Abstrict
A blood pressure monitor including a cuff, a pressure sensor which
detects a pressure in the cuff, a pressure regulating device which
increases the pressure of the cuff, a pulse-amplitude determining
device for determining an amplitude of each of pulses of a pulse
wave which are produced in the cuff and detected by the pressure
sensor while the cuff pressure is increased, a candidate determining
device for determining, as a diastolic BP candidate, a pressure
of the cuff which is detected by the pressure sensor and which corresponds
to an amplitude of a first pulse of the pulses determined by the
pulse-amplitude determining device, by judging whether the amplitude
of the first pulse is not greater than a reference value which is
smaller than an amplitude of at least one second pulse of the pulses,
by a predetermined proportion of the amplitude of the second pulse,
the amplitude of the second pulse being determined by the pulse-amplitude
determining device after the amplitude of the first pulse is determined,
and a BP determining device for determining, as a monitor diastolic
BP value, the cuff pressure corresponding to the amplitude of the
first pulse, when the candidate determining device determines, as
the diastolic BP candidate, the cuff pressure corresponding to the
amplitude of the first pulse, with respect to a predetermined number
of the one or more second pulses.
Claims
What is claimed is:
1. A blood pressure monitor comprising:
an inflatable cuff which is adapted to be wound around a body portion
of a living subject to press said body portion through which an
artery of the subject extends;
a blood pressure measuring device which measures a blood pressure
of the subject by changing a pressure in said cuff;
a pressure pulse wave sensor which is adapted to be pressed against
a distal section of said artery located on a distal side of said
cuff wound around said body portion, so as to detect a pressure
pulse wave which is produced from said distal section of the artery
and is propagated thereto via a skin tissue above said distal section;
relationship determining means for determining a relationship between
blood pressure and magnitude of pressure pulse wave, based on the
blood pressure measured by said blood pressure measuring device
and a magnitude of the pressure pulse wave detected by said pressure
pulse wave sensor;
blood pressure determining means for determining at least a diastolic
blood pressure of the subject according to the determined relationship
based on a magnitude of a lower-peak point of each of successive
first heartbeat-synchronous pulses of said pressure pulse wave detected
by said pressure pulse wave sensor;
cuff-pressure increasing means for increasing said pressure of
said cuff at a predetermined rate;
a cuff pulse wave sensor which detects a cuff pulse wave which
is a pressure oscillation produced in said cuff;
phase-difference determining means for determining a phase difference
of respective lower-peak points of each of successive second heartbeat-synchronous
pulses of said pressure pulse wave and a corresponding one of successive
heartbeat-synchronous pulses of said cuff pulse wave, said second
heartbeat-synchronous pulses of said pressure pulse wave and said
heartbeat-synchronous pulses of said cuff pulse wave being detected
by said pressure pulse wave sensor and said cuff pulse wave sensor,
respectively, when said pressure of said cuff is increased at said
predetermined rate by said cuff-pressure increasing means; and
judging means for judging whether said determined relationship
is accurate, based on at least one diastolic blood pressure determined
by said blood pressure determining means and a pressure of said
cuff corresponding to a time when the phase differences determined
by said phase-difference determining means significantly largely
change.
2. The blood pressure monitor according to claim 1, wherein the
judging means comprises means for differentiating, with respect
to time, the respective phase differences determined by the phase
difference determining means, and determining a time corresponding
to the greatest differential, as the time when the phase differences
significantly largely change.
3. The blood pressure monitor according to claim 2, wherein the
judging means comprises means for judging whether an absolute value
of a difference between the last diastolic blood pressure determined
by the blood pressure determining means and the pressure of the
cuff corresponding to the time corresponding to the greatest differential
is not greater than a reference value.
4. The blood pressure monitor according to claim 3, wherein the
blood pressure measuring device comprises means for measuring a
diastolic blood pressure of the subject by changing the pressure
in the cuff, and wherein the judging means comprises means for judging
whether the absolute value of the difference is not greater than
a modified reference value which is obtained by subtracting, from
the reference value, a difference between the measured diastolic
blood pressure and the pressure of the cuff corresponding to the
time corresponding to the greatest differential.
5. The blood pressure monitor according to claim 1, wherein the
blood pressure measuring device comprises periodic blood pressure
measuring means for periodically measuring the blood pressure of
the subject at a predetermined period, and wherein the cuff pressure
increasing means, the phase difference determining means, and the
judging means are operated at the predetermined period, and the
periodic blood pressure measuring means is operated when a negative
judgment is made by the judging means and is not operated when a
positive judgment is made by the judging means.
6. A blood pressure monitor, comprising:
at least one inflatable cuff which is adapted to be worn on at
least one of a first body portion of a living subject to press the
first body portion through which a first artery ot the subject extends
and a second body portion of the subject to press the second body
portion through which a second artery of the subject extends,
a blood pressure measuring device which measures a blood pressure
of the subject by changing a pressure in the at least one inflatable
cuff,
a pressure pulse wave sensor which is adapted to be worn on a third
portion of the subject to press the third portion through which
a third artery of the subject extends, the pressure pulse wave sensor
detecting a pressure pulse wave which is produced from the third
artery and is propagated thereto via skin tissue,
relationship determining means for determining a relationship between
blood pressure and magnitude of the pressure pulse wave, based on
the blood pressure measured by the blood pressure measuring device
and a magnitude of the pressure pulse wave detected by the pressure
pulse wave sensor,
blood pressure determining means for determining at least a diastolic
blood pressure of the subject, according to the determined relationship,
based on a magnitude of a lower peak point of each of successive
first heartbeat-synchronous pulses of the pressure pulse wave detected
by the pressure pulse wave sensor,
cuff pressure increasing means for increasing a pressure in the
at least one inflatable cuff at a predetermined rate,
a cuff pulse wave sensor which detects a cuff pulse wave which
is a pressure oscillation produced in the at least one inflatable
cuff,
phase-difference determining means for determining a phase difference
of respective lower peak points of each of successive second heartbeat-synchronous
pulses of the pressure pulse wave and a corresponding one of successive
heartbeat-synchronous pulses of the cuff pulse wave, the second
heartbeat-synchronous pulses of the pressure pulse wave and the
heartbeat-synchronous pulse of the cuff pulse wave being detected
by the pressure pulse wave sensor and the cuff pulse wave sensor,
respectively, when the pressure of the at least one inflatable cuff
is increased at the predetermined rate by the cuff pressure increasing
means,
judging means for judging whether the determined relationship is
accurate, based on at least one diastolic blood pressure determined
by the blood pressure determining means and a pressure of the at
least one inflatable cuff corresponding to a time when the phase
difference determined by the phase difference determining means
significantly largely changes.
7. The blood pressure monitor according to claim 6, wherein the
at least one inflatable cuff comprises a single cuff, the cuff pressure
increasing means increases the pressure in the single cuff, and
the cuff pulse wave sensor detects the cuff pulse wave from the
first cuff.
8. The blood pressure monitor according to claim 6, wherein the
at least one inflatable cuff comprises a first cuff adapted to be
wound around an upper arm of one of the two hands of the subject,
and a second cuff adapted to be worn on the wrist of the other hand.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a blood pressure monitor including
an inflatable cuff.
2. Related Art Statement
There is known a blood pressure (BP) monitor which includes an
inflatable cuff adapted to be wound around a body portion, e.g.,
upper arm, of a living subject, e.g., patient, to press the body
portion. The BP monitor functions as an automatic BP measuring device
which periodically measures a BP value of the subject by increasing
the cuff pressure and thereby pressing the body portion of the subject.
However, if the period or interval of the BP measurements effected
by the BP monitor is shortened for improving the accuracy of monitoring
of subject's blood pressure, the frequency of pressing of subject's
body portion is increased, which causes the subject to feel discomfort.
In the above-indicated background, it has been proposed to increase
the pressure of an inflatable cuff being wound around a body portion
of a living subject, up to a predetermined value, detect a pulse
wave that is a pressure oscillation produced in the cuff, and estimate
a BP value of the subject based on the magnitude of the pulse wave.
This technique is disclosed in, e.g., Japanese Patent Application
laid open for inspection purposes under Publication No. 61(1986)-103432,
or Japanese Patent Application laid open for inspection purposes
under Publication No. 60(1985)-241422.
Regarding the above-indicated conventional BP monitor techniques,
however, there are known some cases where it is difficult to detect
a change of magnitudes of pulse waves which reflects a change of
blood pressure of a living subject, if BP values are estimated based
on the pulse waves detected at a considerably low cuff pressure,
which contributes to reducing the discomfort felt by the subject.
More specifically described, respective amplitudes of pulses of
a pulse wave which is detected from an inflatable cuff being wound
around a body portion of a living subject whose blood pressure is
normal, has an envelope indicated at solid line in the graph of
FIG. 6. In contrast, amplitudes of pulses of a pulse wave obtained
from a living subject whose blood pressure is low, has an envelope
indicated at one-dot chain line in FIG. 6. In the case where amplitudes
of pulses of a pulse wave are detected at a considerably low cuff
pressure, e.g., pressure, P.sub.K, in FIG. 6, an amount of change
of the pulse amplitudes with respect to an amount of change of blood
pressure of a living subject may be too small. Thus, when the BP
monitor is used at the low cuff pressure P.sub.K, it may not be
able to monitor the blood pressure of the subject with high accuracy.
There is also known a continuous BP monitor which includes an inflatable
cuff which is adapted to be wound around a body portion of a living
subject to press the body portion; a blood pressure measuring device
which measures a blood pressure of the subject by changing a pressure
in the cuff; a pressure pulse wave sensor which is adapted to be
pressed against a distal section of the artery located on a distal
side of the cuff wound around the body portion, so as to detect
a pressure pulse wave which is produced from the distal section
of the artery; a relationship determining means which determines
a relationship between blood pressure and magnitude of pressure
pulse wave, based on the blood pressure measured by the blood pressure
measuring device and a magnitude of the pressure pulse wave detected
by the pressure pulse wave sensor; a blood pressure determining
means which successively determines a blood pressure of the subject
according to the determined relationship based on a magnitude of
each of successive heartbeat-synchronous pulses of the pressure
pulse wave detected by the pressure pulse wave sensor; and a display
which displays the blood pressure values determined by the blood
pressure determining means. This BP monitor is disclosed in, e.g.,
Japanese Patent Application laid open for inspection purposes under
Publication No. 1(1989)-214338 or Japanese Utility Model Application
laid open for inspection purposes under Publication No. 2(1990)-82309.
In the prior continuous BP monitor, the condition under which the
pressure pulse wave sensor is pressed against subject's artery may
be changed due to, e.g., a physical motion of the subject. Hence,
in order to improve 5 the accuracy of BP values determined by the
BP determining means, the relationship between blood pressure and
magnitude of pressure pulse wave is updated at a predetermined period.
However, the updating of the relationship needs a blood pressure
measurement of the blood pressure measuring device including the
inflation of the cuff. In addition, since the pressure pulse wave
sensor is set on the distal side of the cuff, the continuous BP
determination of the BP determining means is interrupted by the
inflation of the cuff. This problem is exaggerated if the period
of updating of the relationship is shortened for improving the accuracy
of the continuous BP monitoring.
