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BACKGROUND OF THE INVENTION
The present invention relates to a blood pressure measuring equipment more
particularly to a sphygmometer.
Blood pressure measuring equipment described in U.S. Pat. No. 2,827,040
comprises a microphone for detection of the Korotkoff tones generated by
blood flowing through an artery. The microphone is connected via an
amplifier, a bandpass filter and a pulse shaper with a coincidence
circuit. An inflatable sleeve, which is attachable to the arm of the
person to be examined, is connected with an air reservoir having an outlet
nozzle which, during measurement, generates an air jet flowing past a
thermistor. The thermistor, which serves for detection of pressure pulses,
is similarly connected via an amplifier and a pulse shaper with the
coincidence circuit. In addition, a manometer for detection of systolic
pressure and a manometer for detection of diastolic pressure are provided.
Each of the manometers is connected by way of a respective valve with the
air reservoir. The air reservoir is connected by way of a valve with a
compressor and additionally via a venting valve with the ambient
atmosphere. Control equipment for actuation of the different valves is
also included.
During blood pressure measurement, the pressure in the air reservoir is
progressively increased. Pulses are then generated in certain pressure
ranges by Korotkoff tones and by pressure fluctuations and these pulses
are fed to the coincidence circuit. On the first instance of coincidence
of pulses from the two sources, i.e. the coincidence occurring at the
lowest pressure, the manometer for measurement of diastolic pressure is
temporarily connected with the air reservoir so that it measures and
indicates the diastolic pressure. Thereafter, the pressure is further
increased. When the pulse coincidence at the highest pressure occurs, the
manometer serving for measurement of systolic pressure is temporarily
connected with the air reservoir and the systolic pressure is then
measured.
This prior art equipment has the disadvantage that its appliance part
connected to the sleeve must include an air reservoir and four valves,
which make it correspondingly large and awkward and difficult to use as a
manual appliance for single-handed operation by a physician or other
person. A further disadvantage is that detection of pressure fluctuations
by means of a thermistor cooled by an air current is both delicate and
subject to inaccuracies.
Moreover, the temporal course of measurement in this automatic appliance is
significantly different from the measurement procedure usually followed by
physicians when determining blood pressure by conventional manual
appliances. With these appliances, a measuring sleeve is initially rapidly
pumped up to a pressure above systolic pressure. Thereafter, the sleeve is
slowly vented while the onset and conclusion of the production of
Korotkoff tones is determined with the aid of a stethoscope. The duration
of the pressurisation of the sleeve and the temporal course of the
pressure can, however, influence the measurement results. An automatic
appliance in which the temporal pressure course is substantially different
from the above-described traditional measurement process carried out by
physicians can give rise to measurement errors or, at the least, render
more difficult a comparison of measurement results obtained by the
automatic appliance with those obtained by a traditional method.
Another blood pressure measuring equipment described in U.S. Pat. No.
3,450,131 comprises a microphone which is connected via a regulated
amplifier and bandpass filter with a logic circuit. The equipment also
includes an inflatable sleeve and a pressure sensor which is connected
through a switchable analog-digital converter and a gate circuit with a
pressure recording appliance.
For blood pressure measurement, the sleeve is inflated to a pressure above
systolic pressure and then slowly vented. Korotkoff tones are then
generated in a certain pressure range and are converted by the microphone
into electrical signals. The logic circuit connected to the output of the
filter operates in such a manner that it can identify signals with a 1000
Hertz component as noise signals, and signals with a 40 and a 100 Hertz
component, but no 1000 Hertz component, as Korotkoff tones. On each
identification of a signal as a Korotkoff tone, the analog-digital
converter and the gate circuit are controlled by the logic circuit in such
a manner as to cause the instantaneous pressure measured by the pressure
sensor to be recorded in the pressure recording appliance. The first
recorded pressure value then corresponds to systolic pressure and the last
recorded pressure value to diastolic pressures. There is also the
possibility of providing an additional circuit which records only the
systolic and diastolic pressures.
