U.S. patent number 3,618,615 [Application Number 04/854,583] was granted by the patent office on 1971-11-09 for self checking cardiac pacemaker.
This patent grant is currently assigned to Medtronic, Inc.. Invention is credited to Wilson Greatbatch.
United States Patent |
3,618,615 |
Greatbatch |
November 9, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
SELF CHECKING CARDIAC PACEMAKER
Abstract
A cardiac pacer including a pair of output terminals, at least
one of which is adapted to be placed in operative contact with a
patient's heart, coupled to a pulse generator which includes an RC
timing circuit connected to a power supply. A signal responsive
circuit coupled to one of the output terminals provides a signal in
response to each ventricular beat of the patient's heart. The
capacitance of the timing circuit is periodically reduced by a
capacitor connected in series with the timing circuit and in
parallel with a field effect transistor controlled by a binary
counter connected to the pacer signal responsive circuit. The
resulting periodic test pulse of reduced width is utilized to
indicate marginal pacer operation and possible impending failure if
the patient's heart fails to respond to the test pulse.
Inventors: |
Greatbatch; Wilson (Clarence,
NY) |
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
|
Family
ID: |
25319091 |
Appl.
No.: |
04/854,583 |
Filed: |
September 2, 1969 |
Current U.S.
Class: |
607/28 |
Current CPC
Class: |
A61N
1/3708 (20130101) |
Current International
Class: |
A61N
1/362 (20060101); A61N 1/37 (20060101); A61n
001/36 () |
Field of
Search: |
;128/419P,421,422 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Claims
I claim:
1. A cardiac pacer comprising:
a. a pulse generator;
b. a pair of output terminals coupled to said pulse generator, at
least one of which is adapted to be operatively coupled to a
patient's heart;
c. a power supply;
d. said pulse generator including capacitive timing means connected
to said power supply whereby said pulse generator provides output
pulses at a rate and for a duration depending upon the magnitude of
the capacitance in said timing means; and
e. means coupled to one of said terminals for sensing each
ventricular beat of the heart and for reducing the capacitance of
said timing means by a predetermined amount and for a predetermined
time in response to the occurrence of a predetermined number of
ventricular beats.
2. A cardiac pacer as defined in claim 1 wherein said capacitive
timing means comprises an RC circuit.
3. A cardiac pacer as defined in claim 1 wherein said means for
reducing the capacitance of said timing means comprises:
a. counting means having an input and an output, said counting
means being operative to provide an output signal in response to
the occurrence of a predetermined number of signals applied to the
input thereof;
b. means connected to the input of said counting means for
providing a signal thereto in response to each ventricular beat of
the heart;
c. a controlled, normally closed switch connected in controlled
relation to the output of said counting means; and
d. a capacitor connected in series with said capacitive timing
means and in parallel with said controlled switch.
4. A cardiac pacer as defined in claim 3 wherein said controlled
switch comprises a field effect transistor, the gate terminal of
which is connected to the output of said counting means and the
source and drain terminals of which are connected across said
capacitor.
5. A cardiac pacer as defined in claim 3 wherein said means
connected to the input of said counting means includes means for
converting a monopolar signal of either polarity into a bipolar
signal regardless of the polarity of the monopolar signal.
6. A cardiac pacer comprising:
a. a pulse generator;
b. a pair of output terminals coupled to said pulse generator, at
least one of which is adapted to be operatively coupled to a
patient's heart;
c. a power supply;
d. said pulse generator including capacitive timing means connected
to said power supply whereby said pulse generator provides output
pulses at a rate and for a duration depending upon the magnitude of
the capacitance in said timing means;
e. signal-responsive means coupled to one of said terminals for
providing an output signal in response to each ventricular beat of
the heart; and
f. means coupled to said signal-responsive means for reducing the
capacitance of said timing means by a predetermined amount and for
a predetermined time in response to the occurrence of a
predetermined number of output signals.