SUMMARY OF THE INVENTION
It is a first object of the present invention to provide a blood
pressure monitor which includes an inflatable cuff and which monitors
with high accuracy the blood pressure of a living subject without
causing the subject to feel discomfort.
It is a second object of the present invention to provide a continuous
blood pressure monitor which includes an inflatable cuff and which
continuously monitors the blood pressure of a living subject with
reduced discomfort felt by the subject and with reduced interruption
frequency.
The first object may be achieved according to a first aspect of
the present invention, which provides a blood pressure monitor including
an inflatable cuff which is adapted to be wound around a body portion
of a living subject to press the body portion, a pressure sensor
which detects a pressure in the cuff, a cuff-pressure regulating
device which increases the pressure of the cuff, pulse-amplitude
determining means for determining an amplitude of each of pulses
of a pulse wave which are produced in the cuff and detected by the
pressure sensor while the pressure of the cuff is increased by the
cuff-pressure regulating device, candidate determining means for
determining, as a diastolic blood pressure candidate, a pressure
of the cuff which is detected by the pressure sensor and which corresponds
to an amplitude of a first pulse of the pulses determined by the
pulse-amplitude determining means, by judging whether the amplitude
of the first pulse is not greater than a reference value which is
smaller than an amplitude of at least one second pulse of the pulses,
by a predetermined proportion of the amplitude of the second pulse,
the amplitude of the second pulse being determined by the pulse-amplitude
determining means after the amplitude of the first pulse is determined,
and blood-pressure determining means for determining, as a monitor
diastolic blood pressure value, the pressure of the cuff corresponding
to the amplitude of the first pulse, when the candidate determining
means determines, as the diastolic blood pressure candidate, the
pressure of the cuff corresponding to the amplitude of the first
pulse, with respect to a predetermined number of the at least one
second pulse.
In the blood pressure (BP) monitor in accordance with the first
aspect of the invention, the predetermined number may be one, two,
or a greater number. For example, the predetermined number is three.
Thus, the presert BP monitor may determine a monitor diastolic BP
value of a living subject at a pressure level which is higher than
the diastolic BP value and which corresponds to the "third"
one of the subsequent pulses determined after the initial pulse.
The thus determined monitor diastolic BP value enjoys high accuracy.
In addition, since the pressure level where the monitor diastolic
BP value is determined is considerably low, the subject does not
feel discomfort.
According to a preferred feature of the first aspect of the invention,
the candidate determining means comprises judging means for judging
whether the amplitude of the first pulse is not greater than a reference
value which is smaller than an amplitude of each of a plurality
of second pulses of the pulses, by a predetermined proportion of
the amplitude of the each second pulse, the respective amplitudes
of the second pulses being determined by the pulse-amplitude determining
means after the amplitude of the first pulse is determined.
According to another feature of the first aspect of the invention,
the cuff-pressure regulating device comprises pressure increasing
means for stepwise increasing the pressure of the cuff by alternately
increasing the cuff pressure and maintaining the cuff pressure at
each of a plurality of different pressure values, and the pulse-amplitude
determining means determines an amplitude of at least one pulse
which is produced in the cuff and detected by the pressure sensor
while the cuff pressure is maintained at the each pressure value.
The pressure increasing means may increase the cuff pressure by
a constant pressure increase amount, for each time or step, or may
increase the cuff pressure by an increase amount which is variable
depending upon the current cuff pressure. The pulse-amplitude determining
means may determine an amplitude of a single pulse detected by the
pressure sensor while the cuff pressure is maintained at each pressure
value, or an average of respective amplitudes of two or more pulses
detected while the cuff pressure is maintained at each pressure
value. The thus determined pulse amplitude or amplitudes enjoy high
accuracy because they are free from adverse influences resulting
from the increasing of the cuff pressure. Therefore, the monitor
diastolic BP values of the subject are determined with accuracy
based on the pulse amplitudes.
According to another feature of the first aspect of the invention,
the blood-pressure determining means comprises monitor means for
iteratively determining the monitor diastolic blood pressure value.
The monitor means may periodically determine the monitor diastolic
blood pressure value at a predetermined period or interval of time
(i.e., monitor cycle time).
According to another feature of the first aspect of the invention,
the BP monitor further comprises abnormality identifying means for
identifying an abnormality of the monitor diastolic blood pressure
values iteratively determined by the monitor means.
According to another feature of the first aspect of the invention,
the abnormality identifying means comprises means for identifying
the abnormality based on at least one of an amount of change of
a last determined value of the monitor diastolic blood pressure
values from an average of the monitor diastolic blood pressure values,
and a rate of change of the last determined value of the monitor
diastolic blood pressure values from the average of the monitor
diastolic blood pressure values.
According to another feature of the first aspect of the invention,
the BP monitor further comprising a blood pressure measuring device
which increases the pressure of the cuff up to a target pressure
which is higher than a systolic blood pressure of the subject and
measures at least one of a systolic, a mean, and a diastolic blood
pressure value of the living subject based on a variation of respective
amplitudes of pulses of a pulse wave which are produced in the cuff
and detected by the pressure sensor during at least one of the increasing
of the cuff pressure up to the target pressure and a decreasing
of the cuff pressure down from the target pressure. The target pressure
may be, e.g., about 180 mmHg that is estimated to be sufficiently
higher than a normal systolic BP value of a human being.
According to another feature of the first aspect of the invention,
the blood pressure measuring device comprises means for measuring
the at least one of the systolic, the mean, and the diastolic blood
pressure value of the living subject when the abnormality identifying
means identifies the abnormality.
According to another feature of the first aspect of the invention,
the BP monitor further comprising a display which displays the monitor
diastolic blood pressure value determined by the blood-pressure
determining means.
The second object may be achieved according to a second aspect
of the present invention, which provides a blood pressure monitor
comprising an inflatable cuff which is adapted to be wound around
a body portion of a living subject to press the body portion through
which an artery of the subject extends; a blood pressure measuring
device which measures a blood pressure of the subject by changing
a pressure in the cuff; a pressure pulse wave sensor which is adapted
to be pressed against a distal section of the artery located on
a distal side of the cuff wound around the body portion, so as to
detect a pressure pulse wave which is produced from the distal section
of the artery and is propagated thereto via a skin tissue above
the distal section; relationship determining means for determining
a relationship between blood pressure and magnitude of pressure
pulse wave, based on the blood pressure measured by the blood pressure
measuring device and a magnitude of the pressure pulse wave detected
by the pressure pulse wave sensor; blood pressure determining means
for successively determining at least a diastolic blood pressure
of the subject according to the determined relationship based on
a magnitude of a lower-peak point of each of successive first heartbeat-synchronous
pulses of the pressure pulse wave detected by the pressure pulse
wave sensor; cuff-pressure increasing means for increasing the pressure
of the cuff at a predetermined rate; wavetorm-characteristic determining
means for determining a characteristic of a lower-peak portion of
a waveform of each of successive second heartbeat-synchronous pulses
of the pressure pulse wave which are detected by the pressure pulse
wave sensor when the pressure of the cuff is increased at the predetermined
rate by the cuff-pressure increasing means, the lower-peak portion
including a lower-peak point of the each second heartbeat-synchronous
pulse; and judging means for judging whether the determined relationship
is accurate, based on at least one diastolic blood pressure determined
by the blood pressure determining means and a pressure of the cuff
corresponding to a time when the waveform characteristics determined
by the waveform- characteristic determining means significantly
largely change.
In the blood pressure monitor in accordance with the second aspect
of the invention, if the judging means makes a positive judgment,
the relationship need not be updated. Accordingly, the blood pressure
measuring device does not inflate the cuff, and the subject is prevented
from being pressed by the cuff. In addition, although the pressure
pulse wave sensor is set on the distal side of the cuff, the blood
pressure determining means can continue to successively determine
blood pressure values according to the relationship based on the
pressure pulse wave detected by the pressure pulse wave sensor.
The second object may be achieved according to a third aspect of
the present invention, which provides a blood pressure monitor comprising
an inflatable cuff which is adapted to be wound around a body portion
of a living subject to press the body portion through which an artery
of the subject extends; a blood pressure measuring device which
measures a blood pressure of the subject by changing a pressure
in the cuff; a pressure pulse wave sensor which is adapted to be
pressed against a distal section of the artery located on a distal
side of the cuff wound around the body portion, so as to detect
a pressure pulse wave which is produced from the distal section
of the artery and is propagated thereto via a skin tissue above
the distal section; relationship determining means for determining
a relationship between blood pressure and magnitude of pressure
pulse wave, based on the blood pressure measured by the blood pressure
measuring device and a magnitude of the pressure pulse wave detected
by the pressure pulse wave sensor; blood pressure determining means
for determining at least a diastolic blood pressure of the subject
according to the determined relationship based on a magnitude of
a lower-peak point of each of successive first heartbeat-synchronous
pulses of the pressure pulse wave detected by the pressure pulse
wave sensor; cuff-pressure increasing means for increasing the pressure
of the cuff at a predetermined rate; a cuff pulse wave sensor which
detects a cuff pulse wave which is a pressure oscillation produced
in the cuff; phase-difference determining means for determining
a phase difference of respective lower-peak points of each of successive
second heartbeat-synchronous pulses of the pressure pulse wave and
a corresponding one of successive heartbeat-synchronous pulses of
the cuff pulse wave, the second heartbeat-synchronous pulses of
the pressure pulse wave and the heartbeat-synchronous pulses of
the cuff pulse wave being detected by the pressure pulse wave sensor
and the cuff pulse wave sensor, respectively, when the pressure
of the cuff is increased at the predetermined rate by the cuff-pressure
increasing means; and judging means for judging whether the determined
relationship is accurate, based on at least one diastolic blood
pressure determined by the blood pressure determining means and
a pressure of the cuff corresponding to a time when the phase differences
determined by the phase-difference determining means significantly
largely change.
In the blood pressure monitor in accordance with the third aspect
of the invention, if the judging means makes a positive judgment,
the relationship need not be updated. Accordingly, the blood pressure
measuring device does not inflate the cuff, and the subject is prevented
from being pressed by the cuff. In addition, although the pressure
pulse wave sensor is set on the distal side of the cuff, the blood
pressure determining means can continue to successively determine
blood pressure values according to the relationship based on the
pressure pulse wave detected by the pressure pulse wave sensor.
