U.S. patent number 3,870,050 [Application Number 05/413,451] was granted by the patent office on 1975-03-11 for demand pacer.
Invention is credited to Wilson Greatbatch.
United States Patent |
3,870,050 |
Greatbatch |
March 11, 1975 |
DEMAND PACER
Abstract
An artificial cardiac pacer which in the absence of natural
cardiac electrical activity provides stimulating electrical pulses
to the heart at a fixed rate. The stimulating pulses preferably are
obtained from a digital clock integrated circuit including a stable
frequency source such as a quartz crystal and a binary frequency
divider to yield the desired pulse rate. In response to a natural
heart signal, a circuit in the pacer inhibits any stimulating pulse
that occurs at that instant of time, and for a predetermined
interval of time thereafter to provide an adequate safety
margin.
Inventors: |
Greatbatch; Wilson (Clarence,
NY) |
Family
ID: |
23637272 |
Appl.
No.: |
05/413,451 |
Filed: |
November 7, 1973 |
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: |
;128/419P,419R,421,422,423 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Christel & Bean
Claims
I claim:
1. A cardiac pacer comprising:
a. electrode means adapted to be operatively connected to a
patient's heart;
b. a source providing an output train of electrical pulses having a
constant frequency and comprising oscillator means providing output
pulses at a relatively high frequency and means for converting said
high frequency pulses to pulses having a relatively lower frequency
suitable for heart stimulation; and
c. means for coupling said train of pulses from said source to said
electrode means in a manner such that stimulating pulses are fixed
in timing relationship to said source, said coupling means
including means for preventing application to the heart those
pulses in said train which occur at a time during a predetermined
interval in response to the occurrence of a natural heart
signal.
2. A cardiac pacer according to claim 1, wherein said oscillator
means comprises a quartz crystal oscillator and said converting
means comprises a binary frequency divider connected to the output
of said oscillator.
3. A cardiac pacer according to claim 1, wherein said means for
preventing application of pulses to the heart during a
predetermined interval in response to a natural heartbeat includes
means for connecting said converting means to said electrode means
in the absence of a natural heart signal and for temporarily
disconnecting said converting means from said electrode means in
response to the occurrence of a natural heart signal.
4. A cardiac pacer according to claim 1, wherein said coupling
means comprises:
a. AND gate means having a pair of inputs and an output;
b. means for coupling the output of said AND gate means to said
electrode means;
c. means for connecting said converting means to one input of said
AND gate means; and
d. means coupled to said electrode means and connected to the other
input of said AND gate means for applying a logical one signal to
said AND gate input in the absence of a natural heart signal and
for applying a logical zero signal to said AND gate input for a
predetermined time in response to the occurrence of a natural heart
signal.
5. A cardiac pacer according to claim 4, wherein said means coupled
to said electrode means and connected to the other input of said
AND gate means includes means responsive to both positive-going and
negative-going heart signals.
6. A cardiac pacer according to claim 4, wherein said means coupled
to said electrode means and connected to the other input of said
AND gate means includes time delay means for maintaining the
logical zero signal for said predetermined time.
7. A cardiac pacer comprising:
a. electrode means adapted to be operatively connected to a
patient's heart;
b. a source of electrical pulses having a constant frequency;
and
c. control means connected to said electrode means and to said
source for connecting said source to said electrode means in the
absence of a natural heart signal and for temporarily disconnecting
said source from said electrode means in response to the occurrence
of a natural heart signal and thereafter reconnecting said source
to said electrode means, said control means maintaining the
periodicity of said source such that the pulse interval of
stimulating pulses provided by said pacer is fixed to the constant
frequency of said source.
8. A cardiac pacer according to claim 7 wherein said control means
includes timing means for maintaining said source disconnected from
said electrode means a predetermined time after the occurrence of
said natural heart signal.
