pulses in the arterial
blood flow and measure the relative amplitudes of those arterial
pressure pulses.
Typically, the blood pressure cuff 12 is initially inflated to
a suprasystolic pressure level at which blood flow is cut off in
the patient's limb, herein the brachial artery. As an alternative
to a complete cessation of blood flow in the brachial artery, pressure
can be increased in cuff 12 so as to apply sufficient pressure to
impede blood flow in the brachial artery to a point where the pulse
beat is either substantially reduced or can no longer be detected.
Thereafter, pressure applied to the brachial artery by the cuff
12 is reduced by relaxing pressure in cuff 12 in a controlled manner
until the first very weak pulse is detected, and this pressure is
immediately related to a pressure above the patient's systolic blood
pressure level. As the pressure in cuff 12 is reduced, the pressure
is continuously detected by transducer 20 and monitored by microprocessor
30.
After the first faint pulses are reliably detected, cuff pressure
is reduced by a controlled amount and held at the reduced level
for a defined period of time, long enough to acquire additional
pulse information. Most preferably, each "pressure hold"
step is sustained long enough to reliably detect two adequately
discerned blood pressure pulses. Eventually, with a sufficient number
of cuff pressure reductions having been carried out, the amplitude
of the blood pressure pulses is typically observed to rise to a
maximum value and then fall to a point where blood pressure pulses
can no longer be detected, an operating point below the patient's
diastolic pressure reading. As will be seen herein, the present
invention affords a number of significant advantages in obtaining
blood pressure readings in as short an operating time as is practical.
Briefly, the present invention operates early on so as to identify
as quickly as possible the size of the blood pressure cuff in sufficient
time so as to allow calculation of a number of important control
parameters and to thereafter control a substantial portion (and
preferably the major portion) of the cuff's inflation period (i.e.,
the time during which the cuff is brought to a carefully defined
patient-specific suprasystolic pressure level, which is approximately
the maximum pressure experienced by the cuff.
The required sequence of blood pressure readings occurs at points
located below suprasystolic pressures, and taken after the inflation
period, when the cuff is continuously deflated until a sub-diastolic
pressure level is attained. Thereafter, if additional blood pressure
pulse information is desired for the same patient (due, for example,
to artifacts caused by motion), pressure may be increased to a controlled
supradiastolic level. If a complete repetition of the blood pressure
analysis is desired, pressure is elevated once more to a suprasystolic
pressure level to enable a repeated observation of the patient.
It is important that cuff 12 be inflated as quickly as possible
so as to allow the actual blood pressure measurements to be taken
as quickly as possible. However, it has been found that patients
react with alarm to high inflation rates, particularly those carried
out under automatic control of an unattended machine. This could
result in alterations of a patient's vital signs or induce motion
artifacts by agitating the patient. The maximum pressure level is
particularly important for neonatal and other relatively young patients
where a risk of injury may be present if cuff pressures are allowed
to assume elevated levels. Control of neonatal blood pressure cuffs
has traditionally proven to be unusually difficult because of the
smaller volume capacities of the blood pressure cuff used on relatively
young patients. The present invention offers improved protection
while allowing very rapid determination of blood pressure cuff size,
one which can be taken using relatively inexpensive components and
in such a rapid manner that pressure levels even for relatively
small neonatal cuff sizes are well below acceptable elevated pressure
levels.
Once the determination of cuff size has been made, the present
invention determines a number of important operating parameters,
including the "rate" of inflation over the inflation rate
period (preferably the .DELTA.p and .DELTA.t values from beginning
to end of the inflation rate period), the target pressure, the rate
of deflation needed to secure data about the systolic and diastolic
blood pressure levels, and the overshoot control employed in step-wise
deflation, i.e., pressure reduction, over the time period that blood
pressure readings are taken.
