U.S. patent number 3,656,487 [Application Number 05/001,178] was granted by the patent office on 1972-04-18 for electronic demand heart pacemaker with different pacing and standby rates.
This patent grant is currently assigned to Medtronic, Inc.. Invention is credited to David H. Gobeli.
United States Patent |
3,656,487 |
Gobeli |
April 18, 1972 |
ELECTRONIC DEMAND HEART PACEMAKER WITH DIFFERENT PACING AND STANDBY
RATES
Abstract
A demand type heart pacer which provides electronically
generated stimulating pulses to the heart at a first frequency in
the continued absence of natural heartbeats, but which inhibits
output pulses once the natural heart rate exceeds the first
frequency, and allows the heart to beat naturally at any rate above
a second, lower frequency before again providing stimulating
pulses, thereby creating a different standby frequency. The pacer
also reverts to a third frequency in a non-demand type operation in
the presence of an interfering electrical noise pattern.
Inventors: |
Gobeli; David H. (St. Paul,
MN) |
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
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Family
ID: |
26668679 |
Appl.
No.: |
05/001,178 |
Filed: |
January 7, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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832706 |
Jun 12, 1969 |
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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,419D,421,422 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a continuation-in-part of my copending
application, Ser. No. 832,706, filed June 12, 1969, and now
abandoned, entitled ELECTRONIC DEMAND HEART PACER WITH DIFFERENT
PACING AND STANDBY RATES, and assigned to the assignee of the
present invention.
Claims
What is claimed is:
1. In a demand type heart pacer including input and output means
adapted to be connected to a heart, the input means connected to
means for sensing natural heartbeats, the improvement comprising:
pulse generator means connected to the output means and including
timing circuit means operable to control output pulses from said
generator means at a selected frequency; means connecting the means
for sensing natural heartbeats to said pulse generator means for
resetting said timing circuit means on the occurrence of a natural
heartbeat; and further timing circuit means connected to said
timing circuit means for changing the frequency of output pulses
from said generator means for one pulse cycle following a pulse
cycle in which a natural heartbeat occurs.
2. In a demand heart pacemaker including power supply means,
sensing means for detecting a natural heartbeat, pulse generator
means for supplying pulses to stimulate a heartbeat and means for
connecting the sensing means and generator means to a heart, the
improvement comprising: resistor means and first capacitor means
serially connected across the power supply means; said first
capacitor means connected to the pulse generator means for
controlling the frequency of pulses therefrom according to the
charging time of said first capacitor means; and circuit means
including second capacitor means connected to said first capacitor
means for selectively changing the charge time of said first
capacitor means for one pulse cycle following a pulse cycle in
which the natural heartbeat occurs.
3. In a demand type heart pacer including input and output means
adapted to be connected to a heart, the input means connected to
means for sensing natural heartbeats, the improvement comprising:
pulse generator means connected to the output means and including
timing circuit means operable to control output pulses from said
generator means at a selected frequency; and further means
connecting the means for sensing natural heartbeats to said pulse
generator means for resetting said timing circuit means on the
occurrence of a natural heartbeat and including further timing
circuit means operably connected to said timing circuit means on
the occurrence of a natural heartbeat for decreasing the frequency
of output pulses from said generator means for one pulse cycle
following a pulse cycle in which a natural heartbeat occurs.
4. The apparatus of claim 3 in which: said timing circuit means
includes resistor means and capacitor means connected in an RC
timing circuit; and said further timing circuit means includes
further capacitor means and switch means for connecting said
further capacitor means into said RC timing circuit.
5. The apparatus of claim 3 in which: said timing circuit means
includes serially connected resistance means and capacitance means;
and said further timing circuit means includes further capacitance
means, means including diode means connecting said further
capacitance means across said capacitance means, said diode means
poled to prevent said further capacitance means from discharging
into said capacitance means, and switch means connected across said
further capacitance means and connected to said further means for
discharging all said capacitance means on the occurrence of a
natural heartbeat.
6. In a demand heart pacer including power supply means, sensing
means for detecting a natural heartbeat, pulse generator means for
supplying pulses to stimulate a heartbeat and means for connecting
the sensing means and generator means to a heart, the improvement
comprising: resistor means and first capacitor means serially
connected across the power supply means; said first capacitor means
connected to the pulse generator means for controlling the
frequency of pulses therefrom according to the charging time of
said first capacitor means; and circuit means connected to the
sensing means and to said first capacitor means and operable in
response to the occurrence of a natural heartbeat for changing the
charge time of said first capacitor means for one pulse cycle
following a pulse cycle in which the natural heartbeat occurs.
7. The apparatus of claim 6 in which said circuit means comprises:
second capacitor means; switch means connected between the sensing
means and said first and second capacitor means for connecting said
first and second capacitor means in parallel charging paths in
response to the occurrence of a natural heartbeat for increasing
the charging time of said first capacitor means.
