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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a blood pressure measuring appliance, and
particularly, to an appliance having a pulse beat detector responding to
arterial pulse beats, an electrocardiograph signal detector and a blood
pressure measuring and indicating apparatus.
2. Description of Related Art
Blood pressure is ordinarily measured by means of an inflatable bag
connected to a mercury manometer, according to the method of Riva-Rocci.
The inflated bag ties off the upper arm so that the arm artery permits no
more blood to flow therethrough. On reduction of the bag pressure the
blood is again forced through the slowly opening artery. Then, as a result
of turbulent flow, a noise occurs to which it is possible to listen with a
stethoscope. The pressure read off on the manometer corresponds, on
commencement of the turbulent flow, to the systolic blood pressure. When,
upon further pressure reduction in the bag, the artery opens completely,
the turbulent flow changes into a laminar flow and the heard sound or
noise disappears. The pressure then read off on the manometer corresponds
to the diastolic blood pressure. In this measuring method the accuracy of
measurement depends upon the existing physiological conditions of the
patient and the hearing capacity of the physician.
It is known from "Medical & Biological Engineering & Computing", September
1981, pages 671, 672, that the pulse beat transit time varies with the
blood pressure. This is utilized, in combination with a conventional blood
pressure measurement bag, for the determination of the measurement moments
for the systolic pressure and the diastolic pressure. The time is
ascertained between the R-peak of an ECG signal detected by means of an
ECG signal detector and the arrival of the pertinent pulse beat at the
location of a pulse beat detector. The systolic pressure is read off at
the moment of maximum time delay on the manometer of the bag. The
measurement moment for the diastolic pressure is ascertained in dependence
upon the speed of variation of the transit times of successive pulse
beats. The fact is utilized that the transit time of successive pulse
beats remains constant after the diastolic pressure is reached. In the
known blood pressure measuring appliance the actual blood pressure
measurement takes place according to the bag method of Riva-Rocci.
The invention is directed towards provision of a blood pressure measuring
appliance which, after initial calibration, can measure the blood pressure
without an inflatable bag.
SUMMARY OF THE INVENTION
On the basis of the blood pressure measuring appliance as initially
explained, the blood pressure measuring and indicating apparatus is
formed, in accordance with the invention, as a time-measuring apparatus
which ascertains the time intervals between peaks of a predetermined
nature of the ECG signal, detected by the ECG detector, and pulse beats in
each case following the predetermined peaks and detected by the pulse beat
detector, and indicates as blood pressure information, a datum
representing the duration of the ascertained time intervals.
The invention makes use of the fact that a predetermined relationship,
reproducible with adequate accuracy, exists between the transit time of
the pulse beats and the blood pressure. After an initial calibration of
the time measuring apparatus to mean values of systolic and diastolic
blood pressures, which were ascertained on the patient, preferably by
means of a sphygmomanometer by the method of Riva-Rocci, the blood
pressure measurement can thereafter be carried out for this patient
without sphygmomanometer.
It has proved sufficiently accurate if a linear relationship is assumed
between the transit time of the pulse beats and the blood pressure. Thus,
two measured values suffice for the calibration in order that differing
measured values may be interpolated or extrapolated. The calibrated values
can be ascertained mathematically from the previously measured values by
adjustment of scale end values of a mechanical computer dial or by
operation of a computer circuit. It is self-evident that even more
complicated functional relationships between the transit time and the
blood pressure can be taken into consideration, especially if a computer
circuit is used.
The ECG signal detector and the pulse beat detector preferably each
comprise a comparator which compares the amplitude, detected by the
detector, of the ECG signal, especially its R-peak, and/or the pulse beat,
with a threshold value. The ECG signal detector and the pulse beat
detector each generator an impulse when the ECG signal or the amplitude of
the pulse beat, as the case may be, exceeds the threshold value in a
predetermined direction. The time measurement stage then ascertains the
time interval corresponding to the blood pressure, in dependence upon the
impulses of the detectors.
