U.S. patent number 4,005,701 [Application Number 05/585,889] was granted by the patent office on 1977-02-01 for noise rejecting electronic sphygmomanometer and methods for measuring blood pressure.
This patent grant is currently assigned to Whittaker Corporation. Invention is credited to Sol Aisenberg, Ronald W. Chabot.
United States Patent |
4,005,701 |
Aisenberg , et al. |
February 1, 1977 |
Noise rejecting electronic sphygmomanometer and methods for
measuring blood pressure
Abstract
A noise rejecting electronic sphygmomanometer comprises a
conventional cuff-type sphygmomanometer having an inflatable
bladder and pressure monitoring manometer connected thereto. A
first microphone, having a sound pickup directed inwardly to be
toward a patient's arm when the cuff is installed thereabout, is
provided for picking up Korotkoff sounds as the cuff is deflated
and blood flow in a patient's arm resumes. To avoid spurious
indications caused by background sounds, a second microphone,
having a pickup directed to pickup background noises, is also
provided within the cuff. The outputs of both microphones are fed
into electronic discrimination circuitry.
Inventors: |
Aisenberg; Sol (Natick, MA),
Chabot; Ronald W. (Winchester, MA) |
Assignee: |
Whittaker Corporation (Los
Angeles, CA)
|
Family
ID: |
24343389 |
Appl.
No.: |
05/585,889 |
Filed: |
June 11, 1975 |
Current U.S.
Class: |
600/493;
128/901 |
Current CPC
Class: |
A61B
5/02208 (20130101); A61B 7/045 (20130101); Y10S
128/901 (20130101) |
Current International
Class: |
A61B
5/022 (20060101); A61B 7/00 (20060101); A61B
7/04 (20060101); A61B 005/02 () |
Field of
Search: |
;128/2.5A,2.5G,2.5M,2.5S,2.5E |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Howell; Kyle L.
Attorney, Agent or Firm: Bissell; Henry M.
Claims
What is claimed is:
1. In a sphygmomanometer having a cuff, a cuff inflating bladder
and a bladder pressure measuring element, where the cuff is adapted
for positioning around a patient's limb, for measuring the
patient's blood pressure intensity by monitoring Korotkoff sounds
as bladder pressure is slowly reduced from above systolic blood
pressure, the improvement comprising:
a first acoustical pickup disposed within the cuff and positioned
relative to a patient's brachial artery to pick up said Korotkoff
sounds;
a second acoustical pickup disposed within the cuff and positioned
away from the brachial artery to pick up background sounds;
transducing means associated with said pickups for producing a
first electrical signal corresponding to the sounds received by the
first pickup and a second electrical signal corresponding to the
sounds received by the second pickup; and
electronic logic means connected to receive the first and second
signals for providing an output at a first voltage level upon a
comparison of said electrical signals when and only when there is a
first electrical signal corresponding to Korotkoff sounds and no
second electrical signal of comparable magnitude and for providing
an output at a second voltage level except when said first output
signal is provided.
2. The invention as claimed in claim 1 including an indicator
connected to respond to the output at said first voltage level.
3. The invention as claimed in claim 2, wherein said indicator
comprises a light which is lit only when said output is in said
first level.
4. The invention as claimed in claim 1, including first and second
pressure gauges connected to said bladder and means for trapping
pressure in said gauges, said trapping means being responsive to
said output to cause pressure to be trapped in said first pressure
gauge when said output is at said first level and to cause pressure
to be trapped in said second pressure gauge when said logic means
last supplies an output of said first level.
5. The invention as claimed in claim 4, further including means for
automatically inflating and deflating said bladder.
6. The invention as claimed in claim 1, wherein said two pickups
are positioned relatively adjacent to each other and within the
cuff adjacent said bladder.
7. The invention as claimed in claim 1, wherein said logic means
includes filtering means for filtering out said electrical signal
frequencies other than those in a limited range.
8. The invention as claimed in claim 1, wherein said two pickups
are disposed in said cuff at a location remote from cuff
connections for inflating said bladder and for said pressure
measuring element.
9. The invention as claimed in claim 1, wherein said logic means
causes said output to be in said first level when and only when the
actual ratio between magnitudes of sound signals received from said
first pickup and sound signals received from said second pickup
exceed a predetermined ratio, and to be in said second level
whenever said actual ratio is below said predetermined ratio.
10. The invention as claimed in claim 9, including means for adding
an incremental signal to each of said first and second signals
whereby said ratio is never infinity.
11. The invention as claimed in claim 1, wherein non-sound
receiving portions of said pickups are sound insulated.
