U.S. patent number 3,557,796 [Application Number 04/805,714] was granted by the patent office on 1971-01-26 for digital counter driven pacer.
This patent grant is currently assigned to Cordis Corporation. Invention is credited to Gomer L. Davies, John W. Keller, Jr., Reese S. Terry, Jr..
United States Patent |
3,557,796 |
Keller, Jr. , et
al. |
January 26, 1971 |
DIGITAL COUNTER DRIVEN PACER
Abstract
The cardiac pacer disclosed herein employs a digital counter
driven by a relatively high frequency oscillator to time intervals
which simulate various cardiac functions. When allowed to cycle
repetitively, the counter provides a relatively slow, fixed rate
mode of operation. However, the counter may be reset under certain
conditions by ventricular signals to provide a demand mode of
operation and by atrial signals under certain other conditions to
provide a so-called synchronized mode of operation. Noise rejection
circuitry is also disclosed.
Inventors: |
Keller, Jr.; John W. (Miami,
FL), Terry, Jr.; Reese S. (Miami, FL), Davies; Gomer
L. (Fort Lauderdale, FL) |
Assignee: |
Cordis Corporation (Miami,
FL)
|
Family
ID: |
25192314 |
Appl.
No.: |
04/805,714 |
Filed: |
March 10, 1969 |
Current U.S.
Class: |
607/9 |
Current CPC
Class: |
A61N
1/368 (20130101); A61N 1/365 (20130101) |
Current International
Class: |
A61N
1/368 (20060101); A61N 1/365 (20060101); A61m
001/36 () |
Field of
Search: |
;128/2.05,2.06,419--424,419P(Digest) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm;William E.
Claims
We claim:
1. Cardiac pacer apparatus comprising: an oscillator providing a
pulsating signal at a preselected frequency, which preselected
frequency is a relatively large multiple of a normal heart beat
rate; a cyclically operating digital counter means for counting the
pulsations of said pulsating signal; and means controlled by said
counter for generating a cardiac stimulating potential when said
counter reaches a predetermined count.
2. Apparatus as set forth in claim 1 wherein said means for
generating said cardiac stimulating potential includes means for
initiating the generation of said potential at the start of one
cycle of said pulsating signal and for terminating the generation
of said potential at the start of the next cycle of said pulsating
signal.
3. Apparatus as set forth in claim 1 further comprising: means for
detecting cardiac signals generated during a heartbeat; and means
responsive to such detected cardiac signals for setting said
counter to a starting point count which precedes said predetermined
count by a number corresponding to a preselected maximum interval
between successive heartbeats whereby a stimulating potential is
generated only if said preselected maximum interval elapses between
heartbeats.
4. Apparatus as set forth in claim 3 including means controlled by
said counter for inhibiting resetting of said counter when the
count held thereby is between said starting point count and a count
which is intermediate said starting point count and said
predetermined count, the difference between said starting point
count and the intermediate count corresponding to a preselected
refractory delay period.
5. Apparatus as set forth in claim 4 further comprising:
a second digital counter for counting signals detected by said
detecting means;
means controlled by said second digital counter for inhibiting
resetting of the first said counter between said intermediate count
and said predetermined count if more than a predetermined number of
detected signals are counted by said second counter during the
interval between a count which is intermediate said starting point
count and said intermediate count whereby rapidly repeated
electrical noise signals reaching said detecting means are
ineffective to prevent generation of a stimulating potential at
said predetermined count.
6. Apparatus as set forth in claim 1 further comprising means for
detecting atrial signals; and means responsive to such detected
atrial signals for setting said counter to a starting point count,
which precedes said predetermined count by a number corresponding
to a preselected A-V delay whereby stimulating potentials are
generated in synchronously timed relationship to said atrial
signals.
7. Apparatus as set forth in claim 6 including means controlled by
said counter for inhibiting resetting of said counter when the
count held thereby is between said predetermined count and a
subsequent count, the difference between said predetermined count
and said subsequent count corresponding to a predetermined
refractory delay period.
8. Apparatus as set forth in claim 7 further comprising:
a second digital counter for counting signals detected by said
detecting means;
means controlled by said second digital counter for inhibiting
resetting of the first said counter between said subsequent count
and said starting point count if more than a predetermined number
of detected signals are counted by said second counter during the
interval between: a count which is intermediate said predetermined
count and said subsequent count; and said subsequent count, whereby
rapidly repeated electrical noise signals reaching said detecting
means are ineffective to prevent generation of a stimulating
potential at said predetermined count.
