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Description  |
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TECHNICAL FIELD
The present invention relates to an electronic blood presure meter based on
the oscillation method and in articular to an electronic blood pressure
meter which can reduce the time required for measurement by reducing the
cuff that is needed for blood pressure measurement.
BACKGROUND OF THE INVENTION
An electronic blood pressure meter based on the oscillation method
comprises a cuff, a pressurization pump, a vent valve for depressurizing
the cuff, a pressure sensor for detecting the cuff presure, and a micro
computer (MPU).
This MPU is equipped with the functions of detecting a pulse wave component
from the output signal of the pressure sensor, computing a pulse wave
amplitude value from the pulse wave component and determining a systolic
pressure (SYS) and a diastolic pressure (DIA) from the cuff pressure and
the pulse wave amplitude value.
In this electronic blood pressure meter base on the oscillation method, a
threshold value is utilized as a basis for determining the blood pressure
values.
Normally, in measuring blood pressure, with an artery blocked by
pressurizing the cuff, a pulse wave amplitude is detected during the
course of gradually depressurizing the cuff. The pulse wave is an
indication of a deformation of the artery wall and the surround tissue due
to the pulsation in the internal pressure of the artery, and this is
transmitted to the cuff where it is detected as a pressure fluctuation.
This pulse wave amplitude value gradually increases as the cuff pressure
is reduced and can be represented by a curve (envelope) which gradually
increases but after reaching a maximum value diminishes in value.
Thus, a maximum pulse wave amplitude value is detected and a pulse wave
amplitude value which is substantially equal to a threshold value which is
a certain fraction of the maximum pulse wave amplitude value (for instance
50% of the maximum pulse wave amplitude value) is determined during the
gradually increasing process of the pulse wave amplitude value. And the
blood pressure when the pulse wave amplitude value has substantially
coincided with the threshold value is determined as a systolic pressure.
Likewise a pulse wave amplitude value which is substantially equal to
another threshold value which is also a certain fraction of the maximum
pulse wave amplitude value (for instance 70% of the maximum pulse wave
amplitude value) is determined during the gradually decreasing process of
the pulse wave amplitude value, and the blood pressure when the pulse wave
amplitude value has substantially coincided with the threshold value is
determined as a diastolic pressure.
In such a conventional electronic blood pressure meter based on the
oscillation method, the cuff pressure corresponding to the time point when
the pulse wave amplitude value is 50% of the maximum pulse wave amplitude
value is determined as a systolic pressure and the cuff pressure
corresponding to the time point when the pulse wave amplitude value is 70%
of the maximum pulse wave amplitude value is determined as a diastolic
pressure.
According to this process, it is necessary to detect the pulse wave
amplitude value corresponding to 50% of the maximum pulse wave amplitude
value for the purpose of determining the systolic pressure. Therefore, the
cuff pressure must be raised beyond the systolic pressure at which the
pulse wave amplitude value corresponds to 50% of the maximum pulse wave
amplitude value. Thus, not only a considerable time is required for
measurement but also congestion could be cause in blood vessels which are
more periheral than the part of the artery to which the cuff is applied.
Further, because of the need to raise the cuff pressure beyond the systolic
pressure, it can often happen that the cuff presurization is insufficient.
In such case, the cuff pressure at the time point when the pulse wave
amplitude corresponds to 50% of the maximum pulse wave amplitude value
cannot be detected and the measurement has to be repeated all over again.
Furthermore, because it becomes known to the person using the blood
pressure meter that the cuff pressure was insufficient only after the
attempt to measure blood pressure has been concluded, the patient must
endure the discomfort caused by the excessive cuff pressure for an
unreasonably long time.
BRIEF SUMMARY OF THE INVENTION
In view of such problems of the prior art, a primary object of the present
invention is to provide an electronic blood pressure meter which is free
from the problems of the prior art and allows a measurement to be
completed in a very short time.
Another object of the present invention is to provide an electronic blood
pressure meter which minimizes the discomfort of the person whose blood
pressure is to measure through reduction of the maximum cuff pressure.
Yet another object of the present invention is to provide an electrnic
blood pressure meter which is based on a simple arithmetic algorithm and
is therefore easy to implement.