9. A cardiac pacer according to claim 7, wherein said source of
electrical pulses comprises oscillator means providing output
pulses at a fixed and relatively high frequency and means for
converting said high frequency pulses to pulses having a relatively
lower rate suitable for heat stimulation.
10. A cardiac pacer according to claim 7, wherein said source of
electrical pulses comprises a quartz crystal oscillator and a
binary frequency divider connected to the output of said
oscillator.
Description
BACKGROUND OF THE INVENTION
This invention relates to the electronic cardiac pacer art, and
more particularly to a new and improved electronic cardiac pacer of
the demand type.
Early in the development of electronic cardiac pacemaking there
appeared the nonsynchronous pacer which provides fixed-rate
stimulation, and although the stimulation is not automatically
changed in accordance with the body's needs, it has proven
effective in alleviating the symptoms of complete heart block. A
nonsynchronous pacer, however, has the possible disadvantage of
competing with the natural, physiological pacer during episodes of
normal sinus conduction.
As a result, the demand-type pacer was developed having the
capability that artificial stimuli are initiated only when required
and subsequently can be eliminated when the heart returns to the
sinus rhythm. The demand pacer solves the problem encountered with
the nonsynchronous pacer by inhibiting itself in the presence of
ventricular activity but coming "on line" and filling in missed
heart beats in the absence of ventricular activity.
Demand pacer circuits heretofore available include free-running
multivibrators and sense the occurrence of a natural heart beat and
re-time the next pacer impulse to some fixed time following that
beat. This, in turn, requires timing capacitors and high megohm
resistors which are bulky and can be unreliable. In addition, such
arrangements cannot use recently developed digital clock circuitry.
Futhermore, with demand pacers heretofore available the stimulation
impulses are synchronized with the natural heart rhythm, and the
heart will establish a new stimulation time base each time an
irregular cardiac event occurs.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide a new and
improved artificial cardiac pacer of the demand type.
It is a further object of this invention to provide such an
artificial pacer wherein pulse generation is locked in timing
relation to a source of timing signals which operates at a constant
frequency.
It is a further object of this invention to provide such an
artificial pacer which encourages the natural heart rate to conform
to a precise repetitive signal.
It is a further object of this invention to provide such an
artificial cardiac pacer wherein the timing element includes
digital clock circuitry of the integrated circuit type.
The present invention provides an artificial cardiac pacer which in
the absence of natural cardiac electrical activity provides
stimulating electrical pulses to the heart at a fixed rate. The
stimulating pulses preferably are obtained from a digital clock
circuit including a stable frequency source such as a quartz
crystal and a binary frequency divider to yield the desired pulse
rate. In response to a natural heart signal the pacer inhibits any
stimulating pulse that occurs at that instant of time, and for a
predetermined interval of time thereafter to provide an adequate
safety margin. As a result, the pacer provides a train of
stimulating pulses to the heart which are precisely timed at a
fixed frequency and wherein those pulses which would conflict with
a natural heart beat are completely missing from the train.
The foregoing and additional advantages and characterizing features
of the present invention will become clearly apparent from a
reading of the ensuing detailed description thereof together with
the included drawing wherein:
BRIEF DESCRIPTION OF THE DRAWING FIGURES
FIG. 1 is a block diagram of a cardiac pacer according to the
present invention; and
FIG. 2 is a schematic circuit diagram of a portion of the cardiac
pacer of FIG. 1.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
FIG. 1 shows a cardiac pacer according to the present invention
which includes electrode means adapted to be operatively connected
to a patient's heart and comprising a pair of electrodes 11, 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 the circuitry of the cardiac pacer by leads or wires
which are enveloped by a moisture-proof and human body
reaction-free material such as silicone rubber or suitable
plastic.