9. Cardiac pacer apparatus comprising:
an oscillator providing a pulsating signal at a preselected
frequency, which preselected frequency is a relatively large
multiple of a normal heart beat rate;
a cyclically operating digital counter means for counting the
pulsations of said pulsating signal;
means for initiating the generation of a cardiac stimulating
potential starting at a predetermined count and for terminating the
generation of said potential at a subsequent count;
means for detecting cardiac signals generated during a
heartbeat;
means for setting said counter to a starting point count which
precedes said predetermined count by a number corresponding to a
preselected maximum interval between successive heartbeats in
response to cardiac signals which are detected after a count which
is intermediate said starting point count and said predetermined
count, the difference between said starting point count and the
intermediate count corresponding to a preselected refractory delay
period.
10. Apparatus as set forth in claim 9 further comprising:
a second digital counter for counting signals detected by said
detecting means;
means controlled by said second digital counter for inhibiting
resetting of the first said counter between said intermediate count
and said predetermined count if more than a predetermined number of
detected signals are counted by said second counter during the
interval between: a count which is intermediate said starting point
count and said intermediate count; and the first said intermediate
count, whereby rapidly repeated electrical noise signals reaching
said detecting means are ineffective to prevent generation of a
stimulating potential at said predetermined count.
11. Cardiac pacer apparatus comprising:
an oscillator providing a pulsating signal at a preselected
frequency, which preselected frequency is a relatively large
multiple of a normal heartbeat rate;
a cyclically operating digital counter means for counting the
pulsations of said pulsating signal;
means for initiating the generation of a cardiac stimulating
potential starting at a predetermined count and for terminating the
generation of said potential at a subsequent count;
means for detecting atrial signals;
means for setting said counter to a starting point count which
precedes said predetermined count by a number corresponding to a
preselected A-V delay in response to atrial signals which are
detected after a subsequent count which is after said predetermined
count by a number corresponding to a preselected refractory delay
period.
12. Apparatus as set forth in claim 7 further comprising:
a second digital counter for counting signals detected by said
detecting means;
means controlled by said second digital counter for inhibiting
resetting of the first said counter between said subsequent count
and said starting point count if more than a predetermined number
of detected signals are counted by said second counter during the
interval between: a count which is intermediate said predetermined
count and said subsequent count; and said subsequent count, whereby
rapidly repeated electrical noise signals reaching said detecting
means are ineffective to prevent generation of a stimulating
potential at said predetermined count.
Description
BACKGROUND OF THE INVENTION
This invention relates to cardiac pacers and more particularly to
such a pacer employing digital circuitry.
In the normal heart, electrical signals are generated and appear in
the atrium at a rate of approximately 60 to 120 times per minute,
depending on such factors as body size and amount of physical
exertion. Approximately 0.1 second after such a signal has appeared
in the atrium, it is transferred to the ventricle of the heart,
which reacts to the stimulation by contracting. This contraction
forces blood from the ventricle into the arterial system and thence
to the entire body. The delay between the appearance of an
electrical signal in the atrium and its appearance in the ventricle
is called the A-V delay. Following the contraction of the
ventricle, there is an insensitive period lasting about 0.4 second,
during which time the heart is unresponsive to electrical pulses.
This time is referred to as the refractory delay period.
A common type of heart failure is irregularity in the generation of
atrial potentials. In some cases, these potentials appear at only a
low rate; in others, they cease entirely for extended periods
though at other times the signals may be generated with perfect
regularity. It is in persons suffering from this kind of cardiac
disorder that a standby or so-called demand mode pacer is used.
This device is designed to apply stimulating pulses to the
ventricle, by means of an electrode implanted therein, only when
the heart fails to generate pulses spontaneously. When natural
pulses regularly appear, the pacer provides no stimulation; when
they appear irregularly, the pacer adjusts its timing to integrate
its artificial pulses with the natural ones. This type of pacer is
often provided with circuitry which simulates the refractory delay
period of the heart. The reason for including such delay circuitry
is that a spontaneous electrical signal which appears a short time
after delivery of an artificial pulse is ineffective to pump blood,
either because the natural refractory period of the heart caused
the heart to ignore the spontaneous pulse or because the ventricle
has not had time following the previous beat to be refilled with
blood. A simulated refractory period causes the pacer likewise to
ignore these ineffective beats. The device's timing continues just
as if the beats had never occurred.
