 |
Description  |
|
|
BACKGROUND OF THE INVENTION
(a) Field of the Invention
This invention relates to an apparatus for measuring the blood pressure
automatically, continuously and indirectly.
(b) Description of the Prior Art
It is known in the art that the blood pressure can be measured by applying
the external pressure to an artery to be measured by an occluding cuff.
This method is referred to as an auscultatory method. In this case, if the
external pressure is higher than the systolic blood pressure, the
bloodflow in the distal part of the cuff does not exist, and if the
external pressure is kept between the systolic and diastolic blood
pressure, the detection of Korotkoff sounds can be obtained by a proper
transducer which is placed in the distal end of the cuff. Futhermore, if
the external pressure is lower than the diastolic blood pressure, even
though the bloodflow exists, its sound is not heard or is very weak.
Therefore, the generally used sphygmomanometer can continuously measure
only one of the systolic or diastolic blood pressures.
However, the need to measure both the systolic and diastolic blood
pressures for every heart beat could not be fully satisfied, if an
instrument which obtains only either one of the systolic and diastolic
blood pressures, is used.
In view of the above problem, the applicant has proposed an indirectly and
instantaneously measurable sphygmomanometer and filed on Mar. 23, 1979 at
the U.S. Patent Office under the Ser. No. 023,267, now abandoned. This
apparatus uses a volume compensation method and enables to measure
continuously both the systolic and diastolic blood pressures as well as
the instant blood pressure waveform. The method is based on the indirect
unloading of the vascular wall, i.e., the vascular volume per unit length
of blood vessel is maintained constant by applying the external pressure
thereto. The applied external cuff pressure is controlled to balance, with
the intravascular pressure or arterial blood pressure, so that the cuff
pressure indirectly shows the arterial pressure.
The above sphygmomanometer has been found, however, not satisfactorily in
that the operation and adjustment for obtaining the continuous blood
pressure is done manually and it is very complicate to operate, and
moreover it requires fine adjustment for each different subject or person.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide an
automatic, continuous and indirect blood pressure measurement apparatus
which is fully automatically operated under the control of a micro
processor.
The principle upon which the present sphygmomanometer is based, is first
explained. The principle is referred to in the art as a volume
compensation method. In this method, the blood pressure Pb and the
external cuff pressure Pc are controlled to balance with one another in
order to keep the blood wall in an unloaded condition (or natural state
where no external pressure is applied to the blood wall). In this
condition, the external cuff pressure is used as a measure of the blood
pressure. In order to carry out the measurement of the blood pressure by
means of the volume compensation method, it is most important therefore to
detect the unloaded condition of the blood vessel and to maintain such an
unloaded condition.
It is a well known fact that the amplitude of pulsating movement of the
blood vessel wall becomes maximum when average cuff and blood pressures Pc
and Pb equal to each other. Since the amplitude of the blood vessel wall
movement corresponds to the volume per unit length of the blood vessel
(hereinafter referred to as a blood vessel volume), it is possible to
detect the average cuff pressure Pc, which corresponds to the average
blood pressure Pb, by monitoring the blood vessel volume. It is further
understood that in order to maintain the unloaded condition of the blood
vessel wall, first the average cuff pressure Pc (=Pb) at the time of the
maximum volume change is detected, and then a variable component .DELTA.Pc
is superimposed upon the average cuff pressure Pc to cancel out the blood
pressure. In this unloaded condition, no external pressure is applied upon
the blood vessel wall so that the blood vessel volume is held constant.
The blood vessel volume is detected by a photo sensor comprising a light
emitting diode and a photo diode or transistor arranged at opposite sides
of the blood vessel wall to receive light emanating from the light
emitting diode. Hemoglobin contained in a red blood cell absorbs visible
light to a large extent. Therefore, the amount of light received by the
photo diode decreases as the volume of the blood vessel increases, thereby
decreasing an output voltage of the photo diode Sv (hereinafter referred
to as a volume signal) in proportion to the decrease of the cuff pressure
Pc.
In summarizing the above description, the blood pressure is measured by
using the volume compensation method as in the following way: First, an
average cuff pressure Pc is set at a value at which the pulsating
component of the volume signal Sv becomes maximum. Second, a variable
pressure .DELTA.Pc is superimposed upon the average cuff pressure Pc in
such a manner that the volumetric pulse signal Sg becomes zero, i.e., the
volume of the blood vessel becomes constant. The thus obtained cuff
pressure Pc (=Pc+.DELTA.Pc) represents instantaneous blood pressure at a
time.
The foregoing and other objects, the features and the advantages of the
present invention will be pointed out in, or apparent from, the following
description of the preferred embodiment considered together with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit block diagram of a preferred embodiment of the
automatic, continuous and indirect blood pressure measurement apparatus
according to the invention; and
FIG. 2 shows typical waveforms of cuff pressure, volume signal and
volumetric pulse signal, for illustrating the operation of the circuit
shown in FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A preferred embodiment of the present invention will now be described with
reference to the drawings.