Turning now to FIGS. 1-3, operation of the blood pressure monitor
begins with cuff 12 substantially deflated, indicated by pressure
P.sub.0 in FIG. 2. As will be seen herein, the blood pressure reading
cycle is begun later, approximately at time t.sub.R after an elevated,
substantially maximum target pressure P.sub.3 is reached. As indicated
by the operating curve in FIG. 2, the pressure in the cuff must
be substantially increased beyond the initial pressure P.sub.0 and
a substantial amount of time indicated by the interval between t.sub.0
and t.sub.3 is needed to fully inflate the cuff. During the time
required to inflate cuff 12, a brief initial inflation period (t.sub.0
to t.sub.1 is defined, and data characteristic of the cuff is accumulated
and analyzed. Based upon the results, several important factors
are calculated at time t.sub.1, in time to set pump 16 and valve
18 for the remainder of the operation.
As a first step, between time t.sub.0 and t.sub.1, herein the initial
inflation period, cuff 12 is inflated to a relatively low pressure
level, preferably a small fraction of the operating pressure P.sub.3.
As graphically indicated in FIG. 2, this portion of the operating
curve designated C.sub.1 is non-linear and, the curve shape has
been found to be characteristic of the size of the cuff being inflated.
During the initial inflation period, the cuff may be inflated in
a number of different ways. However, the cuff is preferably inflated
with a constant flow rate for a pre-defined period of time. That
is, the time interval of the initial inflation from t.sub.0 to t.sub.1
is preferably fixed as part of the program control loaded into microprocessor
30. In the preferred embodiment, the initial inflation period is
set so as to assure that, for the smallest blood pressure cuff possible
(usually neonatal size) the final pressure at a constant flow rate
is well below the appropriate patient-specific maximum operating
pressure (i.e., approximately P.sub.3 in FIG. 2). At the end of
the initial inflation period, pressure is elevated to level P.sub.1
and the time interval has allowed a pressure difference of P.sub.1
-P.sub.0, herein .DELTA.P. It is preferred that the characteristic
shape of the initial inflation curve C.sub.1 is calculated or otherwise
determined immediately at time t.sub.1 by microprocessor 30, based
upon readings of pressures sensed by transducer 20 and converted
into digital form by converter 44.
Referring to FIG. 3, the initial rate of flow is constant throughout
the initial inflation period. In order to obtain as rapid a processing
time as possible, the rate of flow changes indicated by FIG. 3 are
carried out in a step-wise manner, although sloped or curved flow
rate changes may also be employed.
Referring briefly to FIG. 4, a family of characteristic curves
for three different blood pressure cuff sizes is shown. For example,
for a "size 1" cuff, the smallest cuff size shown, the
inflation curves lie between the ordinate and curve S1.sub.max.
The characteristic curves for the next largest cuff size lie between
boundaries S2.sub.min and S2.sub.max. The next largest cuff size
is associated with an operating region beginning with boundary S3.sub.min
and extending to the right, beyond the area shown in FIG. 4.
As mentioned, a relatively large number of cuff sizes is found
in modern commercial blood pressure reading systems. For the system
shown in the preferred embodiment, nine different cuff sizes are
assumed. FIG. 4 depicts characteristic operating curves for the
three smallest cuff sizes. It has been found that, due to manufacturing
tolerances as well as variations in the conformance of the materials
employed, a single well-defined characteristic curve is not observed
for practical blood pressure cuffs. Rather, as is indicated in FIG.
4, the characteristic curve for a plurality of the same size blood
pressure cuffs falls within a range, lying between minimum and maximum
limits.
Although more precise recognition schemes can be employed, it has
been found expedient for rapid, real time control to pre-define
characteristic curves CC.sub.1 and CC.sub.2 as shown in FIG. 4 lying
within the overlap regions and most preferably at the maximum observed
limits for each particular size. As can be seen in FIG. 4, the first
characteristic curve CC.sub.1 is located slightly to the left of
S1.sub.max, between S2.sub.min and S1.sub.max. Similarly, characteristic
curve CC.sub.2 is located slightly to the left of curve S2.sub.max,
lying between curves S3.sub.min and S2.sub.max. For the purpose
of determining control parameters, it should be understood that
other curves can be employed which are not related to a specific
cuff size. For example, the number of characteristic curves for
the family of adult size cuffs can be reduced in number, since it
has been found that certain control parameters for certain grouped
cuff sizes (neonatal, infant, adult) can be shared for several different
cuff sizes within the same group.
