U.S. patent number 3,693,626 [Application Number 05/086,107] was granted by the patent office on 1972-09-26 for demand pacer with heart rate memory.
This patent grant is currently assigned to Adcole Corporation. Invention is credited to Addison D. Cole.
United States Patent |
3,693,626 |
Cole |
September 26, 1972 |
DEMAND PACER WITH HEART RATE MEMORY
Abstract
A demand controlled cardiac pacer including a pair of electrodes
for connection with the heart, a variable frequency relaxation
oscillator connected to the electrodes, a resetting circuit for
disabling the oscillator when heart pulses are produced at a normal
rate, and a circuit responsive to the rate of reset for modifying
the frequency of the oscillator so that upon heart failure
stimulating pulses will be applied to the electrodes at a rate that
begins somewhat below the last rate of production of natural pulses
and gradually decreases to a fixed minimum rate.
Inventors: |
Cole; Addison D. (Natick,
MA) |
Assignee: |
Adcole Corporation (Waltham,
MA)
|
Family
ID: |
22196314 |
Appl.
No.: |
05/086,107 |
Filed: |
November 2, 1970 |
Current U.S.
Class: |
607/9 |
Current CPC
Class: |
A61N
1/365 (20130101) |
Current International
Class: |
A61N
1/365 (20060101); A61n 001/36 () |
Field of
Search: |
;128/2.6A,2.6F,2.6R,419P,420-424 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Claims
Having thus described my invention, what I claim is;
1. A variable rate demand pacer, comprising:
a pair of electrodes adapted to be connected in electrical circuit
with living tissue;
pulse generating means connected to said electrodes for producing a
first pulse each time an electrical signal above a predetermined
magnitude appears on said electrodes;
pulse rate measuring means controlled by said pulse generating
means for producing an output signal when said first pulses are
produced at a rate below a predetermined maximum rate;
a relaxation oscillator, comprising:
output terminals connected to said electrodes,
a timing circuit including a variable impedance,
a source of voltage, and
a capacitor connected in series, and
means responsive to the voltage across said capacitor for
discharging the capacitor and producing a pulse across said output
terminals when the voltage across the capacitor reaches a
predetermined value;
an electronic switch connected in parallel with said capacitor;
gate means controlled by said pulse generating means and said pulse
rate measuring means for closing said switch when said first pulse
and said output signal are both produced;
voltage detecting means connected to said capacitor to produce a
control signal in accordance with the peak voltage developed across
said capacitor; and
means responsive to said control signal for adjusting said variable
impedance to adjust the period of said oscillator to a rate
approximating the rate at which said electronic switch is
closed.
2. In combination with a pair of sensing and stimulating electrodes
adapted to be connected in electrical circuit with living
tissue,
a resettable variable frequency oscillator comprising
a resistor, a capacitor and a source of voltage connected in
series;
a voltage responsive electron discharge device connected to said
capacitor and responsive to a predetermined voltage across the
capacitor to discharge it;
said resistor and capacitor having a time constant selected to
actuate said discharge device at intervals greater than the
intervals between normal heartbeats;
means connected to said discharge device and responsive to
discharge of said capacitor therethrough to apply a pulse to said
electrodes;
a variable impedance connected in parallel with said resistor;
means responsive to the average voltage across said capacitor for
adjusting said impedance in accordance with said average
voltage;
an electronic switch connected across said capacitor and effective
when closed to discharge said capacitor independently of the
voltage across it; and
means responsive to a pulse across said electrodes for closing said
switch.
3. The apparatus of claim 2, in which said variable impedance
comprises:
a second resistor, and a transistor having an emitter and a
collector connected in series with said second resistor, and a
base;
and in which said average voltage responsive means comprises peak
detecting means connected between said capacitor and said base for
controlling the impedance between said emitter and said
collector.