The cardiac pacer of the present invention further comprises a
source of electrical pulses having a constant frequency. The
source, generally designated 16 in FIG. 1, is a stable frequency
source and includes a pulse source 18 such as an oscillator which
provides output pulses at a relatively high frequency, together
with a divider 20, the input of which is connected to the output of
oscillator 18. Divider 20 is in the form of a binary electronic
frequency divider which divides frequency of the pulses
sequentially down to a relatively lower frequency corresponding to
the desired fixed stimulation rate for the heart. Electrical energy
for operating oscillator 18 and divider 20 is obtained from a
suitable source such as a battery in a known manner. By way of
example, oscillator 18 can comprise a crystal oscillator providing
output pulses at a frequency of 19,661 hertz and commercially
available from the Radio Corporation of America under the product
designation RCA TA5987. Divider 20 likewise can comprise a 14 stage
binary divider available from the Radio Corporation of America
under the commercial designation RCA CD4020A. The output of this
particular oscillator-divider combination is a train of pulses
having a frequency of 72 pulses per minute. The output frequency
can of course be made higher or lower than this example by
substituting a different oscillator. For example, a quartz crystal
oscillator operating at a frequency of 16,384 hertz in combination
with a 14 stage binary divider will yield an output pulse rate of 1
hertz or 60 beats per minute. This train of pulses is present on a
line 22 for a connection to another portion of the pacer circuit in
a manner which will be described.
The cardiac pacer of the present invention further comprises
control means connected to the pacer output electrode means and to
the source of pulses for connecting the source to the electrode
means in the absence of a natural heartbeat and for temporarily
disconnecting the source from the electrode in response to the
occurrence of a natural heartbeat. Electrode 11 is connected by a
line 26 to the input of an amplifier 28 which is similar in
construction and operation to the demand amplifier present in
demand cardiac pacer circuits. Because demand amplifiers are well
known to those skilled in the art, a detailed description of
amplifier 28 is believed unnecessary. Further information and
detailed descriptions of demand amplifiers included in cardiac
pacers can be found in any of the following U.S. Pat. Nos.
3,478,746 issued Nov. 18, 1969; 3,618,615 issued Nov. 9, 1971 and
3,648,707 issued Mar. 14 1972. A heartbeat electrical signal sensed
or picked up by electrode 11 is applied by line 26 to the input of
amplifier 28 which amplifies the heartbeat signal to an amplitude
suitable for use in the subsequent circuitry. The output of
amplifier 28 is connected by a line 30 to the input of a demand
delay switch generally designated 32 in FIG. 1. Switch 32
functions, briefly, to provide a logical one output signal when no
heart signal is present in this circuit branch, and it functions to
provide a logical zero output signal for a predetermined time, such
as about 0.3 seconds, when a heart signal is present in the branch.
The construction and operation of demand delay switch 32 will be
described in detail presently. The logical signals are applied from
the output of switch 32 by line 36 to one input of an AND gate 38.
The other input of AND gate 38 is connected to line 22 from pulse
generator 16. Thus the pulses present on line 22 are transmitted or
passed through gate 38 when a logical one signal is present on line
36, and the pulses on line 22 are inhibited by gate 38 as long as a
logical zero signal is present on line 36. The output of gate 38 is
connected by a line 42 to the input of an amplifier 44, the output
of which is connected to electrode 11. Amplifier 44 like amplifier
28 is of the type commonly found in cardiac pacer circuits for
amplifying the stimulating pulses transmitted to the pacer
electrode and would be provided with suitable bias voltage from a
source (not shown). Because amplifier 44 is of the type well-known
to those skilled in the art, a detailed description thereof is
believed to be unnecessary. Electrode 12 is connected to the ground
or reference portion of the amplifier circuit which is referenced
electrically to the remainder of the cardiac pacer circuit in a
known manner.
FIG. 2 is a schematic circuit diagram of a preferred form of the
demand delay switch 32 of the cardiac pacer of FIG. 1. Demand delay
switch 32 comprises an input or coupling capacitor 40, one terminal
of which is connected to line 30 leading from amplifier 28 and the
other terminal of which is connected to a circuit junction terminal
42. The circuit 32 further comprises a full-wave rectifier in the
form of a first rectifier diode 44, the cathode of which is
connected to terminal 42 and the anode of which is connected to the
circuit reference or ground potential. The rectifier also includes
a second rectifier diode 46, the anode of which is connected to
circuit terminal 42 and the cathode of which is connected to a lead
48.