8. The apparatus of claim 6 including noise responsive means for
blocking repetitive noise signals from said circuit means
comprising: capacitor means having an input plate connected to the
sensing means and an output plate connected to said circuit means;
a first discharge path for said capacitor means connected to said
output plate and including serially connected diode means and
resistor means, said diode means poled to pass current of a first
polarity, said resistor means having a resistance value selected
for generally fast discharge of said capacitor means; and a second
discharge path for said capacitor means connected to said output
plate and including further resistor means having a resistance
value selected for generally slow discharge of said capacitor means
for charging said capacitor means with repeated noise pulses of a
second polarity at a frequency above that of the normal heartbeat
range to block further of said noise pulses of the second
polarity.
9. In a demand pacer having input and output means adapted to be
connected to a heart, pulse generator means connected to the output
means and sensing means for detecting natural heartbeats connected
to the input means, the improvement comprising: resistor means and
first capacitor means connected in an RC timing circuit and to the
pulse generator means for controlling the frequency of the pulse
generator means according to the charge time of said first
capacitor means; first switch means connected to said first
capacitor means and to the pulse generator means, and actuated by
an output pulse from the pulse generator means to discharge said
first capacitor means; second capacitor means; means including
diode means connecting said second capacitor to said first
capacitor means, said diode means poled to permit said first and
second capacitor means to charge in parallel and to prevent the
discharge of said second capacitor means by said first switch
means; and second switch means connected to said second capacitor
means and the sensing means, and actuated by a natural heartbeat to
discharge said first and second capacitor means.
10. The apparatus of claim 9 including noise responsive means
comprising: third capacitor means connected between the sensing
means and said second switch means; second resistor means; further
diode means; third resistor means; means for connecting said second
resistor means across said third capacitor means to provide a first
discharge path, and for connecting said further diode means and
said third resistor means across said third capacitor means to
provide a second discharge path for one electrical polarity; and
said second and third resistor means having resistances selected to
make the time constant of said first discharge path substantially
greater than the time constant of said second discharge path.
11. In a demand type heart pacer including input and output means
adapted to be connected to a heart, the input means connected to
means for sensing natural heartbeats, the improvement comprising:
pulse generator means connected to the output means and including
timing circuit means operable to control output pulses from said
generator means at a selected frequency; further means connecting
the means for sensing natural heartbeats to said pulse generator
means for resetting said timing circuit means on the occurrence of
a natural heartbeat; and further timing circuit means operably
connected to said timing circuit means on the occurrence of an
output pulse from said pulse generator for increasing the frequency
of output pulses from said generator means for one pulse cycle
following the absence of a natural heartbeat.
12. The apparatus of claim 11 in which: said timing circuit means
includes resistor means and capacitor means connected in an RC
timing circuit; and said further timing circuit means includes
further capacitor means connected to said RC timing circuit and
switch means operable on the occurrence of an output pulse from
said pulse generator for charging said further capacitor means.
13. The apparatus of claim 11 in which: said timing circuit means
includes serially connected resistance means and capacitance means;
and said further timing circuit means includes further capacitance
means, means connecting said further capacitance means across said
capacitance means, switch means adapted to connect said further
capacitance means across a source of energy, and means connecting
said switch means to said pulse generator for receiving pulses
therefrom.
14. In a demand heart pacer including power supply means, pulse
generator means for supplying stimulating pulses to a heart and
means for connecting the generator means to a heart, the
improvement comprising: resistor means and first capacitor means
serially connected across the power supply means; said first
capacitor means connected to the pulse generator means for
controlling the frequency of pulses therefrom according to the
charging time of said first capacitor means; and circuit means
connected to the pulse generator and to said first capacitor means
and operable in response to the occurrence of a generated
stimulating pulse for changing the charge time of said first
capacitor means.
15. The apparatus of claim 14 in which said circuit means
comprises: second capacitor means; switch means connecting said
second capacitor means across the power supply; said switch means
connected to and controllable by the pulse generator for charging
said second capacitor means on the occurrence of a generated
stimulating pulse; and means connecting said first and second
capacitor means in parallel for decreasing the charging time of
said first capacitor means.
16. The apparatus of claim 14 including noise responsive means for
blocking repetitive noise signals from said circuit means
comprising: capacitor means having an input plate connected to the
sensing means and an output plate connected to said circuit means;
a first discharge path for said capacitor means connected to said
output plate and including serially connected diode means and
resistor means, said diode means poled to pass current of a first
polarity, said resistor means having a resistance value selected
for generally fast discharge of said capacitor means; and a second
discharge path for said capacitor means connected to said output
plate and including further resistor means having a resistance
value selected for generally slow discharge of said capacitor means
for charging said capacitor means with repeated noise pulses of a
second polarity at a frequency above that of the normal heartbeat
range to block further of said noise pulses of the second
polarity.
17. In a demand pacer having input and output means adapted to be
connected to a heart, pulse generator means connected to the output
means and sensing means for detecting natural heartbeats connected
to the input means, the improvement comprising: resistor means and
first capacitor means connected in an RC timing circuit and to the
pulse generator means for controlling the frequency of the pulse
generator means according to the charge time of said first
capacitor means; first switch means connected to said first
capacitor means and to the sensing means, and actuated by a pulse
from the sensing means to discharge said first capacitor means;
second capacitor means; means connecting said second capacitor to
said first capacitor means for discharging said second capacitor
into said first capacitor; and second switch means connected to
said second capacitor means and the pulse generator means and
actuated by a generated pulse to charge said second capacitor
means.
