Abstrict
An automatic noninvasive blood pressure monitor with a continuous
correction of the setpoint and of the gain without interruptions
of the blood pressure measurement has a pressure vibration generator,
the frequency of vibration being higher than that of the highest
harmonic component of blood pressure wave, and a correction circuit,
the input of which is connected either directly or through at least
one amplifier of the basic servosystem to the sensor of the plethysmographic
gauge and output of which is fed to the input of the circuit for
correction of the setpoint of the basic servosystem.
Claims
What is claimed is:
1. An automatic noninvasive blood pressure monitor for measuring
blood pressure in arteries compressible from the surface, comprising
a pressure means chosen from the group consisting a cuff and a
pelotte, said pressure means equipped with a plethysmographic gauge
of vascular volume;
first summing means, amplifier means, a phase corrector, a first
switch, and a voltage-pressure transducer;
said guage being connected serially through said first summing
means, said amplifier means, said phase corrector and said first
switch to said voltage-pressure transducer, said transducer being
connected to said pressure means; and
a correction circuit having an input and an output, the input of
said correction circuit being connected to said amplifier means,
the output of the said correction circuit being connected to said
first summing means.
2. The monitor as claimed in claim 1 further comprising
a generator of pressure vibrations having a frequency exceeding
the frequency of the highest harmonic component of the blood pressure
beat wave, said generator being connected through connection means
to said transducer.
3. The monitor as claimed in claim 1 further comprising
a generator of pressure vibrations having a frequency exceeding
the frequency of the highest harmonic component of the blood pressure
beat wave, said generator being connected through second summing
means to said transducer;
said generator of pressure vibrations being an oscillator;
the correction circuit comprising a narrow band amplifier tuned
to the frequency of the oscillator and connected to a first detector
the output of which is connected to an evaluating circuit connected
with a first integrator.
4. The monitor as claimed in claim 3 wherein
the amplifier means comprises a first amplifier, and a second amplifier
with electronic gain control;
said second amplifier being connected through a high pass filter
to a second detector and a second integrator, the output of which
is connected to the gain control of the second amplifier.
5. The monitor as claimed in claim 3 wherein
the plethysmographic gauge is a photoelectric reflection plethysmograph
having a light sensor and a light source placed in an inner sheet
of said pressure means.
6. The monitor as claimed in claim 3 wherein
the plethysmographic gauge is a photoelectric reflection plethysmograph
having a light sensor and a light source placed in an opposite rigid
support in the neighbourhood of the measured artery.
7. The monitor as claimed in claim 1 wherein
the amplifier means comprises a first amplifier, and a second amplifier
with electronic gain control;
said second amplifier being connected through a high pass filter
to a second detector and a second integrator, the output of which
is connected to the gain control of the second amplifier.
8. The monitor as claimed in claim 7 wherein
the plethysmographic gauge is a photoelectric reflection plethysmograph
having a light sensor and a light source placed in an inner sheet
of said pressure means.
9. The monitor as claimed in claim 4 wherein
the plethysmographic gauge is a photoelectric reflection plethysmograph
having a light sensor and a light source placed in an opposite rigid
support in the neighbourhood of the measured artery.
10. The monitor as claimed in claim 1 wherein
the plethysmographic gauge is a photoelectric reflection plethysmograph
having a light sensor and a light source placed in an inner sheet
of said pressure means.
11. The monitor as claimed in claim 1 wherein
the plethysmographic gauge is a photoelectric reflection plethysmograph
having a light sensor and a light source placed in an opposite rigid
support in the neighbourhood of the measured artery.
Description BACKGROUND OF THE INVENTION
(a) Field of the Invention
The invention relates to an automatic noninvasive blood pressure
monitor, i.e. an instrument for measuring the blood pressure in
man on arteries being compressible from the surface by means of
a pressure cuff or a pelotte equipped with a plethysmographic gauge,
such as an impedance or a photo-electric one, which gauge is connected
through at least one amplifier and a phase corrector to an electro-pressure
transducer. All these components constitute the closed loop of a
servosystem which continuously and instantaneously changes the pressure
in the cuff and maintains thus the volume of the artery at a value
corresponding to the zero tension of the arterial wall. The pressure
within the cuff thus follows the instantaneous value of intraarterial
pressure. In such instruments automatic setting and correction of
the setpoint and gain is also possible.
(b) Description of the Prior Art
Similar instruments are known from the patent literature, e.g.
Czechoslovak patent specification No. 133205, U.S. patent specification
No. 4,510,940, as well as from articles in medical and technical
journals. The instruments known up to now, however, either have
no automatic initial setting of the setpoint and gain, or have no
correction of these parameters during the measurement. Although
an instrument with such a correction has been proposed, the correction
is performed during short-lasting interruption of the measurement.
Another instrument proposed is equipped with an additional cuff
placed on another circulatory region so that a pair of cuffs is
necessary. The proposed instruments permit measurement of the blood
pressure on arteries of the finger or of another region which can
easily be transilluminated, however continuous blood pressure measuring
is impossible on other arteries especially on larger ones.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide
an automatic noninvasive blood pressure monitor with a continuous
correction of the setpoint and of the gain without interruptions
of the blood pressure measurement.
