U.S. patent number 3,861,387 [Application Number 05/287,608] was granted by the patent office on 1975-01-21 for cardiac arrhythmia detector.
This patent grant is currently assigned to Cardiodynamics, Inc.. Invention is credited to Richard D. Lawhorn, Robert L. Nelson.
United States Patent |
3,861,387 |
Lawhorn , et al. |
January 21, 1975 |
CARDIAC ARRHYTHMIA DETECTOR
Abstract
A selective circuit is provided for passing certain parts of,
and information contained in, a complex input waveform containing
both desired signals and undesired noise components. The circuit is
particularly suited for use in a heart arrhythmia detector. The
circuit operates on the first time derivative of the complex input
waveform. The circuit provides an averaging means wherein the peak
amplitude as well as the duration of the peak amplitude of a number
of beats is averaged for the particular individual to establish a
floating standard and the alarm is sounded when a signal is
received which deviates by a predetermined amount from the thus
established standard. The circuit is essentially self-adjusting to
the norm for a particular individual both as to peak and duration
rather than comparing an individual heartbeat with an arbitrary
standard.
Inventors: |
Lawhorn; Richard D. (Dublin,
CA), Nelson; Robert L. (Dublin, CA) |
Assignee: |
Cardiodynamics, Inc. (Dublin,
CA)
|
Family
ID: |
23103636 |
Appl.
No.: |
05/287,608 |
Filed: |
September 11, 1972 |
Current U.S.
Class: |
600/514; 128/904;
600/515; 600/519 |
Current CPC
Class: |
A61B
5/0245 (20130101); A61B 5/316 (20210101); A61B
5/7239 (20130101); Y10S 128/904 (20130101) |
Current International
Class: |
A61B
5/04 (20060101); A61B 5/024 (20060101); A61B
5/0245 (20060101); A61b 005/04 () |
Field of
Search: |
;128/2.5P,2.5R,2.5T,2.6A,2.6E,2.6F,2.6R,2.1A,2.1R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Slick, Robert G.
Claims
We claim:
1. A cardiac arrhythmia detector comprising in combination:
a. a set of electrodes adapted to be applied to the body,
b. means connected to said electrodes for generating a signal
proportional to the first time derivative of the input signal
applied to said electrodes,
c. means for measuring the peak amplitude of said first time
derivative of each waveform due to either normal or abnormal
vertricular pumping action,
d. means for establishing the running average of the last 10 to 40
of said peak amplitudes,
e. means for establishing an amplitude threshold defined as 1/16 to
1/4 of said running average,
f. means for measuring the time interval spent above said amplitude
threshold by each first time derivative waveform due to either
normal or abnormal ventricular pumping action,
g. means for establishing the running average of the last 10 to 40
of said time intervals,
h. means for comparing each measured peak according to (c) above,
to the running average according to (d) above, to identify whether
or not the individual peak differs from said running average by
more than plus or minus 25 percent, and classifying as normal, on
this basis, those for which the difference is less than plus or
minus 25 percent, and abnormal those for which the different
exceeds plus or minus 25 percent,
i. means for comparing each time interval according to (f) above,
to the running average according to (g) above, to identify whether
or not the individual interval differs from said running average by
more than plus or minus 25 percent, and classifying as normal,
those for which the difference is less than plus or minus 25
percent, and abnormal those for which the difference exceeds plus
or minus 25 percent,
j. means for eliminating from consideration, for purposes of
establishing the running averages of (d) and (g) above, those
derivative waveforms found to be abnormal according to (h) or (i)
above, and
k. alarm means and means for delivering a positive output to said
alarm means whenever a derivative waveform is classified as
abnormal according to either (h) or (i) above.
2. The cardiac arrhythmia detector of claim 1, having in addition
thereto a modulator, sounder and touch plate means and means for
actuating same from said generated signal whereby a wearer can
touch said plate and means whereby said sounder will issue an
audible ECG signal which can be sent over a telephone or the
like.
3. The cardiac arrhythmia detector of claim 1, having means
inhibiting said alarm until a plurality of abnormal indications are
detected within a time period and means for varying said
period.