In order to achieve this object, the electronic blood pressure meter of the
present invention comprises a cuff, a pressurization means for
pressurizing the cuff, a pressure detection means for detecting a fluid
pressure inside the cuff, a pulse wave detection means for detecting a
pulse wave component contained in an output signal of the pressure
detection means, a pulse wave amplitude value computing means for
computing a pulse wave amplitude value from the pulse wave component
detected by the pulse wave component detecting means, and a blood pressure
determining means for determining a systolic pressure and a diastolic
pressure from an output signal of the pulse wave amplitude computing
means, a relative amplitude value computing means for computing at least a
single relative pulse wave amplitude value in relation with a maximum
pulse wave amplitude value obtained by the pulse wave amplitude value
computing means, a reference pressure value computing means for computing
a blood pressure value from the cuff pressure when an amplitude of the
pulse wave signal has coincided with the relative amplitude valve during a
change in the amplitude value of the pulse wave signal, and a blood
pressure computing means for comuting a blood pressure value using the
reference pressure value obtained by the reference pressure value
computing means in accordance with a certain arithmetic formula.
According to the electronic blood pressure meter of this structure, after
the pulse wave amplitude value has been detected and the maximum pulse
wave amplitude value has been detected, the cuff pressure at the time when
the pulse wave amplitude value corresponds to, for instance, 70% of the
maximum pulse wave amplitude value during the decreasing process of the
pulse wave amplitude is determined as a diastolic pressure (DIA). Then,
the cuff pressure (L value) at the time when the pulse wave amplitude
value corresponds to 75% of the maximum pulse wave amplitude value during
the increasing process of the pulse wave amplitude value is determined.
Then, 32% of the difference between the L value of DIA is added to the L
value and this sum is determined as a systolic pressure (SYS).
Therefore, according to this systolic pressure determining means, a
systolic pressure can be computed by finding a pulse wave amplitude value
which corresponds to 75% of the maximum pulse wave amplitude value.
Because the cuff pressure at the time point when the pulse wave amplitude
value is 75% of the maximum pulse wave amplitude value is lower than the
cuff pressure at the time poin when the pulse wave amplitude value is 50%
of the maximum pulse wave amplitude value, the pressure requirement for
the present invention is substantially lower than the prior art electronic
blood pressure meters. Therefore, not only because of the time required
for measurement is reduced but also because the possibility of improper
measurement due to insufficient cuff pressurization can be reduce, the
possibility of causing congestion in the person whose blood pressure is to
be measured can be significantly reduced.
Whereas a conventional electronic blood pressure meters is incapable of any
blood pressure measurement if the initial cuff pressurization was
insufficient, the electronic blood pressure meter of the present invention
is capable of blood presure measuremenet even when the cuff pressure is
insufficient by using an appropriate algorithm for computing the systolic
pressure.
Thus, since it is not necessary to pressurizing the cuff beyond the
systolic pressure of the patient any more, there will be no need for
excessively pressurizing the cuff and the time required for blood pressure
measurement will be significantly reduced.
According to a certain aspect of the present invention, the reference
pressure value used in the step of computing the systolic pressure
includes the diastolic pressure obtained during a same measurement process
according to an arbitrary but different method and a second reference
blood pressure which is different from the former, the systolic pressure
being determined by adding a fraction of a pressure difference between the
diastolic pressure and the second reference blood pressure to either one
of the diastolic pressure and the second reference blood pressure.
According to another aspect of the present invention, the reference
pressure vaue used in the step of computing the diastolic pressure
includes the systolic pressure obtained during a same measurement process
according to an arbitrary but different method and a second reference
blood pressure which is different from the former, the diastolic pressure
being determined by subtracting a fraction of a pressure difference
between the systolic pressure and the second reference blood pressure to
either one of the systolic pressure and the second reference blood
pressure.
According to yet another aspect of the present invention, the relative
amplitude value which is used in the process of obtaining the systolic
pressure is a pulse wave amplitude value at an arbitrary cuff pressure
which is higher than a cuff pressure at which the maximum pulse wave
amplitude is detected, and a ratio of the pulse wave amplitude value at
the arbitrary cuff pressure to the maximum pulse wave amplitude value is
used in the process of computing the diastolic pressure.