In the equipment described in U.S. Pat. No. 3,450,131 the Korotkoff tones
are thus identified exclusively on the basis of their frequencies and
differentiated from interference noises. However, as Korotkoff tones are
very soft in the region of the diastolic pressure, identification of
Korotkoff tones in this manner is susceptible to error.
The sleeve of this equipment is inflated and vented under the control of
control unit which is not described in detail. As there is no electrical
connection between this control unit and the pressure sensor, it evidently
operates independently of the pressure measurement. It is therefore to be
assumed that during the venting phase, the air flows, independently of the
instantaneous pressure, through an outlet which includes a valve and
possibly also a throttle and which has a constant flow cross-section
during the entire venting phase. However, this has the consequence that
the outflow speed at the start of the venting phase, when the pressure is
high, is greater than towards the end of the venting phase. As a result,
the pressure reduction per unit in time is greater during measurement of
systolic pressure than during measurement of diastolic pressure. This in
turn has the consequence that systolic pressure is measured with less
accuracy than diastolic pressure. If, on the other hand, the temporal
pressure reduction is set to take place at such a slow rate that a certain
minimum level of measurement accuracy is achieved during measurement of
systolic pressure, the measurement of both pressures occupies a relatively
long period of time. This is particularly so because the outflow speed
becomes even smaller below the diastolic pressure, so that complete
venting of the sleeve can take a very long time. A further disadvantage of
this equipment is that there is an appreciable risk of under-inflation of
the sleeve, so that the pressure measured on arrival of the first
Korotkoff tone and regarded as systolic pressure in fact lies below the
true systolic pressure.
For the sake of completeness, it is mentioned that U.S. Pat. Nos.
3,903,872, 4,078,551, 4,137,907, 4,140,110 and 4,144,879 also disclose
pressure measuring equipment in which electrical signals dependent on
pressure are formed by a pressure sensor and further signals are formed
from these by differentiation, blood pressure values then being determined
from the different signals. In these known items of equipment, however,
microphones are not used so that it is not possible to utilise Korotkoff
tones for detection of systolic and diastolic pressures. Such items of
equipment therefore operate in a manner substantially different from that
of the previously described known equipment and in general are very
complicated and presumably correspondingly susceptible to errors and
faults.
OBJECTS OF THE INVENTION
The present invention has as its primary object the provision of blood
pressure measuring equipment which avoids the disadvantage of the recited
prior art equipment and by means of which it may be possible to measure
both systolic and diastolic pressure with a high level of accuracy and in
a relatively short measuring time.
A subsidiary object of the invention is the provision of equipment of the
kind described in which pneumatic and electronic components required for
measurement and control require relatively little space and can be
accommodated in a handy appliance capable of being held and operated by
one hand during measurement.
Yet another object of the invention is the provision of blood pressure
measuring equipment whereby signals generated by Korotkoff tones can be
identified with the greatest possible certainty and distinguished from
interference signals.
Other objects and advantages of the invention will be apparent from the
following description.
SUMMARY OF THE INVENTION
According to the present invention there is provided blood pressure
measuring equipment comprising measuring means attachable to a person, the
measuring means comprising a microphone for detection of blood flow
generated Korotkoff tones, and means, for example an inflatable sleeve
worn as an armband, defining a chamber deformable by a fluid, preferably
air. A pump is provided for pumping fluid into the chamber for deformation
thereof and also a valve for controlling fluid outflow from the chamber.
Fluid pressure in the chamber is detected by a pressure sensor which
generates an electrical signal indicative of detected pressure, and a
differentiator is electrically connected to the sensor to provide an
electrical signal indicative of the rate change in said pressure.