4. A detector for missing R waves comprising in combination:
a. a set of electrodes adapted to be applied to the body,
b. circuit means connected to said electrodes, said circuit means
including means to identify a missing R wave, including means for
measuring the interval between each successive pair of normal R
waveforms, and means for declaring a misssing R wave when the
current interval is greater than about 1.5 times the immediately
previous interval, alarm means and means for actuating said alarm
means upon said declaration.
5. A combined cardiac arrhythmia detector and missing R wave
detector comprising in combination:
a. a set of electrodes adapted to be applied to the body,
b. first circuit means connected to said electrodes, said first
circuit means including means whereby the output of the said means
is proportional to the first time derivative of the input signal
applied to said electrodes,
c. means for measuring the peak amplitude achieved by the first
time derivative of each waveform due to either normal or abnormal
ventricular pumping action, any time said derivative waveform
exceeds an amplitude threshold defined as 1/16 1/2 of a running
average,
d. means for measuring the time interval spent above said amplitude
threshold by each first time derivative waveform due to either
normal or abnormal ventricular pumping action,
e. means for establishing the running average of the last 10 to 40
of said time intervals,
f. means for comparing each so measured peak according to said
running average to identify whether or not the individual peak
differs from said running average by more than plus or minus 25
percent, and classifying as normal, on this basis, those for which
the difference is less than plus or minus 25 percent, and abnormal
those for which the difference exceeds plus or minus 25
percent,
g. means for comparing each said time interval to the said running
average to identify whether or not the individual interval differs
from said running average by more than plus or minus 25 percent,
and classifying as normal, those for which the difference is less
than plus or minus 25 percent and abnormal those for which the
difference exceeds plus or minus 25 percent,
h. means for eliminating from consideration, for purposes of
establishing said running averages, those derivative waveforms thus
found to be abnormal,
i. means for producing a first output signal whenever a derivative
waveform is classed as abnormal,
j. a second circuit means connected to said electrodes, said means,
including means to identify an asynchronous R wave, wherein said
means comprises circuitry for measuring the interval between each
successive pair of R waveforms,, and means for declaring an
asynchronous R wave when the current said interval is greater than
about 1.5 times the immediately previous interval, and means for
producing a second output signal upon said declaration, and
k. alarm means and means for actuating said alarm means by the
occurrence of said first output signal followed immediately by said
second output signal.
6. A cardiac R wave detector comprising in combination:
a. a set of electrodes adapted to be applied to the body,
b. circuit means connected to said electrodes for generating a
signal proportional to the first time derivative of the input
signal applied to said electrodes,
c. said circuit means including means for detecting a first time
derivative which is positive in polarity and has a slope on the
heart waveform of about 0.02 volts per second and having a time
duration of about 50 milliseconds, and means for detecting
immediately following said first time derivative, a second first
derivative which is negative in polarity, has a slope on the heart
waveform of about minus 0.02 volts per second and having a time
duration of about 50 milliseconds, and
d. means for producing an output signal each time that both
criteria are met.
7. The cardiac R wave detector of claim 6, having a means whereby
an alarm is sounded when heartbeats are slower than a normal
standard and means for selecting said standard.
8. The cardiac R wave detector of claim 6, having means associated
therewith whereby an alarm is sounded when heartbeats are faster
than a normal standard and means for selecting said standard.
Description
SUMMARY OF THE INVENTION
In many applications of electronics it is desired that only certain
shapes of electrical waveforms be allowed to pass onto subsequent
circuitry or that waveforms of certain characteristics be prevented
from passing. When dealing with relatively simple waves, active or
passive filters will selectively remove or pass selected frequency
components. However, in the case of a complex waveform which has
characteristics of time, amplitude and frequency which
simultaneously occur with variable relationships, a much more
difficult problem is presented. In the past, waveform pattern
recognition has required a large amount of expensive equipment and
resort is frequently had to digital computer systems for such
waveform recognition.
In many applications such as in a heart arrhythmia detector which
may be mounted on the body of an individual, it is desired to
provide extremely compact equipment which has a low power drain.