According to yet another aspect of the present invention, the relative
amplitude value which is used in the process of obtaining the systolic
pressure is a pulse wave amplitude value at an arbitrary cuff pressure
which is lower than a cuff pressure at which the maximum pulse wave
amplitude is detected, and a ratio of the pulse wave amplitude value at
the arbitrary cuff pressure to the maximum pulse wave amplitude value is
used in the process of computing the diastolic pressure.
According to yet another aspect of the present invention, a different
process is selected for determining a blood pressure depending on an
initial cuff pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be shown and described in the following in
terms of concrete embodiments thereof with reference to the appended
drawings, in which:
FIG. 1 is a block diagram showing the circuit structure of an embodiment of
the electronic blood pressure meter of the present invention.
FIGS. 2a and 2b constitute a flow chart showing the processing action of an
embodiment of the electronic blood pressure meter of the present
invention;
FIG. 3 is a flow chart showing an essential part of the processing action
of the systolic pressure in the first embodiment;
FIG. 4 is a graph for illustratingthe process of determining the blood
pressures in the electronic blood pressure meter of the first embodiment;
FIG. 5 is a graph for illustrating the breaks in the pulse wave data;
FIG. 6 is a flow chart showing an essential part of the processing action
of the systolic pressure in the first embodiment according to the second
embodiment of the present invention;
FIG. 7 is a flow chart showing an essential part of the processing action
for determining the systolic pressure in accordance withh the third
embodiment of the present invention;
FIG. 8 is a graph for illustrating the process of determining the blood
pressures in the electronic blood pressure meter of the third embodiment;
FIG. 9 is a graph showing the relationship between X and P;
FIGS. 10a and 10b constitute a flow chart showing the processing actions of
yet another (a fourth) embodiment of the present invention;
FIG. 11 is a flow chart showing an essential part of the processing action
for determining the systolic pressure in accordance with the fourth
embodiment of the present invention; and
FIG. 12 is a graph for illustrating the process of determining the blood
pressures in the electronic blood pressure meter of the fourth embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a block diagram of a concrete embodiment of the pneumatic
system and the measurement circuit of the electronic blood pressure meter
according to the present invention. In this embodiment of the present
invention a cuff 1 is connected to a pressurization pump (pressurizing
means) 4, a vent valve (venting means) 3 and a pressure sensor (pressure
detecting means) 5, by way of a tube 2. The vent valve 3 includes a pair
of valves consisting of a rapid vent valve and a gradual vent valve. The
pressure sensor 5 may consist of any pressure transducer which can covert
pressure into an electric signal such as a diaphragm pressure converter
using a strain gauge and a semiconductor pressure converter element. The
pressurization pump 4 and the vent valve 3 are controlled by a CPU
(central processing unit) 9 which is described hereinafter.
The output (analog) signal of the pressure sensor 5 is amplified by an
amplifier 6 and is then converted into a digital signal by an A/D
converter 7. The CPU receives the output signal of the pressure sensor 5
converted into a digital signal by the A/D converter 7 at a certain
frequency. The output signal of the pressure sensor 5 is also sent to a
band pass filter 8, by way of the amplifier 6, in which a pulse wave
component is extracted from the cuff pressure signal, and the extracted
pulse wave signal (pulse wave component) is supplied to the CPU 9. The CPU
9 is additionally provided with the functions of computing a pulse wave
amplitude value, computing a relative amplitude value from the pulse wave
amplitude value, computing a reference pressure value from the relative
amplitude value and computing a systolic and a diastolic pressure from the
reference pressure value. Further, the CPU 9 is connected to a display
unit 10 for displaying a systolic and a diastolic pressure value.
FIG. 4 is a graph for illustrating the computing process according to which
the electronic blood pressure meter of this embodiment determines a blood
pressure value from a pulse wave envelope.
A pulse wave envelope is normally obtained in the form as shown in FIG. 4.
