U.S. patent number 3,651,798 [Application Number 05/037,543] was granted by the patent office on 1972-03-28 for blood pressure indicator and noise.
This patent grant is currently assigned to Parke, Davis & Company. Invention is credited to Warren W. Brenner, Paul H. Egli, Martin H. Wilcox.
United States Patent |
3,651,798 |
Egli , et al. |
March 28, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
BLOOD PRESSURE INDICATOR AND NOISE
Abstract
An electronic blood pressure indicator for use with conventional
inflatable arm cuff apparatus and a mercury or aneroid manometer,
comprising a microphone for translation of arterial blood flow
sounds into electrical signals, a narrow bandpass amplifier circuit
which filters extraneous frequencies and amplifies a narrow band of
low frequencies containing essential arterial blood flow
information, a discriminator which senses signal strength and
passes only signals exceeding a predetermined value, a pulse
generator which provides a pulse output corresponding to blood flow
pulses passed by the discriminator, the pulse generator cooperating
with switch means to drive an indicator to provide a visual
indication of the arterial blood flow sounds, and a dual lockout
circuit which blocks incoming signals and spurious pulses from the
pulse generator.
Inventors: |
Egli; Paul H. (Collingswood,
NJ), Brenner; Warren W. (Willingboro, NJ), Wilcox; Martin
H. (Castle Rock, CO) |
Assignee: |
Parke, Davis & Company
(Detroit, MI)
|
Family
ID: |
21894899 |
Appl.
No.: |
05/037,543 |
Filed: |
May 15, 1970 |
Current U.S.
Class: |
600/493; 327/227;
381/67; 128/901 |
Current CPC
Class: |
A61B
5/021 (20130101); A61B 7/045 (20130101); Y10S
128/901 (20130101) |
Current International
Class: |
A61B
5/021 (20060101); A61B 7/00 (20060101); A61B
7/04 (20060101); A61b 005/02 () |
Field of
Search: |
;128/2.5A,2.5M,2.5P,2.5Q,2.5R,2.5S,2.5T,2.6A,2.6F,2.6R ;179/1ST
;307/227,273,274,275 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Claims
What is claimed is:
1. Blood pressure indicator apparatus for indicating the blood
pressure of an individual by detecting auscultatory signals
originating in the individual, said blood pressure indicator
apparatus comprising:
a. microphone transducer means, adapted to be placed against an arm
of said individual, for generating electrical signals
representative of blood flow sounds within said arm of said
individual;
b. narrow bandpass amplifier means having input terminals connected
to the output terminals of said microphone transducer means for
amplifying said representative signals, having filter sections
providing for the amplification only of a narrow bandwidth of low
frequency signals corresponding to the appreciable bandwidth of
said blood flow signals;
c. discriminator means connected to and driven by said amplifier
means for providing output responsive only to signal amplitudes
greater than a predetermined amplitude;
d. pulse generator means connected to and driven by said
discriminator means for providing output pulses of fixed amplitude
and time duration responsive to the output signals of said
discriminator means;
e. indicator means for providing a visual output corresponding to
the output pulses of said pulse generator means;
f. switch means connected to and driven by the output of said pulse
generator means and connected to and driving said indicator means
for driving said indicator means in response to the output of said
pulse generator means;
g. lockout means connected to said indicator means and to the input
of said pulse generator means, for locking out the operation of
said pulse generator means for a fixed predetermined time period
subsequent to each pulse operation of said indicator means;
and,
h. power supply means connected to said indicator means, said
switch means, said pulse generator means, said discriminator means,
and said narrow bandpass amplifier means for energizing same.
2. The apparatus as described in claim 1 wherein said lockout means
comprises a capacitor, a low time constant path through which said
capacitor discharges when said switch means switches on said
indicator means, a second and higher time constant path through
which said capacitor charges when said switch means switches said
indicator means off, and a normally open transistor switch
connected in shunting relationship to the input of said pulse
generator means and connected to said second path such that said
pulse generator means is held inactive for appreciably the time
constant of said second path.