FIG. 1 is a block diagram illustrating the construction of a preferred
embodiment of the present invention. In the figure, reference number 1
denotes an annular cuff. The annular cuff 1 has a rigid outer wall and a
resilient membrane 3 which is liquid-tightly sealed to an inwardly
extending wall from the rigid outer wall. The space or chamber defined by
the rigid outer wall, inwardly extending wall and the membrane 3 is filled
with liquid, such as water 5, for imparting external pressure to the
portion to be measured, such as finger 2. A linear pump 6 and shaker 7 are
provided and are communicable with the chamber through a tube 4 so that
the finger segment placed in the occluding cuff 1 can be compressed or
decompressed by the hydraulic pressure in the chamber. The linear pump 6
generates the average cuff pressure Pc and the shaker 7 generates the
variable .DELTA.Pc. In this embodiment, the linear pump 6 is driven by a
geared motor which is available in commerce and is sold under the
merchandise name LC-151G by Copal Electronics Company in Japan. The shaker
7 comprises a driving actuator which is sold under the merchandise name
A110-50S by Hitachi Kinzoku Kabushiki Kaisha in Japan. The cuff pressure
Pc (=Pc+.DELTA.Pc) generated by the linear pump 6 and the shaker 7
compresses the artery of the finger 2 transcutaneously. A pressure sensor
8 is provided in communication with water through the tube 4 for
measurement of the cuff pressure Pc. The pressure sensor 8 measures the
cuff pressure and converts it into an electrical signal having a value
corresponding to the cuff pressure Pc.
A volume sensor 9 is provided between the finger 2 and the membrane 3, the
sensor comprising a light emitting diode 9a and a photo transistor 9b,
both facing with each other through the finger 2. Light emanated from the
light emitting diode 9a transmits through the finger 2 and reaches the
photo transistor 9b. The amount of light transmitted is in proportion to
the volume of the blood vessel concerned. Accordingly, the output voltage
of the photo transistor or the volume signal Sv corresponds to the volume
of the blood vessel.
The volume signal Sv is applied to a DC amplifier 11, the DC component of
the output voltage being changed as desired by the application of an
off-set voltage Vf. Assuming now that the DC component is 10 volts at the
off-set voltage Vf=0, and that the off-set voltage is changed to 7 volts,
then the resultant DC component is rendered to 8 volts. Thus, the DC
amplifier 11 not only amplifies the volume signal Sv, but also eliminates
the DC component from the output signal of the amplifier 11. The off-set
voltage Vf is supplied from a control section 16 described later via a 12
bit digital to analog (D/A) converter 17. The output of the DC amplifier
11 is delivered to a gain control circuit 12 which is composed of a DC
amplifier with a gain controlled variably. More in the concrete, the gain
control circuit 12 includes a feedback circuit comprising a plurality of
pairs of serially connected switch element and resistor, the pairs being
connected between the output and input of the DC amplifier. The switch
element is controlled to open or close depending upon a control signal C1
from the control section 16 in order that the resistance value of the
feedback circuit can be varied and hence the gain can be varied as
desired. The gain control circuit 12 outputs a volumetric pulse signal Sg
which is an amplified signal of the output signal of the DC amplifier 11.
The volumetric pulse signal Sg is then supplied to an analog to digital
(A/D) converter 13 and a phase compensator 14. The 8 bit A/D converter 13
converts the volumetric pulse signal Sg into a digital signal which is
transferred to the control section through a bus 15. The phase compensator
14 is supplied with a control signal (not shown) from the control section
16 and shifts the phase of the volumetric pulse signal Sg. The
phase-shifted signal is then delivered to the gain control circuit 18. The
gain control circuit 18 has a similar construction to the circuit 12, and
generates a driver signal Se by amplifying the signal from the phase
compensator 14 by the degree determined by the control signal C2 from the
control section 16. This driver signal Se is supplied to a driver 19 and
to an (a) terminal of a switch 20. The driver 19 comprises a linear
amplifier 19 and carries out the amplification of a signal having a small
DC variation. While on the other hand, a driver 21 has a hysterises
characteristic and is operative only when a signal having a large DC
variation is applied. The input of the driver 21 is coupled to a (c)
terminal of the switch 20 which constitutes a common terminal of the
switch 20. A linear pump drive signal is supplied to a (b) terminal of the
switch 20. The driver signal Se and linear pump drive signal are subjected
to changeover by a control signal C3 from the control section 16. The
driver 19 drives the shaker 7, while the driver 21 drives the linear pump
6.
The cuff pressure Pc is detected by means of a pressure sensor 8 which
generates a signal proportional to the cuff pressure Pc. The signal is
amplified by an amplifier 24 which outputs a cuff pressure signal Sp. The
cuff pressure signal Sp is converted into a digital signal by an A/D
converter 25, and thereafter it is transferred to the control section 16
through the bus 15. The A/D converter 25 is an 8 bit arrangement. A
display section 27 is also provided which operates under control of the
control section 16 and displays the cuff pressure signal Sp under an
appropriate scale.