The maximum amplitudes of the ECG signal and of the pulse beat can
fluctuate greatly from patient to patient. In order nevertheless to
achieve constant measurement results, it is preferredly provided that the
threshold value indicator of the ECG signal detector or of the pulse beat
detector, as the case, comprises an averaging stage which generates a
threshold value signal following the time mean value of the ECG signal or
the pulse beat sequence. Thus the threshold value indicator adapts the
threshold value, for example, to the direct current mean value of the ECG
signal or the pulse beat. In order to make the threshold value signal
additionally adjustable, for example by hand, an adjustable direct current
level is expediently superimposed upon the threshold value signal.
The impulse generated by the ECG signal detector or the pulse beat detector
can be generated, for example, by means of a monoflop triggerable by the
comparator. In place of a monoflop, it is also possible to use a
differentiation member connected before the signal input of the
comparator. This configuration additionally has the advantage that the
leading edge of the impulse can be allocated more exactly in time to the
leading edge of the R-peak of the ECG signal or the pulse beat, and, on
the other hand, flatter impulse leading edges no longer instigate any
triggering action.
The ECG signal detector is preferably decoupled from the time-measuring
apparatus through an opto-coupler, in order to reliably preclude
stimulation of the patient by interfering impulses fed back to the ECG
electrodes and to minimize artifacts upon the time-measuring apparatus.
For the same reason, the ECG signal detector forms a separate construction
unit with a line independent working voltage source, for example a battery
or the like, which is independent of the voltage supply of the blood
pressure measuring appliance.
Both above and below, it is to be understood that an ECG signal detector
means any apparatus which generates a signal allocated to a predetermined
peak of the ECG signal. As well as by skin electrodes, the ECG signal can
be tapped from the patient by ultra-sonic Doppler methods, resistance
measurements and sound detectors. For the generation of a signal
representative of the pulse beats, it is possible to use photoelectric
methods, ultrasonic Doppler methods, piezoelectric methods, resistance
measurement methods, laser methods with infrared measurement, but also
methods based upon dielectricity measurements, expansion measurements and
upon the detection of infrasonic signals.
Active methods are preferred in which an active sensor delivers through a
measured signal transmitter, a measured signal which varies in dependence
upon blood pressure and is detected by a measured signal receiver. A
preferred pulse beat detector of this kind comprises two measured signal
receivers arranged apart from one another and from the measured signal
transmitter, in order to facilitate the orientation of the pulse beat
detector in relation to an artery. The two measured signal receivers are
coupled to a selector circuit which responds to the signal amplitudes of
the received measured signals and delivers the received measured signal
with the greater amplitude, in each case, for the control of the time
measuring apparatus. The selector circuit can for example be an OR circuit
with biassed diodes. Since the maximum signal amplitudes can fluctuate
greatly in dependence upon the measurement site, each of the two measured
signal receivers comprises an amplification regulation circuit which keeps
constant the mean direct-current voltage level of the measured signal fed
to the selector circuit.
The pulse beat detector and also the time-measuring apparatus are
expediently operated independently of line voltage from a battery or a
rechargeable accumulator. In order to keep the current consumption of the
pulse beat detector as low as possible, the measured signal transmitter of
the sensor is worked in pulsed operation. Pulsed operation further has the
advantage, in photoelectrically working measured signal receivers, that
the influence of artificial light amplitude-modulated in the tempo of the
line voltage frequency, can easily be suppressed without additional
circuitry measures, for example, band-stop filters. The pulsed frequency
is preferably selected equal to twice the line frequency. The output
signal of impulse form of the measured signal receivers is smoothed in an
impulse amplitude demodulator.
The measured signal transmitter is preferably an infrared luminescence
diode, while the measured signal receivers are each formed as infrared
photodiodes. In order to improve the accuracy of measurement of such a
sensor, the infrared luminescence diode is preferably fed from a constant
current source, especially in impulse form. Current/voltage converters are
connected to the infrared photodiodes. Such converters have a very low
input resistance, whereby the photodiode works in short-circuit operation,
its output current is proportional to the received light intensity and
also interference excitations of the supply leads are made ineffective.