3. The apparatus as described in claim 1 wherein said pulse
generator means comprises two transistors of opposite polarity,
each having an output terminal, an input terminal, and a common
terminal, a direct coupling resistance network connecting the
output terminal of the first of said transistors to the input
terminal of said second transistor, a second normally non-signal
coupling network connecting the output terminal of said second
transistor to the input terminal of said first transistor, said
second network containing a capacitor in series with a second
resistance network to provide a fixed time period of signal
coupling from the output terminal of said second transistor to the
input terminal of said first transistor.
4. The apparatus as described in claim 3 wherein each of said
common terminals is connected to a respective voltage reference,
and each of said input terminals is resistively connected to its
respective common terminal, thereby maintaining each of said
transistors non-conducting except when generating a pulse in
response to a signal from said discriminator means.
5. The apparatus as described in claim 4 wherein a diode is in
series with the base-to-emitter junction of said first transistor,
said series combination constituting said discriminator means.
6. The apparatus as described in claim 1 wherein said microphone
transducer means has a circular front housing and a rectangular
back housing, and has a trimming resistor in parallel with its
output terminals for standardizing the microphone output
voltage.
7. The apparatus as described in claim 1 wherein an unregulated
power supply is connected to said indicator means, and a regulated
power supply is connected to said switch means, said pulse
generator means, said discriminator means, and said narrow bandpass
amplifier means.
8. In an electronic instrument for amplifying a pulse train and for
providing an indication of such pulses which exceed a predetermined
lower limit in amplitude, a combined discriminator and pulse
generator circuit comprising:
a. a first diode, a first transistor with its base-to-emitter
junction in series with said first diode, both said diode and said
junction having forward bias voltages cooperating to provide a
discriminator turn-on voltage level;
b. a second transistor having an opposite polarity, and a resistive
network for connecting the collector of said first transistor to
the base of said second transistor;
c. a second diode, and a capacitor and first resistor in series
connected between the collector of said second transistor and the
base of said first transistor through said second diode, thereby
providing a feedback path from the output of said second transistor
to the input of said first transistor;
d. a third diode and a second resistor in series, connected to and
providing a discharge path for said capacitor;
e. said capacitor having a charging path through said second diode
and said first resistor, said charging path cooperating with said
first and second transistors to turn on both said transistors when
an input voltage in excess of said turn-on voltage is applied to
the input of said first diode and to turn off said first and second
transistors after a predetermined time period, thereby providing a
pulse output;
f. said discharge path resetting said combined circuit to a
condition to provide a next pulse output;
g. switch means connected to and driven by the output of said
second transistor for providing a switchable output; and
h. lockout means connected to the output of said switch means and
to the input of said first transistor, for locking out the
operation of said combined discriminator and pulse generator
circuit for a fixed pre-determined time period subsequent to each
of said pulse outputs.
9. The apparatus as described in claim 8 wherein said lockout means
comprises a capacitor, a low time constant path through which said
capacitor discharges when said switch means switches on, a second
and higher time constant path through which said capacitor charges
when said switch means switches off, and a normally open transistor
switch connected in shunting relationship to the input of said
first transistor and connected to said second path such that said
first transistor is held inactive for a second predetermined time
period of about the time constant of said second path.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention lies in the field of electronic sphygmomanometer
apparatus and, more particularly, electronic blood pressure
detection and indicator apparatus.