For clarification, it is noted that the references in the following
description and claims to blood pressure and air chamber pressure are to
be understood as denoting excess pressure measured with respect to ambient
air pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the present invention will now be more particularly
described by way of example and with reference to the accompanying
drawings, in which:
FIG. 1 is a schematic plan view of blood pressure measuring equipment
according to the said embodiment,
FIG. 2 is a schematic block diagram of the principal electronic and
pneumatic components of the equipment of FIG. 1,
FIG. 3 is a circuit diagram of a control unit and a regulator of the
equipment,
FIG. 4 is a cross-section, to an enlarged scale, of an outflow valve of the
equipment, and
FIG. 5 is a diagram illustrating the temporal course of a blood pressure
measurement by the equipment.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the accompanying drawings, in FIG. 1 there is shown blood
pressure measuring equipment comprising a sleeve or cuff 1 attachable to
the arm of a person to be examined and an appliance indicated generally by
3. The sleeve 1 comprises a rubber bag defining a deformable and
inflatable air chamber and contains a microphone. The sleeve 1 is
detachably connected to the appliance 3 by a line 5, which comprises an
air hose connected to the air chamber and a cable connected to the
microphone, the line being provided at the appliance end with a coupling
socket 7. The appliance 3 comprises a housing 9 provided with a threaded
shank 9a to which a pump 13 with a substantially cylindrical rubber pump
bulb is detachably fastened by means of a box nut 11. An air hose
connection nipple 15 and an electrical connection pin 17, formed by a
chassis plug, are provided on the housing 9 for coupling thereto of the
socket 7. A connection element 19, also formed by a chassis plug, is
included for the connection of a headphone. The appliance 3 also comprises
three pushbutton switches 21, 23 and 25, a digital indicating unit 27 and
various pneumatic and electronic components, as will be subsequently
described, accommodated in the interior of the housing 9.
FIG. 2 shows the inflatable air chamber 31, and the microphone 33, of the
sleeve 1 as well as some of the pneumatic and electronic components in the
appliance 3. The air chamber 31 is connected by the air hose in the line
5, and by air lines in the appliance 3, via a check valve 35 with the pump
13, an electrically controllable outflow vent valve 37 and a pressure
sensor 39. The pump 13 is provided with an air inlet having a check valve
41. The two return valves 35 and 41 are so arranged that by alternating
manual compression and release of the pump bellows 13 air can be sucked
from the ambient atmosphere and pumped into the air chamber 31.
The microphone 33 is connected by electrical conductors with the input of a
filter means 51, the output of which is connected with the headphone
connection element 19 and with a discriminator 53, which comprises a
trimming potentiometer 54 for setting of a lower threshold value and a
pulse shaper. The output of the pulse shaper is connected to a control
unit 55.
The pressure sensor 39 comprises a measurement converter bridge circuit
formed by piezo-resistive elements and is connected with the input of an
amplifier 57, the output of which is connected via a differentiator 59,
and via a parallel connection bridging the differentiator, with the
control unit 55. The control unit 55 and the amplifier 57 are also
connected at outputs thereof with a device 75 for automatic zero
balancing, the output of the device 75 being connected to the pressure
sensor 39. The output of the differentiator 59 is also connected to the
control unit 55 and additionally to an input of a regulator 61. The
control unit 55 is connected to another input of the regulator 61, the
output of which is connected with an electromagnetic actuating means of
the outflow valve 37. The control unit 55 additionally has two connections
which are connected to, respectively, two analog memories 63 and 65 each
formed by a respective capacitor, and is connected to an indicating
control device 67. The device 67 includes, amongst other things, an
analog-digital converter and is connected to the indicating unit 27.
A discriminator 69 is connected at its input to the output of the
differentiator 59, and at its output to an input of a heartbeat frequency
meter 71 and an input of the control unit 55. The meter 71 is connected at
a control input thereof to an output of the control part 55 and comprises
an analog memory connected at an output to an input of the control unit
55. The switch 21 is connected to the control unit 55 and the switch 23 to
the indicating control device 67. Also present is a voltage source 73,
which includes a battery and which is connected to supply voltage
connections of the different operative components and to an earth
connection. The switch 25 and also the control unit 55 are connected to
the voltage source 73, which, part from the battery, comprises logic
elements and a regulator for stabilisation of the supply voltage. The
battery is accommodated in a battery compartment closable by a lid.
Certain of the components of the control unit 55 and the connections
thereof to each other and to other components of the equipment will now be
explained with reference to FIG. 3.