The second object may be achieved according to a fourth aspect
of the present invention, which provides a blood pressure monitor
comprising an inflatable cuff which is adapted to be wound around
a body portion of a living subject to press the body portion through
which an artery of the subject extends; a blood pressure measuring
device which measures a blood pressure of the subject by changing
a pressure in the cuff; a pressure pulse wave sensor which is adapted
to be pressed against a distal section of the artery located on
a distal side of the cuff wound around the body portion, so as to
detect a pressure pulse wave which is produced from the distal section
of the artery and is propagated thereto via a skin tissue above
the distal section; relationship determining means for determining
a relationship between blood pressure and magnitude of pressure
pulse wave, based on the blood pressure measured by the blood pressure
measuring device and a magnitude of the pressure pulse wave detected
by the pressure pulse wave sensor; blood pressure determining means
for determining at least a mean blood pressure of the subject according
to the determined relationship based on a mean magnitude of each
of successive first heartbeat-synchronous pulses of the pressure
pulse wave detected by the pressure pulse wave sensor; cuff-pressure
increasing means for increasing the pressure of the cuff at a predetermined
rate; pulse-area calculating means for calculating an area defined
by each of successive second heartbeat-synchronous pulses of the
pressure pulse wave which are detected by the pressure pulse wave
sensor when the pressure of the cuff is increased at the predetermined
rate by the cuff-pressure increasing means; half-area identifying
means for identifying that the pulse areas calculated by the pulse-area
calculating means have decreased to half an initial pulse area obtained
before the cuff-pressure increasing means starts increasing the
pressure of the cuff; and judging means for judging whether the
determined relationship is accurate, based on at least one mean
blood pressure determined by the blood pressure determining means
and a pressure of the cuff corresponding to a time when the half-area
identifying means identifies that the pulse areas calculated by
the pulse-area calculating means have decreased to half the initial
pulse area.
In the blood pressure monitor in accordance with the fourth aspect
of the invention, if the judging means makes a positive judgment,
the relationship need not be updated. Accordingly, the blood pressure
measuring device does not inflate the cuff, and the subject is prevented
from being pressed by the cuff. In addition, although the pressure
pulse wave sensor is set on the distal side of the cuff, the blood
pressure determining means can continue to successively determine
blood pressure values according to the relationship based on the
pressure pulse wave detected by the pressure pulse wave sensor.
The second object may be achieved according to a fifth aspect of
the present invention, which provides a blood pressure monitor comprising
an inflatable cuff which is adapted to be wound around a body portion
of a living subject to press the body portion through which an artery
of the subject extends; a blood pressure measuring device which
measures a blood pressure of the subject by changing a pressure
in the cuff; a pressure pulse wave sensor which is adapted to be
pressed against a distal section of the artery located on a distal
side of the cuff wound around the body portion, so as to detect
a pressure pulse wave which is produced from the distal section
of the artery and is propagated thereto via a skin tissue above
the distal section; relationship determining means for determining
a relationship between blood pressure and magnitude of pressure
pulse wave, based on the blood pressure measured by the blood pressure
measuring device and a magnitude of the pressure pulse wave detected
by the pressure pulse wave sensor; blood pressure determining means
for determining a blood pressure of the subject according to the
determined relationship based on a magnitude of each of successive
first heartbeat-synchronous pulses of the pressure pulse wave detected
by the pressure pulse wave sensor; cuff-pressure regulating means
for increasing the pressure of the cuff up to a predetermined value
and holding the cuff pressure at the predetermined value; pulse-area
calculating means for calculating an area defined by each of successive
second heartbeat-synchronous pulses of the pressure pulse wave which
are detected by the pressure pulse wave sensor when the cuff pressure
is held at the predetermined value by the cuff-pressure regulating
means; and judging means for judging whether the determined relationship
is accurate, based on a ratio of the calculated area of at least
one the second heartbeat-synchronous pulse of the pressure pulse
wave detected by the pressure pulse wave when the cuff pressure
is held at the predetermined value by the cuff-pressure regulating
means, to an initial pulse area obtained before the cuff-pressure
regulating means starts increasing the cuff pressure.
In the blood pressure monitor in accordance with the fifth aspect
of the invention, if the judging means makes a positive judgment,
the relationship need not be updated. Accordingly, the blood pressure
measuring device does not inflate the cuff, and the subject is prevented
from being pressed by the cuff. In addition, although the pressure
pulse wave sensor is set on the distal side of the cuff, the blood
pressure determining means can continue to successively determine
blood pressure values according to the relationship based on the
pressure pulse wave detected by the pressure pulse wave sensor.
The first object may be achieved according to a sixth aspect of
the present invention, which provides a blood pressure monitor comprising
an inflatable cuff which is adapted to be wound around a body portion
of a living subject to press the body portion through which an artery
of the subject extends; a blood pressure measuring device which
measures a blood pressure of the subject by changing a pressure
in the cuff; a cuff pulse wave sensor which detects a cuff pulse
wave which is a pressure oscillation produced in the cuff; a distal
pulse wave sensor which detects a distal pulse wave from a distal
section of the artery located on a distal side of the cuff wound
around the body portion; cuff-pressure increasing means for increasing
the pressure of the cuff at a predetermined rate; first peak-interval
determining means for determining a first interval between an upper-peak
point and a lower-peak point of each of first heartbeat- synchronous
pulses of the distal pulse wave which are detected by the distal
pulse wave sensor when the pressure of the cuff is increased at
the predetermined rate by the cuff-pressure increasing means; second
peak-interval determining means for determining a second interval
between an upper-peak point and a lower-peak point of each of second
heartbeat-synchronous pulses of the cuff pulse wave which are detected
by the cuff pulse wave sensor when the pressure of the cuff is increased
at the predetermined rate by the cuff-pressure increasing means;
difference determining means for determining a difference between
the first interval of the each of the first heartbeat-synchronous
pulses and the second interval of a corresponding one of the second
heartbeat-synchronous pulses; and blood pressure determining means
for determining, as a diastolic blood pressure of the subject, a
pressure of the cuff corresponding to a time when the differences
determined by the difference determining means significantly largely
change.
In the blood pressure monitor in accordance with the sixth aspect
of the invention, the upper-peak and lower-peak points of each pulse
of the cuff pressure wave are not influenced by the increasing of
the cuff pressure, whereas the upper-peak and lower-peak points
of each pulse of the distal pulse wave are influenced by the increasing
of the cuff pressure, because the distal pulse wave sensor is set
on the distal side of the cuff. Therefore, the peak-interval differences
are influenced by the increasing of the cuff pressure. The Inventors
have found that the phase of the distal pulse wave has a certain
relationship with that of the cuff pulse wave and that this relationship
significantly largely changes when the cuff pressure becomes equal
to a diastolic pressure of the subject. Thus, a cuff pressure corresponding
to the time when the peak-interval differences significantly largely
change, can be determined as a diastolic pressure of the subject.
In the case where a physiological change such as arrhythmia occurs
to the heart of the patient, respective waveforms of the cuff pulse
wave and the distal pulse wave change in a similar manner, therefore
the peak-interval differences are not influenced by this change.
Thus, the diastolic BP value of the subject can be determined with
high accuracy. The distal pulse wave sensor may be provided by a
sensor employed for a different purpose from monitoring the blood
pressure of the subject. In this case, the total number of sensors
which are worn on the subject is reduced as compared with the case
where an exclusive distal pulse wave sensor is employed. Although
the distal pulse wave sensor is worn at a position downstream of
the cuff, a measurement using the distal pulse wave sensor, different
from the blood pressure measurement, can be continued without being
interrupted due to the inflation of the cuff, because in a BP monitoring
operation the cuff pressure is not increased to values higher than
the diastolic pressure of the subject.
According to a preferred feature of the sixth aspect of the invention,
the distal pulse sensor comprises a pressure pulse wave sensor which
is adapted to be pressed against the distal section of the artery
via a skin tissue above the distal section, so as to detect a pressure
pulse wave which is produced from the distal section and is propagated
thereto via the skin tissue.
According to another feature of the sixth aspect of the invention,
the distal pulse senzor comprises a photoelectric pulse wave sensor
which emits a plurality of lights having different wavelengths toward
the distal section of the artery via a skin tissue above the distal
section, and detects a photoelectric pulse wave representing respective
intensities of the lights reflected from the distal section via
the skin tissue or transmitted through the body portion. The photoelectric
pulse wave sensor may be employed for measuring a peripheral blood
circulation or a blood oxygen saturation of a living subject. The
manner of measurement of peripheral blood circulation is disclosed
in, e.g., Japanese Patent Application laid open for inspection purposes
under Publication No. 5(1993)-115445, and the manner of measurement
of blood oxygen saturation is disclosed in, e.g., Japanese Patent
Application laid open for inspection purposes under Publication
No. 50(1975)-128387.
The second object may be achieved according to a seventh aspect
of the present invention, which provides a blood pressure monitor
comprising an inflatable cuff which is adapted to be wound around
a body portion of a living subject to press the body portion through
which an artery of the subject extends; a blood pressure measuring
device which measures a blood pressure of the subject by changing
a pressure in the cuff; a pressure pulse wave sensor which is adapted
to be pressed against a distal section of the artery located on
a distal side of the cuff wound around the body portion, so as to
detect a pressure pulse wave which is produced from the distal section
of the artery and is propagated thereto via a skin tissue above
the distal section; relationship determining means for determining
a relationship between blood pressure and magnitude of pressure
pulse wave, based on the blood pressure measured by the blood pressure
measuring device and a magnitude of the pressure pulse wave detected
by the pressure pulse wave sensor; blood pressure determining means
for determining at least a diastolic blood pressure of the subject
according to the determined relationship based on a magnitude of
a lower-peak point of each of successive first heartbeat-synchronous
pulses of the pressure pulse wave detected by the pressure pulse
wave sensor; cuff-pressure increasing means for increasing the pressure
of the cuff at a predetermined rate; a cuff pulse wave sensor which
detects a cuff pulse wave which is a pressure oscillation produced
in the cuff; first peak-interval determining means for determining
a first interval between an upper-peak point and a lower-peak point
of each of first heartbeat-synchronous pulses of the distal pulse
wave which are detected by the distal pulse wave sensor when the
pressure of the cuff is increased at the predetermined rate by the
cuff-pressure increasing means; second peak-interval determining
means for determining a second interval between an upper-peak point
and a lower-peak point of each of second heartbeat-synchronous pulses
of the cuff pulse wave which are detected by the cuff pulse wave
sensor when the pressure of the cuff is increased at the predetermined
rate by the cuff-pressure increasing means; difference determining
means for determining a difference between the first interval of
the each of the first heartbeat-synchronous pulses and the second
interval of a corresponding one of the second heartbeat-synchronous
pulses; and judging means for judging whether the determined relationship
is accurate, based on at least one diastolic blood pressure determined
by the blood pressure determining means and a pressure of the cuff
corresponding to a time when the differences determined by the difference
determining means significantly largely change.
In the blood pressure monitor in accordance with the seventh aspect
of the invention, if the judging means makes a positive judgment,
the relationship need not be updated. Accordingly, the blood pressure
measuring device does not inflate the cuff, and the subject is prevented
from being pressed by the cuff. In addition, although the pressure
pulse wave sensor is set on the distal side of the cuff, the blood
pressure determining means can continue to successively determine
blood pressure values according to the relationship based on the
pressure pulse wave detected by the pressure pulse wave sensor.
Moreover, the accuracy of the relationship is judged by increasing
the cuff pressure up to a value around the diastolic BP value of
the patient, which does not cause the patient to feel discomfort.