2. Description of the Prior Art
The art of measuring blood pressure has been well developed by the
medical profession, resulting in a standard and near-universal
procedure for taking blood pressure measurements. The procedure
involves inflating a rubber bladder in a cloth cuff encircling the
patient's arm, thus forming an occlusion opposing the arterial flow
of blood, the blood being under pressure from the heart. When the
bladder is inflated t a pressure sufficient to cut off the blood
flow entirely, the pressure is slowly released until blood flow
resumes in the artery. As the pressure drops, a point is reached
where the pulsating blood flow first pushes through the cuff,
causing audio sounds normally detectable with a stethoscope. At the
point of first detection, where the decreasing bladder pressure is
matched by the maximum, or systolic blood pressure, the doctor
skilled in auscultation can detect the pulsatile flow in the
artery, and determine the systolic pressure. As the cuff pressure
is further reduced, the blood pulses generate a series of distinct
sounds audible with a stethoscope to persons with average hearing
in an environment of normal background noise. The further reduced
cuff pressure at which the sounds fade away is the pressure which
the skilled doctor detects as the diastolic pressure. Through long
use of the stethoscope, this point in cuff pressure has acquired an
important clinical meaning. Although cuff pressure must decrease
further beyond this point until completely unimpeded flow results,
it is this long used cuff pressure which is clinically important,
and which any instrument must be capable of registering.
The disadvantages of the conventional stethoscope are that the
operator is required to make subjective detections of the sound of
blood pulses which are faint and intermixed with ambiguous external
noises, and that the determination of the end points of the blood
flow pulse train is somewhat subjective and therefore subject to
inaccuracy. To overcome these disadvantages, electric
sphygmomanometers have been devised, utilizing electronic to in
cooperation with a microphone for amplifying and detecting the
auscultatory signals. However, such electronic devices have been
subject to system design deficiencies, particularly in providing
the required sensitivity, avoiding inaccuracy caused by power
supply variations, proper filtering of electronic noise and
extraneous signals derived from muscle movements, efficient
matching of the electronic circuitry with the microphone, and the
problems of power consumption, size and cost.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an improved electronic
blood pressure indicator which will provide an indication of
diastolic and systolic pressure, the indication being free of
background noises and extraneous body signals.
It is a further object of this invention to provide an electronic
blood pressure indicator which can be used with conventional
sphygmomanometer equipment and which will operate with a constant
sensitivity.
It is a further object of this invention to provide a blood
pressure instrument which clearly indicates blood flow signals and
which prevents indication of unwanted signals derived from the
patient as well as generated internally by the instrument.
It is a still further object of this invention to provide a
noise-discriminating pulse amplifier adapted for blood pressure
indication.
Accordingly, the invention provides a microphone sensor which is
resistor-calibrated to provide a standardized output voltage; an
amplifier which filters out signals outside of a narrow bandwidth
of the desired auscultatory signals; a discriminator circuit which
establishes the instrument sensitivity by selecting the passable
signal amplitude; a pulse generator comprised of opposite polarity
transistors to produce a pulse of uniform shape from the variable
signal output of the discriminator; a switch and indicator light
driven by the pulse generator output and powered by an unregulated
power supply; a dual lockout circuit which locks out transmission
of any signals to the indicator until a fixed period after a
previous signal pulse; and a diode regulated power supply. In
operation, the auscultatory signals are detected by the microphone
and fed into the amplifier, the amplitude of the signals is sensed
by the discriminator which permits passage of the signals from the
time cuff pressure is at systolic pressure to the time that it
drops to diastolic pressure. After each pulse, the lockout circuit
prevents any further indication for a period approaching the time
when the next blood pressure pulse is due to arrive.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing the system function of the
different portions of the instrument.
FIG. 2 is a schematic diagram of the electronic circuitry of the
preferred embodiment.
FIG. 3 is a perspective view of the preferred form of microphone
assembly to be used with the instrument of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, the auscultatory sounds caused by the
pulsating blood flow are sensed by a sensor 11, which transforms
the sounds into electrical signals. The signals are coupled into
the bandpass amplifier 12 which filters extraneous signals and
amplifies the desired signals in a narrow frequency range of
approximately 10 to 50 Hz. The significant sounds to be detected
from the occluded blood flow in the artery have a peak intensity at
about 40 Hz. Any microphone capable of responding at this low
frequency would also produce signals from extraneous noise in the
background, as well as from electromagnetic radiations produced by
adjacent equipment, lights, etc. There is thus a requirement for a
sharp frequency cut off filter to provide filtering before and
during amplification of the electrical signal, as will be pointed
out in more detail hereinbelow. The signals generated by the
microphone and coupled into the amplifier are in the range of
nanoamps or lower, and the amplifier accordingly must have a stable
gain of several thousands without introducing appreciable noise
which would result in ambiguous signals.