7. A cardiac pacer as defined in claim 6 wherein said capacitive
timing means comprises an RC circuit.
8. A cardiac pacer as defined in claim 6 wherein said means for
reducing the capacitance of said timing means comprises;
a. counting means having an input connected to the output of said
signal responsive means and an output, said counting means being
operative to provide an output signal in response to the occurrence
of a predetermined number of signals applied to the input
thereof;
b. a controlled, normally closed switch connected in controlled
relation to the output of said counting means; and
c. a capacitor connected in series with said capacitive timing
means and in parallel with said controlled switch.
9. A cardiac pacer as defined in claim 8 wherein said controlled
switch comprises a field effect transistor, the gate terminal of
which is connected to the output of said counting means and the
source and drain terminals of which are connected across said
capacitor.
10. A cardiac pacer as defined in claim 6 wherein said signal
responsive means includes means for converting a monopolar signal
of either polarity into a bipolar signal regardless of the polarity
of the monopolar signal.
11. A cardiac pacer as defined in claim 10 wherein said converting
means comprises differentiating means.
12. A cardiac pacer as defined in claim 6 further including means
connecting the output of said signal responsive means to said pulse
generator for inhibiting said pulse generator for a predetermined
time in response to a ventricular beat of the heart.
13. A cardiac pacer comprising:
a. a pulse generator adapted to be connected to a power supply and
to be coupled to a pair of electrodes, at least one of which is
adapted to be placed in operative contact with a patient's
heart;
b. said pulse generator including capacitive timing means whereby
output pulses are provided at a rate and for a duration depending
upon the magnitude of the capacitance in said timing means;
c. signal-responsive means adapted to be coupled to one of the
electrodes for providing an output signal in response to
ventricular signals of either polarity in the patient's heart;
d. counting means having an input connected to said
signal-responsive means and operative to provide an output signal
in response to the occurrence of a predetermined number of signals
applied to the input thereof;
e. a normally closed semiconductor switch connected in controlled
relation to the output of said counting means; and
f. a capacitor connected in series with said capacitive timing
means and in parallel with said semiconductor switch.
14. A cardiac pacer as defined in claim 13 wherein said capacitive
timing means comprises an RC circuit.
15. A cardiac pacer as defined in claim 13 wherein said
semiconductor switch comprises a field effect transistor, the gate
terminal of which is connected to the output of said counting means
and the source and drain terminals of which are connected across
said capacitor.
Description
BACKGROUND OF THE INVENTION
This invention relates to electronic cardiac pacer and, more
particularly, to a cardiac pacer which may or may not be
implantable within the human body and which is inherently
self-checking.
Artificial cardiac pacer of the implantable variety have come into
widespread use in recent times for patients suffering from complete
heartblock. The acceptance of these pacers by the medical
profession has increased the life expectancy of these patients from
a 50 percent probability of 1 year to nearly the life expectancy of
physically comparable humans not suffering from the same heart
disorder.
While artificial stimulation thus has been successful as a solution
to this primary medical problem, its characteristics have given
rise to a number of secondary problems. A significant secondary
problem is determining when the implanted, battery-operated pacer
should be replaced. A number of solutions to this problem have been
proposed, one being replacement at 30 -month intervals and thus
accepting a 10 percent risk or probability of premature pacer
failure. Another proposed solution is to establish pacer "clinics"
where photographic analysis techniques are used to detect imminent
failure.
None of these proposed solutions is entirely satisfactory for
detecting, simply and positively, a degradation of pacer system
performance. The risk of undetected premature failure associated
with periodic replacement at intervals of about 30 months is
obviously undesirable. Photoanalysis techniques are complicated,
not positive in detection, and obviously would not be readily
available to physician and patient on short notice but rather, as
mentioned previously, would be available only at special
clinics.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide
artificial electrical stimulation of a human organ, such as the
heart, in a way permitting ready evaluation, without surgery, of
the stimulator power supply and also of the condition and placement
of the stimulating electrode.