The demand delay switch 32 further comprises a timing means in the
form of an RC circuit connected between line 48 and the circuit
ground or reference point. In particular, a timing capacitor 50 and
a timing resistor 52 are connected in parallel and between line 48
and the ground or reference terminal. Line 48 also is connected to
one terminal of a resistor 58, the other terminal of which is
connected to the base terminal 59 of an NPN silicon transistor 60.
The collector terminal 61 of transistor 60 is connected to one
terminal of an output resistor 62, the other terminal of which is
connected to a source of positive bias voltage for transistor 60,
preferably having a magnitude of about 6 volts. The emitter
terminal 63 of transistor 60 is connected to the circuit ground or
reference potential point. Collector terminal 61 also is connected
to the input of an inverter schematically designated 66, the output
of which is connected to line 36 in the cardiac pacer of FIG. 1
leading to one input of AND gate 38.
The cardiac pacer of FIG. 1 operates in the following manner. Pulse
generating means 16 produces a train of output pulses on line 22
having a constant frequency or repetition rate of a value or
magnitude which is desired for heart stimulation, preferably at or
near one hertz or 60 pulses or beats per minute. This is
accomplished by the combination of oscillator 18 and divider 20
wherein oscillator 18 provides output pulses at an extremely high
frequency or repetition rate and divider 20 sequentially divides
the pulses to yield the desired lower rate. The quartz crystal
oscillator 18 oscillates at a very precise frequency, for example
19,66l hertz according to the foregoing example. This signal is
then divided sequentially in 14 successive digital binary dividers
included within divider 20 (i.e. first to 9831 then to 4915 and 12
more times until the output is 72 pulses per minute). It is to be
understood that the foregoing quantities are merely exemplary, and
oscillator 18 can operate at different frequencies thereby
providing a different pulse frequency or repetition rate on line
22.
In the absence of a natural heartbeat, the pulses present on line
22 are transmitted or gated through the AND gate 38 and amplified
in amplifier 44 whereupon stimulating pulses are applied to the
heart through electrode 11. On the other hand, when a natural
heatbeat occurs, it is sensed by electrode 11 causing operation of
the circuit branch including demand delay switch 32 to inhibit
stimulating pulses for a predetermined period of time. That period
of time would be about 0.30 seconds so as to prevent the
application of a stimulating impulse to the heart during the T wave
of a natural heatbeat. In particular, the natural heartbeat sensed
by electrodes 11 is applied through line 26 to amplifier 28 wherein
it is amplified and applied to the input of demand delay switch 32.
Switch 32, in turn, functions to apply a logical zero input through
line 36 to AND gate 38 in response to the occurrence of the natural
heartbeat signal and for the aforementioned predetermined time
period. The logical zero input to gate 38 of course causes gate 38
to block passage or transmission of pulses on line 22 to amplifier
44 and electrode 11. After the predetermined time delay and in the
absence of a natural heartbeat sensed by electrode 11, switch 32
applies a logical one input through line 36 to AND gate 38 thereby
causing the gate to allow passage or transmission of pulses from
line 22 to amplifier 44 and electrode 11.