The output of the amplifier 12 is coupled to the discriminator
circuit 13 which serves as the amplitude selecting component of the
system, rejecting smaller level signals generated by extraneous
noise. The signal level required to pass the discriminator is the
design criterion which sets the sensitivity of the overall
system.
The output from the discriminator 13 is coupled into a pulse
generator 14 which produces a pulse of uniform shape and amplitude
to activate switch 15. The signals passed from the discriminator
are highly variable in both frequency and amplitude because of the
irregularity of the arterial sounds during the course of the cuff
pressure drop. Further, there is a considerable difference in
signal strength from patient to patient depending upon the
patient's arm build and blood pressure. Thus, the pulse generator
transforms these variable signals into standardized signals which
contain the required information. The switch 15, driven by the
output of the pulse generator 14, operates the indicator 16,
preferably an incandescent light bulb, which provides the
instrument output. As will be noted hereinbelow, the light bulb
constitutes the largest drain on the battery power source, and it
is accordingly important to provide that it be turned on for the
shortest possible time consistent with a clearly visible light
pulse. Accordingly, the shape of the pulse from the pulse generator
and the switching characteristics of the switch are important
design features.
In practice, the light indicator 16, which provides signals
representing the blood flow sounds, will be used in cooperation
with a conventional pressure gauge marking, so that the operator
can quickly and efficiently read the systolic pressure when the
light first starts to blink, and read the diastolic pressure when
the light ceases to blink. For an aneroid pressure gauge the best
position for the indicator is in the center of the radial dial, and
for a manometer, it is best placed immediately alongside the
calibrations on the mercury column. The indicator light may be of
any conventional form, although the preferred embodiment
incorporates an incandescent bulb having the advantages of
operating on low voltage and being long lived.
Still referring to FIG. 1, a dual lockout circuit 17 is indicated,
which is activated by the output from the switch 15, and which
controls the operation of the pulse generator 14. The regulated
power supply 18 provides power to all of the circuitry except the
indicator light 16, which is supplied by the battery or unregulated
supply 19. Indicator 16 is therefore subject to fading as the
battery becomes exhausted. However, the user is assured of accurate
readings so long as he can see the flashing light, since the supply
18 will maintain sensitivity and accuracy of the measurements with
battery deterioration up to the point where the indicator light is
no longer visible.
Referring now to FIG. 3, the active element is a conventional
piezoelectric crystal 21 such as a Rochelle salt or a ceramic
bimorph. The crystal 21 is mounted from the bottom of the housing
on cork or rubber pads 22 on opposite corners of the crystal 21.
The pads are suitably 0.03 inch thick and one-sixteenth inch wide,
providing a stable platform without unduly restricting the crystal
from bending. On top of the crystal a light metal bridge 23 is
rigidly attached to the two corners opposite from the support pads,
and extends up to a point at the center of the diaphragm 24. The
front of the microphone housing is round so that the diaphragm will
have the symmetrical vibration pattern of a circular plate. The
back of the microphone is made rectangular so that the rubber
bladder within the cuff, which folds over the microphone,
distributes the pressure evenly and holds the microphone into close
contact with the skin.
The microphone is designed to produce an output of about an order
of magnitude larger than necessary to drive the amplifier. The
desired output is trimmed by inserting, in parallel with the
microphone output, a selected resistor 25. The resistor 25 is
chosen of a value to trim the effective output voltage of the
microphone to a standardized value.