It is a further object of the present invention to provide an
implanted medical electronic pulse generator which is inherently
self-checking and thus permits detection of impending generator
failure without surgery and with simple measurement means.
It is a more particular object of the present invention to provide
such a medical electronic pulse generator which can be employed in
an implanted or externally worn artificial cardiac pacer of either
the demand, synchronous or nonsynchronous types or combinations
thereof.
The present invention provides an artificial cardiac pacemaker
wherein the pulse generator thereof at regular intervals produces a
stimulating impulse of significantly lower energy than the other
impulses. In addition, the lowered energy pulse occurs at a time in
the operating cycle earlier than that at which the regular
stimulating impulses occur so as to avoid confusion when the pacer
is operating in a demand mode. If the heart responds to the reduced
energy stimulating pulse, an adequate safety factor remains, but if
the heart does not respond, e.g., no beat is detected in response
to the test pulse, marginal operation and possible imminent failure
is ascertained.
The foregoing and additional advantages and characterizing features
of the present invention will become clearly apparent upon a
reading of the ensuing detailed description of an illustrative
embodiment thereof together with the included drawing depicting the
same.
BRIEF DESCRIPTION OF THE DRAWING FIGURE
The single drawing FIGURE is a schematic diagram of an artificial
cardiac pacer constructed in accordance with the present
invention.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
An artificial cardiac pacer is shown schematically in the drawing
and comprises a pulse generator, designated generally at 10, and a
pair of pacemaker output terminals or electrodes 11 and 12 at least
one of which is surgically placed in contact with the heart of a
patient. In particular, electrode 11 would be placed surgically in
contact with the ventricle of the patient's heart, and electrode
12, which can function as an indifferent or reference electrode,
could be subcutaneously implanted at another part of the patient's
body. Alternatively, electrode 12 also can be placed in contact
with the patient's heart. Electrodes 11 and 12 are connected to
pulse generator 10 by leads or wires 13 and 14, respectively, which
leads would be enveloped by a moistureproof and human body
reaction-free material such as silicone rubber or suitable plastic.
A power supply for the pacer is included in the form of battery 15,
and a capacitor 16 is connected across battery 15 for the purpose
of reducing the peak current drain on the battery and thus
increasing the useful life thereof.
Pulse generator 10 includes a transformer 17 having secondary and
primary windings 18 and 19, respectively. The positive terminal of
battery 15 is connected through a resistor 20 and series-connected
capacitors 21 and 22 to one end or terminal of transformer
secondary winding 18. Resistor 20 and capacitor 21 comprise
capacitive timing means in the form of an RC timing circuit for
pulse generator 10, capacitor 22 being normally shunted out of the
circuit, the operation of which will be described in more detail
presently. Pulse generator 10 also includes a semiconductor
amplifier in the form of transistor 23 having base, emitter and
collector terminals 24, 25 and 26, respectively. Base terminal 24
is connected to one terminal of a biasing resistor 27 which
terminal also is connected by a lead 28 to the other end or
terminal of transformer secondary winding 18. Emitter terminal 25
is connected directly to the other terminal of biasing resistor 27.
Collector terminal 26 is connected to one terminal of a resistor 29
and is coupled through a capacitor 30 and resistor 31 to lead 13
and, hence, terminal or electrode 11. The other terminal of
resistor 29 is connected to lead 14 and thus to electrode 12, and
is connected to one end or terminal of transformer primary winding
19 and through a lead 32 to the positive terminal of battery 15.
The negative terminal of battery 15 is connected to ground.
Pulse generator 10 finally includes a semiconductor oscillator in
the form of transistor 33 having collector, base and emitter
terminals 34, 35 and 36 respectively. Collector terminal 34 is
connected to the other end or terminal of transformer primary
winding 19, and base terminal 35 is connected through a lead 37 to
the junction of resistor 20 and capacitor 21. Emitter terminal 36
is connected to base terminal 24 of amplifier transistor 23.