18. The apparatus of claim 17 including noise responsive means
comprising: third capacitor means connected between the sensing
means and said first switch means; second resistor means; diode
means; third resistor means; means for connecting said second
resistor means across said third capacitor means to provide a first
discharge path, and for connecting said diode means and said third
resistor means across said third capacitor means to provide a
second discharge path for one electrical polarity; and said second
and third resistor means having resistances selected to make the
time constant of said first discharge path substantially greater
than the time constant of said second discharge path.
19. In a heart pacer including input and output means adapted to be
connected to a heart, the input means connected to sensing means
for detecting a natural heartbeat and the output means connected to
pulse generator means for supplying pulses to stimulate a heart,
noise responsive means for preventing repetitive noise signals
sensed by the sensing means from disabling the pulse generator
means comprising: capacitor means having an input plate connected
to the sensing means and an output plate connected to the pulse
generator means; a first discharge path for said capacitor means
connected to said output plate and including serially connected
diode means and resistor means, said diode means poled to pass
current of a first polarity, said resistor means having a
resistance value selected for generally fast discharge of said
capacitor means; and a second discharge path for said capacitor
means connected to said output plate and including further resistor
means having a resistance value selected for generally slow
discharge of said capacitor means for charging said capacitor means
with repeated noise pulses of a second polarity at a frequency
above that of the normal heartbeat range to block further of said
noise pulses of the second polarity.
20. In a demand type heart pacer having pulse generator means
including timing means operable to control output pulses from the
generator means at a selected frequency, having sensing means for
sensing natural heartbeats including means refractory to applied
signals for a predetermined refractory period following each pacer
pulse, having reset means, operatively connected to the pulse
generator means, for resetting the timing means upon occurrence of
each output pulse and having means connected to the pulse generator
means and the sensing means and adapted to be connected to a heart;
the improvement comprising: further means, operatively connecting
the sensing means to the pulse generator means, for resetting the
timing means for a predetermined limited time upon termination of
the refractory period in response to repetitive signals, sensed by
the sensing means, which exceed a predetermined rate greater than a
natural heart rate and including means for preventing the timing
means from continuous resetting in response to such repetitive
signals, thereby decreasing the frequency of output pulses from the
pulse generator means and causing the pacer to operate
asynchronously.
Description
BACKGROUND OF THE INVENTION
This invention is concerned with electronic heart pacing apparatus,
and in particular a demand type heart pacer. Demand pacers are
known in the art to be the type of pacer which provides electrical
pulses to stimulate the heart only in the absence of normal
heartbeats. Prior art demand pacers generate pulses at a fixed
frequency, the generated pulses being prevented if a natural
heartbeat appeared during the period between generated pulses. As
possible complications can occur if external electrical pulses are
provided to a heart which is beating naturally, it would obviously
be advantageous for the heart monitoring pacer to be able to wait a
longer period of time than that provided by its normal pulse
generation frequency, once a natural heartbeat has occurred, to be
certain that it does not provide an unnecessary external pulse.
Yet, the heart monitoring pacer must be certain not to allow the
heartbeat rate to fall below a minimum frequency. The apparatus of
this invention provides, with novel circuitry, the advantages
described above, and in addition provides circuitry for eliminating
dangers due to continuous external noise electrical signals which
may enter the electrical system of the demand pacer.
SUMMARY OF THE INVENTION
Briefly described, the apparatus of this invention comprises a
demand pacer having a pulse generator and an amplifier connected to
the heart through a pair of electrodes. The pulse generator
provides controlled pulses to the heart through the electrodes,
while the amplifier senses signals from the heart through the
electrodes, for controlling the pulse generator. The normal
frequency of the pulse generator is controlled by apparatus
including the charging time of a capacitor. In a first embodiment
of this invention further apparatus including another capacitor is
connected between the pulse generator and the sensing amplifier.
This second capacitor is selectively connected in parallel with the
pulse generator frequency determining capacitor on occurrence of a
natural heartbeat, and thus increases the charging time of the
capacitance for the pulse generator frequency determining
apparatus, to thus lower the frequency of pulses generated. When no
natural heartbeat occurs, the second capacitor is not connected to
the pulse frequency determining capacitor, and the pulse generator
runs at its fixed higher frequency. In a second embodiment of this
invention the further apparatus is connected to the output of the
pulse generator such that the second capacitor is selectively
charged during each stimulating pulse to the heart. The second
capacitor is connected to the pulse generator frequency determining
capacitor so as to add its charge to that of the power supply and
thus decrease the charging time, to increase the frequency of the
pulse generator in the absence of a natural heartbeat.
Therefore, the demand pacer apparatus of this invention will
proceed to supply pulses to the heart at a normal pacing rate of a
first frequency until it senses an ectopic beat, after which it
will wait a longer period than the normal pacing frequency period
before it pulses again. If no further ectopic beats are sensed it
will return to the normal pacing rate after the next generated
pulse. It is important to note that with an irregular heart rate
causing many ectopic beats, the apparatus of this invention will be
alternately inhibited and then pacing. However, the minimum average
pacing rate must be the standby pacing rate. Further, any sinus
rhythm above the standby rate completely inhibits the unit.