Another form of heart disease is the so-called A-V block in which
the patient's heart undergoes normal or near-normal atrial
contraction but the atrial signal is not transferred to the
ventricle. With such a patient, it is desirable to use a so-called
synchronous pacer which detects atrial signals and supplies to the
ventricle a stimulating pulse about 0.1 second later, a period
which constitutes a simulated A-V delay. In the absence of detected
atrial signals, the pacer supplies ventricular pulses at a fixed
rate. The synchronous pacer, like the demand pacer, is often
provided with refractory delay simulation.
A drawback of pacers presently in use is their relatively large
size. While this does not affect the ease of installation in any
but the rarest patients, it is easier to hermetically seal the
pacer package if it is of smaller volume. Such sealing is desirable
because it eliminates the possibility that body fluids will seep
into the device and damage it. Such hermetic sealing employing
metallic members can also be employed to shield the pacer from
electromagnetic interference.
Another problem presented by currently-used pacers is their finite
battery life. Recent improvements have extended battery life to the
neighborhood of 2 years, but increasing this time still constitutes
an important improvement by reducing the frequency of battery
replacements which require surgery.
Among the several objects of the present invention may be noted the
provision of a cardiac pacer which is highly reliable and stable;
which provides very accurate timing; which is protected from
dangerous operation caused by ambient electrical noise; which has
very low power consumption and thus yields long battery life; which
is relatively compact; which will operate in a demand mode
providing either an inhibit demand or synchronous demand operation;
which will operate in a straight synchronous mode; which can
provide a fixed rate operation; and which is relatively
inexpensive. Other objects and features will be in part apparent
and in part pointed out hereinafter.
SUMMARY OF THE INVENTION
Briefly, a cardiac pacer according to the present invention times
various events and delays by means of a digital counter which is
driven by an oscillator operating at a frequency which is a
relatively large multiple of a normal heartbeat rate. A cardiac
stimulating pulse is generated at a predetermined point in the
count. Thus, if the counter cycles repetitively, the heart is
stimulated at a predetermined fixed rate. To provide demand mode
operation, the counter is reset in response to spontaneous cardiac
signals thereby to prevent stimulation when the heart is
functioning normally. To provide synchronous mode operation, the
counter is reset to a point preceding the stimulation count by an
amount which simulates a normal A-V delay.
The use of digital count down circuitry permits both the various
delays and the durations of the stimulating pulses to be accurately
timed. Further, by counting down from a relatively high frequency,
an oscillator having a relatively short duty cycle may be used so
as to reduce battery drain. Further, the use of a relatively short
oscillator period permits timing components, e.g. capacitors, of
relatively small size to be used.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block logic diagram of a cardiac pacer of this
invention;
FIG. 2 is a block logic diagram of another embodiment of the pacer
incorporating noise detection circuitry; and
FIG. 3 is a block logic diagram of still another embodiment.
Corresponding reference characters indicate corresponding parts
throughout the several views of the drawings.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, the apparatus illustrated there is adapted
for use as either a demand or synchronous mode cardiac pacer,
separate output terminals, 6 and 9 respectively, being provided for
the two modes. Depending on the mode of operation desired, an
electrode which is implanted in a location suitable for stimulating
ventricular contraction is connected to one or the other of these
output terminals. A single input terminal, designated 10, is
employed for both modes, the electrode connected to this input
terminal being placed in contact with the ventrical for demand mode
operation and in contact with the atrium for synchronous mode
operation. In demand mode operation, the terminals 6 and 10 may in
fact both be connected to the same lead.
Timing of the different events occurring in the operation of this
apparatus is provided by a digital counter 3. The counter is driven
by an oscillator 1 which establishes the time base. As illustrated,
counter 3 comprises a nine stage binary divider and the oscillator
1 runs at a frequency which is relatively high with respect to the
contemplated range of heartbeat rates or frequencies. Since the
counter 3 requires only a very short triggering pulse, the duty
cycle of oscillator 1 is preferably relatively short to reduce
battery drain.
As is conventional, counter 3 provides a two-state output signal
for each stage of binary division, these signals being designated
C1--C9. The counter also provides signals which are the binary
complements of these signals, these complementary signals being
designated . In the embodiments illustrated herein, only the C1,
C7, C8, C9 and signals are utilized and thus only these signals are
designated on the drawings but, as will be apparent to those
skilled in the digital circuitry arts, other combinations of output
signals may be used if different event timings are desired.
Positive logic is assumed.
As is also conventional, the counter 3 run cyclically, that is, the
states of the binary output signals pass through a sequence which
repeats after all the possible combinations have been utilized.