According to a preferred feat-ire of the seventh aspect of the
invention, the blood pressure monitor further comprises a control
device which controls, when the judging means makes a negative judgment,
the blood pressure measuring means to measure another blood pressure
of the subject, controls the pulse wave sensor to detect another
magnitude of the pressure pulse wave sensor, and controls the relationship
determining means to determine another relationship between blood
pressure and magnitude of pressure pulse wave, based on the another
blood pressure measured by the blood pressure measuring device and
the another magnitude of the pressure pulse wave detected by the
pressure pulse wave sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and optional objects, features, and advantages of the
present invention will better be understood by reading the following
detailed description of the preferred embodiments of the invention
when considered in conjunction with the accompanying drawings, in
which:
FIG. 1 is a diagrammatic view of a blood pressure (BP) monitor
embodying the present invention;
FIG. 2 is an illustrative view for explaining various functions
of the BP monitor of FIG. 1;
FIG. 3 is a flow chart representing a main routine which is executed
by a control device of the BP monitor of FIG. 1;
FIG. 4 is a flow chart representing a BP monitor routine as a step
of the flow chart of FIG. 3;
FIG. 5A is a time chart showing a relationship between time and
cuff pressure, P.sub.c, or pulse amplitude, A.sub.m ;
FIG. 5B is a table showing a manner in which a cuff pressure, P.sub.c,
is determined as a monitor diastolic blood pressure, MBP.sub.DIA
;
FIG. 6 is a graph showing the envelope of pulse amplitudes obtained
by changing cuff pressure applied to a living subject having a normal
blood pressure, and the envelope of pulse amplitudes obtained by
changing cuff pressure applied to a living subject having a low
blood pressure;
FIG. 7 is a diagrammatic view of a continuous BP monitor as a second
embodiment of the present invention;
FIG. 8 is a graph showing an example of a pressure pulse wave (PPW)
detected by a PPW sensor of the BP monitor of FIG. 7;
FIG. 9 is a graph showing a relationship determined by a control
device of the BP monitor of FIG. 7;
FIG. 10 is an illustrative view for explaining various functions
of the control device of the BP monitor of FIG. 7;
FIG. 11 is a flow chart representing a control routine according
to which the BP monitor of FIG. 7 operates;
FIG. 12 is a graph showing a relationship between waveform characteristic
L and cuff pressure that is determined by the control device of
the BP monitor of FIG. 7;
FIG. 13 is an illustrative view corresponding to FIG. 10, for explaining
various functions of a control device of a continuous BP monitor
as a third embodiment of the present invention;
FIG. 14 is a flow chart corresponding to FIG. 11, representing
a control routine according to which the BP monitor of FIG. 13 operates;
FIG. 15 is a graph showing an example of a cuff pulse wave (CPW)
detected by a CPW sensor, and an example of a pressure pulse wave
(PPW) detected by a PPW sensor, of the BP monitor of FIG. 13;
FIG. 16 is a graph showing a relationship between phase different
T and cuff pressure that is determined by the control device of
the BP monitor of FIG. 13;
FIG. 17 is an illustrative view corresponding to FIG. 10, for explaining
various functions of a control device of a continuous BP monitor
as a fourth embodiment of the present invention;
FIG. 18 is a flow chart corresponding to FIG. 11, representing
a control routine according to which the BP monitor of FIG. 17 operates;
FIG. 19 is an illustrative view corresponding to FIG. 10, for explaining
various functions of a control device of a continuous BP monitor
as a fifth embodiment of the present invention;
FIG. 20 is a flow chart corresponding to FIG. 11, representing
a control routine according to which the BP monitor of FIG. 19 operates;
FIG. 21 is a cross-section view of a cuff pulse wave (CPW) sensor
employed by a continuous BP monitor as a sixth embodiment of the
present invention;
FIG. 22 is an illustrative view corresponding to FIG. 10, for explaining
various functions of a control device of a continuous BP monitor
as a seventh embodiment of the present invention;
FIG. 23 is a graph showing an example of a cuff pulse wave (CPW)
detected by a CPW sensor, and an example of a pressure pulse wave
(PPW) detected by a PPW sensor, of the BP monitor of FIG. 22;
FIG. 24 is a flow chart corresponding to FIG. 11, representing
a control routine according to which the BP monitor of FIG. 22 operates;
FIG. 25 is a graph showing a relationship between peak-interval
difference t and cuff pressure that is determined by the control
device of the BP monitor of FIG. 22;
FIG. 26 is a diagrammatic view corresponding to FIG. 7, showing
a part of a BP monitor as an eighth embodiment of the present invention;
FIG. 27 is an illustrative view corresponding to FIG. 10, for explaining
various functions of a control device of the BP monitor of FIG.
26; and
FIG. 28 is a flow chart representing a control routine according
to which the BP monitor of FIG. 26 operates.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 through 4 and FIGS. 5A and 5B, there will
be described a blood pressure (BP) monitor to which the present
invention is applied.
In FIG. 1, reference numeral 10 designates an inflatable cuff which
is adapted to be wound around an upper arm of a living subject,
such as a patient. The cuff 10 is provided by an inflatable bag
10a formed of a resilient sheet such as a rubber sheet or a vinyl
sheet, and a non-stretchable arm belt 10b in which the bag 10a is
accommodated. The bag 10a is connected via air piping 18 to a pressure
sensor 12, an air pump 14, and a pressure regulator valve 16.
The pressure sensor 12 includes a pressure-sensing semiconductor
element which detects an air pressure in the cuff 10 (i.e., bag
10a), generates a pressure signal, SP, representing the detected
cuff pressure, and supplies the pressure signal SP to each of a
low-pass filter 20 and a band-pass filter 22. The low-pass filter
20 extracts, from the pressure signal SP, a direct-current (DC)
component representing a static pressure, P.sub.c, of the cuff 10,
and supplies a cuff-pressure signal, SK, representing the static
pressure P.sub.c, to an analog-to-digital (A/D) converter 24. The
band-pass filter 22 extracts, from the pressure signal SP, an alternate-current
(AC) or frequency (e.g., 1 to 10 Hz) component representing pulses
of a pulse wave which are produced in the cuff 10. The band-pass
filter 22 supplies a pulse-wave signal, SM, representing the pulse
wave, to the A/D converter 24. The pulse wave is a pressure oscillation
which is transmitted from the arteries (e.g., brachial artery) of
the living subject to the cuff 10 in synchronism with the heartbeat
of the subject and is produced in the cuff 10.
The band-pass filter 22 functions as a pulse-wave sensor, and has
a frequency characteristic that the band width thereof is sufficiently
narrow to be able to extract, freely from noise such as motion artifact
noise, the respective amplitudes of pulses of a pulse wave, i.e.,
pressure oscillation that is produced in the cuff 10 in synchronism
with the heartbeat of the living subject, while the pressure of
the cuff 10 is slowly decreased at the rate of 2 to 3 mmHg/sec.
The A/D converter 24 includes a multiplexer which processes the
two input signals SK, SM by time sharing, and has the function of
concurrently converting the two analog signals into digital signals
SK, SM, which are supplied to a central processing unit (CPU) 28
of a control device 26.
The control device 26 is provided by a microcomputer including
the CPU 28, a random access memory (RAM) 30, a read only memory
(ROM) 32, an output interface 34, and a display interface 36. The
CPU 28 processes the input signals SK, SM supplied from the A/D
converter 24, by utilizing a temporary-storage function of the RAM
30, according to control programs pre-stored in the ROM 32. The
CPU 28 controls the air pump 14 and the pressure regulator valve
16 via the output interface 34, and controls a display 38 via the
display interface 36. The display 38 includes an image display panel
(not shown) which displays an image, such as BP values and waveforms,
consisting of a number of picture elements, and a printer (not shown)
which records, on a recording sheet of paper, using an ink, the
image currently displayed on the image display panel. In the present
embodiment, the air pump 14, the pressure regulator valve 16, and
the control device 26 cooperate with one a nother to provide a cuff-pressure
regulating device 52 (FIG. 2) which will be described later.
A mode switch 40 is manually operable by a user for selectively
establishing a BP measure mode and a BP monitor mode. The mode switch
40 supplies a mode signal indicative of the selected mode, to the
CPU 28. A Start/Stop switch 42 is manually operable by the user
for inputting a start command or a stop command to start or stop
an operation of the present BP monitor, and supplies a command signal
indicative of the input command, to the CPU 28.
FIG. 2 illustratively shows the various functions of the present
BP monitor. When the mode switch 40 is operated to select the BP
measure mode, the cuff-pressure regulating device 52 is operated
to increase the pressure of the cuff 10 (hereinafter, referred to
as the "cuff pressure P.sub.c ") up to a target pressure
higher than a systolic BP value of the subject. While the cuff pressure
is increased, or subsequently decreased from the target pressure
at a low rate of 2 to 3 mmHg/sec, a BP measuring device 50 carries
out an oscillometric BP measuring method in which a BP value of
the subject is measured based on the variation of respective amplitudes,
A.sub.n (n are natural numbers), of a series of pulses of a pulse
wave which is detected from the cuff 10. The BP measuring device
50 is provided by the pressure sensor 12, the low-pass filter 20,
and the control device 26. The BP measuring device 50 performs a
BP measurement in the case where the operation of the BP monitor
is started in the BP measure mode. The BP measuring device 50 also
performs a BP measurement when abnormality identifying means 60
(described later) identifies an abnormal change of monitor diastolic
BP values determined by diastolic-BP determining means 58 (described
later). The control device 26 functions as both the diastolic-BP
determining means 58 and the abnormality identifying means 60.
In the BP monitor mode, the cuff-pressure regulating device 52
operates, at a predetermined interval of time, for increasing the
cuff pressure P.sub.c at a predetermined rate of change. Pulse-amplitude
determining means 54 determines, by calculation, respective amplitudes,
A.sub.m (m are natural numbers), of pulses of a pulse wave which
is produced in the cuff 10 in synchronism with the heartbeat of
the subject while the cuff pressure is increased by the cuff-pressure
regulating device 52. Candidate determining means 56 judges whether
an amplitude A.sub.m (m.ltoreq.i-1) of each of prior pulses determined
by the pulse-amplitude determining means 54 is not greater than
a reference value, A.sub.a (=A.sub.m.times.(1-R), where m=i and
0<R<1), which is smaller than an amplitude A.sub.m (m=i) of
a subsequent pulse determined by the pulse-amplitude determining
means 54, by a predetermined proportion, R, of the amplitude A.sub.m
of the subsequent pulse. The amplitude of each of the prior pulses
is determined before the amplitude of the subsequent pulse is determined.
If a positive judgment is made, the candidate determining means
56 determines, as a diastolic BP candidate, P.sub.ck, of the subject,
a cuff pressure P.sub.c detected when each prior pulse is detected
in the cuff 10. The control device 26 functions as both the pulse-amplitude
determining means 54 and the candidate determining means 56. The
diastolic-BP determining means 58 determines, as a monitor diastolic
BP value, MBP.sub.DIA, of the subject, the cuff pressure P.sub.c
corresponding to the amplitude of a prior pulse, if the candidate
determining means 56 determines, as a diastolic BP candidate, the
cuff pressure P.sub.c corresponding to the amplitude of that prior
pulse, with respect to each of a predetermined number, N.sub.0,
of the subsequent pulses.