Therefore, the apparatus of this invention will allow normal sinus
rhythm to control at a rate below or above its own normal pacing
frequency, yet provides a minimum rate determined by the standby
frequency unless repetitive interference is present. When such
interference occurs, the pacer circuitry causes the device to
revert to asynchronous operation at a rate somewhat lower than the
standby rate.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic diagram showing the electronic circuitry of a
first embodiment of the apparatus of this invention; and
FIG. 2 is a variation of a portion of the schematic of FIG. 1
showing a second embodiment of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1 there is shown a source of energy here shown as a power
supply comprising batteries 11. The positive terminal of supply 11
is connected to a positive bus 12, while the negative terminal of
supply 11 is connected to a negative bus 13. A capacitor 14 is
connected between busses 12 and 13 for supply regulation purposes,
in a manner well known to those skilled in the art. A transistor 15
has an emitter connected to negative bus 13, and a collector
connected through a resistor 16 to positive bus 12.
There are also shown a pair of transistors 19 and 20, each having
an emitter connected through a resistor 23 to negative bus 13. The
collector of transistor 19 is connected to positive bus 12, while
the base of transistor 19 is connected through a resistor 21 to bus
12 and through a resistor 22 to bus 13. The collector of transistor
20 is connected through a resistor 24 to bus 12, and through a
capacitor 25 to bus 12.
A transistor 26 has an emitter connected directly to bus 12, a base
connected to the collector of transistor 20, and a collector
connected to a junction 10. Junction 10 is connected through a
serial combination of a resistor 27 and a capacitor 28 to the base
of transistor 20.
A transistor 30 has an emitter connected to negative bus 13, a
collector connected through a resistor 31 to the base of transistor
20, and a base connected through a serial combination of a resistor
33 and a diode 32 to junction 10. Junction 10 is also connected
through a resistor 29 to the base of transistor 15.
The collector of transistor 30 is also connected to a variable
resistor 36. The wiper arm of resistor 36 is connected through a
resistor 37 to positive bus 12. The collector of transistor 30 is
connected to negative bus 13 through each of a pair of capacitors
34 and 35.
A transistor 40 has an emitter connected to negative bus 13, a
collector connected through a diode 38 to the collector of
transistor 30, and a base connected through the serial combination
of a diode 41 and a resistor 42 to negative bus 13. The collector
of transistor 40 is also connected through a capacitor 39 to bus
13, while the base of transistor 40 is also connected to bus 13
through a resistor 43.
A transistor 48 has an emitter connected to positive bus 12, and a
collector connected through a resistor 46 to negative bus 13. The
collector of transistor 48 is also connected to negative bus 13
through a capacitor 45, and is connected to the base of transistor
40 through a capacitor 44. A resistor 51, a capacitor 52 and a
resistor 53 are serially connected between positive bus 12 and
negative bus 13. A field effect transistor 55 has a gate connected
to a junction between capacitor 52 and resistor 53, a source
connected to positive bus 12, and a drain connected to the base of
transistor 48. A diode 57 is connected between the source and drain
of field effect transistor 55.
A transistor 60 has an emitter connected to positive bus 12 through
each of a resistor 61 and a capacitor 62. The collector of
transistor 60 is connected through a resistor 63 to negative bus
13, and through a capacitor 58 to the base of transistor 48. The
base of transistor 60 is connected through a resistor 66 to
positive bus 12.
A field effect transistor 65 has a drain connected to the base of
transistor 60, and a source connected to negative bus 13 through
each of a resistor 67 and a capacitor 68. The gate of field effect
transistor 65 is connected to negative bus 13 through each of a
pair of oppositely poled diodes 71 and 72.
The collector of transistor 15 is connected through a capacitor 74
to an electrode 75. Electrode 75 is connected through the serial
combination of a capacitor 76 and a resistor 78 to the gate of
field effect transistor 65. A resistor 77 is connected from a point
intermediate capacitor 76 and resistor 78 to negative bus 13.
Another electrode 80 is connected to negative bus 13. Finally,
junction 10 is connected through a diode 79 to the junction between
capacitor 52 and resistor 53.
To best understand the operation of the apparatus shown in FIG. 1,
assume that no natural heartbeats have been present and that the
pulse generator of the pacer has been running at its normal
repetition rate. Assume also that an output pulse has just been
completed.
The pulse generator can be generally described as including
transistors 19, 20, 26 and 30, along with their associated
components including in particular capacitors 28 and 35. Transistor
15 comprises the output transistor.
During the last output pulse, capacitor 74 will have discharged for
a period of time determined by the pulse generator. With the pulse
generator off during the period between pulses, transistor 15 will
be off, and capacitor 74 will recharge through the path comprising
battery 11, bus line 12, resistor 16, capacitor 74 itself, the
heart through electrodes 75 and 80, and through bus line 13 to
battery 11. This will cause the plate of capacitor 74 connected to
the collector of transistor 15 to be positive with respect to its
opposite plate.
As capacitor 74 is charging, so will be capacitor 35. Capacitor 35
is charged through variable resistor 36 and resistor 37. As
capacitor 39 will have charged at some previous time, and will not
have discharged during the last pulse in the absence of a natural
heartbeat, for reasons more fully described below, capacitor 39
will have no significant affect on the time for the charge of
capacitor 35. Capacitor 34 is selected to be an RF bypass capacitor
to prevent turn-on of the pulse generator from stray pickup, and
will, therefore, be sufficiently smaller than capacitor 35 so as to
be essentially ignored in considering the timing of the pulse
generator.