Referring now to FIG. 2, the narrow bandpass amplifier component of
the circuitry is comprised of transistors 51, 52, 53, 54 and 55,
with associated circuitry. The filtering function is distributed
throughout the amplifier. The intrinsic capacitor of the microphone
in combination with resistor 42 provides a first low frequency
rejection circuit. Additional low frequency filtering is
accomplished by capacitor-resistor combinations 41, 43 and 44
connected to the base terminals of transistors 51, 53 and 54
respectively, and combination 45 connected between ground and the
emitter of transistor 55. High frequency rejection is obtained by
capacitor-resistor combinations 56, 57 and 58 connected between
ground and the collectors of transistors 51, 53 and 54
respectively. These networks thus provide an effective narrow
bandwidth of frequencies having a peak amplification occurring at
approximately 25 Hz, and providing adequate amplification at 40 Hz
to detect the desired arterial signals. The bandwidth is peaked at
25 Hz rather than 40 Hz to improve the rejection of 60 Hz noise
signals.
The signal is amplified by transistors 51, 53 and 54 and associated
circuits acting in cascade. Transistor 52, in emitter-follower
configuration, acts to isolate transistor 51 from transistor 53.
Transistor 55 provides additional power amplification. Amplifier
voltage gain is established and maintained at a known value by
holding the base bias voltage of transistors 51, 53 and 54
independent of battery voltage, and by the negative feedback
provided by the high value emitter resistors of those
transistors.
The discriminator circuit 13 is coupled to the output of the
amplifier through capacitor 61, and is comprised of diode 62 and
the base-emitter junction of transistor 63. Both diode 62 and the
base-emitter junction of transistor 63 have forward bias, or
"turn-on" voltages. The voltage coupled through capacitor 61 must
attain the magnitude of the two combined turn-on voltages before
signal current will flow into the base of transistor 63. If the
signal amplitude supplied through capacitor 61 is less than the sum
of the turn-on voltages of diode 62 and transistor 63, no signal
current flows into the base of transistor 63, and consequently
pulse generator 14 remains in its stable state. By this circuitry,
amplitude selection is obtained, with the smaller level noise
signals and extraneous body signals being rejected. It is noted
that the turn-on voltages of diode 61 and transistor 63 are
uninfluenced by battery voltage changes and vary negligibly within
component production tolerances.
The pulse generator 14 is comprised of transistors 63 and 64 and
associated elements, and operates in the following manner. Between
pulses, transistors 63 and 64 are non-conducting, and the capacitor
73 is discharged, having no voltage across it. When the signal
exceeds the threshold level and turns on transistor 63, a large
current flows through resistors 74 and 75 which are series
connected between the collector of transistor 63 and the power
supply, developing a forward bias voltage at the base of transistor
64, thus turning it on. With transistor 64 switched on, current is
supplied to resistor 71, generating a positive voltage which is
impressed across capacitor 73 and resistors 77 and 78 in series
with diode 79. Capacitor 73 charges exponentially through diode 79
to a voltage which is approximately the voltage across resistor 71.
During the rise in voltage across capacitor 73 diode 80 is
non-conducting, and the charging current flows through resistor 77,
adding to the current initially flowing through diode 62, thus
providing a greater input to transistor 63 and driving it further
into an on condition. Since the output of transistor 63 is coupled
to the input of transistor 64, transistor 64 is also driven into a
greater on condition, in turn increasing the input to transistor
63, and so on. Thus, transistors 63 and 64, through this
regenerative action, are quickly driven into saturation and appear
as essentially short circuits, with each supplying the driving
current required by the other. This condition is maintained so long
as the current passing through resistor 77 is sufficient to
maintain saturation of transistor 63, and is a function of the time
constant by which capacitor 73 is charged. It is noted that this
time constant is determined solely by capacitor 73 and resistors 77
and 78, and not by the amplitude of the incoming signal. When
capacitor 73 approaches its fully charged condition, the charging
current tapers off and a reverse or degenerative process sets in,
quickly taking both transistor 63 and 64 out of conduction and
turning them off, thus terminating the voltage pulse appearing
across resistor 71. Capacitor 73 at this time discharges rapidly
through diode 80 and resistor 81, diode 79 being non-conducting
during this discharge.