12. A noise rejecting electronic sphygmamanometer which
comprises:
a. A flexible cuff adapted to be wrapped and fastened around a
patient's limb,
b. an inflatable bladder disposed within a portion of said
cuff,
c. Means for inflating and deflating said bladder, whereby blood
circulation in portions of a limb about which said cuff is wrapped
and fastened may be stopped and may be subsequently restored,
d. means connected to said bladder for measuring pressure
therein,
e. first and second microphones disposed in said cuff,
said first microphone having a directional pickup portion directed
toward a patient's limb when said cuff is wrapped and fastened
therearound, said first microphone pickup portion being adapted for
positioning over a brachial artery, whereby Korotkoff blood sounds
in said artery may be picked up thereby,
said second microphone having a directional pickup portion directed
to pick up background noises,
f. electronic discriminating means for receiving electrical signals
from said first and second microphones and for discriminating
between the electrical means responsive to an output from the
discriminating means to provide a detectable indication when and
only when a sound is picked up by said first microphone pickup
portion and is not picked up by the second microphone pickup
portion.
13. The invention as claimed in claim 12, wherein said
discriminating means includes filtering means for allowing passage
of electrical signals from said first and second microphones having
frequencies within a limited band range.
14. The invention as claimed in claim 13, wherein said
discriminating means further includes means for determining the
ratio between the magnitude of signals received from said first and
second microphones, and for causing said indicating means to
provide said detectable indication when and only when said
determined ratio exceeds a predetermined ratio.
15. The invention as claimed in claim 14, wherein said
predetermined ratio is about 1.4, whereby said indicating means
provides said detectable indication only when said magnitude of
said first microphone signal is about 1.4 times greater than said
magnitude of said second microphone signal.
16. The invention as claimed in claim 12, wherein said
discriminating means further includes means for shaping said
filtered electrical signals from said first and second microphones,
and means for comparing said shaped signals and for causing an
output of said comparing means to be at a first predetermined
level, having a detectable indication, when and only when a shaped
signal from said first microphone is not coincident with a shaped
signal from said second microphone, and for causing said output of
said comparing means to be at a second predetermined level for
other microphone signal conditions.
17. The invention as claimed in claim 16, wherein said comparing
means comprises a signal inverter connected to receive said shaped
signal from said second microphone and a logic gate connected to
separately receive the output of said inverter and said shaped
signal of said first microphone.
18. The invention as claimed in claim 16, wherein said
discriminating means further includes integrating means for
integrating said output of said comparing means and for causing
said detectable indication only when an out-of-coincidence
condition of said shaped signals from said first and second
microphones exists for a predetermined time interval.
19. The invention as claimed in claim 18, wherein said
predetermined time interval is about 50 milliseconds.
20. The invention as claimed in claim 12, wherein said indicating
means includes systolic and diastolic pressure reading gauges
connected with said bladder and means for trapping pressure in said
gauges in response to said indicating means, when said cuff is
attached about a patient's limb and pressure in said bladder is
slowly released from above systolic blood pressure levels.
21. The invention as claimed in claim 20, wherein said trapping
means includes a first valve connected to said systolic gauge and a
second valve connected to said diastolic gauge, said first valve
being caused to close at a first instance of Korotkoff sounds being
picked up by said first microphone, said second valve being caused
to open momentarily at each instance of a Korotkoff sound being
picked up by said first microphone.
22. The invention as claimed in claim 20 including means for
automatically inflating and deflating said bladder.
23. The invention as claimed in claim 12, wherein said first and
second microphones are located remotely from connections to said
bladder for said inflating and deflating means for said pressure
measuring means.
24. A method of indicating when systolic and diastolic blood
pressures are to be read from a pressure monitoring device
connected to a cuff-type sphygmomanometer having an inflatable
bladder, which comprises the steps of:
a. installing a first microphone under said bladder and in a
position to be over a brachial artery to pick up Korotkoff blood
sounds when said sphygmomanometer is positioned about a patient's
limb,
b. installing a second microphone, having a sound pickup positioned
to pick up background noises,
c. installing said sphygmomanometer on a patient's limb and
inflating said bladder until blood flow is stopped in the patient's
limb under and below said sphygmomanometer,
d. releasing the pressure in said bladder slowly while monitoring
the pressure therein on said pressure monitoring device,
e. converting sounds picked up by said first and second microphones
into electrical signals,
f. comparing said electrical signals from said first and second
microphones and causing a detectable indication when and only when
said comparison indicates a strong probability that sounds picked
up by said first microphone are Korotkoff blood sounds and not
background sounds,
g. reading a first pressure from said pressure monitoring device at
the initial onset of said detectable indication, said first
pressure being the systolic blood pressure, and
h. reading a second pressure from said pressure monitoring device
as pressure is further reduced in said bladder at the first
cessation of said detectable indications, said second pressure
being the diastolic blood pressure.
25. The method as claimed in claim 24, including the step of
filtering said electrical signals from said two microphones so that
only frequencies in a limited range are passed and shaped.