Transistors 19 and 20 are connected in a well known differential
amplifier manner to provide temperature compensation for the
turn-on point of transistor 20, to maintain substantially constant
timing for the pulse generator. Resistors 21, 22, 23 and 24 act as
bias resistors for transistors 19 and 20. Capacitor 25 is a bypass
capacitor connected across resistor 24.
When capacitor 35 becomes sufficiently charged, the positive
potential felt across it will be felt at the base of transistor 20
to turn it on. The turn-on of transistor 20 will cause a drop in
its collector voltage which will be felt at the base of transistor
26 to turn it on. The turn-on of transistor 26 will cause a
positive potential from bus line 12 to be felt at junction 10. This
positive potential at junction 10 will cause several actions.
First, the positive potential at junction 10 will be felt through
capacitor 28 to provide positive feedback to the base of transistor
20 to turn it on stronger and keep in on. Second, the positive
potential at junction 10 will be felt through diode 32 and resistor
33 on the base of transistor 30 to turn it on, causing the
discharge of capacitor 35. Thirdly, the positive potential at
junction 10 will be felt through resistor 29 on the base of output
transistor 15 to turn it on, causing capacitor 74 to discharge
through the collector-emitter path of transistor 15, and through
electrode 80 to the heart and back through electrode 75 to
capacitor 74. Finally, the positive potential at junction 10 will
be felt through diode 79 to cause an inhibit signal to prevent the
preamplifier from sensing the pacemaker pulse as though it were a
natural heartbeat, or R wave, in a manner to be more fully
described below.
The turn-on of transistor 30 will quickly discharge capacitor 35,
and transistor 30 will stay on until the positive potential
disappears at junction 10. The positive potential to junction 10
will remain as long as transistor 26 is on, which is determined by
the on time of transistor 20. Transistor 20 will remain on for a
period of time determined by the charge time of capacitor 28,
through which the positive potential at junction 10 is felt.
Therefore, the charge time of capacitor 28 determines the on time
of the pulse generator, thus determining the on time of output
transistor 15 and the output pulse width.
When capacitor 28 has charged sufficiently so that the potential on
the base of transistor 20 is no longer sufficient to keep it on,
transistor 20 will turn off to turn off transistor 26 and remove
the positive potential from junction 10, thus causing the turnoff
of transistor 30 and output transistor 15. Capacitors 74 and 35
will then again begin to charge in the manner described above, and
the pulse generator cycle will be repeated, provided no natural
heartbeat occurs, as described below.
The amplifier of the pacer of FIG. 1 comprises field effect
transistor 65, transistors 60 and 48, and their associated circuit
components. Resistors 46, 61, 63, 66 and 67 are bias resistors for
their respective transistors. Capacitors 62 and 68 are bypass
capacitors. Field effect transistor 65, and transistor 60, are so
biased as to be class A amplifiers and to amplify input signals of
either polarity. Resistor 77 is a DC bias reference for the gate of
field effect transistor 65. Resistor 78 is a limiting resistor and
works with diodes 71 and 72 to limit the input voltage to the gate
of field effect transistor 65.
When a pulse of either polarity appears at electrode 75, it will be
felt through capacitor 76 and resistor 78 across the parallel
combination diodes 71 and 72. As field effect transistor 65 is
class A biased, a pulse of either polarity, limited by diodes 71
and 72 along with resistor 78, will be amplified by it. This will
cause a change of potential on the drain electrode of field effect
transistor 65, which will in turn be felt on the base of class A
amplifier connected transistor 60 and further amplified by it.
Transistor 60 will then have a change of potential at its
collector, which will be differentiated and passed through
capacitor 58 to the base of transistor 48. In the embodiment shown
in FIG. 1, transistor 48 is class C connected, and will turn on
when the negative portion of the differentiated signal through
capacitor 58 reaches the base of transistor 48. Diode 57 will
prevent capacitor 58 from eventually charging due to a series of
negative going potential changes at the collector of transistor
60.
The turn-on of transistor 48 causes its collector to go positive.
This positive signal passes through a reversion to fixed rate
circuit which will be more fully described below. For the present,
it is sufficient to recognize that the positive signal will pass
through capacitor 44 to the base of transistor 40 to turn it on.
The turn-on of transistor 40 causes an interruption of the normal
charge cycle of capacitor 35 and changes the pacer to its standby
or sentinel frequency which is lower than that of the normal
frequency of the pulse generator. This circuitry will also be more
fully described below.
The above-described amplifier has a refractory period primarily
caused by bypass capacitors 62 and 68. These capacitors are chosen
to be of a value so that they will charge during the presence of an
input signal passing through the amplifier, and the discharge time
of capacitors 62 and 68 will constitute a refractory period for the
amplifier. The capacitors must be chosen with care as they will
also affect the gain of the amplifier.