The output of the discriminator 53 and the output of the discriminator 69
are connected with different inputs of Korotkoff tone identifier 81 of the
control unit 55. The identifier 81 includes AND-gates, which are connected
together and one of which serves as a coincidence circuit as will be
subsequently explained, and at least one flipflop. The identifier 81 also
comprises a detector which, amongst other things, serves for recognition
of the last Korotkoff tone and which is composed of elements for exercise
of logic functions and determination of at least one time interval. One
output of the identifier 81 is connected to an electronic multiple
switching device 87. Another output of the identifier 81 is connected to
the control input of the heartbeat frequency meter 71. The outputs of the
amplifier 57 and meter 71 are also connected to inputs of the multiple
switching device 87. The switching device includes connections which are
connected to components not illustrated in FIG. 3, namely the memories 63
and 65 and the indicating control device 67. The output of the
differentiator 59 is connected to the input of a level monitor 89.
As will be explained, the purpose of the discriminator 69 is to provide
pulses indicative of pressure fluctuations generated by heartbeats. The
discriminator 69 is accordingly constructed in such a manner that it
generates pulses on those pressure fluctuations at which the differential
quotient dp/dt is positive and exceeds a given threshold value of at least
100 Pascals per second, for example about 300 to 400 Pascals per second.
The level monitor 89 also responds when the temporal pressure rise, i.e.
the differential quotient dp/dt, exceeds a predetermined upper limit
value. This limit value, however, is greater than the differential
quotients arising as a consequence of the heartbeats and is at least about
2000 Pascals per second, for example 3000 to 7000 Pascals per second.
The output of the level monitor 89 is connected with an input of a flipflop
91 and with the identifier 81 both directly and via a time delay element
93. A further output of the identifier 81 is connected to another input of
the flipflop 91 and also to the switching device 87. One output of the
flipflop 91 is connected to the identifier 81 and another output of the
flipflop to the multiple switching device 87.
The regulator 61, the circuit diagram of which is also shown in FIG. 3,
comprises a differential amplifier 101 having its inverting input
connected via a resistor 103 with the output of the differentiator 59. In
addition, the output of the level monitor 89 and an output of the
identifier 81 are connected, via a diode 105 with a parallel-connected
capacitor 107, to the inverting input of the amplifier 101. The
non-inverting input of the amplifier 101 is connected via a resistor 109
with ground, via a resistor 111 with the positive pole of the voltage
source 73, and via a diode 113 with the output of the flipflop 91
connected to the identifier 81. The output of the amplifier 101 is
connected via a capacitor 117 with the inverting input and, via a switch
119, with the voltage source 73. The switch 119 is arranged in the
interior of the battery compartment, which must be opened if it is
intended to actuate the switch 119. The output of the amplifier 101 is
also connected with one terminal of a coil 131 of the outflow valve 37,
the other terminal of which is connected to the positive pole of the
voltage source 73.
The electronic components illustrated in FIGS. 2 and 3 can be constructed
as integrated circuits.
As shown in FIG. 4, the outflow valve 37 comprises a housing with two parts
133 and 135. The housing part 133 is provided at the centre of its outer
end face with a connection 133a having an inlet passage 133b. The
connection 133a has an extension projecting into the interior of the
housing and is connected by way of the lines shown in FIG. 2 with the air
chamber 31 and with the other previously described components. The
interior space of the housing is in communication through at least one
outlet passage (not shown) with the environment. A yoke 137 with a
ferromagnetic ring 139 is fastened to the housing part 135. Arranged to be
axially displaceable in a hollow cylindrical portion of the yoke 137 is an
armature 141, to which the coil 131 is fastened. The armature 141 is
provided with a rubber sealing member 143 at its end facing the outlet end
of the inlet passage 133b. A compression spring 145 presses the armature
41 away from the outlet end of the inlet opening 133b. The coil 131 is
connected by way of a cable 147 with the output of the amplifier 101 and
with the positive pole of the voltage source 73.
When the coil 131 is free of current, the armature 141 is disposed in the
illustrated opening setting in which air under pressure, fed through the
inlet passage 133b, can flow into the interior of the housing and out
through the oulet passage or passages into the environment. When a voltage
is applied to the coil 131, the armature 141 is displaced towards the
outlet end of the inlet passage 133b to reduce the flow cross-section of
the valve in correspondence with the magnitude of the applied voltage or
to close the valve entirely.