With the nine stage binary counter illustrated, the number of
possible states is 2.sup.9 or 512. Further, the counter may at will
be reset to a predetermined starting point by the application of a
reset signal to a reset terminal, designated R. The starting point
of the counter is considered herein to be the zero count and the
various possible states or counts are considered to be zero through
511. At zero count, the output signals C1--C9 are low and the
output signals are positive or high.
The output signal from the counter 3 is applied to the clock input
terminal, designated C, of a so-called D-Type flip-flop 16 and the
C1 signal is applied to the reset terminal R of this flip-flop.
D-type flip-flops are available commercially in integrated circuit
form from several manufacturers and their operation is generally as
follows. The device responds to a positive-going voltage transition
on the clock input terminal, designated C, by making the logic
level output on an output terminal, designated Q, identical to the
logic level present at an input terminal designated D, at the time
of the positive-going transition at the C terminal. Thereafter the
Q output signal remains the same, ignoring changes in the D input
signal, until another positive-going transition is applied at the
clock input C. The device may also be reset by the application of a
high or positive signal to a reset terminal designated R. The reset
function overrides all others; as long as a high level is present
at the R terminal, the output signal at Q is low regardless of
changes at the D or C terminals. A signal which is the binary
complement of the signal present at terminal Q is provided at a
terminal designated .
Cardiac signals applied to the input terminal 10 are amplified and
shaped by means of an amplifier 11 so as to be squared into
waveforms suitable for use with digital circuitry, as is understood
by those skilled in the art. The square signals thereby obtained
are applied to the clock terminal C of a second D-type flip-flop
17. The C9 signal is applied to the D input of this flip-flop and
the output signal from the oscillator 1 is applied to the reset
terminal R. The Q output signal from flip-flop 17 is applied to the
reset terminal R of the counter 3 while the output signal from this
flip-flop is applied to the D input terminal of the first flip-flop
16.
The Q output signal from the flip-flop 16 is applied to the input
terminal of an amplifier 21 which responds to a positive input
signal by applying to the output terminal 6 a voltage level which
is suitable for cardiac stimulation.
The portions of the apparatus thus far described are those employed
in providing operation in the demand mode and that operation is
substantially as follows. Assuming initially that no cardiac
signals are applied to the input terminal 10 and that flip-flop 17
is in its reset condition so that the respective output signal is
high, the counter 3 will run indefinitely in its cyclic mode. Thus,
when the count changes from 511 to 0, the output signal will
experience a positive-going transition and the positive output
signal present at the D input terminal of flip-flop 16 will be
transferred to its Q output terminal. On the next count, however,
the flip-flop 16 will be reset by the C1 signal as the C1 signal
goes positive on count 1. It can thus be seen that, with no
detected cardiac signals, the pacer will generate a stimulating
pulse having a duration of one oscillator cycle for every 512
oscillator cycles. Assuming that the oscillator 1 operates at 590
c.p.s., the apparatus will thus deliver 1.7 millisecond pulses at
the rate of approximately 70 pulses per minute which is an
appropriate timing for nonsynchronous or demand mode cardiac
stimulation.
During the counting from zero to the intermediate count of 255, the
C9 output signal is low and thus, even if cardiac signals are
received, the flip-flop 17 will not change state. This insensitive
period simulates the refractory delay of the heart and prevents any
interaction between the output and input circuits of the pacer.
From count 256 through count 511, however, the C9 signal is high
and thus a cardiac signal received during this latter interval will
cause the flip-flop 17 to change states and the resulting high
signal applied to the terminal R of the counter 3 will cause the
counter to be reset to its zero count. Simultaneously with the
resetting of the counter 3, the Q signal from flip-flop 17 goes low
so that, even though the resetting of the counter produces a
positive-going transition in the C9 signal, no change of state is
produced in the flip-flop 16 and thus no output signal is
generated. On the next count after the resetting of counter 3, the
flip-flop 17 is reset by the oscillator output signal. Received
cardiac signals are typically of sufficient duration to span more
than one cycle of the oscillator so that the flip-flop 17 will
change state at least once even if the cardiac signal is initiated
during a period when the oscillator output signal is high.