26. The method as claimed in claim 25, including the steps of
shaping said filtered and shaped electrical signals from said first
and second microphones into square wave pulses, comparing the
coincidence of said square wave pulses from said first and second
microphones and causing said detectable indication only when a
pulse from said first microphone is not coincident with a pulse
from said second microphone.
27. The method of claim 25, including the step of determining the
ratio between the magnitude of filtered signals from said first
microphone and the magnitude of filtered signals from said second
microphone, and causing said detectable indication only when said
value exceeds a predetermind ratio.
28. A method for measuring systolic and diastolic blood pressures,
using a cuff tupe sphygmomanometer having an inflatable bladder,
which comprises the steps of:
a. installing a first, blood-sound microphone, having a pickup
under said bladder and in a position to be over a brachial artery
to pick up Korotkoff blood sounds when said sphygmomanometer is
positioned about a patient's limb,
b. installing a second noise microphone, under said bladder to pick
up background noises,
c. connecting a first pressure gauge and a first pressure shutoff
valve to said bladder,
d. connecting a second pressure gauge and a second pressure shutoff
valve to said bladder,
e. installing said sphygmomanometer about a patient's limb and
inflating said bladder until blood flow in the limb under and below
said sphygmomanometer is stopped,
f. releasing the pressure from said bladder slowly,
g. converting sounds picked up by said first and second microphones
into electrical signals,
h. causing said first valve to close, thereby trapping pressure in
said first gauge, the first time an electrical signal received from
said first microphone is not coincident with an electrical signal
received from said second microphone,
i. causing said second valve to momentarily open, thereby admitting
pressure to said second gauge, each time an electrical signal
received from said first microphone is not coincident with an
electrical signal received from said second microphone, and
j. reading, when pressure in said bladder is substantially reduced,
systolic blood pressure from said first gauge and diastolic blood
pressure from said second gauge.
29. A method for measuring systolic and diastolic blood pressures,
using a cuff-type sphygmomanometer having an inflatable bladder,
which comprises the steps of:
a. installing a first blood-sound microphone under said bladder and
in a position to be over a brachial artery to pick up Korotkoff
blood sounds when said sphygmomanometer is positioned about a
patient's limb,
b. installing a second, noise microphone under said bladder to pick
up background noises,
c. connecting a first pressure gauge and a first pressure shut-off
valve to said bladder,
d. connecting a second pressure gauge and a second pressure
shut-off valve to said bladder,
e. installing said sphygmomanometer about a patient's limb and
inflating said bladder until blood flow in the limb under and below
said sphygmomanometer is stopped,
f. releasing the pressure from said bladder slowly,
g. converting sounds picked up by said first and second microphones
into electrical signals,
h. causing said first valve to close, thereby trapping pressure in
said first gauge, the first time the ratio between magnitudes of
electrical signals received from said first and second microphones
exceeds a preselected level,
i. causing said second valve to momentarily open, thereby admitting
pressure to said second gauge, each time said ratio between
magnitudes of electrical signals received from said first and
second microphones exceeds said preselected level, and
j. reading, when pressure in said bladder is substantially reduced,
systolic blood pressure from said first gauge and diastolic blood
pressure from said second gauge.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of blood pressure
measuring instruments or sphygmomanometers; more particularly, it
relates to such instruments utilizing acoustical pickups or
microphones for determining, by blood sounds, the pressure levels
at which systolic and diastolic blood pressure are to be
measured.
2. Description of the Prior Art
Periodic and accurate blood pressure determinations are essential
to proper diagnosis of heart function, and particularly, to
discover hypertension in humans. Instruments used to measure blood
pressure are called sphygmomanometers, the most common type of such
instruments being an inflatable cuff which is secured around a
patient's upper arm. The operator listens, normally via a
stethoscope positioned against the patient's artery in the lower
arm, to the heartbeat and inflates the cuff until the heartbeat no
longer can be heard. Blood flow is then effectively cut off to the
lower arm. The pressure in the cuff is at all times registered on a
pressure gauge, generally a mercury manometer. When the blood flow
to the lower arm is completely occluded, pressure is slowly
released from the cuff. The cuff pressure at which the operator
again just begins to hear characteristic blood flow noises
(Korotkoff sounds) is termed the systolic blood pressure of the
patient. Additional pressure is released from the cuff until the
Korotkoff sounds momentarily get louder and then fade. The point at
which the Korotkoff sounds first disappear is termed the diastolic
blood pressure.
The described method requires relatively subjective determination
of sound levels by the operator. Various extraneous factors, such
as ambient noise and the operator's hearing, may affect the
determination of exactly when the Korotkoff sounds can first be
heard as the cuff is deflated and exactly when they later
disappear. As accurate and repeatable blood pressure determinations
may be essential to a proper determination of the patient's well
being, improved and less subjective methods for determining blood
pressure are desirable.