For nine different cuff sizes in the blood pressure monitor of
interest, nine characteristic curves will be predetermined and stored
in microprocessor 30. As cuff pressure data is taken in the initial
inflation period t.sub.0 to t.sub.1, curve data represented as a
solid continuous curve portion C.sub.1 is accumulated in microprocessor
30 and is compared against the pre-defined characteristic curves.
The closest curve fit lying immediately to the right of the observed
curve portion indicates the determined cuff size. In practice, cuff
size determinations can be made very quickly without substantial
delay, at time t.sub.1. A series of formulas or look-up tables are
then employed by microprocessor 30 to determine a number of important
operating parameters which control system operation beyond time
t.sub.1. An example will be given below.
Before proceeding with a further discussion of operating parameters
determined by the present invention, it should be understood that
relatively inexpensive equipment can be employed to acquire and
interpret enough pressure data points between times t.sub.0 and
t.sub.1 to form a substantially solid curved portion as indicated
in FIG. 2. It has been found sufficient in practicing the present
invention to forego expensive computer control equipment and to
rely instead on data collected as a series of spaced apart operating
points. In its simplest form, the present invention looks at the
pressure difference over the initial inflation period and scans
a table of characteristic values to determine blood pressure cuff
size. An adjustment must be made, however, for different initial
pressure levels P.sub.0. Other types of "curve fitting"
can be employed using "least squares fit" and other known
techniques.
Once the blood pressure cuff size is determined, a number of operating
parameters are determined and are loaded by the control program
of microprocessor 30. One parameter determined is the inflation
rate between time t.sub.1 and time t.sub.2, represented by the step
increase at time t.sub.1 in FIG. 3. It is generally preferred that
the operating curve portion C.sub.2 during this time period be substantially
linear in shape, although other shaped inflation curves could be
employed as well. Preferably, the slope of curve portion C.sub.2
is pre-defined in a look-up table where other data, based upon observed
patient response for the particular size blood pressure cuff, is
stored.
It is desirable to shorten the inflation time period t.sub.1 -t.sub.2
as much as possible. However, excessive rates of inflation are known
to startle patients if the inflation is perceived as being near
instantaneous, or if the rate of rapid constriction of the patient's
appendage is perceived as being surprisingly steep. All of these
factors tend to alarm certain patients, with expected physiological
reactions resulting. Accordingly, the stored inflation rate values
for curve portion C.sub.2 are a trade-off between speed and unintended
patient response. With the present invention, the differing slope
values for differing cuff sizes can be tailored for optimal results.
Referring again to FIGS. 2 and 3, another important parameter determined
by cuff size is the maximum operating pressure needed for blood
pressure readings, identified as pressure P.sub.3 in FIG. 2. In
an effort to avoid expensive inflation equipment with more elaborate
overshoot control, and to allow the use of single hose cuffs, the
automatic control provided by the present invention provides an
over-shoot control time period t.sub.2 -t.sub.3 during which the
rate of inflation is reduced a small amount as indicated by the
step drop at time t.sub.2 in FIG. 3. As can be seen in FIG. 2, the
attendant pressure-time response of the cuff is non-linear but,
with prior testing of known cuffs, the time interval of the over-shoot
control period can be accurately determined and stored as a control
parameter in a cuff-size related look-up table accessible by microprocessor
30. As can be seen in FIG. 2, the pressure reaches an absolute maximum
P.sub.x during the over-shoot control period although, with sufficient
passage of time the pressure level is stabilized at time t.sub.3
in preparation for the beginning of a blood pressure reading cycle.