From the foregoing, it can be seen that, if the patient's heart is
beating normally at a rate which is more than the free running rate
of the pacer, i.e. about 70 beats per minute, and not more than
twice that rate, i.e. about 140 beats per minute, the counter 3
will be reset to its zero count by each natural heartbeat before a
count of 511 is reached. Thus, the patient's heart will not be
stimulated at all if it is beating spontaneously within this 2 -to-
1 range of rates. However, if no spontaneous heartbeat is detected
between count 256 and count 511, the pacer will then stimulate the
patient's heart at the end of the full count period, that is, after
a period which corresponds to the 70 pulse per second free running
rate. In other words, the difference between the starting point
count and the end of the counting sequence establishes a maximum
interval between heartbeats. Accordingly, if the spontaneous heart
signals disappear intermittently, the pacer will integrate its
operation with the normal heartbeat.
The generation of a synchronous pacing signal is controlled by
means including a third D-type flip-flop 18. The C8 and C9 signals
are combined in a NOR gate 27 to provide a signal which is high
from count 0 through count 127. The resultant signal is applied to
the D input terminal of flip-flop 18. The C7 signal is applied to
the clock input terminal C of this flip-flop and the C1 signal is
applied to its reset terminal R. The output signal provided at the
Q terminal of flip-flop 18 is amplified by an amplifier 33 to
provide at output terminal 9 a signal level suitable for cardiac
stimulation.
The C7 signal experiences a positive-going transition at counts 64,
192, 320 and 448 but only the transition at count 64 can produce a
change in state of the flip-flop 18 since the D input is low at the
other three transition points. Accordingly, it can be seen that
flip-flop 18 will be put into its so-called set state each time the
counter 3 reaches a count of 64. Further, the flip-flop 18 will be
reset on the next count, i.e. count 65, by the C1 signal. Thus,
each time the counter reaches a count of 64, a cardiac stimulating
pulse having a duration of one oscillator cycle will be provided at
the output terminal 9.
When the apparatus illustrated in FIG. 1 is to be utilized in the
synchronous mode, the input terminal 10 is connected to an
electrode which is implanted so as to detect atrial signals. The
resetting of counter 3 is controlled in response to detected
signals as described previously. Thus, the counter is reset to its
zero count if an atrial signal is detected from count 256 through
count 511. A stimulating pulse is then generated at output terminal
9 when count 64 is reached. The delay provided by the interval
between the resetting and the 64 count is about 108 milliseconds
which satisfactorily simulates the normal A-V delay. Thus the heart
is stimulated with timing appropriate for synchronous pacer
operation.
From the time that the counter 3 is reset until count 256 is
reached, the input circuit is insensitive to detected cardiac
signals just as it was in the demand mode of operation described
previously. Thus, the interval from count 64 to count 256 provides
a simulated refractory period during which the apparatus will not
respond to input signals. Accordingly, interaction between the
input and output circuits is prevented, as is the overly rapid
stimulation of the heart due to premature detected signals.
If no atrial signals at all are detected, the counter 3 will run
cyclically as described previously and stimulating pulses will be
generated at a fixed rate, one pulse being generated each time the
counter 3 passes the 64 count. Accordingly, if there is failure of
the heart to generate atrial signals or if the input lead should
break, the generation of stimulating pulses will not fail
altogether but will lapse into the relatively slow free-running
rate.
As noted previously, electrical noise present in certain
environments can interfere with cardiac pacer operation by
introducing false input signals which improperly modify the
operation of the pacer. The embodiment illustrated in FIG. 2
includes circuitry for identifying interfering electrical noise and
preventing it from causing a dangerous mode of operation. The
embodiment of FIG. 2 is essentially the same as the apparatus of
FIG. 1 except for the addition of those components which condition
the effect of input signals. The amplifier 11 also drives a counter
35 which, as illustrated, comprises a two stage binary divider.
Counter 35 provides, at an output terminal designated Q35, a signal
which has a positive-going transition after four input signals have
been counted. The signal from gate 27 is applied to a reset
terminal R on counter 35 for resetting counter 35 to its zero count
during counts 0 through 128 of counter 3.
The output signal from counter 35 is applied to the clock input
terminal C of a fourth D-Type flip-flop 38. A Logic High Signal is
applied to the D input terminal of flip-flop 38 and the Q output
signal from flip-flop 16 is applied to the reset terminal R of
flip-flop 38. The Q output signal from flip-flop 38 is combined
with the signal from counter 3 in a NOR gate 44 and the resultant
signal is applied to the D input terminal of the flip-flop 17 in
place of the straight C9 signal which was employed in the
embodiment of FIG. 1.