The control section 16 is preferably made of a central processor unit Z80,
parallel input and output Z80-PIO, read-only memory and random access
memory all available from Zilog, Inc., in the U.S.A.
The operation of the automatic, continuous and indirect sphygmomanometer
thus constructed will be described with reference to FIGS. 1 and 2. A
finger 2 is inserted into the cuff 1 as shown in FIG. 1. A start button
(not shown) is depressed to start the operation of the control section 16
at the timing to as shown in FIG. 2. The control section 16 delivers a
control signal C3 to make the common terminal (c) get contact with the (b)
terminal. The driver 21 is supplied with logical "1" signals as a linear
pump drive signal. Accordingly, the driver 21 drives the linear pump 6 in
such a manner that the cuff pressure Pc is linearly increased as shown in
FIG. 2 at line (a). In compliance with the increase of the cuff pressure
Pc, the volume signal Sv rises up gradually as shown at line (b). When the
cuff pressure Pc goes up to around the average blood pressure Pb, a
pulsating movement appears on the blood vessel wall which movement
generates a volumetric pulse signal Sg on the volume signal Sv. Line (c)
shows the waveforms of the volumetric pulse signal Sg. When the cuff
pressure Pc reaches the value equal to the average blood pressure Pb, the
amplitude of the volumetric pulse signal Sg becomes maximum. At this
condition, the averaged value of the volume signal Sv and the value of the
cuff pressure Pc are stored in a memory device of the control section 16,
the former value being referred to as a servo target value Svs, the latter
as a servo initial pressure Pcs. The actuation of the linear pump 6 is
further continued in order to obtain the cuff pressure Pc larger than the
blood pressure Pb. Then, the volumetric pulse signal Sg gradually
decreases and finally fades out to zero. When the cuff pressure Pc reaches
a determined upper limit value such as 180 mm Hg, the control section 16
this time delivers logical "0" signals to the driver 21. As a result, the
linear pump 6 is driven oppositely to thereby linearly decrease the cuff
pressure Pc. At the time t1 shown in FIG. 2 when the cuff pressure Pc
falls down to the servo target value Pcs, the linear pump 6 is stopped and
the (a) terminal of the switch 20 is used for supplying the driver signal
Se. The DC component of the driver signal Se is substantially zero at this
condition, and has only the pulsating component of the volume signal Sv,
that is, the volumetric signal Sg, so that the driver 21 is in a disabled
state. The foregoing operation has been carried out in order to set the
cuff pressure Pc at the servo target value Pcs and to obtain the average
blood pressure Pb.
Upon setting the cuff pressure Pc at the servo target value Pcs at the time
t1, the gain control circuit 12 is supplied with a control signal C1. The
gain control circuit 12 is adjusted to have such a gain as the volumetric
pulse signal Sg can have a predetermined amplitude. This predetermined
amplitude is illustratively shown at line (c) between the time t1 and t2.
By setting such a gain, the amplitude difference of volumetric pulse
signals between persons can be eliminated. The DC level deviation of the
volume signal Sv from the servo target value Svs is detected in such a
manner that a change of the predetermined amplitude of the volumetric
pulse signal Sg is detected, and if the amount of change excesses a preset
value, then the control section 16 controls the off-set voltage Vf to
restore to its original servo target value Svs.
Then, at the time t2 as shown in FIG. 2, the control section 16 instructs
to execute a next step where the shaker 7 is energized in accordance with
the volumetric pulse signal Sg. In this operation, a feedback control is
effected by controlling the variable Pc of the cuff pressure so that the
volumetric pulse signal Sv is rendered zero, that is, the volume of the
blood vessel is made constant. More in detail, the gain control circuit 18
is supplied with a control signal C2 in order that the amplitude of the
drive signal Se is adjustably controlled to make the volumetric pulse
signal Sg smaller by energizing the shaker 7 through the driver 19. In
addition to the above, the phase of the volumetric pulse signal Sg is
shifted by the phase compensator 14. Thus, the variable .DELTA.Pc of the
cuff pressure generated by the shaker 7 is cancelled out by the blood
pressure, and at the time t3 when the signal Sg becomes zero, the cuff
pressure Pc equals to the blood pressure Pb. The blood pressure or
intravascular pressure Pb can be obtained continuously and instantaneously
from the amplifier 24 as a cuff pressure signal Sp which is displayed on
the display section 27 in real time under a suitable blood scale.
It is to be noted here that after the time t2, the off-set voltage Vf and
the gain of the gain control circuit 12 are maintained constant. In the
case when the average blood pressure Pb changes after the time t2 and a DC
component is superimposed upon the volumetric pulse signal Sg, the DC
component is supplied through the phase compensator 14 and the gain
control circuit 18 to the driver 21 to drive the linear pump 6.
Accordingly, the average cuff pressure Pc restores to the average blood
pressure Pb.
It may be possible to display a cardiogram on the display section 27, or
display other suitable physiological information.
* * * * *
|
|
 |
|
 |
Description |
|