The present invention provides a compact circuit of low power drain
of the analog type which can be adjusted to selectively pass or
reject selected types of complex electrical waveforms. The
circuitry of the present invention can be incorporated in a small
self-contained unit which can be worn on the person of the user
such as that shown in U.S. Pat. Nos. 3,138,151 and 3,547,107.
The present invention was specifically developed for the use in
such a self-contained heart arrhythmia detector which can be
mounted on the chest, carried in a connecting or placed on a
table.
One problem in such devices is that electrical noise signals may be
generated in various manners, such as by imperfect contact between
the input electrodes and the skin of a wearer as well as in other
manners, such as muscle tremor and, since the desired signal is
extremely minute, these extraneous signals can mask the desired
signal, either preventing an alarm when it should be given, or
triggering a false alarm. This is undesirable, not only as a
nuisance, but also because it causes misdirection of efforts in
emergency situations and leads to erroneous diagnosis which can be
serious to the extent of being fatal. Such electrical noise can be
generated in various fashions, both within and without the body of
the individual being monitored. The circuitry of the present
invention makes it unlikely for the alarm to be triggered by
electrical noise.
The present invention discloses circuitry which analyzes the heart
waveform by processing the first time derivative of the waveform,
rather than the waveform itself. This unique treatment results in
greatly improved rejection of noise-type waveforms such as may stem
from electrode movement or muscle actions of the patient whose
heart is being monitored. The basic circuitry detects abnormal
ventricular activity of the heart. Also disclosed is circuitry for
a time-derivative-actuated R-wave detector and a missing-R-wave
detector. These circuits may be employed either individually or in
coordination to provide the optimum processor for a given patient's
heart. A further improvement allows for addition of a heart
bradycardia or tachycardia detector and means for transmission of
the patient ECG over the telephone.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings forming a part of this invention:
FIGS. 1A, 1B and 1C are a schematic diagram of an electrical
circuit embodying the present invention.
FIGS. 2A, B, C and D and 3A, B, C and D are diagrams of approximate
waveforms in various parts of the circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The input electrodes are connected to the circuit at the terminals
labeled 1, 2 and 3. The differential signal of interest is sensed
by the electrodes connected to terminals 1 and 3. The electrode
connected to terminal 2 provides a ground return to the signal
source to provide a common-mode signal return. The electrodes may
be of any conventional design, such as those shown in U.S. Pat.
Nos. 3,138,151 and 3,547,107. The signals from the terminals 1 and
3 are applied to compound emitter followers 4 and 5 where the
signal impedance level is reduced for delivery to the differential
amplifier 6. The latter amplifier in conjunction with components 7,
8 and 9 provides a voltage gain of about 120, providing a typical
output signal of about 120 millivolts, peak. Components 10 and 11
form a high-pass filter which differentiates the signal from
amplifier 6. The 3-dB cutoff frequency of this filter is about 35
Hz. The differentiated signal is applied to the
automatic-gain-controlled amplifier 12 for further increase in
signal voltage level. Components 13 and 14 provide for a minimum
voltage gain of about 20. The voltage gain is increased as
necessary by the shunting effect of the control element,
field-effect-transistor 15. Controlling gate voltage for the
field-effect-transistor is derived from the negative peaks of the
output signal from amplifier 12. These peaks cause conduction
pulses through the PNP transistor 16, which produce a negative
charge on capacitor 17. This charge is filtered by components 18,
19 and 20 and applied to the gate of transistor 15. Thus, as the
negative peaks at the output of amplifier 12 tend to grow in
magnitude, the voltage at the gate of transistor 15 becomes more
negative, increasing the effective resistance of this control
element, decreasing the gain of amplifier 12, and decreasing the
output at point 2. The electrodes are placed on the patient's body
so that the voltage waveforms at points 1 and 2 are polarized as
shown in FIG. 2. Note that a positive first time derivative of the
waveform at point 1 produces a positive waveform at point 2, and
vice versa.
Components 21-77 form the abnormal-ventricular-activity (AV)
detector. Significant waveforms in this circuitry are shown in FIG.
2. Transistors 21 and 22 and capacitors 23 and 24 form two positive
peak detectors, one with an output (at point 3) equal to the full
positive peak (less one base-emitter voltage drop) of the output of
amplifier 12, and one with output equal to approximately six-tenths
of this value (less one base-emitter voltage drop).