In order to eliminate subjective sound determinations by a
stethoscope, cuff-type sphygmomanometers have sometimes been
modified by insertion of a microphone in the cuff, the microphone
being positioned over a restricted brachial artery when the cuff is
fastened about a patient's arm. The microphone, which replaces the
stethoscope, is electronically connected to an indicator, for
example a light, and the cuff is inflated as above described until
the blood supply is cut off to the patient's lower arm; then
pressure is slowly released from the cuff. At the systolic
pressure, when blood flow just resumes, the microphone detects the
Korotkoff sounds and causes the light (or other indicator) to be
actuated. As the cuff is further deflated, the Korotkoff sound is
picked up at each pulse beat and, in typical systems, a light
flashes at each beat. When diastolic pressure is reached, there is
no longer a restriction of the blood vessels, and consequently no
further Korotkoff sounds are picked up. At this point, flashing of
the light stops. Systolic blood pressure is read from the manometer
when the light first starts flashing, and the diastolic pressure is
read when the light stops flashing.
A principal difficulty with the use of a microphone to pick up
blood sounds is that background or extraneous noises arising, for
example, from movement of the patient's arm or from the surrounding
environment are also picked up by the microphone. Erroneous
indications, and hence erroneous blood pressure readings, thus
often occur.
Gilford, U.S. Pat. No. 2,827,040, discloses apparatus wherein blood
sounds picked up by a microphone are correlated with blood pressure
pulses caused by the heartbeat. An indicator is to be actuated only
when picked-up sounds occur simultaneously with pressure pulses.
This is intended to screen out extraneous noise sources. If,
however, a background noise is picked up coincidentally with a
pulse beat an erroneous indication will be given. Further
improvement over such a system is needed.
SUMMARY OF THE INVENTION
A noise rejecting electronic sphygmomanometer, in accordance with
the invention, comprises a flexible cuff, adapted for fastening
around a patient's limb, together with an inflatable bladder
installed therein and means for monitoring pressure in the bladder.
A first, blood-sound acoustical pickup and a second, noise
acoustical pickup are installed in the cuff, the first pickup
having a sound receiving portion being directed inwardly to be over
a brachial artery to pick up Korotkoff sounds when the cuff is
installed. The sound receiving portion of the second acoustical
pickup is directed to pick up background noises, but not blood
sound noises. Discriminating means are connected to the pickups for
comparing electrical signals received therefrom. An output is
caused to be at a first predetermined level when and only when
there is strong assurance that the sounds picked up by the first
pickup are Korotkoff sounds and not background noises, and is at a
second level for all other sound conditions.
More particularly, an indicator is connected to the discriminating
means output and is caused to give an indication only when the
discriminating means output is at the first level; that is, when
Korotkoff blood sounds are being picked up by the first microphone.
In this manner, as pressure is released from the cuff bladder, an
operator records, from a manometer, systolic blood pressure at the
onset of the indications and records diastolic blood pressure at
the cessation of the indications.
The discriminating means includes means for filtering out
electrical signals from the two pickups, which may be microphones,
other than those in a band for example about 5 Hz wide and centered
about 90 Hz, a frequency representative of strong turn-on and
turn-off blood sounds and not representative of common background
noises. Within the discriminating means, the filtered signals are
digitized and shaped into square waves which are fed into a
discrimination circuit which compares signals from both
microphones. The discriminator output is at the first level when
there is a signal from the first microphone and no coincident
signal from the second microphone, and is at the second level for
all other signal conditions. The output of the discrimination
circuit is integrated and then fed into a threshold detector, both
of which cooperate to determine if any out-of-coincidence time is
sufficiently great to provide an indication of blood sound, such
time being predetermined to be about 50 milliseconds.
In a variation, according to one aspect of the invention, two
pressure gauges are separately connected to the cuff bladder, the
first through a normally open valve and the second through a
normally closed valve. Circuitry causes the normally open valve to
close, trapping pressure in the first gauge at the onset of the
indications and causes the normally closed valve to momentarily
open at each indication to admit pressure into the second gauge.
When the indications cease, pressure is trapped in the second
gauge. Systolic blood pressure is then read from the first gauge
and diastolic blood pressure is read from the second gauge.
In another variation, according to a second aspect of the
invention, indication is provided only when the ratio between the
signal magnitude from the first (blood sound) microphone and that
of the second (noise) microphone is greater than a predetermined
level. To this end, filtered microphone signals are fed into a
ratio circuit which controls operation of the indicator light.
Corresponding methods for measuring a patient's systolic and
diastolic blood pressure are thereby provided.