The time-measuring circuit is preferably a microprocessor circuit. The
measurement results are expediently indicated in a liquid crystal display,
in order to save energy. The microprocessor circuit stores in its memory
the measured values ascertained in the initial calibration and converts
the measured transit times of the pulse beats into blood-pressure values.
Further data, for example the pulse frequency, can be ascertained from the
signals delivered by the detectors and displayed. Furthermore, with the
aid of the microprocessor it is possible to determine upward and downward
limit values of the blood pressure and the pulse frequency on overstepping
of which the passed limit value appears on the display and in addition, in
the case of prior permission of the user, an alarm is activated.
In order to keep the current consumption of the microprocessor circuit low,
the time normally required for the time comparison is preferably realized
not in the form of a program routine of the microprocessor but rather, by
an additional quartz-stabilized timer circuit which the microprocessor
governs in interrupt-request operation. The signals of the ECG signal
detector and of the pulse beat detector are temporarily stored in a
temporary store, likewise separate from the microprocessor, until they are
processed by the microprocessor.
The various features of novelty which characterize the invention are
pointed out with particularity in the claims annexed to and forming a part
of this disclosure. For a better understanding of the invention, its
operating advantages and specific objects attained by its use, reference
should be had to the accompanying drawings and descriptive matter in which
there are illustrated and described preferred embodiments of the invention
.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a block circuit diagram of a blood-pressure measuring
appliance according to the invention; and
FIGS. 2a to 2e show time diagrams of signals which appear at different
points of the block circuit diagram in FIG. 1.
DETAIL DESCRIPTION OF THE DRAWINGS
The blood pressure measuring appliance comprises an ECG signal detector 1
with skin-contact electrodes 3 which generates at its output 5, for each
R-peak of the ECG signal, an impulse in a chronologically defined position
in relation to the R-peak. A pulse beat detector 7 generates at its output
9, for each pulse beat following upon the R-peak, an impulse with fixed
time relationship to the leading edge of the pulse beat. The pulse beat
detector 7, possibly merely its sensor 11, is preferably held on an arm
band and responds to arterial pressure fluctuations of the lower arm or
another suitable part of the body. The impulses generated by the ECG
signal detector 1 and the pulse beat detector 7 are fed to a
time-measuring circuit 13 which ascertains the time interval between
mutually associated impulses of the two detectors 1, 7 and, in dependence
upon this time interval, displays a value representative of the blood
pressure in a digital display apparatus 15, for example a liquid crystal
display unit. The blood pressure measuring appliance here makes use of the
fact that the blood pressure, which can be measured by the
sphygmomanometer method of Riva-Rocci, is dependent in a predetermined
manner upon the transit time of the pulse beat triggered by the R-peak of
the cardiac action potential.
The ECG signal detector 1 comprises an input amplifier 17 connected with
the ECG electrodes 3 and having an amplification factor of about 1000, and
a band-pass filter 19 connected to the amplifier 17. The band-pass filter
19 is formed as an active filter, its pass band extending from about 7 to
40 Hz. The band-pass filter 19 eliminates artefacts and noise of the
electrodes 3 and other interference signals received through the
electrodes. A comparator 21, for example a Schmitt-Trigger, is connected
with its one input through a differentiating member 23 to the band-pass
filter 19 and with its other input to a threshold value generator 25. The
differentiating member 23 generates impulses according to the edges of the
ECG signal peaks, the amplitude of these impulses being compared with the
signal generated by the threshold value generator 25. The threshold value
is set so that only the differentiated signal of the leading edge of each
R-peak exceeds the threshold value. The comparator 21 thus gives off a
narrow impulse, for each leading edge of the R-peak, which is shortened in
time by means of a differentiator 26 and fed through an opto-coupler 27 to
the output 5. Since the input amplitude of the amplifier 17 can fluctuate
greatly in dependence upon the fitting site of the electrodes, the
threshold value generator 25 contains an averaging stage which adapts or
readjusts the threshold value signal fed to the comparator 21 for
comparison to the direct current mean value of the ECG signal. The ECG
signal fed to the threshold value generator 25 for averaging is
superimposed in a summing circuit 29 upon an adjustable reference direct
current voltage level.