In a conventional electronic blood pressure meter, a systolic pressure
(SYS) and a diastolic pressure (DIA) were determined as the cuff pressures
corresponding to the time points when the pulse wave amplitude corresponds
to 50% and 70% of the maximum pulse wave amplitude value before and after
the occurrence of the maximum pulse wave amplitude value (peak value),
respectively, as mentioned earlier. On the other hand, a primary feature
of the present embodiment exists in finding a cuff pressure value L (mmHg)
at the time point when the pulse wave amplitude value corresponds to 75%
of the maximum pulse wave amplitude before the occurrence of the maximum
pulse wave amplitude value instead of finding the pulse wave amplitude
value (SYS) corresponding to 50% of the maximum pulse wave amplitude
value. Note that the pressure value L is substantially lower than the
systolic pressure or the cuff pressure at the time point when the pulse
wave amplitude value corresponds to 50% of the maximum pulse wave
amplitude.
Then, the diastolic pressure (DIA) is determined as the cuff pressure at
the time point when the pulse wave amplitude value corresponds to 70% of
the maximum pulse wave amplitude value in a conventional manner when the
pulse wave amplitude value is decreasing with decreasing cuff pressure.
And the systolic pressure is computed as follows:
SYS=L+0.32(L-DIA) (mmHg)
In other words, the systolic pressure (SYS) is computed by using the cuff
pressure L at the time when the pulse wave amplitude corresponds to 75% of
the maximum pulse wave amplitude value as a reference value, adding 32% of
the difference between the L value and DIA to the L value and determining
this sum as a systolic pressure.
FIGS. 2a and 2b constitute a flow chart showing the actual processing
action of the electronic blood pressure meter of the first embodiment.
When the action of the electronic blood pressure meter of the present
embodiment is initiated, the pressurization pump 4 is activated by a
signal b (refer to FIG. 1) from the CPU 9 and the cuff 1 is pressurized
(ST1 or step 1). When the cuff pressure has been increased to a
predetermined pressure level, it is detected by the CPU 9 (ST2) and the
pressurization process is completed by stopping the action of the
pressurization pump 4 (ST3). Thereafter, the vent valve 3 begins a gradual
venting action (ST4) controlled by a control signal a from the CPU 9
according to a certain program stored in the permanent memory incorporated
in the CPU 9 and the system flow advances to the succeeding stage of
determining blood pressure values which is described hereinafter.
First of all, variables n and Hmax are cleared in an initializing step
(ST5). The variable n is a count of the pulse wave which is incremmented
by the CPU 9 for each pulsation of the pulse wave while the variable Hmax
is a hold value of the current maximum pulse wave amplitude value for
detecting the ultimate maximum pulse wave amplitude value.
Then, variables i, Pmax and Pmin are cleared in another initialization step
(ST6). The variable i is an index value of the cuff pressure data A(i) and
the pulse wave data P(i) which are inputted to the CPU 9 from the A/D
converter 7 while the variables Pmax and Pmin retain the maximum and
minimum pulse wave amplitude values for computing the pulse wave amplitude
value for each pulsation of the pulse wave.
Thereafter, after the variable n is incremented (ST7) and the variable i is
likewise incremented (ST8), the pulse wave data P(i) is transmitted from
the A/D converter 7 to the CPU 9 according to a control signal c given to
the A/D converter 7 from the CPU 9 (ST9). The value of the pulse wave data
P(i) is then compared with Pmax (ST10) and, if P(i) is greater than Pmax,
Pmax is updated by replacing the value of Pmax with that of P(i) in ST11.
Then, the system flow advances to ST12. Otherwise or if P(i) is not
greater than Pmax, the system flow advances directly from ST10 to ST12.
In ST12, the value of P(i) is compared with Pmin and if P(i) is smaller
than Pmin the value of P(i) is substituted into Pmin for purpose of
updating Pmin (ST13) before the system flow advances to ST14. Otherwise,
the system flow advances directly from ST12 to ST14.
In ST14 it is determined whether a break in the pulse wave data has been
detected or not. A break of the pulse wave data is defined as a point of
intersection between the pulse wave data P(i) with a certain threshold
level TH2 as the pulse wave amplitude increases as indicated by each of
the arrows shown in FIG. 5 and gives a dividing point in the pulse wave
signal for each pulsation thereof.