The abnormality identifying means 60 identifies an abnormality
of the monitor diastolic BP values MBP.sub.DIA determined by the
diastolic-BP determining means 58, such as an abrupt decrease of
the blood pressure of the subject. If the identifying means 60 identifies
an abnormality, the BP measuring device 50 is immediately operated
for carrying out an oscillometric BP measurement.
Next, there will be described the operation of the present BP monitor
by reference to the flow charts of FIGS. 3 and 4.
First, at Step S1, the CPU 28 judges, based on the command signal
supplied from the Start/Stop switch 42, whether the Start/Stop switch
43 has been operated to input a start command to start the operation
of the BP monitor. If a negative judgment is made at Step S1, the
control of the CPU 28 repeats Step S1 while waiting for a positive
judgment to be made at Step S1. Meanwhile, if a positive judgment
is made, the control of the CPU 28 proceeds with Step S2 to judge
whether the mode switch 40 has been operated to select the BP monitor
mode. In the case where the BP measure mode is in use, a negative
judgment is made at Step S2, so that the control of the CPU 28 goes
to Step S3, i.e., BP measure routine according to which a known
oscillometric BP measurement is carried out to measure a systolic,
a diastolic, and a mean BP value, BP.sub.SYS, BP.sub.DIA, BP.sub.MEAN,
of the subject. When the BP measurement is finished, the pressure
regulator valve 16 is opened to quickly deflate the cuff 10, thereby
releasing the upper arm of the subject from the cuff pressure P.sub.c,
i.e., pressing force of the cuff 10. Step S3 is followed by Step
S4 to store the measured BP values in the RAM 30 and operate the
display 38 to indicate numerals representing the measured BP values.
More specifically described, in the oscillometric BP measurement
effected at Step S3, the air pump 14 and the pressure regulator
valve 16 are operated to quickly increase the cuff pressure P.sub.c
up to a predetermined target pressure, P.sub.cm, e.g., 180 mmHg.
Subsequently, the air pump 14 is stopped and the degree of opening
of the regulator valve 16 is regulated, so that a slow deflation
of the cuff 10 is started. That is, the cuff pressure P.sub.c is
decreased at a low rate of 2 to 3 mmHg/sec which is suitable for
BP measurements. During this slow cuff-pressure decrease, the control
device 26 determines BP values according to a well known oscillometric
BP determining algorithm. That is, the CPU 28 determines, as a systolic
BP value BP.sub.SYS, a cuff pressure at the time when the pulse
amplitudes A.sub.n significantly change in a phase in which the
amplitudes A.sub.n increase; determines, as a mean BP value BP.sub.MEAN,
a cuff pressure at the time when the amplitudes A.sub.n take a maximum
value, i.e., when a pulse having a maximum amplitude is produced;
and determines, as a diastolic BP value BP.sub.DIA, a cuff pressure
at the time when the pulse amplitudes A.sub.n significantly change
in a phase in which the amplitudes A.sub.n decrease.
In the case where the mode switch 40 has been operated to select
the BP monitor mode, a positive judgment is made at Step S2 and
accordingly the control of the CPU 28 goes to Step S5 to judge whether
a timer has counted up a predetermined monitor cycle time from the
time when a monitor diastolic BP value MBP.sub.DIA had been determined
at Step S6 in the preceding control cycle in accordance with the
main routine of FIG. 3. The monitor cycle time may fall in the range
of from several minutes to ten and several minutes. If a negative
judgment is made at Step SS, the CPU 28 repeats Step S5 until a
positive judgment is made. Meanwhile, if a positive judgment is
made at Step S5, the control of the CPU 28 goes to Step S6, i.e.,
BP monitor routine of FIG. 4.
At Step S6-1 of FIG. 4, the CPU 28 judges whether the CPU 28 has
received, from the band-pass filter 22, a pulse-wave signal SM representing
one pulse of a pulse wave. If a negative judgment is made at Step
S6-1, the current control cycle in accordance with the routine of
FIG. 4 is ended. On the other hand, if a positive judgment is made
at Step S6-1, the control of the CPU 28 goes to Step S6-2 to increase
the cuff pressure P.sub.c by a predetermined pressure increase amount,
.DELTA.P.sub.c, and subsequently to Step S6-3 to determine, by calculation,
an amplitude A.sub.m of the pulse received at Step S6-1 and store,
in an appropriate area of the RAM 30, the determined amplitude A.sub.m
together with the cuff pressure P.sub.c at the time when the pulse
is detected through the band-pass filter 22. The cuff pressure P.sub.c
is read from the cuff-pressure signal SK supplied from the low-pass
filter 20. While Steps S6-2 and S6-3 are repeated, the cuff pressure
P.sub.c is increased at a predetermined rate of change, more specifically,
stepwise by the respective pressure amounts .DELTA.P.sub.c, as shown
in FIG. 5. While the cuff pressure P.sub.c is held for a short duration
at each of the pressure steps, the CPU 28 reads in one pulse, determines
the amplitude A.sub.m of the pulse, and stores the pulse amplitude
A.sub.m in the RAM 30. Step S6-3 corresponds to the pulse-amplitude
determining means 54.
At Step S6-4, the CPU 28 judges whether the amplitude A.sub.m (m.ltoreq.i-1)
of each of the prior pulses determined at Step S6-3 in the prior
control cycles before the current control cycle is not greater than
a reference value A.sub.a (e.g., 0.7.times.A.sub.m) which is smaller
than the amplitude A.sub.m (m=i) of the current pulse determined
at Step S6-3 in the current control cycle, by a predetermined proportion
R (e.g., 30% (R=0.3)) of the amplitude A.sub.m of the current pulse.
If a positive judgment is made, the CPU 28 determines, as a diastolic
BP candidate P.sub.ck Of the subject, the cuff pressure P.sub.c
corresponding to the pulse amplitude A.sub.m of each prior pulse.
While the cuff pressure P.sub.c is increased up to a mean BP value
BP.sub.MEAN of the subject, the pulse amplitudes A.sub.m continue
to increase as indicated at broken line in FIG. 5A. For example,
in the case where the pulse amplitude A.sub.2 is determined at Step
S6-3, the only prior amplitude A.sub.1 is compared with 0.7.times.(the
amplitude A.sub.2). If the amplitude A.sub.1 is greater than 0.7.times.(the
amplitude A.sub.2), the cuff pressure P.sub.c1 corresponding to
the amplitude A.sub.1 is not determined as a diastolic BP candidate
P.sub.ck. In the example shown in FIG. 5B, a negative judgment is
made at Step S6-4 for each of the amplitudes A.sub.1 to A.sub.4.
Once a negative judgment is made for a pulse amplitude A.sub.m at
Step S6-4, the CPU 28 never makes a judgment for that amplitude
at Step S6-4 in the following control cycles.
On the other hand, if the CPU 28 makes a positive judgment at Step
S6-4, the control of the CPU 28 goes to Step S6-5 to determine the
cuff pressure P.sub.c corresponding to the pulse amplitude A.sub.m,
as a diastolic BP candidate P.sub.ck, and store the pressure P.sub.c
in an appropriate area of the RAM 30. For example, in the case where
the pulse amplitude A.sub.6 is determined at Step S6-3, the prior
amplitude A.sub.5 is compared with 0.7.times.(the amplitude A.sub.6)
and, if the amplitude A.sub.5 is not greater than 0.7.times.(the
amplitude A.sub.6), the cuff pressure P.sub.c5 corresponding to
the amplitude A.sub.5 is determined as a diastolic BP candidate
P.sub.ck. Thus, a positive judgment is made at Step S6-4 for the
amplitude A.sub.5. Regarding the example shown in FIG. 5B, a positive
judgment is made for the amplitude A.sub.5, when the amplitude A.sub.5
is compared with the amplitude A.sub.7 in the next cycle, and with
the amplitude A.sub.8 in the cycle after that next cycle. Steps
S6-4 and S6-5 correspond to the candidate determining means 56.
Step S6-5 is followed by Step S6-6 to judge, regarding each of
the prior amplitudes A.sub.m, whether a number, N, of the positive
judgments made for each prior amplitude A.sub.m at Step S6-4 becomes
not smaller than a reference number, N.sub.0, e.g., 3. If a positive
judgment is made for any of the prior amplitudes A.sub.m, a positive
judgment is finally made at Step S6-5. For example, in the example
shown in FIG. 5B, when the amplitude A.sub.8 is determined at Step
S6-3, a positive judgment is made for the amplitude A.sub.5 and
a negative judgment is made for each of the amplitudes A.sub.6 and
A.sub.7. Accordingly, a positive judgment is finally made at Step
S6-6. The reference number N.sub.0 is empirically determined as
being suitable for obtaining a diastolic BP value. In the case where
the above-indicated pressure increase amount .DELTA.P.sub.c is about
5 mmHg, the reference number N.sub.0 is determined at 3.
If a negative judgment is finally made at Step S6-6, i.e., if a
negative judgment is made for all the prior amplitudes A.sub.m,
the control of the CPU 28 goes back to Step S6-1. On the other hand,
if a positive judgment is finally made at Step S6-6, the control
goes to Step S6-7 to determine, as a monitor diastolic BP value
MBP.sub.DIA, the cuff pressure P.sub.c corresponding to the pulse
amplitude A.sub.m for which three positive judgments are made at
Step S6-4. The thus determined cuff pressure P.sub.c is stored in
the RAM 30. Regarding the example shown in FIG. 5B, the cuff pressure
P.sub.c5 is determined as a monitor diastolic BP value MBP.sub.DIA.
In the present embodiment, Steps S6-6 and S6-7 correspond to the
diastolic-BP determining means 58. Step S6-7 is followed by Step
S6-8 to display the newly determined monitor diastolic BP value
MBP.sub.DIA in place of the old value MBP.sub.DIA determined in
Step S6 in the preceding control cycle in accordance with the main
routine of FIG. 3.
After the monitor diastolic BP value MBP.sub.DIA is determined
at Step S6, the control of the CPU 28 goes to Step S7 to judge whether
the Start/Stop switch 42 is operated to stop the BP monitoring operation.
If a positive judgment is made at Step S7, the control goes back
to Step S1. On the other hand, if a negative judgment is made at
Step S8, the control goes to Step S8 to judge whether an abnormality
has occurred to the monitor diastolic BP values MBP.sub.DIA. For
example, the CPU 28 identifies an abnormality of the diastolic BP
values MBP.sub.DIA, if an amount, or a rate, of change of the current
value MBP.sub.DIA from a moving average of prior values MBP.sub.DIA
exceeds a reference value, which indicates that the blood pressure
of a living subject has abruptly decreased. Step S8 corresponds
to the abnormality identifying means 60.
If a negative judgment is made at Step S8, the control goes back
to Step S5 and repeats Steps S5 to S8. Meanwhile, if a positive
judgment is made at Step S8, the control of the CPU 28 goes to Step
S5 to carry out an oscillometric BP measurement using the cuff 10
like at Step S3. Step S9 is followed by Step S10 to operate the
display 38 to display the measured BP values BP.sub.SYS, BP.sub.MEAN,
BP.sub.DIA.