The operation of the blood pressure measuring equipment will now be
explained in detail with reference to FIG. 5.
For performance of a measurement, the sleeve 1 is connected by the line 5
with the appliance 3 and is attached to the arm of the person to be
examined. The dimensions of the appliance 3 are such that it can
conveniently be held by one hand, for which purpose the pump 13 also
serves as a handgrip. When necessary, all three pushbutton switches 21, 23
and 25 can be actuated by the hand holding the appliance.
To begin with, the change in pressure p in the air chamber 31 in relation
to the course of time t will be discussed. The temporal course of the
pressure p is represented by the curve 151 of the diagram illustrated in
FIG. 5. The pressure sensor 39 during measurement generates a voltage
which is proportional to the pressure p.
When the sleeve is attached, the appliance is made operationally ready at
the instant t.sub.0 through a brief depression of the ON/OFF switch 25.
The valve 37 is fully open at this instant and in the time interval from
the instant t.sub.0 to the instant t.sub.1, whereupon it automatically
closes. During this time interval, the pressure sensor 39 is automatically
balanced to zero by the zero balance device 75. The end of this balance at
the instant t.sub.1 is signalled by the indicating unit 27 indicating the
value zero.
Air is now pumped into the air chamber 31 by means of manual actuation of
the pump 13. As a result, the pressure in the chamber 31 rises in steps so
that the differential quotient dp/dt has a relatively high positive value
which exceeds the limit value monitored by the level monitor 89. The level
monitor 89 and generally exceeding any rate of change on pressure in the
cuff 1 that could be created by the blood pressure of the limb to which
this cuff is attached therefore feeds to the inverting input of the
amplifier 101 a voltage which has the effect of causing the regulator 61
to completely close the valve 37. This state of affairs is illustrated by
the curve 153 of FIG. 5, which represents the temporal course of the flow
cross-sectional area q of the valve 37.
During the inflation process, the pressure in the chamber 31 is
continuously indicated by the indicating unit 27 and the feed of
measurement values through the switching device 87 is controlled in such a
manner that the values are indicated at constant time intervals of, for
example 0.3 seconds. When the pressure has risen to a level sufficiently
above the expected systolic pressure p.sub.S, the inflation process is
terminated at the instant t.sub.2.
The regulator 61 is constructed as integral regulator and the capacitors
107 and 117 are so dimensioned that the valve 37 remains closed after the
last pump stroke for a defined time of at least one second, for example
two to three seconds. At the instant t.sub.3, the regulator 61 starts to
open the valve 37 so that air can flow out of the air chamber 31. During
this regulating stage, the regulator 61 compares the voltage fed it from
the differentiator 59 with the reference voltage formed by the stabilized,
positive supply voltage of the voltage divider consisting of the resistors
109 and 111. The resistors 109 and 111 are such that the mean air pressure
reduces at a constant rate of about 300 to 500 Pascals per second.
With regard to the significance of the expression "mean pressure", when the
pressure in the chamber 31 lies within a certain range, heart activity has
the consequence, as already briefly mentioned, that a small pressure
fluctuation occurs with each heartbeat. As will be explained, these
pressure fluctuations are required for the measurement and should not be
eliminated by the regulation or, at most, reduced by anything more than a
small amount. The regulator 61 is accordingly constructed in such a manner
that the regulating time constant of the entire regulating circuit, to
which the amplifier 57, the differentiator 59 and the valve 37 also
contribute, is at least 0.15 seconds. However, in order for the temporal
decrease of the mean pressure to be regulated to a substantially constant
value during the measurement time interval of interest, the regulating
time constant is preferably at most 2 seconds, for example about 0.5
seconds.
When the pressure p is now reduced from its maximum lying above the
systolic pressure p.sub.S, the pressure fluctuations caused by heartbeats
occur from the instant t.sub.4. These pressure fluctuations are detected
by the differentiator 59 and converted into corresponding signals. The
discriminator 69 then generates a pulse which is fed to the coincidence
circuit of the identifier 81 on each pressure fluctuation generated by a
heartbeat. These pulses form a pulse sequence which is designated by 155
in FIG. 5.