4. A demand pacer, comprising:
a pair of electrodes adapted to be connected in electrical circuit
with a heart,
amplifying means connected to said electrodes to produce an output
signal in response to a voltage on said electrodes having an
amplitude characteristic of an R wave,
a pulse generator connected to said amplifying means to produce a
pulse in response to each signal,
a first timing circuit comprising a first resistor, a battery, and
a first capacitor connected in series;
a first electronic switch connected across said first capacitor to
discharge it when said first switch is closed;
means responsive to the trailing edge of each pulse for closing
said switch to discharge said first capacitor;
voltage sensing means controlled by said capacitor for producing a
first control signal when the voltage across said capacitor reaches
a predetermined value;
gate means responsive to said pulses and said first control signals
to produce a second control signal when said first control signal
occurs during a pulse;
a second electronic switch controlled by said gate means and closed
by said second control signal;
a second capacitor connected across said switch;
a second resistor and a source of voltage connected in series with
said capacitor;
an electrically variable impedance connected in parallel with said
second resistor;
means responsive to the peak voltage developed across said second
capacitor for adjusting said variable impedance;
a voltage responsive electronic discharge device connected across
said capacitor to discharge it in response to a predetermined
voltage across the capacitor, and means responsive to the discharge
of said capacitor to apply a pulse to said electrodes of sufficient
amplitude to cause said amplifying means to produce an output
signal.
5. A variable rate demand pacer, comprising:
a pair of electrodes adapted to be connected in electrical circuit
with living tissue;
pulse generating means connected to said electrodes for producing a
pulse each time a signal above a predetermined amplitude appears on
said electrodes;
a resettable relaxation oscillator having an adjustable period
connected to said electrodes;
means controlled by said pulse generating means and responsive to
said pulses to reset said oscillator;
means responsive to the rate at which said pulses are produced for
adjusting the period of said oscillator;
said oscillator comprising:
a source of voltage, a resistor, and a capacitor connected in
series;
an electron discharge device connected across said capacitor and
responsive to a predetermined voltage across the capacitor to
discharge the capacitor;
an electronic switch connected across said capacitor and effective
when closed to discharge the capacitor and thereby reset the
oscillator; and
a variable impedance connected in parallel with said resistor;
and
in which said rate responsive means comprises peak detecting means
responsive to the peak voltage reached by said capacitor prior to
discharge for producing a control signal, and means responsive to
said control signal for adjusting said variable impedance.
6. A cardiac pacer normally responsive to natural heart stimulating
pulses in its operation and adapted to provide artificial heart
stimulating pulses in the event of the absence of a natural pulse,
comprising:
means for receiving natural heart stimulating pulses;
means for generating an artificial heart stimulating pulse;
means responsive to said stimulating pulses for controlling said
artificial pulse generating means to produce an artificial pulse
only after a variable time interval, after the last said
stimulating pulse;
means responsive to a plurality of previously naturally occurring
pulses for defining said variable time interval as a function of
said plurality of previously occurring natural pulses; and
means for coupling artificial stimulating pulses to a natural
heart.
7. The The pacer of claim 6, wherein:
said previously naturally occurring pulses responsive means
includes means for storing a selected number of the last received
natural pulses and producing a signal which is a function of the
average interval between said selected pulses.
8. The pacer of claim 6, wherein:
said artificial pulse generating means tends to provide artificial
pulses in sequence and said means for controlling said artificial
pulse generating means includes means inhibiting said artificial
pulse generating means from producing an artificial pulse during
said variable time interval after a received natural pulse.
9. The pacer of claim 6, wherein:
said artificial pulse generating means includes a resettable,
variable rate, pulse generator.
10. The pacer of claim 7, wherein:
said artificial pulse generator is operative selectively to supply
artificial pulses in sequence at controllable intervals, said
controllable intervals, at least initially, being determined as a
function of the most recent of a predetermined sequence of natural
heart stimulating pulses.
11. The pacer of claim 6, wherein:
said artificial pulse generator includes a resettable variable
frequency relaxation oscillator, comprising:
a source of voltage, a variable impedance and a capacitor connected
in series;
an electronic switch connected in parallel with said capacitor and
effective to discharge said capacitor when closed;
means responsive to the rate of discharge of said capacitor for
adjusting said variable impedance; and
a voltage responsive electron discharge device connected in
parallel with said capacitor and responsive to a predetermined
voltage across the capacitor to discharge it.