Field-effect-transistors 25 and 26 provide means for discharging
the capacitor 23, and transferring its charge to capacitor 27,
respectively. Gate control signals for these transistors are
developed from circuitry described below. Transistor 28 is used as
an emitter follower to drive the integrating circuit formed by
components 29, 30, 31 and 32. The latter network produces, at point
4, an output voltage equal to approximately three-fourths of the
running mean of the positive peaks stored on capacitor 27 taken
over approximately a 22 period. Resistors 31 and 32 divide the
point 4 voltage by about six. This voltage E.sub.t, is applied to
transistor 33, which is part of the voltage comparator circuit
comprised of components 33-41. The other input to this comparator
circuit comes from point 2 through current-limiting resistor 42.
Thus, whenever the voltage at point 2 exceeds the voltage at the
junction of resistors 31 and 32 (i.e., exceeds one-sixth of the
average of the positive peaks of the voltage at point 2,) the
voltage at the collector of transistor 36 is caused to approach the
positive supply voltage. This action is reinforced by the positive
feedback furnished by the resistor 38 between transistor 36
collector and transistor 35 base. This positive-going excursion of
the transistor 36 collector voltage declares a significant
ventricular action has occurred and begins a sequence of electronic
events which evaluate whether or not the ventricular action should
be classified as normal or abnormal.
The positive-going voltage transition at the transistor 36
collector causes a positive-going pulse to be transmitted to the
gates of field-effect-transistors 25, 202, 43 and 203, in turn
causing their effective resistance to be sharply reduced and thus
discharging capacitors 23, 24, 44 and 77, respectively. The
discharging pulse is transmitted to the gates of the
field-effect-transistors through the pulse forming network network
of elements 45 through 49. This pulse is approximately 1
millisecond wide; the discharge of elements 23, 24, 44 and 77 is
very rapid. The discharge of these capacitors prepares them to
receive new information in the form of a measured amount of charge
from their driving elements. At this time, the voltage at the
output of amplifier 12 (point 2) has passed a threshold of
approximately one-eights (i.e., three-fourths divided by six) of
its previous positive peaks and presumably is approaching another
positive peak. Since capacitor 23 has been quickly discharged and
released, it will follow the positive excursion at point 2 and hold
the peak value attained (less a base-emitter voltage drop).
Capacitor 44 will charge through resistor 50 and diode 51 so long
as the collector voltage of transistor 36 is near the positive
supply voltage (i.e., as long as the voltage at point 2 stays above
the threshold of one-eighth of the running mean of its previous
positive peak). The time constant of resistor 50 and capacitor 44
is long compared to the normal amount of time over which the latter
charges. Thus the voltage on capacitor 44 is an approximately
linear function of how long the collector voltage on transistor 36
stays near the positive supply voltage. In summary, then, capacitor
23 gives a voltage proportional to the peak of the positive first
derivative of the significant ventricular action appearing at point
1 and capacitor 44 gives a voltage proportional to the time
duration of the this positive first derivative.
Field-effect-transistor 52 acts as a switch to transfer the charge
from capacitor 44 to capacitor 53 at the appropriate time.
Gate-control voltage for this transistor is derived from the same
source as that for transistor 26; the circuitry is described below.
Transistors 54 and 55 are emitter followers which couple the
voltage on capacitor 53 to the running-mean network 56-59 without
significant offset voltage. The running-mean network 56-59 produces
a voltage equal to approximately three-fourths of the average of
the voltage on capacitor 53 taken over approximately a 20-second
period.
When the voltage at point 2 passes its peak positive value and
decreases below the threshold value described above, the voltage at
transistor 36 collector is caused to switch to a highly negative
value, approaching the negative supply voltage. This negative
voltage transition is sensed by the pulse-forming network of
components 60-63, which in turn transmits a positive pulse
(hereafter sometimes referred to as an interrogation pulse)
approximately one millisecond long to the bias terminals h on
operational amplifiers 64-67 which are configured as voltage
comparators. If, during the 1 millisecond period, the voltage at
the positive input terminal of any comparator is more positive than
that at its negative input terminal, the output terminal of that
comparator will produce a positive voltage which will turn on one
of the emitter followers 68A through D and produce a positive
voltage at the emitter of that transistor. Thus, the transistors 68
form an "OR" function; an emitter will go high if any of the
voltage comparator 64-67 outputs produces a positive voltage (i.e.,
the OR function will produce a positive output if the particular
significant ventricular action just completed is detected to
deviate more than a prescribed amount from either the mean of its
normal positive peak values, or from the mean of its normal
duration).