The basic servosystem loop is completed by a pressure vibration
generator, the frequency of vibration being higher than that of
the highest harmonic component of blood pressure wave, and by a
correction circuit, the input of which is connected either directly
or through at least one amplifier of the basic servosystem to the
sensor of the plethysmographic gauge and output of which is fed
to the input of the circuit for correction of the setpoint of the
basic servosystem.
The instrument thus contains, in difference from similar known
instruments, a generator producing pressure vibrations which are
superimposed on the basic pressure wave in the cuff or the pelotte,
and a correction circuit which continuously observes the oscillations
of the vascular volume which are brought about by the pressure vibrations
and which circuit derives a correction signal from their amplitude
and/or phase during each pulse interval. The correction signal is
fed, after its integration, to the basic servosystem where it adjusts
its setpoint, i.e. the extent of compression of the vessel on which
the measurement is being performed. In fact, the correction circuit
measures, parallely with the blood pressure, the so called dynamic
vascular compliance which is in a close relation to the vascular
wall tension and which adjusts this tension to a value which is
optimal for the transmural pressure transmission. Also, the circuit
for the automatic gain control works on a similar basis, i.e. measurement
of volume oscillations produced by pressure vibrations. The plethysmographic
gauge of the pressure cuff or pelotte uses, in contrast to known
similar instruments, the reflexion photoelectric plethysmography.
When compared with known instruments of this kind, the inventive
instrument presents many advantages. The correction of the setpoint
and of the gain is continuous, i.e. the blood pressure measurement
runs without periodical interruptions. The correction is derived
from the signal of the same gauge which provides the function of
the basic servosystem and does not need therefore another gauge.
The instrument is thus simpler from the standpoint both of the user
and the producer. The possibility of using another gauge remains,
however, as shown in the alternative embodiment of the instrument.
The correction is very effective and fast so that the instrument
does not need any algorithm for finding the initial setpoint. Using
the reflexion photoelectric plethysmograph as a sensor for measuring
the arterial volume makes it possible to measure, in a noninvasive
way, the blood pressure not only in the finger but also in other
arteries accessible from the surface.
BRIEF DESCRIPTION OF THE DRAWING
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 embodiments considered
together with the accompanying drawings in which:
FIG. 1 is a circuit block diagram of the basic embodiment of the
automatic noninvasive blood pressure monitor according to the invention;
FIG. 2 is a circuit block diagram of a preferred embodiment of
the automatic noninvasive blood pressure monitor according to the
invention; and
FIG. 3 shows waveforms and curves for illustrating the operation
of the circuit shown in FIGS. 1 and 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The basic embodiment and 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 o the basic
embodiment of the present invention. In the figure, reference number
1 denotes a pressure cuff. The pressure cuff 1 is provided with
a plethysmographic gauge 2 of the arterial volume and surrounds
the measured zone 3. The plethysmographic gauge 2 is connected through
a summing member 4 to an amplifier 5, the output of which amplifier
5 is connected to a voltage-pressure transducer 6. The pressure
cuff 1 is connected fluidically on the one hand to the voltage-pressure
transducer 6 on the other hand to a generator 7 of pressure vibrations.
The output of the amplifier 5 is also connected to a correction
circuit 8 the output of which is connected to the summing member
4.
The pressure cuff 1, the summing member 4, the amplifier 5 and
the voltage-pressure transducer 6 make up the loop of the basic
servosystem which servosystem keeps the artery volume of the measured
zone 3 at a constant value by immediate pressure changes in the
pressure cuff 1. The artery is compressed so that the vascular wall
has a zero tension. Only under this condition does the pressure
in the pressure cuff 1 correspond to the intraarterial pressure.
Such a state is tested by means of pressure vibrations produced
by the generator 7 of pressure vibrations, the correction circuit
8 producing the necessary correction signal. This correction signal
is brought in the corresponding polarity to the summing member 4,
adjusting continuously the compression degree of the vascular volume
so that the condition of the zero tension of the vascular wall is
fulfilled.
The preferred embodiment of the automatic noninvasive blood pressure
monitor according to the invention is shown in FIG. 2. The pressure
cuff 1 is applied in a suitable region, such as, e.g. finger, forearm,
temporal region, etc., where the artery 12 lies in a soft tissue
against a natural background e.g. the bone 13 or an artificial support.
A light source 10 and a light sensor 11 are placed on the inner
sheet of the pressure cuff 1 in such a manner that the artery 12
is in their neighborhood or between them.
The light sensor 11 is connected through a first summing member
14 to a first amplifier 15, the output of which is connected through
a second switch 21 and a first integrator 22 with changed polarity
back to the first summing member 14. The output of the first amplifier
15 is connected, in addition, to second amplifier 16 with an electronic
gain control.
The second amplifier 16 is connected through a phase corrector
17 and through a first switch 18 and a second summing member 19
to the voltage-pressure transducer 6 which transducer is connected
to the pressure cuff 1 and an electromanometer 20. The output of
the first amplifier 15 is connected also to a narrow band amplifier
25 the output of which is connected through a first detector 26
to an evaluating circuit 27 connected to the first switch 21.