The artificial pacer provided by the present invention further
comprises signal responsive means, designated generally at 40 in
the drawing, for providing an output signal in response to
ventricular beats of the heart. More specifically, signal
responsive means 40 provides a bipolar output signal in response to
a monopolar signal of either polarity appearing at electrode 11 and
on line 13 indicative of each ventricular beat of the heart.
Signal responsive means 40 includes a first semiconductor amplifier
in the form of transistor 41 having base, emitter and collector
terminals 42, 43 and 44, respectively. Base terminal 42 is
connected by a lead 45 to a positive bias voltage tap of battery 15
which preferably is taken at half the maximum battery voltage
whereby transistor 41 is in common base configuration. Emitter
terminal 43 is connected to one terminal of a coupling capacitor
46, the other terminal of which is connected to lead 13 and thus
electrode 11. Emitter terminal 43 of transistor 41 also is
connected through a resistor 47 and a lead 48 to the positive
terminal of battery 15. Collector terminal 44 is connected through
a resistor 49 to ground. Signal responsive means 40 further
includes a second semiconductor amplifier in the form of transistor
50 having base, emitter and collector terminals 51, 52 and 53,
respectively. Base terminal 51 is connected to collector terminal
44 of amplifier transistor 41, and emitter terminal 52 is connected
to ground through the parallel combination of resistor 54 and
capacitor 55. Collector terminal 53 of transistor 50 is connected
through a resistor 56 to lead 48 and hence the positive terminal of
battery 15.
Signal responsive means 40 also includes a first semiconductor
switch in the form of transistor 57 having base, emitter and
collector terminals 58, 59 and 60, respectively. Base terminal 58
is coupled through a capacitor 61 to collector terminal 53 of
transistor 50 and is connected through a resistor 62 to lead 48 and
hence the positive terminal of battery 15. Emitter terminal 59 is
connected directly to lead 48, and collector terminal 60 is
connected to ground through a resistor 63. Signal responsive means
40 finally comprises a second semiconductor switch in the form of
transistor 64 having base, emitter and collector terminals 65, 66
and 67, respectively. Base terminal 65 is connected to collector
terminal 60 of transistor 57, and emitter terminal 66 is connected
directly to ground. Collector terminal 67 of transistor 64 is
connected by a lead 68 to base terminal 35 of oscillator transistor
33.
The artificial pacer provided by the present invention finally
comprises means coupled to signal responsive means 40 for reducing
the capacitance of the capacitive timing means or RC timing circuit
in pulse generator 10 by a predetermined amount and after a
predetermined time interval in response to the occurrence of a
given number of output signals provided by means 40. The means for
reducing the capacitance includes a counting means in the form of
an 8:1 binary counter 70 having input and output terminals 71 and
72, respectively. Counter 70 functions to provide an output pulse
on terminal 72 in response to the occurrence of a predetermined
number of input signals, in this particular example eight,
appearing on input terminal 71. Inasmuch as binary counters such as
counter 70 are well understood by those skilled in the electronics
art and are readily available commercially, a detailed description
of the construction and operation of counter 70 is believed
unnecessary. Moreover, such counters are now available in
integrated circuit form and thus occupy an extremely small volume
so as to make feasible the inclusion of such a binary counter in an
implantable cardiac pacer. The input terminal 71 of counter 70 is
connected by a lead 73 to the output of signal responsive means 40,
in particular to the collector terminal 67 of switching transistor
64. As a result, counter 70 will provide an output signal on
terminal 72 after a predetermined number, here eight, of "R" waves
produced in the heart and sensed by means 40.
Capacitor 22 in signal generating means 10 is connected in parallel
with a normally closed, controlled switch in the form of field
effect transistor 74 having control electrode 75 and source and
drain electrodes 76 and 77, respectively. Control electrode 75 is
connected to output terminal 72 of counter 70 by a lead 78, and
source and drain electrodes 76 and 77 are connected by leads 79 and
80, respectively, to the terminals of capacitor 22. A high megohm
resistance 81 can be connected across capacitor 22 to insure an
equitable long term distribution of total charge between the two
capacitors 21 and 22.