The positive pulse formed by the network of elements 60-63 is also
transmitted to the gates of transistors 26 and 52 through the delay
network of resistor 69 and capacitor 70 and current-limiting diode
71, so that the charge on capacitors 23 and 44 is made to transfer
to capacitors 27 and 53, respectively. However, if any of the
comparator (64-67) outputs goes positive during the interrogation
period, transistor 72 is made to turn on by the positive voltage
appearing at the emitter of transistor 68, thus clamping out the
charge-transfer pulse to the gates of elements 26 and 52 preventing
the capacitors 27 and 53 from receiving voltage information from
capacitors 23 and 44 respectively. This inhibiting function only
occurs for ventricular actions which are classed as abnormal, so
that the averaging networks 29-32 and 56-59 do not receive input
information due to abnormal heart beats.
During approximately the first minute of operation it is necessary
to allow all ventricular activity to load the running-average
networks made up of elements 29-32 and 56-59. Thus, it is necessary
to lock out the charge-transfer inhibiting capability of transistor
72. This is accomplished by the action of transistor 73. When the
entire circuit is first actuated with power, the touch-plate
monostable multivibrator (discussed more completely below) is
caused to produce a negative voltage at the base of transistor 73,
turning this transistor off and defeating the charge-transfer
inhibit function of transistor 72.
The voltage on the positive input of voltage comparator 64 is equal
to approximately six-tenths of the positive peak value achieved by
the output of amplifier 12 during a given ventricular action less
one base-emitter voltage drop. The voltage on the negative input is
equal to approximately three-fourths of the running mean of the
positive peaks achieved by amplifier 12 output during heart beats
which are classed as normal, less one base-emitter voltage drop.
Thus, the output of the comparator 64 will become positive during
the interrogation pulse received at its bias terminal if the
positive peak at amplifier 12 output exceeds 125 percent of the
running mean of the normal positive peaks.
The voltage on the positive input of comparator 65 is equal to that
on the negative input of comparator 64. The voltage on the negative
input is equal to the full positive peak achieved by the amplifier
12 output, less one base-emitter voltage drop. Thus, the output of
comparator 65 will become positive during the period of the
interrogation pulse received at its bias terminals if the positive
peak of the amplifier 12 output is less than 75 percent of the
running mean of the normal positive peaks.
The positive input of voltage comparator 66 is derived from
resistance-capacitance network 75-77. This network charges the
capacitor 77 during the same time period that capacitor 44 is being
charged through resistor 50, except that the RC time-constant is
chosen to be somewhat longer than that of elements 50 and 44, so
that capacitor 77 will charge to only about six-tenths of the
charge on capacitor 44. The voltage at the negative input of
comparator 66 is equal to approximately three-fourths of the
running means of the voltage achieved by capacitor 44 during normal
heart beats. Thus, comparator 66 will produce a positive voltage at
its output during the period when the interrogation pulse is
applied to its bias terminal if the time period that the amplifier
12 output spends above the threshold voltage is longer than 125
percent of the running mean of the time spent above the threshold
during normal heart beats.
The positive input of comparator 67 is equal to the negative input
of comparator 66. The negative input is equal to the voltage on
capacitor 44. Thus, the output of comparator 67 will produce a
positive voltage during the period of the interrogation pulse if
the period spent by the amplifier 12 output above the threshold
voltage is less than 75 percent of the running mean of the time
spent above the threshold during normal heart beats.