A reliable apparatus and corresponding method for measuring a
patient's systolic and diastolic blood pressure are provided which
utilize, rather than a conventional stethoscope, a pair of
microphones, one of which is directed to pick up blood sounds and
the other of which is directed to pick up background noises. By
comparing the output signals from both microphones, and rejecting
all signals from the blood-sound microphone which may be caused by
background noises, an indication is given only when there is a very
strong assurance that Korotkoff blood sounds are actually being
picked up. An operator is thus assured that the onset of the
indications signals the onset of Korotkoff blood noises and that
systolic blood pressure should be read at this point. A cessation
of the indications informs the operator that the Korotkoff noises
have stopped, and at this point the diastolic blood pressure is
read. Also, means may be provided for automatically registering
systolic and diastolic blood pressure on pressure gauges such that
an operator need not watch for indications or make instantaneous
readings from a pressure indicator.
BRIEF DESCRIPTION OF THE DRAWING
A better understanding of the present invention may be had from a
consideration of the following detailed description, taken in
conjunction with the accompanying drawings, in which:
FIG. 1 is a perspective view, showing the sphygmomanometer
apparatus of the present invention with the cuff unrolled;
FIG. 2 is a perspective view, showing the sphygmomanometer
apparatus with the cuff wrapped and fastened around a patient's
arm;
FIG. 3 is a block diagram schematic of the electronic
circuitry;
FIG. 4 is a logic diagram of the electronic circuitry;
FIGS. 5, 5a and 5b are waveform diagrams showing gate output vs.
multivibrator output for the condition of partially coincident
signals from both microphones;
FIG. 6 is a schematic diagram of a variation of the invention
showing connection of the apparatus to a "pneumatic memory";
and
FIG. 7 is an electrical schematic of a second variation of the
invention, showing an alternative means for microphone signal
discrimination.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIGS. 1 and 2, the noise rejecting sphygmomanometer,
described briefly, comprises a conventional blood pressure cuff 10
having an inflatable bladder 12 positioned at one end thereof, a
hand pump 14 connected to the bladder by a tube 16, and a pressure
measuring instrument, for example, a mercury manometer, 18
connected to the bladder by a tube 20. A first, bloodsound
acoustical pickup or microphone 22 and a second noise acoustical
pickup or microphone 24, disposed between the bladder 12 and an
inner layer of a cuff, preferably along a cuff end edge 26, are
connected to an electronic control box 28 by electrical lines 30
and 32 respectively. An indicator light 34 on the box 28,
responsive to sounds picked up by the microphones 22 and 24,
indicates only the presence of blood flow or Korotkoff sounds
picked up by the first microphone, as more particularly described
below, thereby enabling an operator to determine when systolic and
diastolic blood pressure readings should be read from the
instrument 18.
More particularly, the first microphone 22, which replaces the
stethoscope conventionally used with a cuff type sphygmomanometer,
is positioned having its sound pickup portion directed inwardly so
that it may easily pick up blood sounds from a brachial artery 36
over which it is positioned when the cuff 10 is installed around a
patient's arm 38 (FIG. 2). When the bladder 12 is inflated and
blood flow to the portion of the artery 36 over which the first
microphone 22 is positioned is blocked, no blood sounds are picked
up. As pressure in the bladder 12 is released, by a valve 40 on the
pump 14, blood starts flowing in the artery 36 and the distinctive
Korotkoff sounds are picked up by the microphone 22. At the onset
of such sounds, a systolic blood pressure is to be measured. When
more pressure is released from the bladder 12, the Korotkoff
sounds, heard at every heartbeat, diminish as arterial restriction
is reduced. Diastolic blood pressure is to be measured at the
pressure where the Korotkoff sounds first disappear.
There exists, however, a possibility that extraneous sounds--room
noises, patient movements noises, etc.--may be picked up by the
microphone 22. If the indicator light 34 were triggered by all
sounds picked up by the first microphone 22, erroneous indications,
and hence erroneous blood pressure readings, might result. To
prevent this, sound discrimination is provided so that the
indicator light 34 lights only when there is strong assurance that
the sounds picked up by the first microphone 22 are actually
Korotkoff sounds. To this end, the second microphone 24 is
preferably positioned with its sound pickup portion directed
outwardly away from the brachial artery 36 to pick up background
noises but not blood flow sounds. However, if the second microphone
24 is not positioned to be over an artery when the cuff 10 is
fastened about a patient's arm, the microphone sound pickup portion
thereof may alternatively be directed inwardly as is the pickup
portion of the microphone 22. To prevent as much extraneous sound
pickup as possible, the backs of both microphones 22 and 24 may be
sound insulated. Further, the microphones 22 and 24 are remotely
located from the bladder connection ends of the tubes 16 and 20,
and the electrical lines 30 and 32 are brought out of the cuff
remotely from the tubes 16 and 20.