Referring now to FIG. 2 the detailed operation of demand delay
switch 32 is as follows. When a natural heartbeat does not occur,
there is no signal on line 30 and transistor 60 in the circuit of
FIG. 2 is off or non-conducting. Therefore, the voltage at the
collector terminal 61 is high corresponding to a logical zero
level. This, in turn, is converted by inverter 66 to a logical one
signal level which is applied through line 36 to an input of AND
gate 38. When the natural heartbeat occurs, the corresponding
signal is present on line 30 and is rectified by the combination of
diodes 44, 46 and applied through line 48 and resistor 58 to base
terminal 59 of transistor 60. The signal level is sufficient to
turn transistor 60 on thereby allowing current to flow through the
collector-emitter path thereof with the result that the voltage at
collector terminal 61 falls relatively instantaneously to a low
voltage level. This corresponds to a logical one level and is
converted by inverter 66 to a logical zero voltage level which then
is applied through line 36 to the input of AND gate 38. Capacitor
50 and resistor 54 comprise an RC time delay circuit which serves
to hold the signal level above the base-emitter threshold level of
transistor 60 for the predetermined time during which it is desired
to maintain a logical zero voltage level on line 36. In particular,
transistor 60 is of the silicon type having a base-emitter voltage
drop of about 0.5 volts, and the magnitudes of capacitor 50 and
resistor 54 together with magnitude of base resistor 58 are
selected so that transistor 60 remains on or conducting for a
predetermined time period, for example about 0.3 seconds after the
occurrence of a heart signal. The voltage waveform present on base
terminal 59 has a relatively straight, vertical leading edge and a
gradual decay which maintains the waveform amplitude above the 0.5
volt threshold level for the predetermined time period as set by
the values of capacitor 50 and resistor 54. The provision of the
full-wave rectifier comprising diodes 44, 46 connected as shown in
FIG. 2 insures that both negative-going and positive-going heart
signals will activate the demand delay switch 32.
The cardiac pacer of the present invention is a demand-type pacer
providing stimulating impulses which are locked or fixed in timing
relationship to a fixed oscillator rate and are not in time step
with the natural heart rhythm. In particular, the pacer pulse
interval is precisely locked to a stable frequency source, such as
the quartz crystal oscillator 18, which oscillates to provide
signals or pulses at a relatively high frequency which then is
divided sequentially a number of times in a binary digital
electronic divider 20 to yield the desired heart stimulation rate
which is at or near one hertz or 60 beats per minute. The cardiac
pacer of the present invention utilizes the amplified natural heart
signal to inhibit any pacer impulse that occurs between that
instant and a time about 0.3 seconds later so as to avoid firing
into the T wave of a natural heartbeat. Therefore, the pacer
impulse train is a string of precisely time impulses at the
frequency of the oscillator-frequency divider combination such as
one hertz but with some impulses completely missing if those
impulses would conflict in time with a natural heartbeat. In other
words, the cardiac pacer of the present invention inhibits the
train of pulses in time-step with the natural rhythm of the heart
as compared to conventional demand-type pacemakers which deliver a
stimulating impulse in time-step with the natural rhythm of the
heart.
The cardiac pacer of the present invention advantageously utilizes
integrated circuit components which are at a high level of
sophisticated development. In particular, the oscillator 18 and
frequency divider 20 are provided on integrated circuit chips and
therefore provide all the advantages associated therewith. The
utilization of such a timer in a cardiac pacer permits elimination
of the bulky timing capacitors and high megohm resistors which can
be unreliable and are not amendable to integrated circuitry. These
capacitors and resistors can be eliminated because the timing
element in the digital clock or stable frequency source 16 is a
quartz crystal chip which operates at a relatively high frequency,
i.e. around 19,000 hertz, rather than bulky RC components which
operate at 1 hertz.
Furthermore, there is believed to be a medical advantage in
encouraging the heart to conform to a precise repetitive signal
rather than allowing the heart to reestablish a new time base each
time an untoward event like an extrasostole occurs. Thus there is
believed to be an advantage in using a cardiac pacer impulse not in
time-step with the natural rhythm of the heart. In other words, the
cardiac pacer of the present invention inhibits an existing pacer
impulse if it conflicts with a natural heart stimulus but does not
disturb the periodicity of the basic rhythm of the oscillator.
It is therefore apparent that the present invention accomplishes
its intended objects. While a single embodiment of the present
invention has been described in detail, this is for the purpose of
illustration, not limitation.
* * * * *