When a break in the pulse wave has been detected, the system flow advances
to ST15 but if it is not detected the process of ST8 through ST14 is
repeated on the succeeding pulse wave data. Suppose that a break in the
pulse wave data has been detected. Then, the difference between Pmax and
Pmin is computed and this difference is substituted into the pulse wave
amplitude value H(n) (ST15).
The cuff pressure data is obtained from the A/D converter 7 and the cuff
pressure value A(n) corresponding to the pulse wave amplitude value H(n)
is supplied to the CPU 9 (ST16).
In ST17, it is determined whether H(n) is greater than Hmax or not. If H(n)
is greater than Hmax, then, the determination result of ST 17 is
affirmative and Hmax is updated by substituting the value of H(n) into
Hmax and the count n if the pulse wave counter is substituted into the
variable N to be retained therein (ST18). Thereafter, the system flow
returns to ST6 and the process in ST6 through ST18 is repeated. If the
pulse wave amplitude value has not reached the maximum value and is still
increasing, H(n) is always greater than Hmax and this updating process is
repeated until the maximum value of the pulse wave amplitude value has
been reached.
Suppose that H(n) has become smaller than Hmax. Then, the determination
result of ST17 is negative and the system flow advances to ST19. In ST19,
it is determined whether the pulse wave amplitude value H(n) is smaller
than 0.7 Hmax or not. In other words, it is determined whether the pulse
wave amplitude value has reached the level of 70% of the maximum value
while the pulse wave amplitude value is decreasing after reaching the
maximum value nor not. Here, when the pulse wave amplitude value
corresponds to 70% of the maximum pulse wave amplitude value the cuff
pressure represents the diastolic pressure.
Suppose that the pulse wave amplitude value H(n) is greater than 70% of the
maximum pulse wave amplitude value Hmax. Then, the determinatiion result
of ST19 is negative. Therefore, the system flow returns to ST6 and the
process in ST7 through ST19 is repeated. Suppose that the pulse wave
amplitude value has reached 70% of the maximum pulse wave amplitude value.
Then, the determination result of ST19 is affirmative and the cuff
pressure A(n) at that moment is determined to be the diastolic pressure
(DIA) (ST20). Thereafter, the systolic pressure is determined in ST21
through the process which is described hereinafter and the measurement is
completed after rapidly venting the cuff (ST22) and displaying the
systolic and diastolic pressure value on the display unit 10 (ST23).
FIG. 3 is a flow chart of the subroutine for actually determining the
systolic pressure in ST21.
First of all, the number N of the maximum pulse wave amplitude value is
substituted into the variable j (ST101) and the variable j is decremented
(ST102). In succeeding ST103, it is determined whether the pulse wave
amplitude value H(j) is smaller than 0.75 Hmax or not. In other words, the
cuff pressure L (at the time point when the pulse wave amplitude value
corresponding to 75% of the maximum pulse wave amplitude value) which
serves as a reference value for determining the systolic pressure as shown
in FIG. 4 is now going to be obtained.
If the pulse wave amplitude value H(j) is greater than 75% of the maximum
pulse wave amplitude value, the determination result of ST103 is negative
and the system flow returns to ST102. In other words, as long as the pulse
wave amplitude value H(j) indexed by the variable j is not smaller than
0.75 Hmax, the process in ST102 and ST103 is repeated.
Suppose that the pulse wave amplitude value H(j) has become smaller than
75% of the maximum pulse wave amplitude value, then, the determination
result of ST103 is affirmative and the cuff pressure A(j) when the H(j)
has fallen below the critical value is set as the pressure value L
(ST104).
Then, the systolic pressure value is computed according to the following
formula (ST105) and the system flow returns from the subroutine for
computing the systolic pressure to the main flow.
SYS=L+0.32(L-DIA)
In the above described embodiment, the systolic pressure (SYS) was derived
by finding the cuff pressure value L at the time point when the pulse wave
amplitude value corresponds to 75% of the maximum pulse wave amplitude
value. But, according to the broadest concept of the present invention, it
is also possible to derive the systolic pressure normally from the cuff
pressure at the time point when the pulse wave amplitude corresponds to
50% of the maximum pulse wave amplitude value but use the cuff pressure at
the time point when the pulse wave amplitude value corresponds to 75% of
the maximum pulse wave amplitude value for deriving the systolic pressure
only when the initial cuff pressure was so low that the pulse wave
amplitude value corresponding to 50% of the maximum pulse wave amplitude
value pressure was not detected.