Signal discrimination is provided by electronics within the control
box 28 which compare for coincidence electrical impulses received
from the first and second microphones 22 and 24, so that only when
there is an impulse from the first or bloodsound microphone and no
simultaneous impulse from the second or noise
microphone--indicating that a true Korotkoff sound has been picked
up--is the light 34 triggered.
As depicted schematically in FIG. 3, impulses from each of the
microphones 22 and 24 are separately filtered by conventional first
and second bandpass filters 50 and 52, respectively, which
preferably allow signals within a band width of only about 5 Hz,
and centered about 90 Hz, to pass. The 90 Hz level is selected
because at this frequency there is a rapid turn-on and turn-off of
Korotkoff sounds and muscle noises, which are a common cause of
extraneous sounds, are fairly low.
Outputs from the two filters 50 and 52 are separately amplified by
conventional amplifiers 54 and 56 respectively. Outputs from these
amplifiers 54 and 56 are formed into square waves by conventional
comparators 58 and 60. Each series of filter, amplifier and
comparator 50, 54 and 58 or 52, 56 and 60 may comprise separate
portions of a single integrated circuit, for example a type 3900
quad operational amplifier.
Square waves from the comparators 58 and 60 are separately fed into
conventional, astable multivibrators 62 and 64 for conversion into
square wave pulses having pulse widths of about 200 milliseconds.
The two multivibrators 62 and 64 may, for example, comprise
portions of a single, type 555 integrated circuit timer.
From the multivibrators 62 and 64, the square wave pulses are fed
into a discrimination circuit 66 in which coincidence of pulses
from the separate multivibrators is determined, as described below.
The output of the discrimination circuit 66 is fed into an
integrating circuit 72 and thence to a threshold detector 74. From
the detector 74, the signal is amplified by a driver 76 which
drives or lights the indicator light 34, when and only when there
is a signal from the first microphone 22 and no coincident signal
from the second microphone 24.
Logic of the discriminator circuit 66 is illustrated in FIG. 4
which shows the four possible combinations of signals and
nonsignals from the multivibrators 62 and 64, corresponding to
sounds or no sounds picked up by the microphones 22 and 24. In the
diagram and for purposes of discussion, a signal corresponding to a
sound is identified by a digital "1"; absence of sound is
identified by a digital 0. Pulses from the multivibrators 62 and 64
thus comprise either 1's or 0's. When a sound is picked up by the
first microphone 22 (output of the multivibrator 62 = 1) and no
coincident sound is picked up by the second microphone 24
(coincident output of the multivibrator 64 = 0)--a condition
strongly indicative of a Korotkoff sound having been picked up--the
output of the discriminator circuit 66, indicated by Column A in
FIG. 4, is in a first preselected state, for example a 0 state,
which will cause the indicator light 34 to be turned on. For all
other combinations of coincident sounds (1's) and no sounds (0's),
the output of the discriminator circuit 66 is in a second state,
for example a 1 state, and light 34 remains unlit. Column B
indicates the condition of no sound picked up by the microphone 22
(output of the multivibrator 62 = 0) and sound picked up by the
microphone 24 (output of the multivibrator 64 = 1), indicative of
only a background noise; thus, the output of the discriminator
circuit 66 is a 1. Column C indicates sounds picked up by both
microphones 22 and 24 (1's from both multivibrators 62 and 64)
indicating the possibility that the sound picked up by the
microphone 22 may be noise and not a Korotkoff sound; again, the
discriminator circuit 66 output for such is a 1. Column D
illustrates the situation in which there is no sound from either
the microphone 22 or 24 (0's from both multivibrators 62 and 64);
the output of the discriminator 66 remains at the 1 level.
A condition of importance remains to be examined: that condition
illustrated in FIGS. 5, 5a and 5b in which there are only partially
coincident 1 pulses from the multivibrators 62 and 64. Such a
condition indicates that not entirely coincident sounds have been
picked up by the first and second microphones 22 and 24. In the
situation illustrated in FIG. 5, the 1 output of the multivibrator
62 leads the 1 output of the multivibrator 64 by a time .tau.
(corresponding to the first microphone 22 picking up a sound a
short time before the second microphone 24 picks up a sound). The
output of the discrimination circuit 66 thus is caused to remain at
the 1 level (coincidence of 1 signals) except for a short time
.tau. during which it falls to a 0 level. The integrator 72
integrates the discriminator output, and if the time .tau. is
sufficiently long (FIG. 5a) the output of the integrator 72 will
reach a predetermined threshold level V.sub.t of the threshold
detector 74, and the light 34 will be triggered on. If, however,
the time .tau. is too short, the output of the integrator 72 will
not reach the threshold level V.sub.t, and the light 34 will not be
triggered (FIG. 5b). The threshold voltage V.sub.t can easily be
preselected by varying resistances and capacitances within the
detector 74, and is preferably adjusted so that the light 34 will
be triggered for the time .tau. equal to or greater than about 50
milliseconds. This assures sufficiently non-coincident signals to
be indicative of the presence of a true Korotkoff sound.