In operation, pulse generator 10 of the pacer provided by this
invention, when no natural cardiac activity is present, produces
regular periodic pulses at a rate of approximately one per second
between electrodes 11 and 12 at least one of which is surgically
placed in contact with the heart of a patient. During this
free-running or nonsynchronous operation, a sawtooth waveform
exists at base terminal 35 of oscillator transistor 33 which
waveform falls quickly to nearly zero volts immediately upon
cessation of the stimulating pulse, which as a duration of about 2
milliseconds. The waveform at base terminal 35 then rises
exponentially in about 1 second to a voltage level, for example 0.6
volts, sufficient to drive transistor 33 into conduction and
initiate another pacer stimulating pulse. In particular, current
flows from battery 15 through resistor 20 and charges capacitor 21,
capacitor 22 being shorted out of the circuit by the normally
closed switch in the form of field effect transistor 74. The time
interval required for the voltage at base terminal 35 of transistor
33 to rise to the critical value thus is determined by the values
of resistor 20 and capacitor 21 which constitute an RC timing
circuit for pulse generator 10.
When transistor 33 turns on, current flows from battery 15, through
transformer primary winding 19, and through resistor 27. The flow
of current through winding 19 induces a flow of current in
secondary winding 18 which, in turn, elevates the voltage on base
terminal 24 of transistor 23 to a level sufficient to turn on
transistor 23. The voltage on collector terminal 26, as a result,
drops to nearly ground potential, and since the voltage across
capacitor 30 cannot change instantaneously, electrode 11 is driven
negatively to a value nearly equal to the supply voltage.
The induced flow of current in transformer winding 18 causes
capacitor 21 to discharge and recharge in the opposite direction.
Transformer 17 saturates, the field in winding 19 begins to
collapse, which immediately reverses the polarity and direction of
current in winding 18. This, in turn, immediately drives
transistors 23 and 33 into cutoff and ends the pacer stimulating
pulse. The reverse charge on capacitor 21 holds transistor 33
cutoff until the charge is reversed again by current through
resistor 20. It is apparent, therefore, that both the duration or
width as well as the time interval between output pulses produced
by pulse generator 10 are determined in part by the magnitudes of
capacitor 21 and resistor 20 which constitute an RC timing circuit
for pulse generator 10. It should be apparent also, that had
capacitor 22 not been switched out of the circuit, the fact that it
is connected in series between transformer winding 18 and capacitor
21 would change the duration of and time interval between pacer
stimulating pulses. The role of capacitor 22 will be explained
hereafter.
Capacitor 16 is charged slowly by battery 15 in the time interval
between pulses generated by transistor 33, the pulsed saturation of
transistor 23 rapidly discharging capacitor 16 over a very short
interval to provide large amplitude peak pulse currents for
electrodes 11, 12 through capacitor 30. Capacitor 30 acts as a
coupling capacitor which charges in one polarity sense by the pacer
pulses and discharges between pacer pulses to provide a reversed
current. It is believed that such reversal of current through the
patient's heart is beneficial in that it prevents any harmful
effects which might result from the passage of a unidirectional
current through the heart.
Signal responsive means 40 responds to the natural "R" voltage wave
of a normal heartbeat, when present, to inhibit or disable pulse
generator 10 so as to prevent the occurrence of generated pacer
electronic pulses upon electrodes 11 and 12 when the heart is
functioning normally. The means 40 is designed to respond to "R"
waves of either polarity because some patients can generate
successive "R" waves of opposite polarity.