FIG. 6 shows an alternative subroutine for determining the systolic
pressure in accordance with a second embodiment of the present invention
which is different from the first embodiment in the way ST21 is carried
out. This embodiment is characterized by the process of determining the
systolic pressure, and the diastolic pressure may be determined in the
same manner as in the first embodiment.
In this embodiment, the systolic pressure is computed from the cuff
pressure at the time point when the pulse wave amplitude corresponds to
75% of the maximum pulse wave amplitude value in accordance with the above
mentioned formula only when the pulse wave amplitude corresponding to 50%
of the maximum pulse wave amplitude was not detected, for instance, due to
insufficient initial pressurization of the cuff. Otherwise, the cuff
pressure at the time point when the pulse wave amplitude corresponds to
50% of the maximum pulse wave amplitude value is directly determined to be
the systolic pressure.
First of all, the number N of the maximum pulse wave amplitude value is
substituted into the variable j (ST201). Then, in ST202, it is determined
whether the ratio H(1)/Hmax is less than 0.5 or not. If the ratio is less
than 0.5, the system flow advances to the process of ST203 through ST205
and the systolic pressure is determined from the cuff pressure at which
the pulse wave amplitude value H(j) corresponds to 50% of the maximum
pulse wave amplitude value Hmax. If the ratio H(1)/Hmax is equal to or
greater than 0.5 in ST202, the process in ST206 through ST209 is carried
out and after the pressure value L (the cuff pressure at which the pulse
wave amplitude value corresponds to 75% of the maximum pulse wave
amplitude value) is determined the systolic pressure is computed from this
pressure value L in accordance with the above mentioned formula.
When the system flow advances from ST202 to ST203, the variable j is
decremented (ST203) and it is then determined whether the pulse wave
amplitude H(j) specified by the variable j is less than 0.5 Hmax (ST204)
or not. If H(j) is not less than 0.5 Hmax, the determination result of
ST204 is negative and the system flow returns to ST203. Thus, the process
of ST203 and ST204 is repeated until the value of H(j) specified by the
variable j becomes less than 0.5 Hmax. Suppose that the pulse wave
amplitude value H(j) has become less than 0.5 Hmax. Then, since the
determination result of ST204 is affirmative, the current cuff pressure
A(j) specified by the variable j is determined to be the systolic pressure
(SYS) (ST205) and the system flow returns to the main flow from this
subroutine for determining the systolic pressure.
Meanwhile, when the system flow advances from ST202 to ST206, the variable
j is decremented (ST206) and it is determined whether the pulse wave
amplitude value H(j) is less than 0.75 Hmax or not (ST207). When H(j) is
not less than 0.75 Hmax, the system flow returns to ST206 and the process
in ST206 and ST207 is repeated until H(j) gets less than 0.75 Hmax. When
the H(j) has become less than 0.75 Hmax, the determination result of ST207
is affirmative and the cuff pressure A(j) which is specified by the
variable j is set as the pressure value L (ST208). Thus, the systolic
pressure is computed from the following equation (ST209) and the system
flow returns to the main flow from this subroutine for determining the
systolic pressure.
SYS=L+0.32(L-DIA)
FIG. 8 illustrates the process of computing blood pressure values according
to the third embodiment of the present invention. In this graph, P (%)
denotes the ratio of the pressure difference between a pressure value M
and the systolic pressure SYS to the pressure difference between the
pressure value M and the diastolic pressure DIA, the pressure value M
being the cuff pressure at the same point when the pulse wave amplitude
value corresponds to X% of the maximum pulse wave amplitude value. And
Table 1 given below shows the relationship between X(%) and P(%) which was
obtained from a statistical study made on 225 persons.