From the above, it is seen that pulse duration can be determined by
design of capacitor 73 and resistors 77 and 78. Further,
transistors 63 and 64 are chosen as opposite polarity transistors,
one being an NPN and the other a PNP, so that both transistors can
be in the off state during the relatively long period between
pulses, thereby avoiding energy drain from the battery.
The indicator 72 is driven by an indicator switch 15, comprised of
transistors 91 and 92 in cascade. When the pulse generator delivers
a voltage across resistor 71, the signal is transmitted through
resistor 93, turning on transistors 91 and 92. Transistor 92 is
driven heavily into conduction, thereby effectively closing the
circuit through the power supply 94 and the indicator 72, for the
duration of the pulse generator output.
Still referring to FIG. 2, the dual lockout circuitry 17 is
comprised of the feedback loop from the collector of transistor 92
to the base of transistor 63, comprising resistor 101, capacitor
102, and resistor 104 in series, resistor 104 being tied to the
base of transistor 106, the collector of which is connected to the
base of transistor 63. Diode 103 is connected between ground and
the node joining resistor 104 and capacitor 102. Resistor 105 is
connected between the base of transistor 106 and ground. In
operation, between output pulses capacitor 102 is charged to
appreciably the battery voltage. When an output pulse drives
transistor 92 into conduction, capacitor 102 discharges through
resistor 101, diode 103 and transistor 92 until the voltage across
its terminals approaches zero. At the end of the output voltage
pulse, transistor 92 turns off again, impressing the battery
voltage across capacitor 102. The capacitor 102 charges through
resistors 101, 104 and 105, with the voltage across resistor 105
appearing on the base of transistor 106, driving it into
conduction. As transistor 106 is driven into conduction, it clamps
the base of transistor 63 appreciably to ground, disenabling it
from passing any signals through to the output. Thus, the pulse
generator will remain inactive as long as transistor 106 remains in
its conductive condition, which condition will be maintained for a
duration set by the time constant of the charging path for
capacitor 102, i.e., capacitor 102, resistor 101, 104, and 105.
When the charging current becomes insufficient to maintain
transistor 106 in conduction, it switches to an off state, thereby
enabling transistor 63 to pass signals which appear at its
input.
The above described lockout circuit serves a dual function. The
first function is that of eliminating all extraneous signals other
than the anticipated arterial sounds. As the approximate time
period between such auscultatory sounds is well known, the time
constant of the lockout circuit can be designed to eliminate most
extraneous signals which might occur between the desired arterial
pulses. The second function of the lockout circuit relates to the
power drain of the indicator light. When the indicator light 72 is
flashed on, it initially draws an appreciable amount of current as
compared to the current load of the entire circuit. Consequently,
the voltage source 94 will suffer a drop in voltage, which drop
will cause a transient signal to be amplified through the circuit,
thereby driving one or more of the transistors into saturation. Due
to the relatively large amount of capacitance in the circuit for
frequency selectivity purposes, the saturated transistors would not
return to their linear state until appreciably after the pulse, at
which time additional undesired transients would be generated and
result in multiple lamp output flashes, with consequent user
confusion and additional battery drain.
Still referring to FIG. 2, the regulated power supply is comprised
of battery 94, capacitors 111, 112 and 113, diodes 115, 116, 117
and 120, and resistors 118 and 119. Capacitor 113 provides the
initial current necessary to drive indicator 72, which requires an
initial current about eight times the steady state current of the
circuit. Capacitor 112, connected to battery means 94 through a
diode 120 maintains an adequate power supply for the amplifier
stages during the indicator on period, when light 72 is loading
down the battery. Capacitor 111 and resistor 118, connected between
capacitor 112 and ground, provide additional filtering of the power
supply to transistors 51 and 53, the two most sensitive amplifier
stages. The series combination of diodes 115, 116 and 117 in
combination with resistor 119 provide an amplifier bias voltage
supply substantially independent of the battery voltage. This
provision of a stable bias voltage is combined with large value
emitter resistors to render the instrument sensitivity relatively
immune from battery voltage variations and transistor production
tolerances.
* * * * *