As is apparent from the foregoing description, the present BP monitor
operates such that in the BP monitor mode the amplitudes A.sub.m
of pulses of a pulse wave which are produced in the cuff 10 while
the cuff pressure P.sub.c is increased by the cuff-pressure regulating
device 52, are determined by the pulse-amplitude determining means
54. In addition, the candidate determining means 56 judges whether
the amplitude A.sub.m (m.ltoreq.i-1) of each of the prior pulses
determined in the prior control cycles before the current control
cycle is not greater than the reference value A.sub.a which is smaller
than the amplitude A.sub.m (m=i) of the current pulse determined
in the current control cycle, by the predetermined proportion R
of the amplitude A.sub.m of the current pulse and, if a positive
judgment is made, determines, as a diastolic BP candidate P.sub.ck
of the subject, the cuff pressure P.sub.c corresponding to the amplitude
A.sub.m of each prior pulse. The diastolic-BP determining means
58 judges, regarding each of the prior amplitudes A.sub.m, whether
the number N of the positive judgments made for each prior amplitude
A.sub.m becomes not smaller than the reference number N.sub.0 and,
if a positive judgment is made for any of the prior amplitudes A.sub.m,
determines, as a monitor diastolic BP value MBP.sub.DIA, the cuff
pressure P.sub.c corresponding to the prior amplitude A.sub.m for
which the predetermined number N.sub.0 of positive judgments are
made. The present BP monitor has been developed based on the fact
that a diastolic BP value BP.sub.DIA does not correspond to a pulse
having an amplitude not smaller than an amplitude, A.sub.max.times.(1-R),
smaller than a maximum amplitude, A.sub.max, of the last pulse that
is detected in the current control cycle, by the predetermined proportion
R of the amplitude A.sub.max of the last pulse. The last or current
pulse has a maximum amplitude A.sub.max of all the amplitudes A.sub.m
of the prior pulses which have been detected prior to the last or
current pulse while the cuff pressure P.sub.c is increased, as indicated
in FIG. 5A. In addition, the present BP monitor has been developed
based on the fact that an amplitude of a pulse correctly corresponding
to a diastolic BP value BP.sub.DIA does not change even if the cuff
pressure P.sub.c is increased.
Therefore, the present BP monitor can determine a monitor diastolic
BP value at a pressure level higher than the diastolic BP value
by only the product of the pressure increase amount .DELTA.P.sub.c
and the reference number N.sub.0. This monitor diastolic BP value
enjoys high accuracy. In addition, since the pressure level where
the BP value is determined is considerably low, the living subject
does not feel discomfort.
In the present embodiment, the cuff-pressure regulating device
52 stepwise increases the cuff pressure P.sub.c, by alternately
increasing it by the increment amount AP.sub.c and holding it at
each increased level. The pulse-amplitude determining means 54 determines,
by calculation, the amplitude A.sub.m of the pulse which is produced
when the cuff pressure P.sub.c is held at each increased level.
The thus determined pulse amplitude A.sub.m enjoys high accuracy
because it is free from the adverse influence resulting from the
increasing of the cuff pressure P.sub.c. Therefore, the monitor
BP values are determined with accuracy based on the pulse amplitudes
A.sub.m.
In addition, in the present embodiment, when the abnormality identifying
means 60 identifies an abnormality of the monitor diastolic BP values
MBP.sub.DIA, the BP monitor automatically carries out an oscillometric
BP measurement by increasing the cuff pressure P.sub.c up to a high
level which is estimated to be higher than a systolic BP value of
a living subject. Thus, the BP monitor provides accurate BP values
upon identification of an abnormality of the subject. Therefore,
a doctor or a nurse can take an appropriate medical treatment on
the subject.
Although in the illustrated embodiment the pressure P.sub.c of
the cuff 10 is stepwise increased at a predetermined rate in the
BP monitor mode, it is possible that the cuff pressure P.sub.c be
continuously increased at a predetermined rate.
While in the illustrated embodiment the pressure increase amount
.DELTA.P.sub.c is a constant value, it is possible that the pressure
increase amount .DELTA.P.sub.c be variable depending upon the current
cuff pressue P.sub.c.
Although in the illustrated embodiment the predetermined proportion
R used at Step S6-4 is a constant value, it is possible that the
CPU 28 determine a value R based on the variation of the amplitudes
of pulses of a pulse wave obtained in the oscillometric BP measurement
effected at Step S3. In the latter case, the BP monitor can determine
a value R suitable for each individual subject.
While in the illustrated embodiment the reference number N.sub.0
used at Step S6-6 is a constant value, it is possible that the CPU
28 determine a number N.sub.0 based on the variation of the amplitudes
of pulses of a pulse wave obtained in the oscillometric BP measurement
effected at Step S3. In the latter case, the BP monitor can determine
a number N.sub.0 suitable for each individual subject.
In the illustrated embodiment, the BP measuring device 50 performs
an oscillometric BP measurement at Step S3 when the operation of
the BP monitor is started in a state in which the BP measure mode
has been selected, or the abnormality identifying means 60 identifies
an abnormality of the monitor diastolic BP values determined by
the diastolic-BP determining means 58. However, it is possible to
adapt the BP measuring device 50 to periodically perform an oscillometric
BP measurement at a predetermined cycle time, i.e., at a predetermined
interval of time.
Referring next to FIGS. 7 to 12, there will be described a continuous
blood pressure (BP) monitor 100 as a second embodiment of the present
invention. The BP monitor 100 may be used to monitor BP values of
a patient who is undergoing, or has undergone, a surgical operation.
In FIG. 7, the BP monitor 100 includes an inflatable cuff 110 including
a rubber bag and a band-like cloth bag in which the rubber bag is
accommodated. The cuff 110 is wound around, e.g., an upper arm 112
of a patient. The cuff 110 is connected via piping 120 to a pressure
sensor 114, a selector valve 116, and a first air pump 118. The
selector valve 116 is selectively placed, under control of an electronic
control device 128, in a first state in which the valve 116 permits
pressurized air to be supplied from the air pump 118 to the cuff
110 to increase quickly the air pressure of the cuff 110 (hereinafter,
referred to as the "cuff pressure"), a second state in
which the valve 116 causes the cuff 110 to be deflated slowly, and
a third state in which the valve 116 causes the cuff 110 to be deflated
quickly.
The pressure sensor 114 detects the cuff pressure (i.e., air pressure
in the cuff 110), and generates a pressure signal, SP, representing
the detected cuff pressure. The pressure signal SP is supplied to
each of a static-pressure filter circuit 122 and a pulse-wave filter
circuit 124. The static-pressure filter circuit 122 includes a low-pass
filter which extracts, from the pressure signal SP, a cuff-pressure
signal, SK, representative of a static or direct-current component
of the pressure signal SP. The cuff-pressure signal SK is supplied
via a first analog-to-digital (A/D) converter 126 to the control
device 128.
The pulse-wave filter circuit 124 includes a band-pass filter which
extracts, from the pressure signal SP, a pulse-wave signal, SM.sub.1,
representative of an oscillating or alternating-current component
of the pressure signal SP. The pulse-wave signal SM.sub.1 is supplied
via a second A/D converter 130 to the control device 128. The alternating-current
component represented by the pulse-wave signal SM.sub.1 corresponds
to an oscillatory pressure wave, i.e., pulse wave which is produced
from a brachial artery (not shown) of the patient in synchronism
with the heartbeat of the patient and is propagated via skin tissue
to the cuff 110. This pulse wave is referred to as the "cuff
pulse wave (CPW)" to be distinguished from a "pressure
pulse wave (PPW)" which will be described later. In the present
embodiment, the cuff 110, the pressure sensor 114, and the pulse-wave
filter circuit 124 cooperate with one another to provide a cuff
pulse wave sensor.
The control device 128 is provided by a microcomputer including
a central processing unit (CPU) 129, a read only memory (ROM) 131,
a random access memory (RAM) 133, and an input and output (I/O)
port (not shown). The CPU 129 processes input signals, including
the signals SK, SM.sub.1, by utilizing a temporary-storage function
of the RAM 133, according to control programs pre-stored in the
ROM 131. In addition, the CPU 129 supplies drive signals via the
I/O port to drive circuits (not shown) associated with the selector
valve 116 and the air pump 118, respectively. Thus, the CPU 129
controls respective operations of the valve 116 and the pump 118.
For example, when an oscillometric BP measurement using the cuff
110 is carried out to calibrate the present BP monitor 100, the
CPU 129 controls the valve 116 and the pump 118 to increase quickly
the cuff pressure up to a predetermined target value and subsequently
decrease the cuff pressure at a low rate of 2 to 3 mmHg/sec. Based
on the variation of the cuff pulse wave represented by the pulse-wave
signal SM.sub.1 provided by the pulse-wave filter circuit 124 during
the low-rate decreasing of the cuff pressure, the CPU 129 determines
a systolic, a mean, and a diastolic BP value of the patient, according
to a known oscillometric BP measuring method. In addition, the CPU
129 controls a display 132 to display the thus determined BP values.
A pressure-pulse-wave (PPW) detecting probe 134 includes a container-like
sensor housing 136, and a fastening band 140 connected to the sensor
housing 136. With the help of the fastening band 140, the PPW detecting
probe 134 is detachably attached to a wrist 142 of the same arm
112 of the patient on which the cuff 110 is worn, such that an opening
of the sensor housing 136 is opposed to a body surface 138 of the
patient. A PPW sensor 146 is secured via an elastic diaphragm 144
to inner surfaces of the sensor housing 136 such that the PPW sensor
146 is movable relative to the housing 136 and is advanceable through
the opening of the housing 136 toward the body surface 138 of the
patient. The sensor housing 136 and the diaphragm 144 cooperate
with each other to define a pressure chamber 148, which is supplied
with pressurized air from a second air pump 150 via a pressure regulator
valve 152. Thus, the PPW sensor 146 is pressed on the body surface
138 with a pressing force, P.sub.HD, corresponding to an air pressure
in the chamber 148. In the present embodiment, the pressing forces
of the PPW sensor 146 applied to the body surface 138 or a radial
artery 156 are indicated in terms of pressure values (mmHg) in the
chamber 148. The sensor housing 136, the diaphragm 144, the pressure
chamber 148, the second air pump 150, the pressure regulator valve
152, etc. cooperate with one another to provide a pressing device
which presses the PPW sensor 146 against the radial artery 156 via
the body surface or skin tissue 138.