At time t.sub.R the blood pressure reading cycle is initiated along
with deflation of the blood pressure cuff. According to the present
invention, it is preferred that deflation during the blood pressure
reading, i.e., between times t.sub.3 and t.sub.4, have a constant
rate of pressure change, that is, a linear dP/dt characteristic
shape. The pressure level P.sub.3 is defined at the beginning of
the blood pressure reading cycle and its value is determined based
upon the blood pressure cuff size. Preferably, the beginning reading
pressure P.sub.3 is obtained by consulting a look-up table stored
within microprocessor 30. Generally, the pressure level P.sub.3,
although different for each cuff size or perhaps cuff group (i.e.,
neonatal, infant, adult), is associated with a suprasystolic level
for the appendage involved. As indicated in FIGS. 2 and 3, it is
preferred that pressurization of the cuff is terminated at approximately
time t.sub.3 and thereafter the cuff is deflated by operation of
valve 18 under control of microprocessor 30.
As mentioned, at approximately time t.sub.1 the observed curve
data is compared against stored values to determine the blood pressure
cuff size. This result is used to determine a number of important
operating parameters and the first parameter needed is the inflation
rate or inflation curve shape between times t.sub.1 -t.sub.2. As
a practical matter, the initial target pressure P.sub.3 is also
determined about time t.sub.1, along with the deflation rate between
times t.sub.3 and t.sub.4, as well as the individual overshoot controls
and valve duty cycles for deflation pressure drops indicated in
the stair step curve portion C.sub.4. If desired, the determination
of the maximum pressure can be delayed until a time shortly prior
to t.sub.2, the end point of the over-shoot control period t.sub.3
can be delayed until a time after t.sub.2 and the deflation rate
and step size can be delayed until time t.sub.3, if desired. However,
it has been found expedient to perform all necessary parameter determinations
approximately at time t.sub.1 and this is found to be readily achievable
using modestly priced components.
As will be apparent, a wide variety of step configurations can
be employed to accommodate the same deflation rate slope. It is
preferred in practicing the present invention that the deflation
control parameters be pre-defined for each cuff size and stored
in memory, available to microprocessor 30. Upon identification of
the cuff size, the number of steps defined either by the pressure
drop of each step or the time period between steps is then used
to control, preferably in a duty-cycle fashion, the driver 34 of
valve 18 to achieve the performance desired.
Only a few deflation steps are shown in FIG. 2 for purposes of
illustration. It is preferred in practicing the present invention
that the time period between adjacent pressure drop steps be chosen
to allow for artifact and motion rejection for the data collected
in the appendage being monitored. In the example given, the brachial
artery is monitored, and the pressure P.sub.3 applied to the patient's
upper arm is high enough to assure either that blood flow is substantially
occluded in the brachial artery or is otherwise reduced as required.
Each step during deflation allows for matching arterial pressure
pulses based upon pre-determined data for patients monitored by
the particular cuff size employed. The blood pressure pulses are
preferably monitored by the pressure transducer 20, but, as mentioned,
may also be monitored by microphone 52 and a conventional audio
processor 54, such as one of the Korotkoff pulse determination type,
doppler (ultrasonic) or tonometry (strain gage array) techniques.
Over the evaluation period t.sub.3 -t.sub.4, after a certain number
of deflation steps are carried out, blood pressure pulses will be
detected and the history of the blood pressure pulse data will be
stored in micro-processor 30 for future reference.
Throughout the deflation period, the amplitude of the blood pressure
pulses will change over time, typically rising to a maximum value
at the mean arterial pressure, thereafter falling to a minimum,
final value toward the end of the blood pressure reading, at a time
before time t.sub.4. The number of steps during the deflation period
t.sub.3 -t.sub.4 are chosen to allow accurate detection of the systolic,
diastolic and MAP values, while assuring that at least two matching
pressure pulses are observed for each step interval. If desired,
the blood pressure drop at each step can be held constant throughout
the deflation period or can be varied throughout the deflation period
according to pre-defined values stored in microprocessor 30 for
the particular blood pressure cuff size. In any event, the ending
point t.sub.4 of the blood pressure reading cycle is chosen to be
substantially beyond the time when the final detectable blood pressure
pulse is detected.