Description
My invention relates to cardiac pacers, and particularly to a novel
cardiac pacer with which the rate at which stimulating pulses is
produced is determined by the user so long as natural pulses are at
least intermittently produced. The term stimulating pulses as used
herein is in accordance with well-known terminology meaning that
the stimulating pulse functions as a stimulus to evoke the
characteristic physiologic activity of a nerve or muscle, more
particularly as used herein, to the heart. The stimulating pulse
may be either naturally derived or artificially produced.
Implantable cardiac pacers have been developed for the purpose of
supplying stimulating pulses to actuate the heart when the body
mechanism fails to provide such pulses. One such pacer comprises a
pair of electrodes adapted to be connected in electrical circuit
with the myocardium, and a fixed rate relaxation oscillator
connected to the electrodes to supply stimulating pulses to the
heart at a rate lower than the normally expected rate of natural R
waves. The apparatus generally comprises means responsive to the
signals appearing on the electrodes for resetting the oscillator to
inhibit its operation when natural pulses occur at a normal rate.
Such apparatus is reasonably well suited for bed patients whose
heart rates may be expected to be more or less constant. However,
for ambulatory patients, there is the problem that the normal heart
rate varies considerably depending upon the level of activity of
the patient, as well as on emotional and digestive factors. Should
an episode of heart block occur in a patient whose heart was
beaking rapidly, the transition to the relatively low rate
necessary with a fixed rate pacer could have unpleasant and
possibly dangerous consequences. The object of my invention is to
improve the fidelity with which pacers supplement the action of the
natural heart.
Briefly, the above and other objects of my invention are attained
by a novel pacer in which a variable frequency, resettable
relaxation oscillator is employed. Heart sensing and stimulating
electrodes are provided for connection to the body of the host. The
output terminals of the oscillator are connected to these
electrodes. A pulse generator is connected to the electrodes to
produce a control pulse each time an electrical signal of
sufficient magnitude appears across the electrodes. Preferably, a
pulse rate measuring circuit is provided that is controlled by the
pulse generator to produce a signal indicative of the rate at which
such pulses occur on the electrodes. The oscillator is reset by
signals produced when both the pulse generator produces an output
pulse and the pulse rate measuring circuit indicates that the rate
of pulse production is within a predetermined range. A timing
circuit is provided which measures the rate at which the oscillator
is reset, and adjusts the frequency of the oscillator to correspond
with that rate. If natural heart pulses fail to occur over a
relatively long period, the oscillator rate falls to a minimum rate
that is selected to be below the rate at which normal pulses would
occur. The pulse rate measuring circuit is provided to protect
against interference from stray signals that may be coupled into
the circuits. Thus, in the absence of any pulses, either of noise
or of natural physiological origin, the apparatus of my invention
functions as a conventional demand pacer. However, in the presence
of intermittent pulses or only occasional stoppages, the pacer acts
to supplement the natural heart at a rate determined by the
patient's needs.
The manner in which the apparatus of my invention is constructed,
and its mode of operation, will best be understood in the light of
the following detailed description, together with the accompanying
drawings, of a preferred embodiment thereof. In the drawings;
FIG. 1 is a schematic wiring diagram of a variable rate pacer in
accordance with my invention;
FIG. 2 comprises a composite graph of waveforms encountered in the
operation of my invention;
FIG. 3 is a composite graph illustrating other waveforms
encountered in the operation of the apparatus of my invention;
and
FIG. 4 is a composite graph of waveforms illustrating still further
features of the operation of the apparatus of my invention.
Referring now to FIG. 1, I have shown a cardiac pacer incorporating
a pair of electrodes 1 and 2. One of the electrodes is to be
connected to the myocardium of the patient. The other may be
connected to any convenient location elsewhere in the body to serve
as a reference and return electrode. The electrode 2 has been shown
at a reference ground potential, for convenience of exposition, and
not to imply any particular requirement of the circuit. The
electrodes 1 and 2 serve to detect naturally occurring heart waves,
and at times to apply stimulating electrical pulses to the heart to
substitute for missing natural heart signals.