Between generated pacer output pulses, a natural heartbeat, if it
occurred, would generate an "R" wave of from about 2 to 20
millivolts which would be conducted back over the electrodes 11, 12
and leads 13, 14 to signal responsive means 40. Transistors 41 and
50, together with associated circuitry, amplify the "R" wave
voltage signal of either polarity appearing on electrodes 11, 12,
as produced by a normal heartbeat, and transform the "R" wave
signal into a bipolar signal by mathematical differentiation. The
differentiation is performed by capacitor 46 and resistor 47, by
capacitor 55 and resistor 54, as well as by capacitor 61 and
resistor 62. The particular pair or pairs of resistor and capacitor
which perform the differentiation will depend upon the values
selected for the components, as is readily apparent to those
skilled in the art.
Transistors 57 and 64 are switching transistors for selectively
disabling or inhibiting oscillator transistor 33 whenever a natural
"R" wave appears in the normal heart. In other words, the natural
and normal "R" wave will be amplified by transistor 41, again by
transistor 50 to a bipolar signal with adequate amplitude in each
polarity portion so that transistor switch 57, which is normally
cutoff, is driven into conduction for 20 milliseconds or more, by
that portion of the signal having the proper polarity for
delivering a saturation pulse to switch on transistor 64.
Transistor 64, functioning as an on-off switch, provides a
low-impedance path between collector 67 and emitter 66, the latter
being grounded. This, in turn, grounds base terminal 35 of
oscillator transistor 33 long enough to discharge capacitor 21 and
thereby reinitiate the pacer pulse generating cycle. In other
words, switching transistor 64, by discharging capacitor 21,
prevents the voltage on base terminal 35 from reaching the level
sufficient to forward bias the base-emitter junction of transistor
33 which level in this particular example was previously mentioned
to be about 0.6 volts. As a result, pulse generator 10 cannot
provide an output or stimulating pulse until about 1 second
following the last previous natural heartbeat. One second is
approximately the normal frequency rate of oscillator transistor 33
under control of the R-C timing network of resistor 20 and
capacitor 21.
Signal responsive means 40 will, of course, provide an output
signal in response to a stimulated "R" wave occurring as a result
of the operation of pulse generator 10. This, however, does not
have the effect of inhibiting pulse generator 10 because amplifier
40 saturates for about 100- 300 milliseconds following a pacer
pulse, providing a refractory period during which amplifier 40 is
ineffective. In other words, actual inhibiting of pulse generator
10 by a signal from means 40 occurs only when a natural heartbeat
follows a stimulated heartbeat at a time less than the interval
determined by the R-C timing network but greater than the
refractory period.
In accordance with the present invention, each output signal
provided by signal responsive means 40, in addition to being
applied to base terminal 35 of oscillator transistor 33, is applied
to the input terminal 71 of binary counter 70. After a
predetermined number of signals each indicative of either a natural
or a stimulated heartbeat, counter 70 provides an output signal
which is impressed upon base terminal 75 of field effect transistor
74 which, in turn, responds by behaving like an open semiconductor
switch.
Capacitor 22 then is no longer shorted out of the RC timing circuit
of pulse generator 10. Instead, capacitor 22 now is connected in
series with resistor 20, capacitor 21 and transformer winding 18.
As a result, the net capacitance of the RC timing circuit is
reduced. This reduction in capacitance has a two-fold effect on the
next stimulating pulse provided by pulse generator 10. One is a
reduction in pulse width which should be apparent as a result of
the earlier discussion wherein it was explained how the stimulating
pulse width is dependent upon the magnitude of the capacitance
connected between resistor 20 and transformer winding 18. The
second effect is this reduced-energy pulse occurs at an earlier
time relative to the occurrence of the regular pulses provided by
generator 10.
This pacer impulse of significantly shorter duration occurring
periodically, such as after every eight regular stimulating
impulses, can be utilized to evaluate the operation of the
artificial pacer and its power supply. If the heart can respond to
the test pulse, then an adequate safety factor remains and the
implanted pacer is left in place. If the heart does not respond to
the lower energy test pulse, this indicates that the artificial
pacer is operating in the marginal range and that failure may be
imminent. The surgeon then will consider either replacement of the
system or correction of the defect. Satisfactory operation of this
testing procedure can be obtained if an energy reduction in the
range of about 30 to 50 percent is made in the test pulse relative
to a regular stimulating pulse.