As has been described above, a pulse of either polarity appearing
at electrode 75 will be felt through the amplifier to affect the
frequency of the pulse generator. To prevent this from occurring
when the pulse at electrode 75 is the pacemaker generated pulse
rather than a natural heartbeat or R wave, an inhibit circuit is
provided which comprises field effect transistor 55 and its
associated components resistor 51, capacitor 52 and resistor 53. As
has been described above, the positive potential at junction 10
which turns on output transistor 15 will also be felt through diode
79 to the inhibit circuit. This positive potential will be felt on
the gate of field effect transistor 55 to raise the gate potential
to substantially that of the source potential of field effect
transistor 55, thus removing the reverse bias and turning on field
effect transistor 55. This will cause a positive potential to be
felt on the base of transistor 48 thus holding it off and
inhibiting any pulse from the amplifier from passing through to the
pulse generator. The amplifier will have a small recovery pulse
appearing at the base of transistor 48 following its refractory
period, due to the charge and discharge of capacitors 62 and 68,
but this recovery pulse is not of sufficient magnitude to turn on
transistors 48 and 40 unless repetitive interference is present as
will be more fully described below. Capacitor 52 is provided to
hold on field effect transistor 55 for a period of time sufficient
to prevent the pacemaker pulse from turning on transistor 48 but to
allow the recovery pulse to be transmitted to the base of
transistor 48. Capacitor 52 will be discharged by the pulse
appearing from junction 10 through diode 79, and in recharging will
keep a positive potential on the gate of field effect transistor 55
to keep it on for a period of time determined by the resistance
present in the charging path of capacitor 52. Thus, the pacer of
FIG. 1 will prevent its normal pulse generator output frequency
from being affected by sensing of its own output pulse.
There has been described above the operation of the apparatus by
which the pulse generator of FIG. 1 provides heart stimulating
pulses at a fixed frequency, and how the sensing portion of the
apparatus is inhibited from interrupting the operation of the pulse
generator at the fixed frequency when the generated pulses
themselves are sensed. However, as has been stated above, it is the
purpose of the apparatus of this invention, once a natural
heartbeat or R wave has been sensed, to lower the pulse generator
frequency, that is, increase the cycle period between pulses, to
give the heart an opportunity to beat naturally without stimulus
pulses from the pacer, even though the heart rate should be lower
than that of the normal pulse generator frequency. Also, the
standby or lower frequency used when a heartbeat is detected
becomes substantially the lowest frequency at which the heart will
be allowed to beat in the absence of repetitive noise signals. That
is, the standby frequency normally sets a lower limit of heartbeat
rate, though the pacer as a whole will allow the heart to beat
naturally between that lower standby rate and the higher normal
pulse generator frequency rate.
For purposes of example, assume the normal pulse generator
frequency to be such as to provide pulses for an equivalent heart
rate of 72 bpm. If the apparatus of FIG. 1 is pacing, as in
complete heart block, and the heart produces an ectopic beat after
a previous pacer pacemaker pulse and the refractory period of the
amplifier, but before the next pacer pacemaker pulse, the pulse
generator will be reset. The pacer pacemaker will then pulse again
at a time after the ectopic beat determined by the standby rate. If
the heart continues natural beating at a rate greater than the
standby rate, the pulse generator of the pacer pacemaker will be
prevented from providing further pulses. However, if after the
ectopic beat no other beat occurs within the standby rate period,
for example 1,000 ms for a standby rate of 60 bpm, the pacer
pacemaker will again provide a pulse at the end of this period.
Then, if no further natural heartbeats are sensed, the unit will
pulse again at the normal pulse generator frequency of 72 bpm,
until another natural heartbeat is sensed. Thus, the apparatus of
this invention, after sensing a natural heartbeat, waits for a
period of time longer than the normal pacing frequency before it
provides another heart stimulating pulse. If no more natural
heartbeats are sensed, the pacer returns immediately, and during
the next period of time between pulses, to the normal pacing
rate.
The operation of the apparatus of FIG. 1 to achieve the standby
rate will now be explained. Assume that a natural heartbeat or R
wave appears at electrode 75. In the manner described above, this
pulse will be felt through capacitor 76 and resistor 78 to pass
through the amplifier of the apparatus of this invention. If this
natural heartbeat or R wave appears at electrode 75 at a time when
a generated pulse is not present and when the refractory period of
the amplifier is completed, transistor 48 will be turned on, to
turn on transistor 40 in the manner described above. When
transistor 40 turns on, it provides a discharge path for capacitor
39. Note that in normal operation of the pulse generator, capacitor
39 will have charged through resistors 36 and 37 and diode 38.