As shown in FIG. 3, the discrimination circuit 66 may comprise
first and second NAND gates 78 and 80. The output of the
multivibrator 62 is fed into one input of the first gate 78 and the
output of the multivibrator 64, which is inverted by the second
gate 80, is fed into a second input of the first gate. This creates
the logic illustrated in FIG. 4 and described above. Other types of
circuitry may be employed in the discriminator 66, so long as the
logic is preserved. It is to be understood, however, that the logic
states of 1's and 0's may be reversed, the only requirement being,
as above stated, that the discrimination circuit 66 have a
predetermined first output level corresponding to a sound being
picked up by the first microphone 22 and no coincident sound picked
up by the microphone 24, and a second predetermined level for all
other conditions.
In the manner described, the light 34 is triggered when true
Korotkoff sounds are picked up by the first microphone 22 and at no
other time. An operator need only watch the pressure indicated on
the manometer 18 and the light 34 as he slowly releases pressure
from the bladder 12. Systolic blood pressure is recorded when the
light 34 starts flashing and diastolic pressure is recorded when
the light stops flashing.
Although the output of the driver 76 has been illustrated and
described as triggering an indicator light 34, the driver can be
used to develop other output indications. For example, as
illustrated in FIG. 6, the output of the driver 76 is used to
trigger a "pneumatic memory" system. The pneumatic memory comprises
a systolic pressure gauge 100 and a diastolic pressure gauge 102.
Associated with the systolic gauge 100 is a normally open pressure
shutoff valve 104 which is connected in series with a portion of
the tube 20, through pressure lines 106 and 108, to the gauge 100.
A normally closed pressure valve 110 is connected to the diastolic
gauge 102 through pressure lines 112 and 114 from the tube 20. The
output of the control box 28 (that is, the output of the driver 76)
is fed, via an electrical line 116 to a monostable flip-flop 118,
the output of which is in turn connected to the valve 104 by a line
120. The output of the control box 28 is also fed, via a line 122,
to one input of an OR gate 124, the output of which is fed by a
line 126 to a pulse generator 128, and thence, via a line 130, to
the valve 110. A reset OR gate 134 is connected by a line 136 to
the flip-flop 118 and by line 138 to a second input of the gate
124. One input 140 of the reset gate 134 may be connected to a
"push-to-reset" switch 142. A second input 144 of the reset gate
134 is connected to an automatic reset pulse generator 146 (as
described below).
The cuff 10 is fastened about a patient's arm and the bladder 12 is
inflated to a pressure above the anticipated systolic blood
pressure. The valve 104 is open and the pressure in the bladder 12
is directed to the gauge 100; the valve 110 is closed and no
pressure is read on the gauge 102. Pressure is slowly released from
the bladder, and a triggering output is provided by the driver 76
in the control box 28 at the first true Korotkoff sound picked up
by the microphone 22, as described above. This triggering pulse
causes the flip-flop 118 to change state and close the valve 104,
trapping pressure, which is the systolic blood pressure, in the
gauge 100. The flip-flop 118 then remains in the flipped state and
the valve 104 remains closed, so that pressure of the gauge 100
need not be immediately read. However, the valve 110 is caused to
open momentarily at each triggering output of the box 28, the pulse
generator 128 shaping the output of the gate 124 to accomplish
this. As pressure in the bladder 12 is reduced and at each
successive Korotkoff sound, a lesser pressure is registered on the
gauge 102. When the last triggering pulse has been provided,
indicating a cessation of Korotkoff sounds, the valve 110 will
remain closed and the pressure trapped in the gauge 102 will be the
diastolic pressure. An operator merely reads the systolic pressure
from gauge 100 and the diastolic pressure from the gauge 102. After
the reading has been completed, the system may be returned to its
initial condition by depressing the switch 142 or by supplying a
signal from the reset generator 146.
The above-described variation is easily adaptable to an automatic
inflation-deflation system permitting the systolic and diastolic
pressures to be taken with the press of a single switch. A start
switch 150 turns on a timer 152, preset for about 10-20 seconds,
the timer turning on a small air pump 154 which inflates the
bladder 12 and at the same time closing a normally open bleed valve
156 to hold the air in the bladder. At the end of the cuff
inflation period, the cuff pressure will be about 30 mm of mercury
above systolic pressure. When the timer 152 shuts off, the air pump
154 is stopped, the bleed valve 156 is opened and a reset pulse is
applied by the generator 146 to the reset gate 134. The bleed valve
156 is metered so that the cuff pressure decreases about 3 mm of
mercury per second. The operation is otherwise as described before,
with the systolic pressure being displayed upon the gauge 100 and
the diastolic pressure being displayed upon the gauge 102 after
pressure in the bladder 12 has been released.