TABLE 1
______________________________________
X (%) P (%)
______________________________________
80.0 44.31
75.0 33.42
70.0 22.17
65.0 15.95
60.0 10.33
55.0 4.96
50.0 0.00
______________________________________
In the first embodiment the pulse wave amplitude value (L) corresponding to
75% of the maximum pulse wave amplitude value was used as a reference
value in computing a systolic pressure, but in the present embodiment, as
shown in FIG. 8, the pulse wave amplitude value corresponding to X% of the
maximum pulse wave amplitude value is first determined in order to find
the difference between the cuff pressure M at the time point when this X%
pulse wave amplitude value was detected and DIA which may be derived in
any conventional method, and the systolic pressure is determined as a sum
of P% of this difference and the cuff pressure M corresponding to the X%
pulse wave amplitude value.
In other words, in the present embodiment, it is evaluated when a first
pulse wave amplitude value which may vary depending on the degree of the
initial cuff pressure has occurred in relation with the occurrence of the
maximum pulse wave amplitude value and the determination of SYS is based
on a selection process in which if the first pulse wave amplitude value
(X% pulse wave amplitude value) is less than 50% of the maximum pulse wave
amplitude value, then, SYS is determined in the same way as in the
conventional process and if the first pulse wave amplitude value is
greater than 50%, then, SYS is determined according to a process which is
described hereinafter.
Since the cuff pressure when the ratio of the pulse wave amplitude value to
the maximum value is X(%) is M and the ratio of the pressure difference
between M and the systolic pressure SYS to the pressure difference between
M and the diastolic pressure DIA is P(%) by definition, the systolic
pressure SYS can be expressed as follows:
SYS=M+(P/100) (M-DIA) (mmHg)
In this embodiment, the relationship between P and X is found with a
statistical technique and is introduced into the process of computing the
systolic pressure. Table 1 shows the results obtained from the statistical
data of the relationship between P and X of 225 samples. FIG. 9 shows this
P-X relationship in the form of a graph. In this graph, the statistically
obtained P-X curve is shown by the solid line while the broken line is an
approximation of this curve which can be expressed by the following
formula:
P=(x-36).sup.2 /40-5
FIG. 7 shows another alternative subroutine for determining the systolic
pressure in accordance with a third embodiment of the present invention
which is different from the previous embodiments in the way ST21 is
carried out. And the working principle of this embodiment was described
above with reference to FIGS. 8 and 9. This embodiment is characterized by
the process of determining the systolic pressure, and the diastolic
pressure may be determined in the same manner as in the first embodiment.
First of all, the means for determining the systolic pressure (which may be
implemented by the functions of the CPU 9) in this embodiment detects what
fraction X(%) of the maximum pulse wave amplitude value the first detected
pulse wave amplitude value H(1) corresponds to, immediately after the
start of blood pressure measurement. This fraction X% is computed with the
following formula (ST306).
X=H(1).100/Hmax
This fraction X(%) is the ratio of the first pulse wave amplitude value
H(1) to the maximum pulse wave amplitude value Hmax. In succeeding ST307,
it is determined whether X is greater than 50% or not. In this embodiment,
the method for determining the systolic pressure is selected depending on
whether the first detected pulse wave amplitude value is less than 50% of
the maximum pulse wave amplitude value or not.
Suppose that the cuff pressurization was sufficient and a weak pulse wave
was obtained from the beginning, or, in other words, that the value of
H(1) is equal to or less than 50% of the maximum pulse wave amplitude.
Then, the system flow advances to ST308 where the initial value of 1 is
substituted into the pointer j, and j is incremented by 1 (ST309). Then,
the pulse wave amplitude value H(j) specified by the value of j is
compared with 50% of the maximum pulse wave amplitude value (ST310). If
H(j) is greater than 0.5 Hmax the determination result in ST310 is
affirmative and the cuff pressure value A(j-1) preceding the current cuff
pressure specified by the pointer j is determined to be the systolic
pressure (ST311). In other words, when the pulse wave amplitude value H(j)
is less than 50% of the maximum pulse wave amplitude value, the systolic
pressure is determined in the same way as in the conventional method. On
the other hand, if the pulse wave amplitude value H(1) is found to be
greater than 50% of the maximum pulse wave amplitude value, the
determination result oof ST307 is negative and the system flow advances to
ST312.
In ST312, P is computed with the following formula:
P=(X-36).sup.2 /40-5
and the systolic pressure is computed as given by the following formula:
SYS=A(1)+[P/100][A(1)-DIA]
FIG. 12 illustrates the principle of | | |