When the pressure in the air chamber 31 is reduced to be in a certain
range, blood flowing through the artery enclosed by the sleeve 1 generates
noises, the so-called Korotkoff tones, on each blood stroke generated by a
heartbeat. These Korotkoff tones are converted by the microphone 33 into
electrical signals and transmitted through the filter means 51, which
preferably also amplifies the signals, to the discriminator 53. When the
voltage of the Korotkoff tone signals exceeds the lower threshold value
determined by the discriminator 53, the pulse shaper of the dirciminator
53 feeds a respective pulse to the identifier 81 of the control unit 55.
This pulse sequence is designated by 157 in FIG. 5 and extends from the
instant t.sub.5 to the instant t.sub.6.
As is evident from FIG. 5, the pulses generated through the pressure
fluctuations are wider than the pulses generated through the Korotkoff
tones. The widths of the pulses of the pulse sequences 155 and 157 are
dimensioned in such a manner that the pulses are still superimposed when
the rising edges of the pressure fluctuations and of the tones are
somewhat displaced in time relative to each other. The Korotkoff tone
identifier 81 opens a window for the pulses of the pulse sequence 157
during each pulse of the pulse sequence 155. Signals from the microphone
are thus further processed when they fall into a window opened by a
pressure fluctuation, i.e. when a coincidence exists between the tone
signals and the pressure fluctuations. As a result, the Korotkoff tones
can be identified as well as distinguished from interference noises and
the latter can be suppressed.
It is to be mentioned that the coincidence circuit in the identifier 81 is
blocked for the Korotkoff tone signals during the inflation process by the
signal fed to the identifier from the level monitor 89. The time delay
element 93 in that case has the effect that the Korotkoff tone signals are
still blocked for a time duration of 2 to 3 seconds even after the end of
the rapid pressure rise, i.e. after the termination of the inflation
process.
The electronic multiple switching device 87 connects the output of the
amplifier 57 from the instant t.sub.0, or at least from the instant
t.sub.4, with the memory 63 so that the instantaneous pressure is thus at
first stored continuously in the memory 63. When the coincidence circuit
of the identifier 81 identifies a signal from the microphone as the first
Korotkoff tone in the manner previously described, a flipflop of the
identifier feeds a corresponding control signal to the switching device
87. The switching device 87 at first provisionally separates the memory 63
from the amplifier 57. The instantaneous pressure present on the arrival
of the first signal identified as a Korotkoff tone is therefore
provisonally stored in the memory 63 and provisionally identified as
systolic pressure.
The detector also present in the identifier 81 checks whether a second
pulse coincidence follows the first provisionally identified Korotkoff
tone within a predetermined time interval of at least 2 seconds and at
most 10 seconds, for example 5 seconds. Only if this is the case are both
pulse coincidences identified as genuine Korotkoff tones. When two genuine
Korotkoff tones are thus identified within the predetermined time
interval, the memory 63 for the blood pressure measurement process
concerned is finally separated from the amplifier 57 and the pressure
stored on the first Korotkoff tone is stored as systolic pressure.
If, on the other hand, the identifier 81 does not identify a second
Korotkoff tone within the predetermined time interval, the pulse
coincidence provisionally identified as a Korotkoff tone is now identified
as interference. The memory 63 is then again connected with the amplifier
57 until the next coincidence occurs.
As the pressure in the air chamber 31 drops, further Korotkoff tones follow
the first Korotkoff tone. The identifier 81 first checks on whether each
pulse from the discriminator 53 of the sound recording channel is
coincident with a pulse from the discriminator 69 of the pressure
measuring channel. The identifier 81 also checks whether coincidences
follow one another within the predetermined time interval of 2 to 10, for
example, 5 seconds. As long as this is the case, the switching device 87
briefly connects the output of the amplifier 57 with the memory 65 on each
Korotkoff tone, i.e. on each pulse of the pulse sequence 157. A new
pressure value is thus stored in the memory 65 on each Korotkoff tone,
these pressure values progressively reducing. As already mentioned, the
pulse sequence 157 extends to the instant t.sub.6. As no further pulses
occur after the instant t.sub.6, the value of the pressure p measured at
the instant t.sub.6 remains stored in the store 65 until the appliance is
switched off. This storage value therefore represents the diastolic
pressure.