However, when transistor 30 is turned on to discharge capacitor 35,
diode 38 will prevent the discharge of capacitor 39. Thus in normal
operation only capacitor 35 has a substantial affect on the timing
for the pulse generator. However, when transistor 40 is turned on
to discharge capacitor 39, capacitor 35 will also discharge through
diode 38 and transistor 40 and the pulse generator timing cycle
will be completely reset. Further, current flowing through
resistors 36 and 37 will now be divided between capacitors 35 and
39 until such time as diode 38 becomes reverse biased by the charge
on capacitor 39. As the capacitance is increased by the addition of
capacitor 39, and as the amount of current available through
resistors 36 and 37 remains essentially the same, it will obviously
take longer for capacitor 35 to charge to the potential where it
will turn on transistor 20 of the pulse generator as described
above. Therefore, it is apparent that each time a pulse passes
through the amplifier to turn on transistor 40, the entire pulse
generator cycle will be reset, and the next timing cycle will be at
the standby rate which is a longer period than that of the normal
pulse generator frequency. If another natural heartbeat appears
during the standby period, the pulse generator will again be reset
and prevented from providing a stimulating pulse, and the standby
rate will again commence. Thus, as long as a natural heartbeat has
appeared during the standby period, no stimulating pulse will be
provided from the pacer, and following each natural heartbeat the
pacer pulse generator will operate at the standby frequency. If a
standby frequency period is completed without the appearance of a
natural heartbeat, the pacer will provide a stimulating pulse and
immediately return to the normal pacing frequency, provided no
further natural heartbeats appear.
The apparatus of this invention also includes circuitry for
automatically reverting to asynchronous operation at a third
frequency of the pulse generator different from either the first or
second frequencies when it is subjected to interference from
electrical signals of a repetitive nature. To activate the
reversion circuitry, such repetitive noise signals must have a
repetition rate in excess of a designed minimum rate greater than
the upper limit of normal heartbeat rates. In FIG. 1, this
circuitry comprises diode 41, resistor 42 and resistor 43 as
combined with coupling capacitor 44. The refractory period of the
amplifier contributes to determination of the pulse generator
frequency in the presence of such interference.
Assume, for example, that the apparatus of FIG. 1 is exposed to
external electrical noise of a periodic nature, such as a sine
wave, and that the sine wave enters the amplifier after its
refractory period and so is felt at the collector of transistor 48.
If such were the case and no noise protective circuitry were
present, then the sine wave would be felt across coupling capacitor
44 and would turn on transistor 40 to continuously reset the pulse
generator, which, of course, is not desirable. To prevent this
continuous resetting of the pulse generator and consequent
continuous inhibition of the pacer regardless of heart activity due
to external noise, diode 41 serves to let the positive portion of
the sine wave pass through resistor 42 to ground. The negative
portion of this sine wave will see resistor 43, which is chosen to
be of substantially larger resistance than resistor 42. Therefore,
if a series of sine waves passes through capacitor 44, then
capacitor 44 will charge with the plate connected to the collector
of transistor 48 being positive and the other plate negative. This
positive charge facing the source of the sine waves will eventually
block out further positive portions of the sine wave and thereafter
prevent the turn-on of transistor 40. However, if the time between
positive pulses is great enough, such as in the range of normal
heartbeat frequencies or during the refractory period of the
amplifier, capacitor 44 will have sufficient time to discharge
through resistor 43, to avoid blocking the next incident pulse and
subsequent normally spaced heartbeat pulses.
It is thus apparent that whenever a source of noise, such as 60
cycle AC, is picked up by the apparatus of this invention, the
pulse generator of this invention is immediately reset to the
standby period and begins charging to fire as soon as capacitor 44
is substantially fully charged. When a pulse is fired by the pulse
generator it is sensed by the amplifier but blocked by the inhibit
circuit. Sensing of the pacemaker pulse makes the amplifier block
other signals for its refractory period, during which capacitors 35
and 39 begin charging at the standby rate and capacitor 44
discharges. At the end of the refractory period, if repetitive
interference is still present, it is superimposed on the recovery
pulse of the amplifier and causes portions of the pulse to have a
magnitude sufficient to rapidly and repetitively turn on
transistors 48 and 40 until capacitor 44 becomes fully charged.
Each turn-on of transistors 48 and 40 resets the pulse generator to
its standby period again, but since capacitor 44 charges quickly
and blocks subsequent interference pulses, the timing cycle begins
again almost immediately. Consequently, the resetting action in the
presence of repetitive noise (once on output pulse generation and
possibly several times in rapid succession on termination of the
refractory period) has been simply called "double reset."
The pulse firing and double reset repeat so long as interference is
present. Thus, in the presence of repetitive interference or noise,
the pacer runs at a rate slower than either the normal rate or the
standby rate and is not synchronized with natural heartbeats. The
pacer pulse repetition period in the presence of repetitive noise
signals is approximately the sum of the standby period and the
amplifier refractory period. For example, if the standby period is
1,000 ms and the refractory period is 150 ms, the pacer period in
the presence of repetitive interference is about 1,150 ms or a rate
of about 52 bpm. This slowdown of the pacer accompanying its
reversion to asynchronous operation in the presence of repetitive
noise signals is very useful in testing the functioning of the unit
after implantation. It can also be seen that the slowdown
accompanying a reversion to asynchronous operation can be achieved
in a demand pacer without the standby rate feature in a similar
manner by double reset of the simpler timing circuit used in such
devices.
Referring now to FIG. 2 there is shown a portion of the schematic
of FIG. 1 with necessary changes pertaining to a second embodiment
of the invention. In FIG. 2, those circuit elements which are the
same as in FIG. 1 bear the same number.
Therefore, in FIG. 2 there is again shown the pulse generator
generally described as including transistors 19, 20, 26 and 30,
along with their associated components. Transistor 15 again
comprises the output transistor. However, in FIG. 2, junction 10 is
connected to the base of transistor 20 through a capacitor 100, and
to the base of a transistor 110. It is thus apparent that the on
time of the pulse generator will be determined by the charge time
of capacitor 100 of FIG. 2, as opposed to capacitor 28 of FIG.