Thus, the emitter of any one of transistors 68a, 68b, 68c or 68d
will produce a positive voltage if during a significant ventricular
action, the peak of the positive first derivative of the voltage at
point 1 deviates in magnitude by more than 25 percent from the
running mean of the peaks during normal heart beats, or if the time
period spent by the positive first derivative above a threshold
value equal to one-eighth of the running mean of its normal peaks
deviates more than 25 percent from the running mean of the same
time periods for normal heart beats. Anytime any one of the
emitters of transistors 68a, 68b, 68c or 68d becomes positive, an
abnormal ventricular action is declared. The positive emitter
voltage is transferred to the alarm criteria circuitry, described
below, through operational-amplifier follower 204.
Components 76'-114 form the R-wave detector. The detector operates
by determining whether or not the elapsed time between a strong
negative voltage deflection and a strong positive voltage
deflection at point 2 is within a certain allowable limit set by an
internal monostable multivibrator. The R wave is identified by
requiring a first time derivative which is positive in polarity,
indicating a slope on the heart waveform of about 0.02 volts per
second and having a time duration of about 50 milliseconds,
followed immediately by a first time derivative which is negative
in polarity, indicating a slope on the heart waveform of about
-0.02 volts per second and having a time duration of about 50
milliseconds and includes means for indicating each time this
criterion is met. If properly sequenced negative and positive
voltage excursions do occur, the collector of transistor 109 is
caused to become strongly positive for a period of time set by a
second monostable multivibrator, normally about 65 milliseconds.
For voltage excursions which are not of sufficient magnitude, or
for which the strong negative excursions are not followed within
the allowable time limit by a sufficiently strong positive
excursion, transistor 109 will not produce a strongly positive
collector voltage.
Components 76' and 78 couple the signal from amplifier 12 output
into the base of transistor 80. The latter element and elements
81-87 form a monostable multivibrator with a time-out period of
approximately 100 milliseconds. When a strong negative voltage
excursion is coupled into the base of transistor 80, the collector
of this transistor becomes strongly positive, furnishing base
current to transistor 113 and turning this element on for about 100
milliseconds. If a strong positive voltage excursion is coupled
from point 2 through elements 77' and 79 into the base of the
transistor 89 during this time period, sufficient current will be
drawn through resistors 100 and 101 to fire the monostable
multivibrator formed by elements 102-108. This causes the collector
of transistor 102 to become strongly positive, causing sufficient
current flow through resistor 111 to activate transistor 114.
Current drawn by transistor 114 through resistor 110 causes
transistor 109 to be turned on, thereby causing its collector
voltage to become strongly positive. This condition continues for
about 65 milliseconds, the time-out period of the monostable
multivibrator. The output of the R-detector circuitry (i.e., the
collector of transistor 109) is made available to the
missed-R-detector and the abnormal-rate circuitry discussed
below.
Elements 115-134 form the abnormal-heart-rate detector. Each time
an R-wave is detected by the R-wave detector, a precise (both in
amplitude and duration) positive pulse is applied by transistor 109
to doide 115. Current then flows through diode 115 and resistor 116
to charge capacitor 117. The time constant of resistor 116 and
capacitor 117 is chosen such that the capacitor is only partially
charged during an individual input pulse. However, discharge is at
a relatively constant rate through resistor 118. Thus, if the heart
rate increases, the voltage on capacitor 117 increases, while if
the rate decreases the voltage on the capacitor decreases. The
transistor pair 121-122 and associated resistors form a
differential voltage comparator which detects abnormally low heart
rates; if the voltage on capacitor 117 becomes less than the
threshold voltage on the base of transistor 122 (set by the
adjustable voltage divider formed by resistors 130-134) the
transistor 121 will conduct. This will cause sufficient current
flow in resistor 119 to turn on transistor 124. The resultant
current drawn through resistor 129 is sufficient to cause a voltage
drop across this element which will turn on transistor 136. The
latter element then delivers current to the tone oscillator circuit
described below. Similarly, transistors 126 and 127 form a
differential voltage comparator which detects abnormally high heart
rates. When the voltage on capacitor 117 increases above the level
set at the base of transistor 127 by the voltage divider network of
elements 130-134, transistor 126 draws sufficient current through
resistor 129 to turn on transistor 136. The latter element then
delivers current to the tone oscillator circuit described
below.