Referring to FIGS. 2a, naturally occurring heart waves have the
general waveform shown, and are divided for purposes of description
and analysis into the P, Q, R, S and T components familiar to those
skilled in the art. Of these, the R wave is normally of
considerably higher amplitude, and is selected for purposes of
marking the interval between beats and to time the application of
stimulating substitute pulses.
Returning to FIG. 1, the electrodes 1 and 2 are connected to the
input terminals of a conventional amplifying and limiting network
A1 which may comprise conventional circuits for detecting the R
waves selectively, and producing corresponding pulses H as
indicated in FIG. 2b.
Referring again to FIG. 1, the pulse produced by the amplifier A1
serves to trigger a conventional one-shot multivibrator OS and
cause it to produce an output pulse of predetermined duration. The
pulse so produced is assumed to be a positive, rectangular
pulse.
The active output terminal of the one-shot multivibrator OS is
connected to one input terminal of a conventional AND gate G that
is arranged to produce a positive output signal when and only when
two positive signals are applied to its input terminals. The output
terminal of the multivibrator OS is also connected through a
capacitor C1 to the base of a conventional pnp transistor Q1 that
serves as an electronic switch.
The collector of the transistor Q1 is returned to ground through a
resistor R1. The emitter of the transistor Q1 is connected to
ground through a capacitor C2, and through a resistor R2 to the
positive terminal of battery B.
The resistor R2 and the capacitor C2 comprise a timing circuit
producing a maximum voltage across the capacitor C2 that is a
function of the time between discharges of that capacitor. When the
transistor Q1 is biased into conduction, the capacitor C2 is
discharged through the transistor Q1 and the resistor R1. The
resistor R1 is selected to be small relative to the resistor R2; it
is included merely to protect the transistor Q1 against excessive
currents. As indicated, the transistor Q1 is turned on at the
trailing edge of each pulse produced by the multivibrator OS.
The voltage across the capacitor C2 is compared with a reference
voltage developed across a resistor R4. The resistor R4 is
connected between ground and one terminal of a resistor R3 that has
its other terminal connected to the positive terminal of the
battery B.
The junction between the resistor R2 and the capacitor C2 is
connected to the non-inverting input terminal of a conventional
operational amplifier A2. The junction of the resistors R3 and R4
is connected to the inverting input terminal of the amplifier A2.
The amplifier A2 is provided with a conventional feedback resistor
R5.
The output terminal of the amplifier A2 is connected to the second
input terminal of the AND gate G. The amplifier A2 will produce a
positive output signal to enable the gate G when the voltage across
the capacitor C2 has reached a predetermined value.
The output terminal of the gate G is connected to the base of an
npn transistor Q2 that serves as an electronic switch. The emitter
of the transistor Q2 is connected to ground. The collector of the
transistor Q2 is returned to the positive terminal of the battery B
through a transistor R6. The collector of the transistor Q2 is
returned to ground through capacitor C3 and a resistor R7 connected
in series. The resistor R7 is selected to be small with respect to
the resistor R6, and serves primarily to protect the transistor Q2
against excessive currents when it is biased into conduction to
discharge capacitor C3.
The collector of the transistor Q2 is connected to the emitter of a
unijunction transistor Q4 connected as a relaxation oscillator. The
base of the transistor Q4 that is more remote from the emitter,
commonly termed "base one" is returned to ground through a resistor
R14. The other base, commonly termed "base two," is connected to
the positive terminal of the battery B through a resistor R13.