The controlling input of the evaluating circuit 27 is connected
to the output of the second amplifier 16. The output signal of the
second amplifier 16 is fed also through a high pass filter 28 and
a second detector 29 to a second integrator 30 the output tension
of which controls the gain of the second amplifier 16.
The first switch 18 and the second switch 21 are controlled by
an external signal fed to a controlling input 31.
The first switch 18 is connected to a voltage source 23, the second
summing member to an oscillator 24.
The function of the instrument is controlled by the first switch
18 and the second switch 21. Before starting the measurement it
is necessary to put both switches 18,21 to the position a. In this
position a of the switches 18,21, the loop of the basic servosystem
is open, the pressure in the pressure cuff 1 corresponds to the
voltage set in the voltage source 23 and the automatic zeroing loop
is closed, i.e. the first integrator 22 compensates gradually the
d.c. component of the photoelectric voltage of the light sensor
11 which corresponds to the volume of the artery 12. After a few
seconds the output voltage of the first amplifier 15 is put to zero
and the switches 18,21 are set to position b.
If the correction signal of the output of the evaluating circuit
27 is zero, then the output voltage of the first integrator 21 will
not change and servosystem will keep the photoelectric signal from
the light sensor 11 corresponding to the volume of the artery 12
at its original value. In this situation any change of vascular
volume produces an instantaneous change of pressure in the pressure
cuff 1 which compensates the volume change almost without any rest
error, if the gain of the second amplifier 16 has been properly
set and thanks to the phase corrector 17 which has the properties
of a PID controller.
If the initial volume was chosen in a way that the arterial wall
had a zero tension, then the pressure in the pressure cuff 1 and
thus even the output voltage of the electromanometer 20 corresponds
in each instance to the intraarterial pressure.
In practice however one can estimate the initial vascular volume,
i.e. the setpoint of the servosystem only with difficulties and
moreover, it is known that its value can change during the measurements.
Therefore, in the instrument the required setpoint is found automatically
and corrected continuously so that small pressure vibrations are
superimposed on the recorded pressure course, the frequency of which
exceeds any frequency naturally contained in the pressure wave.
The oscillator 24 is the source of these vibrations, the output
periodical voltage of which oscillator is added to the output voltage
of the phase corrector 17 in the second summing member 19.
The voltage pressure transducer 6 generates then the required pressure
course. The superimposed pressure vibrations in the pressure cuff
1 then produce small but measureable oscillations of the volume
of the artery 12 which are recorded together with the original photoelectric
signal by the light sensor 11, amplified by the first amplifier
15, then separated and further amplified by the narrow band amplifier
25 which is tuned on the frequency of the oscillator 24 and then
detected by the first detector 26. The output signal of this detector
26 corresponds to the physiological variable called dynamic vascular
compliance (DVC) which markedly depends on the arterial wall tension.
As shown in FIG. 3, the rest error of the photoelectric signal
i.e. a small increase of vascular volume PG in the systolic part
of each beat is accompanied by a characteristic change of DVC in
dependence on setting the servosystem setpoint. As illustrated in
section A of FIG. 3, in case the setpoint is set incorrectly so
that the vascular volume PG is kept by the servosystem at too high
a value, then the recorded pressure CP is lower than the real intraarterial
pressure BP (dashed line) and DVC decreased at the increase of vascular
volume during the rest error deviation. The opposite case is illustrated
in section C of FIG. 3. If the vessels are too much compressed by
the servosystem which then measures an incorrectly high pressure,
then DVC increases markedly at the onset of the beat. When correctly
setting the pressure monitor (section B), the vascular volume PG
has a medium value and the registered pressure CP is identical with
the intraarterial pressure BP.
The evaluating circuit 27 (FIG. 2) of the instrument which in principle
is a gated amplifier triggered by the rest error signal obtained
at the output of the second amplifier 15, detects just this transient
deviation of DVC at the onset of each beat, and derives from it
the error signal E (see FIG. 3) and feeds it to the first integrator
22 the output voltage of which estimates the setpoint of the servosystem
by means of the first summing member 14. In this way, the degree
of arterial compression is automatically and continuously corrected
so that the arterial wall tension is minimal.
A continual control of amplification is provided by the second
amplifier 16 (FIG. 2) with electronic gain control and by the circuit
comprising the high pass filter 28, the second detector 29 and the
second integrator 30. From the output signal of the second amplifier
16, the high pass filter 28 selects higher frequencies than is the
lowest presumed frequency of the servosystem's own oscillations
and also the oscillations produced by intentionally induced pressure
vibrations in the cuff. Such frequency components are a.c. amplified
and detected by the second detector 29 and integrated by the second
integrator 30 to which is fed also a constant voltage estimating
the necessary gain of the servosystem. The output voltage of the
second integrator 30 thus keeps the gain of the second amplifier
16 at a value which is safely lower than the value at which undamped
oscillations of the servosystem would arise. |