Components 138-171 form the missed-R-wave (MR) detector. The
purpose of this circuit is to detect a sudden elongation in the
period between R-waves. Such elongation often accompanies heart
arrhythmias, and is manifest typically as a missing QRS wave
complex due to the heart's substitution of an ectopic beat for
normal activity. FIGS. 3A, B, C and D show typical approximate
waveforms at four significant points in the MR detector circuitry.
These points are labeled on the schematic diagram.
Field-effect-transistor 143 and resistor 144 form a constant
current source which charges capacitor 142 at a rate of
approximately 1.5 volts per second. The resultant voltage ramp is
shown in FIG. 3A. Capacitor 142 is suddenly discharged each time an
R-wave is declared by the R-wave detector. The discharge is
accomplished by transistor 141 which receives a strong forward bias
from transistor 109 through the resistance-capatance network of
components 138-140 each time an R-wave is detected. If an R-wave is
skipped (i.e., the period between R-waves suddenly elongates), the
discharge fails to take place and the voltage ramp on capacitor 142
continues upward, as shown between the 3- and 5-second points on
FIG. 3A. Transistors 145 and 147 in association with resistors 146,
148 and 149, form a compound emitter follower without significant
offset voltage between input and output. Thus, when the voltage at
point (6) becomes greater than that at point (7), point (7) will
follow point (6), but when the voltage at point (6) decreases below
that at point (7), point (7) will not track point (6), but instead
capacitor 150 will slowly discharge through resistors 148 and 149.
The approximate voltage waveform at point (7) is shown in FIG. 3A.
Components 153-156 form a simple dc-supply which holds the cathode
of diode 158 about 6 volts more positive than the drain of
transistor 143. This produces a strong reverse bias on the junction
of field-effect-transistor 159, thus inducing a high resistance
between source and drain in this element and isolating point (8)
from point (7). However, when the collector of transistor 109
becomes strongly positive, transistor 152 is forward biased,
pulling the gate of transistor 159 to a low voltage and greatly
reducing the source-drain resistance of transistor 159, thereby
connection capacitor 150 to capacitor 160. Since the latter
capacitor is much smaller in value than the former, the voltage on
capacitor 160 becomes very nearly equal to the voltage on capacitor
150 at the instant it was connected to capacitor 160. When the
collector of transistor 109 becomes strongly negative once again,
transistor 159 again goes to the high-resistance state and
capacitor 160 is isolated from capacitor 150. Thus, the voltage
waveform at point (8) is shown in FIG. 3C. Transistors 162 and 163
form a differential voltage comparator; whenever the voltage at the
base of transistor 163 (point 9) becomes less than that at the base
of transitor 162, the latter element will draw sufficient current
through resistor 165 to forward bias transistor 167, in turn
forward biasing transistor 170 and causing its collector to become
strongly positive. The voltage at the base of transistor 162 is
established by that at point (8) through resistor 161. The voltage
waveform at point (9) is about two-thirds of that at point (7).
When a missing R-wave is encountered, capacitor 142 will not be
discharged at the normal point in time, and will continue to
charge, as shown at the 4-second point in FIG. 3A. Thus, the
voltage at point (9) will also continue to increase as shown in the
corresponding point on FIG. 3D. Meanwhile, point (8) has remained
isolated from point (7) since transistor 109 has not produced a
positive pulse to turn on transistor 159. Consequently the voltage
at point (8) will remain at a nearly constant level. Finally, at
the point labeled X on FIGS. 3C and 3D, the voltage at point (9)
will exceed that at point (8), forward biasing transistor 170 by
the process described above. A strongly positive voltage at the
collector of transistor 170 declares a missing R-wave.
It is very important to note that this circuitry will detect a
mising R-wave regardless of the rate of arrival of R-waves from the
heart. Faster rates will reduce the peak height of the waveform in
FIGS. 3A and 3B and decrease the period of this sawtooth waveform.
The other waveforms in FIG. 3 will be scaled proportionately since
they are all dependent on the waveform at point (7). But at the
occurrence of a missing R-wave, a crossover point for the voltages
at points (8) and (9) will be caused by the same train of events as
detailed above for a heart rate of one beat per second. Slower
heart rates will cause the waveform in FIG. 3A to grow in magnitude
and lengthen in period, but once again, the crossover between the
voltages at points (8) and (9) will be established as above.