FIG. 2a shows the ECG signal U.sub.1 fed to the differentiating member 23
and supplied by the band-pass filter 19. The R-peaks are designated by 31
in FIG. 2a. FIG. 2b shows, with a curve 33 shown in dashed lines, the ECG
signal U.sub.1 from FIG. 2a after superimposition of the adjustable
reference level in the summing circuit 29. The threshold value signal
U.sub.2, obtained by averaging the signal 33 and still recognizably
following the ECG signal U.sub.1 in order to ensure a sufficiently rapid
speed of adaptation of the threshold value, is represented in solid lines
in FIG. 2b. FIG. 2c shows in solid lines the differentiated ECG signal
U.sub.3 and shown in dashed lines, in comparison therewith, the threshold
value signal U.sub.2 fed together with the signal U.sub.3 to the
comparator 21. The needle impulses of the signal U.sub.3, corresponding to
the leading edges of the R-peaks 31, generate needle impulses U.sub.4 at
the output of the comparator 21.
The sensor 11 of the pulse beat detector 7 comprises an infrared
luminescence diode 35 and two infra-red photodiodes 37, 37' arranged on
mutually opposite sides of the diode 35 and spaced therefrom. The
photodiodes 37, 37' detect the infra-red light emitted by the luminescence
diode 35 and reflected according to the state of filling of the artery.
The pulse beat travelling past beneath the sensor 11 momentarily increases
the state of filling of the artery and leads to a fluctuation of intensity
of the reflected light. The output signals of the photodiodes 37 and 37'
are processed further in separate channels 39 and 39' and the signal of
greater amplitude in each case is automatically selected at the output of
the channels 39, 39'. Thus the orientation of the sensor 11 in relation to
the artery is facilitated.
The luminescence diode 35 is operated from an impulse generator 41 through
a current regulating circuit 45 with current of impulsed form of constant
impulse amplitude. A comparator circuit 47 controlling the
current-regulating circuit 45 compares the actual value of the impulse
amplitudes with a preferably adjustable ideal or nominal value. Due to the
impulsed operation of the luminescence diode 35, the current consumption
of the blood pressure measuring appliance, preferably operated from a
battery or rechargeable accumulator, is reduced. The current-regulating
circuit 45 ensures constant luminous intensity of the transmitted infrared
light impulses. The impulse frequency of the impulse generator 41 is made
equal to twice the line voltage frequency, that is 100 Hz for a line
frequency of 50 Hz. In this way, without need to use a notch filter, the
influences of artificial light upon the photo-diodes 37, 37' can be
eliminated by ordinary band-pass filters.
A current-to-voltage converter 49, for example an amplifier with very low
input impedance, is connected to the photodiode 37. The converter 49
short-circuits the photodiode 37 for interference voltages and thus is
controlled in dependence upon the output current of the photodiode 37. In
short-circuit operation, the output current of the photodiode 37 is in
linear proportion to the received light intensity. To the current/voltage
converter 49, there is connected a pulse amplitude demodulator 51 which
delivers to a buffer amplifier 53 an impulse output signal of the
photodiode 37 amplitude-modulated according to the envelope curve of the
pulse beat. The buffer amplifier 53 has an amplification factor of about
60. To the buffer amplifier 53, a band-pass filter 55 is connected, the
pass band of which reaches from about 4 to 45 Hz. The band-pass filter 55
eliminates interference deriving from the connecting cable of the sensor
11 and from extraneous light influences. To the band-pass filter 55, there
is connected, through a multiplier 57, a rationalizer 59, for example a
two-way rectifier, which, irrespective of the sign of the signals
delivered through the band-pass filter 55, delivers a signal following the
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