It has been found desirable at time t.sub.4 to reduce the pressure
in the cuff to a value approximately equal to the initial pressure
level P.sub.0 or below. Given the speed and ease of operation made
possible by the present invention, a repeat of the entire operation
t.sub.0 -t.sub.4 may be elected in which case the initial inflation
of the cuff can be immediately begun for the subsequent operation.
A substantially instantaneous pressure drop is indicated at time
t.sub.4. Typically, final depressurization occurs over time with
a sloped or curved operating characteristic. In a subsequent immediately
consecutive operating sequence, the systolic, MAP and diastolic
values that were previously observed can be used to adjust the beginning
of the deflation period so as to reduce the overall reading times.
An example of initial parameter determination will now be given.
The following is an example of the cuff detection process according
to the present invention, assuming a simplified system having five
different cuff sizes. Reference is made to the following table showing
pressure-time relationships empirically determined for each particular
pneumatic control system of interest. The pressure thresholds in
the following table are determined from pressure versus time curves
for each cuff size. In the following table, three initial pressure
ranges and four cuff size thresholds are considered.
Sample Table Of Cuff Determination Pressure Thresholds Initial
Pressure Pressure Pressure Pressure Threshold Threshold Threshold
(mmHg) @ 11 @ 12 @ 13 0-4 58 74 80 4-7 80 119 130 7-10 94 132 151
Cuff Determination Process
1. The valve is open at the beginning of a blood pressure determination
cycle and the pressure in the cuff is monitored. If the pressure
in the cuff is >10 mmHg, the valve is left open until the pressure
drops below 10 mmHg.
2. The initial pressure in the cuff is determined and the pressure
thresholds are obtained from the corresponding row of the table
above (i.e., if the initial pressure is 0-4 mmHg, the pressure thresholds
at 11, 12 and 13 will be 58, 74 and 80 mmHg, respectively.
3. The valve is closed, and the pump is turned on at a fixed slow,
flow rate. The cuff pressure/time relationship (dP/dt) is measured
and is compared to the threshold for the smallest cuff first and
then for increasingly larger cuffs until a cuff size is determined.
4. The pressure in the cuff is monitored for up to 11 seconds while
inflating. If the pressure in the cuff exceeds the pressure threshold
for 11 (58 mmHg in this example) prior to 11, the smallest cuff
size has been detected. The initial target inflation pressure, inflation,
parameters, and deflation parameters are then set for this cuff.
5. If the pressure threshold for 11 has not been exceeded, the
pressure in the cuff is monitored until either 12 seconds has been
reached, or the pressure threshold for 12 has been exceeded. If
the pressure threshold for 12 has been exceeded prior to 12, the
next larger cuff size has been detected and he parameters are set
for this cuff size.
6. If the pressure threshold for 12 has not been exceeded, step
5 is repeated while monitoring for subsequent pressure and time
thresholds until a cuff size has been determined.
7. Once a cuff size has been determined, the inflation parameters
are set and the cuff is inflated until the initial target inflation
pressure has been reached. At this time the blood pressure reading
cycle begins and the cuff is deflated using the cuff deflation parameters
for the detected cuff size until a blood pressure determination
is made.
The following parameter table was developed for the five cuff sizes
studied.
Sample Cuff Size Parameter Table Initial Target Inflation Inflation
Deflation Deflation Cuff Inflation Duty Overshoot Duty Overshoot
Size Pressure Cycle Parameter Cycle Parameter 1 100 30% -3 30% +4
2 125 40% -1 40% +1 3 148 70% 0 50% 0 4 170 100% +1 70% 0 5 170
100% +2 70% -1
The drawings and the foregoing descriptions are not intended to
represent the only forms of the invention in regard to the details
of its construction and manner of operation. Changes in form and
in the proportion of parts, as well as the substitution of equivalents,
are contemplated as circumstances may suggest or render expedient;
and although specific terms have been employed, they are intended
in a generic and descriptive sense only and not for the purposes
of limitation, the scope of the invention being delineated by the
following claims. |