One significant advantage is that the testing procedure in the case
of a patient in complete block is quite simple and can be done
anytime, anywhere and even by the patient himself. The patient's
pulse is felt by hand, either by the doctor or by the patient
himself. The manually sensed heartbeats are counted and if a beat
is missing at a constant rate, as in this example every eighth
beat, marginal operation of the pacer is indicated. Heart beats
could, of course, be sensed by an electrocardiograph and a count
taken therefrom.
Another advantage of the arrangement of the present invention is
that it is readily adaptable to pacers of the demand type which
type has achieved widespread use. A problem in determining whether
or not a pacemaking system of the demand type is in good operating
condition exists because an inserted test pulse can be confused
with a normal heartbeat and thus be inhibited. The arrangement of
the present invention, however, advantageously solves this problem
by causing the lowered energy pulse to override the demand system
and to be applied to the heart at a safe time regardless of whether
the heart is in block or is beating normally in sinus rhythm. In
other words, the reduced-energy test pulse occurs at an earlier
time relative to the occurrence of the regular pulse provided by
generator 10. To accomplish this, "R" waves both natural and
stimulated are counted and the lowered energy test pulse is applied
to the heart at a short but safe time delay after the last "R" wave
in the counted train. The delay time must be short enough so that
the test pulse occurs before the next possible natural "R" wave but
long enough to occur well after the "T" wave which typically
follows the "R" wave by about 0.3 seconds. A safe delay time would
be 500 milliseconds.
While the arrangement of the present invention advantageously
prevents confusion of a test pulse with a normal heartbeat in a
demand-type pacer, it can of course be employed in a pacer of the
nondemand or free-running type. In the latter case, the signal
responsive means 40 would be replaced by a simpler means connecting
one of the pacer terminals to the input of counter 70 for providing
a signal thereto in response to each ventricular beat of the
heart.
Since a synchronous pacer is always in operation at its
nonsynchronous rate or faster, the arrangement of the present
invention is applicable to a synchronous pacer in the same manner
as it is applicable to a nonsynchronous pacer.
Another advantage of the present invention is the simplicity of the
means for accomplishing the stated objectives. A periodic reduction
in the capacity of the timing capacitor is a simple and effective
means of accomplishing at the same time both a reduction in
duration and, hence, energy of the test pulse and a reduction in
delay time after the last preceding "R" wave. The fact that such
reduction is applied periodically to one of the normal stimulating
pulses in the train rather than inserting an extra pulse renders
unnecessary the provision of any extra or additional
pulse-generating means.
Power for operating counter 70 and for biasing transistor 74 can be
obtained from a separate battery or from battery 15 as by a
connection to lead 48. If the latter alternative is selected,
battery 15 may have to be of a slightly larger rating. Minor
additional biasing arrangements may be necessary to insure that the
output from counter 70 is at a level sufficient to activate
transistor 74. In addition, the output of counter 70 should be
applied to the input of transistor 74 only for a time sufficient to
reduce the capacity of the R-C timing circuit and thereby to insert
a reduced energy pulse between two normally occurring pulses from
generator 10. The value of capacitor 22 relative to the value of
capacitor 21 is selected so as to provide the previously mentioned
energy reduction of 30 to 50 percent in the test pulse as compared
to the regular stimulating impulses. To be implanted, the entire
pacer, including its battery which can be rechargeable, is encased
in an envelope of a moistureproof and human body reaction-free
material such as silicone rubber or suitable plastic so as to
permit long term implantation within the human body.
It is thus apparent that the present invention accomplishes its
intended objects. While the present invention has been described in
detail, this has been done by way of illustration without thought
of limitation.
* * * * *