The PPW sensor 146 includes a semiconductor chip formed of a monocrystalline
silicon which has a press surface 154, and a number of pressure-sensing
semiconductor elements (not shown) which are arranged, in the press
surface 154, in an array at a regular interval of distance (about
0.2 mm), such that the array of pressure-sensing elements extends
in the direction of width of the radial artery 156. When the PPW
sensor 146 is pressed against the radial artery 156 via the body
surface 138 of the wrist 142, the PPW sensor 146 detects an oscillatory
pressure wave, i.e., pressure pulse wave (PPW) which is produced
from the radial artery 156 in synchronism with the heartbeat of
the patient and is propagated via the body surface 138 to the PPW
sensor 146. The PPW sensor 146 generates a PPW signal, SM.sub.2,
representing the detected PPW, and supplies the PPW signal SM.sub.2
to the control device 128 via a third A/D converter 158. An example
of the PPW (i.e., PPW signal SM.sub.2) detected by the PPW sensor
146 is illustrated in the graph of FIG. 8.
The CPU 129 of the control device 128 processes the input signals,
including the PPW signal SM.sub.2, by utilizing the temporary-storage
function of the RAM 133, according to the control programs pre-stored
in the ROM 131, and supplies drive signals to drive circuits (not
shown) associated with the second air pump 150 and the pressure
regulator valve 152, respectively. Thus, the CPU 129 controls respective
operations of the pump 150 and the valve 152 to regulate the air
pressure of the pressure chamber 148 applied to the PPW sensor 146,
i.e., the pressing force P.sub.HD of the PPW sensor 146 applied
to the radial artery 156 via the body surface or skin tissue 138.
When a continuous BP monitoring operation is carried out, the CPU
129 determines an optimum pressing force, P.sub.HDO, of the PPW
sensor 146 applied to the radial artery 156, based on the PPW (signal
SM.sub.2) detected by the PPW sensor 146 while the pressure of the
pressure chamber 148 is slowly changed, and controls the pressure
regulator valve 152 to maintain the pressure of the chamber 148
at the determined optimum pressing force P.sub.HDO. In addition,
the CPU 129 determines a relationship between BP values and PPW
magnitudes P.sub.M (i.e., voltage values of the signal SM.sub.2),
based on a systolic and a diastolic BP value, BP.sub.SYS, BP.sub.DIA,
measured using the cuff 110 according the oscillometric BP measuring
method, and a maximum and a minimum magnitude, P.sub.Mmax, P.sub.Mmin,
of one heartbeat-synchronous pulse of the PPW detected by the PPW
sensor 146 being pressed on the body surface 138 with the optimum
pressing force P.sub.HDO. According to the thus determined relationship,
the CPU 129 determines a systolic and a diastolic BP value (i.e.,
monitor BP values), MBP.sub.SYS, MBP.sub.MEAN, MBP.sub.DIA, of the
patient, based on a maximum magnitude (i.e., upper-peak magnitude)
P.sub.Mmax, a mean magnitude (described later), P.sub.Mmean, and
a minimum magnitude (i.e., lower-peak magnitude), P.sub.Mmin, of
each of successive heartbeat-synchronous pulses of the PPW detected
by the PPW sensor 146 being pressed with the optimum pressing force
P.sub.HDO. Subsequently, the CPU 129 controls the display 132 to
successively display, for each heartbeat- synchronous pulse, the
thus determined monitor BP values MBP.sub.SYS, MBP.sub.MEAN, MBP.sub.DIA,
in digits, and continuously display the waveform of the PPW detected
by the PPW sensor 146. This waveform represents the instantaneous
monitor BP values MBP of the patient.
FIG. 9 shows an example of a relationship between BP values MBP
(monitor BP values) and PPW magnitudes PM that is determined by
the control device 128 or the CPU 129. This relationship is expressed
by the following linear function:
where A is a constant corresponding to the slope of the linear
function and B is a constant corresponding to the intercept of the
axis of ordinate indicative of the monitor BP values MBP.
FIG. 10 illustrates various functions of the electronic control
device 128 of the continuous BP monitor 100. The pressing pressure
of the cuff 110 is detected by the pressure sensor 114. The static-pressure
filter circuit 122 cooperates with the control device 128 to provide
a BP measuring device 172 which measures, according to an oscillometric
BP measuring method (JIS T 1115; JIS is Japanese Industrial Standard),
a systolic BP value BP.sub.SYS, a mean BP value BP.sub.MEAN, and
a diastolic BP value BP.sub.DIA of a living subject based on the
variation of respective amplitudes of heartbeat-synchronous pulses
of the cuff pulse wave (CPW) detected by the CPW sensor 114, 124,
130 while the pressure of the cuff 110 is slowly increased or decreased
at the rate of 2 to 3 mmHg/sec. The cuff pulse wave is represented
by the pulse-wave signal SM.sub.1 obtained through the pulse-wave
filter circuit 124. The PPW sensor 146 is worn on the wrist 142
of the same arm 112 of the patient on which the cuff 110 is worn,
and detects the PPW produced from the radial artery 156 downstream
of the brachial artery being pressed by the cuff 110. The control
device 128 functions as a relationship determining means 174 which
determines a MBP-P.sub.M relationship between monitor BP values
MBP and PPW magnitudes P.sub.M that is expressed by the linear function
shown in FIG. 9, based on the PPW detected by the PPW sensor 146
and the BP values measured by the BP measuring device 172. The control
device 128 also functions as a monitor-BP (MBP) determining means
176 which successively determines, according to the MBP-P.sub.M
relationship, a monitor BP value MBP of the subject based on a magnitude
of each of heartbeat-synchronous pulses of the PPW detected by the
PPW sensor 146. The selector valve 116 and the first air pump 118
cooperate with the control device 128 to provide a cuff-pressure
regulating device 178 which regulates the air pressure of the cuff
110 (i.e., cuff pressure) that is detected by the pressure sensor
114. The cuff-pressure regulating device 178 changes the cuff pressure
according to a well-known procedure, so that the BP measuring device
172 can measure BP values of the patient using the cuff 10 and the
relationship determining means 174 calibrates the MBP-P.sub.M relationship
based on the BP values measured using the cuff 110. For example,
the regulating device 178 increases the cuff pressure up to a target
value, e.g., 180 mmHg, which is higher than an estimated systolic
BP value of the patient and subsequently decreases the cuff pressure
slowly at the rate of 2 to 3 mmHg/sec, during a measurement period
in which BP values of the patient are determined by the BP measuring
device 172 according to a well-known oscillometric BP determining
algorithm. After the BP measuring operation, the regulating device
178 quickly deflates the cuff 110. In addition, the cuff-pressure
regulating device 178 provides a cuff-pressure increasing means
178 which continuously or stepwise increases the cuff pressure at
a predetermined rate.
Moreover, the control device 128 functions as a waveform-characteristic
determining means 180 which determines a characteristic of a lower-peak
portion of a waveform of each of successive heartbeat-synchronous
pulses of the pressure pulse wave (PPW) which are detected by the
PPW sensor 146 when the cuff pressure is increased at the predetermined
rate by the cuff-pressure increasing means 178. The lower-peak portion
of the waveform of each pulse includes a lower-peak point of each
pulse. In addition, the control device 128 functions as a judging
means 182 which judges whether the relationship determined by the
relationship determining means 174 is accurate, based on at least
one diastolic BP value BP.sub.DIA determined by the blood pressure
determining means 176 and a cuff pressure corresponding to a time
when the waveform characteristics detected by the waveform-characteristic
determining means 180 significantly largely change.
Next, there will be described the operation of the BP monitor 100
constructed as described above, by reference to the flow chart of
FIG. 11 representing a control program pre-stored in the ROM 131.
First, at Step S101, the CPU 129 of the control device 128 controls
the second air pump 150 and the pressure regulator valve 152 to
increase slowly the pressure of the pressure chamber 148, and determines,
as an optimum pressing force P.sub.HDO, a pressure P.sub.HD of the
chamber 148 when the PPW sensor 146 detects a maximum pulse having
the greatest amplitude of respective amplitudes of all the pulses
detected thereby during the slow increasing of the pressure of the
chamber 148. Subsequently, the CPU 129 maintains or holds the pressure
of the chamber 148 at the thus determined optimum pressing force
P.sub.HDO. Thus, the optimum pressing force P.sub.HDO is applied
to the PPW sensor 146 to press the radial artery 156 via the body
surface 138.
Next, the control of the CPU 129 proceeds with Step S102 to judge
whether a relationship as shown in FIG. 9 has been determined for
a particular patient on which the cuff 110 is worn. If a positive
judgment is made at Step S102, the control of the CPU 129 goes to
Step S103. Since, however, initially a negative judgment is made
at Step S102, the control goes to Step S107 corresponding to the
BP measuring device 172. Specifically described, the selector valve
116 is switched to the first state and the first air pump 118 is
operated, so that the cuff pressure is increased up to a target
pressure (e.g., 180 mmHg) higher than an estimated systolic BP value
of the patient. Subsequently, the air pump 118 is stopped and the
selector valve 116 is switched to the second state, so that the
cuff pressure is decreased at a predetermined low rate (e.g., about
3 mmHg/sec). Based on the variation of respective amplitudes of
heartbeat-synchronous pulses of the cuff-pulse-wave (CPW) signal
SM.sub.1 obtained during this slow decreasing of the cuff pressure,
the CPU 129 determines a systolic, a mean, and a diastolic BP value
BP.sub.SYS, BP.sub.MEAN, BP.sub.DIA of the patient according to
a known oscillometric BP determining algorithm. More specifically,
the CPU 129 determines, as the systolic BP value BP.sub.SYS, a cuff
pressure at the time when the pulse amplitudes significantly largely
increase, determines, as the diastolic BP value BP.sub.DIA, a cuff
pressure at the time when the pulse amplitudes significantly largely
decrease, and determines, as the mean BP value BP.sub.MEAN, a cuff
pressure at the time when the pulse amplitudes become maximum. In
addition, the CPU 129 determines a pulse rate of the patient based
on the time interval between respective upper-peak points of two
successive heartbeat-synchronous pulses of the CPW signal SM.sub.1.
The thus measured BP values and pulse rate are stored in the RAM
133 and displayed by the display 132. Then, the selector valve 116
is switched to the third state, so that the cuff pressure is quickly
decreased or deflated.
Subsequently, the control of the CPU 129 goes to Step S108 to determine
a relationship between monitor BP value MBP and magnitude P.sub.M
of pressure pulse wave (i.e., voltage of the pressure-pulse-wave
(PPW) signal SM.sub.2) as shown in FIG. 9. More specifically, the
CPU 129 newly reads in one heartbeat-synchronous pulse of the PPW
signal SM.sub.2 supplied from the PPW sensor 146, determines a maximum
and a minimum magnitude P.sub.Mmax, P.sub.Mmin of the one pulse,
and determines the previously-indicated linear function based on
the systolic and diastolic BP values BP.sub.SYS, BP.sub.DIA of the
patient measured at Step S107 and the thus determined maximum and
minimum magnitudes P.sub.Mmax, P.sub.Mmin of the one pulse of the
PPW signal SM.sub.2. Step S108 corresponds to the relationship determining
means 174.
After the MBP-P.sub.M relationship shown in FIG. 9 is determined
at Step S108, the control of the CPU 129 goes to Step S109 and the
following steps to carry out a continuous BP monitoring operation.