When the detector of the identifier 81 ascertains that no further Korotkoff
tone has occurred during the above-mentioned predetermined time interval
of 2 to 10 seconds, it feeds a corresponding signal at the instant t.sub.7
to the switching device 87 and the flipflop 91. As a result, the measuring
of pressure is terminated. In addition, the flipflop 91 now feeds a
voltage, which is substantially greater than the previously supplied
reference voltage, to the non-inverting input of the amplifier 101. This
has the effect of causing the regulator 61 to fully open the valve 37. The
pressure p then drops very rapidly and at the instant t.sub.8 is again at
the value zero, i.e. the ambient air pressure.
The measurement phase is thus terminated only when no further Korotkoff
tone occurs during the time interval of 2 to 10 seconds. By this means it
is ensured that the measurement is not terminated during an auscultatory
gap, i.e. on the absence of the Korotkoff tones during one or possibly
several heartbeats, before the pressure in the air chamber has dropped to
the real diastolic pressure.
The control unit 55 is so constructed that is connects the output of the
amplifier 57 with the indicating control device 67 at regular time
intervals of, for example, 0.3 seconds from the zero balance up to the
instant t.sub.8. The indicating unit 27 then indicates the instantaneous
pressure each time. The control unit 55 could, however, also be
constructed in such a manner that the pressure would be indicated on each
pulse of the pulse sequence 155 in the time interval between the instants
t.sub.4 and t.sub.7.
During the time between the instants t.sub.3 and t.sub.6 or t.sub.7 the
identifier 81 also switches on the heartbeat frequency meter 71 so that
this measures the heartbeat frequency during the occurrence of the pulse
sequence 155 and averages it over a few pulses. The mean value is stored
in the store of the meter 71.
The nature of the control 55 is such that the memory 63, the memory 65 or
the memory of the meter 71 can be cyclically interrogated by a brief
depression of the switch 21. The relevant storage value stored in analog
form is then fed to the indicating control device 67 and converted by this
into a digital signal. This is fed to the indicating unit 27 so that the
unit thus selectably indicates systolic or diastolic pressure or heartbeat
frequency. In addition, the indicating unit 27 indicates by a symbol which
of the three measurement values is just being displayed.
The indicating control device 67 includes a network which can be switched
over by the switch 23 and connected between the feed lines of the memories
63 and 65 and the analog-digital converters. This makes possible
selectable presentation of the pressure indication in kilopascals or
torrs, the switching between these alternative forms of indication being
effected by a brief depression of the switch 23.
When all three storage values have been read off, the appliance 3 can be
switched off by depressing the ON/OFF switch 25, whereby the measurement
is concluded.
Now that the general mode of operation of the equipment has now been
explained, a further function of the level monitor 89 will be described.
As already mentioned, the level monitor 89 responds when the differential
quotient dp/dt exceeds an upper limit value. Such excessively rapid
pressure changes can occur not only during the inflation phase, but also
when the examined person perhaps moves the arm to which the sleeve 1 is
attached. When such a movement takes place between the instants t.sub.3
and t.sub.7, it could cause a measurement error. However, measurement
errors of that kind can be prevented by means of the level monitor 89 and
associated elements. When the differential quotient dp/dt exceeds the
predetermined limit value as the result of an arm movement, for example at
a time between the instants t.sub.5 and t.sub.6, this has the effect that
an AND-gate of the identifier 81 is blocked for the Korotkoff tone signals
and the valve 37 is closed. The time delay element 93 in that case has the
effect that the Korotkoff tone signals remain blocked for a time duration
of 2 to 3 seconds even after the end of the rapid pressure change, in
analogous manner to the blocking at the end of the inflation process.
As already explained, the valve is opened again only after a certain
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