Components 172-192 form a logical network with which any of several
different modes of operation of the overall circuitry may be
selected by manipulation of switches 190-192. Description of the
circuitry will not be given in great detail, since similar logic
has been used by many others in the past and the circuitry employed
here is not unique in principle. Switch 190 controls the operation
of the abnormal-heart-rate detector circuitry: when the switch is
closed the circuit is active, when it is open, the circuit is an
abnormal When the circuit is active, anabnormal heart rate will
turn on the tone alarm 205 by forward-biasing transistor 194. When
switches 191 and 192 are both closed, both the MR detector and the
AV detector are deactivated. When switch 191 is open and switch 192
is closed, the MR detector is activated and the AV detector is
deactivated. Transistor 188 is forward-biased so if a missed R-wave
is detected, the collector of transistor 170 will go strongly
positive, forward-biasing transistor 187 and pulling its collector
strongly negative. Each missed R-wave will deliver a negative pulse
to the alarm criteria circuitry, discussed below, through capacitor
193. When switch 191 is closed and switch 192 is open, the AV
detector is activated and the MR detector is deactivated.
Transistor 184 is forward-biased so if an abnormal ventricular
action is detected, transistor 183 will be forward-biased and its
collector pulled stongly negative. Each AV will deliver a negative
pulse to the alarm criteria circuitry through capacitor 193. When
both switches 191 and 192 are open, an AV will produce a positive
charge on capacitor 177 and forward-bias transistor 180. A strong
positive voltage on the collector of transistor 109, caused by a
detected R-wave, will forward-bias transistor 175 and discharge
capacitor 177, turning off transistor 180. However, if a positive
pulse is received from the MR detector before a positive pulse from
the R-wave detector, transistor 181 will be forward biased and the
collector of transistor 180 will be pulled strongly negative,
delivering a negative pulse to the alarm criteria circuitry through
capacitor 193.
Transistors 195-197, and their associated passive components form a
touch-plate-activated monostable multivibrator. This circuit
provides a means by which the patient himself can turn on the tone
oscillator discussed below. The basic principles of this
multivibrator have been previously described in the patent
application of one of us, Ser. No. 260,775, filed June 8, 1972.
Output from the multivibrator causes transistor 136 to be strongly
forward-biased which in turn causes transistor 194 to be turned on,
activating the tone.
Unijunction transistors 198 and 199, and associated passive
components, form the alarm criteria circuitry. Through the use of
this circuitry it is possible to require that more than one
occurrence of an abnormal heart activity take place in a certain
programmable time period before the alarm itself will be activated.
The basic circuitry for this alarm criteria operation has been
previously described.
The actual alarm is manifest through the operation of a tone
oscillator which drives a loudspeaker or sounder. The oscillator
may be activated through the forward-biasing of either transistor
200 or transistor 194. The former is activated by the alarm
criteria circuitry, and the latter by the abnormal-rate circuit or
touch-plate circuit. The tone oscillator itself is of conventional
design. When the oscillator is keyed on, its frequency is modulated
by the voltage waveform appearing at the output of amplifier 6.
This waveform is amplified by operational amplifier 201 and
delivered to the base of transistor 205 which controls the period
of oscillation of the tone oscillator. The advantage of modulating
the tone oscillator with the patient's ECG is that the ECG may
thereby be transmitted over the telephone for recording and
analysis in real-time at the doctor's office.
With reference to the abnormal-ventricular-activity detector, it
was stated that the peak amplitudes of the positive first
derivative is averaged over approximately a 20-second period. In
actuality the optimum averaging period varies between about 10 and
about 40 seconds, depending on the ECG characteristics of a given
patient. The same considerations apply to the averaging period for
the amount of time spent by the positive first derivative above the
adaptive threshold; the optimum period may vary from about 10 to
about 40 seconds. Similarly, the adaptive threshold described as
one-eighth of the average peak amplitude of the positive first
derivative may be optimized anywhere between about one-sixteenth
and one-fourth of the average peak amplitude.
* * * * *