The minimum rate of oscillation of the relaxation oscillator
comprising the transistor Q4 is determined by the time constant of
the resistor R6 and the capacitor C3. The resistor R6 is connected
in shunt with a second circuit for modifying the period of the
oscillator. That circuit extends from the emitter of the
unijunction transistor Q4 through the collector-to-emitter path of
a pnp transistor Q3, and thence through a resistor R12 of the
positive terminal of the battery B. When the transistor Q3 is cut
off, the relaxation oscillator comprising the unijunction
transistor Q4 oscillates at the rate set by the resistor R6 and the
capacitor C3. When the transistor C3 is biased into conduction,
depending on the extent of the bias, more or less current flows
through the resistor R12 and increases the frequency of
oscillation.
A control circuit for adjusting the extent of conduction of the
transistor Q3 is responsive to the rate at which the transistor Q2
is turned on to discharge the capacitor C3. For that purpose, a
non-inverting amplifier A3 has its active input terminal connected
to the collector of the transistor Q2. The active output terminal
of the amplifier A3 is connected to a peak detecting circuit
comprising diode D1, and a capacitor C4 connected in parallel with
a resistor R10 between the cathode of the diode D1 and ground.
The cathode of the diode D1 is also connected to the non-inverting
input terminal of a conventional operational amplifier A4. The
inverting input terminal of the amplifier A4 is connected to the
junction of a pair of resistors R8 and R9. The other terminal of
the resistor R9 is connected to ground, and the other terminal of
the resistor R8 is connected to the positive terminal of the
battery B. The amplifier A4 is provided with a conventional
feedback resistor R11.
The active output terminal of the amplifier A4 is connected to the
base of the pnp transistor Q3. The positive potential across the
resistor R9 tends to produce a negative potential biasing the
transistor Q3 into conduction. A positive potential across the
capacitor C4 tends to resist this reference potential, and turn off
the transistor Q3. Thus, if the capacitor C4 is charged to a
sufficiently high voltage, the transistor Q3 will be cut off.
The voltage across the capacitor C4 will depend upon the period
between discharges of the capacitor C3. If it is discharged at the
minimum rate; i.e., under conditions when the transistor Q3 remains
cut off and the oscillator comprising the unijunction transistor Q4
oscillates at the minimum rate, the peak voltage reached across the
capacitor C3 will be relatively high, and the voltage across the
capacitor C4 will be high enough to keep the transistor Q3 turned
off. On the other hand, if the capacitor C3 is more frequently
discharged, a lower voltage will be reached across the capacitor
C4, and the transistor Q3 will conduct more heavily, increasing the
frequency of oscillation of the oscillator comprising the
transistor Q4.
Base one of the transistor Q4 is connected to the base of a npn
transistor Q5 that serves as an electronic switch. The emitter of
the transistor Q5 is connected to ground. The collector of the
transistor Q5 is returned to the positive terminal of the battery B
through a resistor R15. The collector of the transistor Q5 is
connected to the stimulating electrode 1 through a capacitor C5.
Thus, during the period between the time when the unijunction
transistor Q4 is fired, and the time that the capacitor C3 has
discharged sufficiently to cut off the transistor Q4, the collector
of the transistor Q5 will go essentially to ground potential and
thereby apply a rectangular negative pulse to the electrode 1.
The polarity of natural electrical waves produced in the myocardium
with respect to the ground connection provided elsewhere in the
body will determine whether it is the electrode 1, connected to the
collector of the transistor Q5, or the electrode 3, that is
actually connected to the myocardium. As that matter of polarity
will not trouble the myocardium. As that matter of polarity will
not trouble the artisan, it will be ignored in the following
discussion, and in the graphs of FIG. 2 through 4, where all pulses
have been indicated as positive to simplify the discussion.
The apparatus of FIG. 1 is preferably enclosed in a suitable
biologically compatible container for implantation. As will be
apparent to those skilled in the art, the circuits of FIG. 1 are
well adapted to construction by miniature circuit techniques
familiar to those skilled in the art. Thus, the pacer need occupy
but little space.
Having thus described the construction of the apparatus of my
invention, its operation will next be described in connection with
the diagrams of FIG. 2 through 4. As noted above in connection with
FIG. 2, the pulses H are produced by the amplifier A1 in response
to each naturally occurring R wave. Fig. 3a shows a series of such
pulses H, followed by a heart block episode in which no pulses are
produced. As illustrated in FIG. 3b, when a pulse H fails to occur,
a pulse S is produced a short time after it should have occurred.