It is to be appreciated that the triggering output of the driver 76
may be used in still other ways to indicate when systolic and
diastolic blood pressure should be read. For example, if the
pressure in the bladder 12 is recorded on a recording oscillograph
(not shown), the output of the driver 76 may be supplied to one
channel of the recorder. After depressurizing the bladder 12, the
systolic and diastolic blood pressures can be read from the
recording by reference to the presence or absence of the triggering
pulses.
Other means for microphone signal discrimination may also be
provided. For example, FIG. 7 illustrates circuitry in which is
electronically computed the ratio between the magnitudes of signals
from the first and second microphones 22 and 24. Background sounds
simultaneously picked up by the two microphones 22 and 24 can be
expected to have about an equal magnitude -- particularly if both
of the microphones have pick-up portions directed inwardly toward a
patient's arm--hence, the ratio of signal magnitudes in such a case
will be about one. However, Korotkoff sounds picked up by the first
microphone 22 over the brachial artery are expected to be greater,
by a factor of about two or more, than Korotkoff sounds normally
picked up by the other microphone. That is, if the ratio of
simultaneous signal magnitudes (dividing the magnitude of the first
microphone signal by the magnitude of the second microphone signal)
is substantially greater than one, for example, about 1.4 or
greater, there is strong assurance that the sound picked up by the
first microphone was a true Korotkoff sound. If a sound is picked
up by the first microphone 22 and none is picked up by the second
microphone 24 (an indication, as described above, that a true
Korotkoff sound has been picked up), the signal ratio will also be
much greater than one and an indication will also be given. On the
other hand, if no sound is picked up by the first microphone 22 and
sound is picked up by the second microphone 24 (an indication of
background noise) the ratio of signal magnitudes will be much less
than one, and no indication will be given.
To this end, signals from the filters 50 and 52 are fed into a
conventional ratio determining circuit 164, in which the ratio
between signal magnitudes from the first and second microphones 22
and 24 is determined. The output from the circuit 164 is fed into a
comparator 166 which compares the output of the circuit 164 to a
predetermined ratio or level, and thence to a one-shot
multivibrator 168. If the ratio is greater than the predetermined
ratio (for example about 1.4), the comparator 166 causes the
multivibrator 168 to light the indicator light 34.
Since it is important that the polarity of the output signal remain
constant, outputs of the filters 50 and 52 are fed through diodes
172 and 174 respectively to eliminate negative output signals to
the circuit 166. Ground path resistors 176 and 178 are associated,
respectively, with diodes 172 and 174. Other circuits can be used
to insure the positive polarity of the two signals. It is also
important that the input to the circuit 164 from the filter 50
(V.sub.22) and from the filter 52 (V.sub.24) never be zero, so that
the circuit can calculate the ratio. Hence, very small electrical
signals -.delta.1 and +.delta.2 are added, respectively, to
V.sub.22 and V.sub.24 through resistors 180 and 182. The actual
mathematical operation performed by the circuit 164 is thus
(V.sub.22 -.delta.1) divided by (V.sub.24 +.delta.2).
Such circuitry as just described has the advantage of enabling
discrimination even when simultaneous Korotkoff sounds are picked
up by both microphones 22 and 24.
Additional signal discrimination may be provided by use of
conventional signal pulse width discriminators (not shown) in
conjunction with the frequency bandpass provided by the filters 50
and 52 to take advantage of the fact that Korotkoff sound pulses
(picked up by the first microphone 22) will generally have a
different pulse width than extraneous sounds (picked up by either
of the two microphones). To some extent, the pulse width is
determined by the band width of the filters 50 and 52, and if the
band width is too low, the pulse widths may be significantly
distorted and both the Korotkoff sound signals and the extraneous
noise signals will have similar pulse widths. A trade-off is thus
necessary between pulse width discrimination and frequency
bandpass: the frequency pass band must be narrow enough to
eliminate noise, but wide enough not to distort the pulse widths
excessively.
Corresponding methods for measuring systolic and diastolic blood
pressures, either automatically or non-automatically are thereby
provided. Although there have been described hereinabove specific
arrangements of a noise rejecting electronic sphygmomanometer and
methods for measuring blood pressure in accordance with the
invention for the purpose of illustrating the manner in which the
invention may be used to advantage, it will be appreciated that the
invention is not limited thereto. Accordingly, any and all
modifications, variations or equivalent arrangements which may
occur to those skilled in the art should be considered to be within
the scope of the invention as defined in the appended claims.
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