First, at Step S109, the CPU 129 judges whether the CPU 129 has
read in one heartbeat-synchronous pulse of the PPW signal SM.sub.2
supplied from the PPW sensor 146 being pressed at the optimum pressing
force P.sub.HDO. If a negative judgment is made at Step S109, the
CPU 129 waits for detecting one pulse of the PPW signal SM.sub.2.
Meanwhile, if a positive judgment is made at Step S109, the control
of the CPU 129 goes to Step S110 to determine a maximum (upper-peak)
magnitude P.sub.Mmax and a minimum (lower-peak) magnitude P.sub.Mmin
of the one pulse of the PPW signal SM.sub.2. In addition, the CPU
129 determines a mean magnitude, P.sub.Mmean, of the one pulse in
a known manner. For example, the CPU 129 determines, as the mean
magnitude P.sub.Mmean of one pulse, a signal-related one of the
varycentric coordinates of an area defined by the waveform of the
one pulse and a base line passing through the lower-peak point of
the one pulse, the base line being indicated at two-dot chain line
in FIG. 8. The pulse area is calculated by first subtracting the
magnitude of the lower-peak point from the magnitude of each sampling
point on the waveform of the one pulse of the signal SM.sub.2 and
then summing up the thus obtained values.
Step S110 is followed by Step S111 to determine a systolic, a mean,
and a diastolic BP value MBP.sub.SYS, MBP.sub.MEAN, MBP.sub.DIA
(monitor BP values) of the patient, based on the maximum, mean,
and minimum magnitudes P.sub.Mmax, P.sub.Mmean, P.sub.Mmin of the
one pulse of the PPW signal SM.sub.2 determined at Step S110, according
to the MBP-P.sub.M relationship determined at Step S108. The CPU
129 controls the display 132 to display, on its image display panel,
not only the thus determined monitor BP values MBP but also the
waveform of the one pulse that is continuous with the respective
waveforms of the prior pulses. Steps S110 and S111 correspond to
the monitor-BP determining means 176.
Subsequently, the control of the CPU 129 goes to Step S112 to judge,
based on a timer, whether a predetermined period of 10 to 20 minutes
has passed after the current MBP-P.sub.M relationship is determined
at Step S108. If a negative judgment is made at Step S112, the control
goes back to Step S109 and the following steps to continue the continuous
BP monitoring routine. Thus, the present BP monitor 100 successively
determines, for each heartbeat-synchronous pulse of the signal SM.sub.2,
a systolic, a mean, and a diastolic BP value MBP.sub.SYS, MBP.sub.MEAN,
MBP.sub.DIA of the patient and displays the determined BP values
on the display 132. On the other hand, if a positive judgment is
made at Step S112, the CPU 129 resets the timer to zero, and the
control of the CPU 129 goes back to Step S102.
In the current control cycle, a positive judgment is made at Step
S102 because a MBP-P.sub.M relationship has been determined at Step
S108 in the preceding control cycle. Then, the control of the CPU
129 goes to Step S103 to continuously or stepwise increase the cuff
pressure from atmospheric pressure at a predetermined rate of 5
to 20 mmHg/sec and determine a characteristic of a lower-peak portion,
F (FIG. 8), of the waveform of each of successive heartbeat-synchronous
pulses of the signal SM.sub.2 which is detected by the PPW sensor
146 when the cuff pressure is increased at the predetermined rate.
This characteristic may be a length, L, of the lower-peak portion
F defined by the magnitude P.sub.Mmin of the lower-peak point and
a magnitude, P.sub.1, greater by a predetermined amount than the
magnitude P.sub.Mmin, as illustrated in FIG. 8. Step S103 corresponds
to the cuff-pressure increasing means 178 and the waveform-characteristic
determining means 180. Step S103 is followed by Step S104 to judge
whether the CPU 129 has identified a point or a time when the characteristic
values L have significantly largely changed. For example, the CPU
129 differentiates the characteristic values L by subtracting, from
each value L.sub.i, the preceding value L.sub.i-1 and determines,
as an inflection point, K.sub.1, a point corresponding to the greatest
differential, as illustrated in FIG. 12.
When the cuff pressure takes values between the systolic and diastolic
BP values of the patient, the lower-peak portion of the waveform
of each pulse of the signal SM.sub.2 is cut off, because the transmission
of the PPW (i.e., blood flow) from the upstream side of the cuff
110 to the downstream side of the same is partially interrupted
by the cuff 110. As the cuff pressure increases, the respective
lengths of the cut-off portions of the pulses increase and accordingly
the respective lengths L of the lower-peak portions of the pulses
increase. Therefore, the above-indicated point K.sub.1 is indicative
of a time when the cuff pressure is equal to an actual or true diastolic
BP value of the patient. Initially, a negative judgment is made
at Step S104, and the control of the CPU 129 goes back to Step S103.
If a positive judgment is made at Step S104 while Steps S103 and
S104 are repeated, the control goes to Step S105 to determine a
cuff pressure, P.sub.CD1, corresponding to the point K.sub.1 identified
at Step S104 and store it in the RAM 133. The cuff pressure P.sub.CD1
is indicative of a true diastolic BP value of the patient. Step
S105 functions as a diastolic BP determining means.
Step S105 is followed by Step S106 to judge whether the current
MBP-P.sub.M relationship determined at Step S108 is accurate, based
on the last diastolic BP value MBP.sub.DIA determined at Step S111
and the cuff pressure P.sub.CD1 (i.e., true diastolic BP value)
stored at Step S105. For example, the CPU 129 judges whether the
absolute value of the difference of the last diastolic BP value
MBP.sub.DIA and the cuff pressure P.sub.CD1, i.e., .vertline.MBP.sub.DIA
-P.sub.CD1.vertline., is not greater than a reference value, .DELTA.P.sub.1.
This reference value is employed for guaranteeing the accuracy of
the MBP-P.sub.M relationship: The reference value is, e.g., 5 mmHg.
However, in the case where there is a difference between the cuff
pressure P.sub.CD1 and the diastolic BP value BP.sub.DIA measured
at Step S107, the first difference .vertline.MBP.sub.DIA -P.sub.CD1.vertline.
is compared with a modified reference value obtained in advance
by subtracting, from the reference value .DELTA.P.sub.1, the second
difference between the cuff pressure P.sub.CD1 and the diastolic
BP value BP.sub.DIA measured using the cuff 110. Step S106 corresponds
to the relationship-accuracy judging means 186.
If a positive judgment is made at Step S106, the current MBP-P.sub.M
relationship is accurate and appropriate, therefore need not be
updated. Therefore, the control of the CPU 129 skips Steps S107
and S108 and goes to Step S109 and the following steps, i.e., the
continuous BP monitoring routine. On the other hand, if a negative
judgment is made at Step S106, the control goes to Steps S107 and
S108 to carry out an oscillometric BP measurement and determine
a new MBP-P.sub.M relationship and subsequently goes to the continuous
BP monitoring routine.
As is apparent from the foregoing description relating to the second
embodiment shown in FIGS. 7 to 12, the CPU 129 of the control device
128 determines, at Step S103, a length L of a lower-peak portion
F of the waveform of each of successive heartbeat-synchronous pulses
of the PPW signal SM.sub.2 which is detected while the cuff pressure
is increased at a predetermined rate. At Step S105, the CPU 129
determines a cuff pressure P.sub.CD1 corresponding to a point K.sub.1
or time when the determined lengths L significantly largely change.
At Step S106, the CPU 129 judges whether the current MBP-P.sub.M
relationship determined at Step S108 is accurate, based on the determined
cuff pressure P.sub.CD1 and the last diastolic BP value MBP.sub.DIA
last determined at Step S111. If a positive judgment is made at
Step S106, an oscillometric BP measuring operation is not carried
out at Step S107 and the current relationship is not updated at
Step S108, i.e., is maintained. Thus, the patient is prevented from
being pressed by the cuff 110. In addition, although the PPW sensor
146 is worn at a position downstream of the cuff 110, the continuous
BP monitoring operation is continued at Steps S109-S111, without
being interrupted due to the inflation of the cuff 110.
In the second embodiment, since the judgment about whether the
current MBP-P.sub.M relationship is accurate is made based on the
cuff pressure P.sub.CD1 and the last monitor diastolic BP value
MBP.sub.DIA determined at Step S111, it is more accurate than a
judgment made based on a monitor diastolic BP value MBP.sub.DIA
determined at Step S111 a predetermined time before, or the last
diastolic BP value BP.sub.DIA measured at Step S107.
Referring next to FIGS. 13 through 16, there will be described
a third embodiment of the present invention. The third embodiment
relates to a continuous BP monitor 200 having the same hardware
construction as that of the second embodiment shown in FIG. 7. The
same reference numerals as used in the second embodiment are used
to designate the corresponding elements or parts of the third embodiment
and the description thereof is omitted.
However, as shown in FIG. 13, the BP monitor 200 has different
functions from those of the BP monitor 100 as the second embodiment.
A control device 128 of the BP monitor 200 functions as a phase-difference
determining means 298 which determines a phase difference, T (msec),
of respective lower-peak points of each of successive heartbeat-synchronous
pulses of a PPW signal SM.sub.2 and a corresponding one of successive
heartbeat-synchronous pulses of a CPW SM.sub.1, as illustrated in
the graph of FIG. 15. Those pulses of the signal SM.sub.2 and those
pulses of the signal SM.sub.1 are detected by a PPW sensor 146 and
a CPW sensor 114, 124, 130, respectively, when a pressure of a cuff
110 is increased at a predetermined rate by a cuff-pressure increasing
means 178. The control device 128 also functions as a judging means
286 which judges whether an MBP-P.sub.M relationship determined
by a relationship determining means 174 is accurate, based on one
or more diastolic blood pressure values determined by a monitor-BP
determining means 176 and a cuff pressure corresponding to a point,
K.sub.2, (FIG. 16) or a time when the phase differences T determined
by the phase-difference determining means 298 significantly largely
change.
FIG. 14 shows a flow chart representing a control program according
to which the control device 128 controls the operation of the present
BP monitor 200. The flow chart of FIG. 14 is different from that
of FIG. 11 only in that Step S104 of FIG. 11 is replaced by Step
S214 of FIG. 14 that corresponds to the phase-difference determining
means 298. At Step S103, a CPU 129 controls the cuff-pressure increasing
means 178 to start increasing the cuff pressure. At Step S214, first,
the PPW sensor 146 and the CPW sensor 114, 124, 130 obtain the PPW
signal SM.sub.2 and the CPW signal SM.sub.1, respectively, and the
CPU 129 determines a phase difference T of respective lower-peak
points of each of successive heartbeat-synchronous pulses of the
PPW signal SM.sub.2 and a corresponding one of successive heartbeat-synchronous
pulses of the CPW SM.sub.1, as shown in FIG. 15. Then, the CPU 129
judges whether the CPU 129 has identified a point or a time when
the phase differences T have significantly largely changed. For
example, the CPU 129 differentiates the phase-difference values
T by subtracting, from each value T.sub.i, the preceding value T.sub.i-1
and determines, as an inflection point, K.sub.2, a point corresponding
to the greatest differential, as illustrated in FIG. 16. When the
cuff |