Assuming that the flip-flop 38 is in its reset condition and that
the counter 3 has just reached a count of 128 so that the counter
35 has just been reset and the signal applied to the D input of
flip-flop 38 is high, the operation of this noise detection
circuitry is as follows. Detected input signals advance the counter
35. If more than four input signals are received before the counter
3 reaches a count of 256, the flip-flop 38 will be caused to change
states, that is, to be put into its so-called set condition. In
this set condition, the flip-flop 38 applies a high signal to the
NOR gate 44. Thus, the NOR gate will provide a low signal to the D
input of flip-flop 17 regardless of the state of the signal.
Accordingly, detected input signals which are passed by the
amplifier 11 to the clock input C of flip-flop 17 cannot cause that
flip-flop to change states and thereby reset counter 3 as described
previously. If the counter 3 is not reset, the apparatus then
provides stimulating pulses at the relatively slow free running
rate in the same manner as if no input signals were received. If
fewer than four input signals are received between the count of 128
and the count of 256, the flip-flop 38 does not change states and
the flip-flop 17 becomes responsive to input signals received after
counter 3 reaches a count of 256 as described previously.
In summary, it can be seen that this noise detection circuitry
operates by counting received signals for a predetermined period
which precedes the normal sensitive period and by providing a fixed
rate or free-running mode of operation if more than a predetermined
number of input signals are detected during this period. In other
words, the detection of input signals at a relatively high
repetition rate is taken as an indication that interfering
electrical noise is present and the pacer then ignores all detected
signals for the remainder of the fixed rate cycle. While genuine
cardiac signals may be ignored as a result of this mode of
operation, the reversion to fixed rate operation is deemed
preferable to the random rate of stimulation which might otherwise
occur in the case of synchronous mode operation or the complete
cessation of stimulation which might otherwise occur in the case of
demand mode operation. This noise detection circuitry thus provides
an advantageous protective feature in either mode of operation.
The standby demand mode operation provided by the circuits of FIGS.
1 and 2 is of the so-called inhibit type, that is, no output pulses
are produced if normal heart signals are detected. The embodiment
illustrated in FIG. 3, which is basically similar to the FIG. 1
embodiment, is arranged to provide so-called synchronous demand
operation. A NAND gate 45 combines the output signal from flip-flop
16 with the output signal from the reset flip-flop 17. The NAND
gate then drives the output amplifier 21.
The NAND gate 45 reinverts the inverted or complemented output
signal from flip-flop 16 so that, when no input signal is received,
this circuit operates as the FIG. 1 circuit to provide stimulation
at a fixed rate. When a ventrical signal is detected, causing the
counter 3 to be reset as before, an output pulse is also generated
immediately due to the signal from flip-flop 17. However, since the
ventrical is already starting to contract, the stimulus from the
pacer is effectively disregarded. This type of operation avoids the
possible conflicts of rhythm that may be present with a fixed-rate
pacer, for example, when A-V mode conduction is restored. If
desired, switching means can be incorporated to permit both inhibit
demand and synchronous demand modes.
A form of synchronous demand operation can also be provided using
the circuit of FIG. 1 without change by connecting the output
terminal 9 to the input terminal 10 and to a ventrical lead. With
this arrangement a pulse is generated a short interval after a
normal ventricle signal is detected but this pulse is again
ineffective since it occurs within the refractory period. If no
input pulses are detected the apparatus lapses into a fixed rate
mode of operation as in the other arrangements discussed.
It should be understood that certain variations on the preferred
embodiments of these pacers are within the spirit of the invention.
The binary dividers, for example, could be replaced by other binary
logic devices such as ring counters, shift registers, or the like.
Similarly, the NOR gates could be replaced by other gates. If
desired, the stimulating pulse duration could be determined by a
one-shot multivibrator. These changes could be made with only
relatively minor alterations in the circuitry as well as changes in
the logic.
An advantage of the use of digital circuitry for providing the
various timing functions is that the various components, e.g., the
counters, gates and flip-flops, can be constructed in integrated
circuit form using presently available devices. Further, by custom
designing the integrated circuitry using presently available
fabrication techiques, the entire pacer can be constructed on a
single semiconductor substrate thereby further reducing size,
eliminating hand-wired circuit interconnections and facilitating
hermetic sealing. Further, such integrated circuits have relatively
low power consumptions.
In view of the above, it will be seen that the several objects of
the invention are achieved and other advantageous results
attained.
As various changes could be made in the above constructions without
departing from the scope of the invention, it is intended that all
matter contained in the above description or shown in the
accompanying drawings shall be interpreted as illustrative and not
in a limiting sense.
* * * * *