Pulses S continue to be produced at a rate that is initially
slightly below the rate at which H pulses had been produced. That
rate gradually tapers to a minimum rate below the rate at which
natural pulses should be produced. When heart beats are resumed,
the pulses S will cease.
The details of the manner in which the apparatus in FIG. 1
functions to produce the operation illustrated in FIG. 3, together
with its mode of operation in the presence of noise, will next be
described in connection with FIG. 4.
Referring to FIG. 4a, I have shown two initial naturally produced
pulses H that are assumed to be at the end of a train of such
pulses occurring over a sufficiently long interval that the
operation of the apparatus in FIG. 1 has become stabilized. Prior
to the first of the pulses H shown in FIG. 4, the capacitor C2 has
been charging, as illustrated in FIG. 4b. When the first pulse H
occurs, the one-shot multivibrator OS is triggered to produce a
pulse as illustrated in FIG. 4d.
It is assumed that the capacitor C2 has charged to a value that
will overcome the reference voltage appearing across the resistor
R4, and thereby enable the gate G to produce a positive signal in
response to the pulse from the one-shot multivibrator OS. That
pulse will turn on the transistor Q2 and discharge the capacitor C3
through the resistor R7. The capacitor C3 will remain discharged
during the pulse produced by the multivibrator OS. At the trailing
edge of that pulse, the capacitor C2 will be discharged, as shown
in FIG. 4b. That capacitor will begin to charge again, and the
cycle will be repeated when the next pulse H is produced.
Next, it is assumed that a heart block episode occurs in which an R
wave does not occur within the current period of oscillation of the
multivibrator comprising the unijunction transistor Q3. The
capacitor C2 will continue to charge, and the capacitor C3 will be
charged until it reaches the unijunction firing potential, shown as
Vt in FIG. 4c.
During the charging of the capacitor C3, the time constant is
determined both by the resistor R6 and by the resistor R12 in
series with the conducting transistor Q3. The discharge of the
capacitor C3 thus occurs more rapidly than it would if the
transistor Q3 was cut off, and a stimulating pulse S is produced
after an interval longer than but close to the interval between
previously appearing pulses H. When the pulse thus is produced, it
will retrigger the one-shot multivibrator OS. At the trailing edge
of the multivibrator pulse, the capacitor C2 will be
discharged.
Since the interval between the last pulse H and the stimulating
pulse S was longer than the interval between the previous pulses H,
the peak detecting circuit, comprising the diode D1, the capacitor
C4 and the resistor R10, will apply a higher voltage to the
amplifier A4, reducing the current in the emitter-to-collector path
of the transistor Q3, and thereby lowering the rate at which the
capacitor C3 will charge. Accordingly, assuming no intervening
pulse H, the next pulse S produced by the circuit in FIG. 1 will
follow the previous pulse S by a somewhat longer interval than that
pulse followed the last H pulse. If succeeding pulses S continue to
be produced in the absence of pulses H, the interval would
gradually increase until the transistor Q3 was completely cut off,
whereupon the pulses S would continue to be produced at a fixed
rate.
FIG. 4a illustrates a noise pulse N occurring shortly after the
second pulse S. That pulse will trigger the one-shot multivibrator
OS and cause the capacitor C2 to be discharged before it reaches
the voltage Vg at which the gate G could be enabled. Accordingly,
the transistor Q2 will remain cut off the capacitor C3 will
continue to charge until it reaches the unijunction firing
potential Vt. Thus, the oscillator comprising the unijunction
transistor Q3 will continue to produce pulses at a rate declining
towards the minimum rate so long as either natural pulses H or
noise pulses N are produced at a rate greater than the maximum
expected rate for natural pulses.
While I have described my invention with respect to the details of
the preferred embodiment thereof, many changes and variations will
occur to those skilled in the art upon reading my description, and
such can obviously be made without departing from the scope of my
invention.
* * * * *