1.
Transistor 110 has its collector connected directly to bus 12,
while its emitter is connected to a capacitor 111 to bus 13. A
bypass capacitor 112 is connected across capacitor 111. The
collector of transistor 30 is connected through a resistor 114 to
bus 12, and through a resistor 113 to the emitter of transistor
110. The collector of transistor 30 is also connected through a
resistor 102 to the base of transistor 20, and through a capacitor
103 to bus 13. A bypass capacitor 104 is connected across capacitor
103. The base of transistor 30 is connected (not shown) to receive
an input signal from the sensing amplifier whenever a natural
heartbeat occurs, for example with circuitry similar to that shown
in FIG. 1.
In operation, assume again that no natural heartbeats have been
present and that the pulse generator of the pacer has been running
at its normal repetition rate. Assume also that an output pulse has
just commenced.
That means that the charge on capacitor 103 has built to a
sufficiently positive level to turn on transistor 20. As described
in the operation of the circuitry of FIG. 1 above, the turn-on of
transistor 20 will cause the turn-on of transistor 26 causing a
positive voltage rise at junction 10. This positive voltage rise
will be felt at the base of transistor 15 to turn it on and cause
capacitor 74 to discharge through the heart through terminals 75
and 80. The positive pulse at junction 10 will also be felt through
capacitor 100 to keep transistor 20 and thus transistor 26 turned
on. This voltage will be affective for the charge time of capacitor
100, which thus determines the output pulse width.
The positive pulse at junction 10 will also be felt through diode
32 and resistor 33 on the base of transistor 30 to turn it on and
thus discharge capacitor 103 to prepare it for the next timing
sequence. In addition, the positive pulse at junction 10 will be
felt on the base of transistor 110 to turn it on causing capacitor
111 to charge.
Now assume that the output pulse has been completed and that
transistor 20 turns off, thus turning off transistor 26 which
removes the positive voltage at junction 10 to thus turn off
transistors 15, 30 and 110. Capacitor 103 will have been generally
completely discharged by the turn-on of transistor 30, and will
commence its normal charge through the path comprising positive bus
12, resistor 114, through capacitor 103 to negative bus 13.
However, because transistor 110 was on during the generated output
pulse capacitor 111 will have received a charge. When the generated
pulse is completed and transistor 110 turns off, the charge on
capacitor 111 will discharge through the path comprising resistor
113, capacitor 103 and bus 13. Thus, in addition to its normal
charge path, capacitor 103 received an additional charge from
capacitor 111. This results in a faster charge time for capacitor
103 with a resulting increase in the frequency of the pulse
generator. It is thus apparent that when there is a generated or
stimulating output from the pulse generator, transistor 110 and
capacitor 111 will operate to cause an increased generating
frequency.
Assume now that there is a natural heartbeat felt at terminals 75
and 80. This natural heartbeat will be sensed at the sensing
circuitry (not shown in FIG. 2) which can be the same as that shown
in FIG. 1, for example. The resulting input from the sensing of the
natural heartbeat will be felt on the base of transistor 30 thus
discharging capacitor 103 with a resulting restart of the pulse
generator off period. However, inasmuch as transistor 110 did not
again turn on, there was no additional charge to capacitor 111, and
therefore no extra charge available for capacitor 103. Thus, the
charge time for capacitor 103 will be greater, causing a decreased
pulse frequency following the natural heartbeat.
Should there be no further natural heartbeats before the next pulse
generator output pulse, the generated pulse would again result in
the turn-on of transistor 110 and an increase of the frequency of
the pulse generator back to the stimulating rate.
It is thus apparent that the embodiment of FIG. 2 operates to
provide the same function as the apparatus of FIG. 1. FIG. 1
provides a decrease in the pulse generator frequency in the
presence of a natural heartbeat, while FIG. 2 provides an increase
in the pulse generator frequency when there is no natural
heartbeat, both embodiments thus providing a stimulating pulse at a
first frequency, and a standby rate of a lower frequency, as well
as a minimum frequency.
The apparatus of both of the embodiments of the drawings are
intended to be totally implantable within the body. The embodiments
carry their own source of power, battery 11, and are adapted to be
so constructed that all of the components are packaged in a
substance or substances substantially inert to body fluids and
tissue. Electrodes 75 and 80, shown schematically in the drawings,
are adapted to be connected directly to the heart muscle in one of
several manners well known in the art, and are made of an
electrically conductive material which is also substantially inert
to body fluids and tissue.
In FIG. 1 resistor 36 is shown to be a variable resistor which may
be used to vary the normal frequency of the pulse generator. In
addition, capacitor 39 may be a variable capacitor or comprises a
plurality of capacitors which are selectively connected in the
circuit by a manual switch or the like, to thus provide means for
selecting the standby or sentinel frequency of the pacer.
It is thus apparent that the novel apparatus of this invention
provides a demand type pacer having circuitry for providing heart
stimulating pulses at first rate and a lower pulse generating or
standby rate in the presence of a natural heartbeat, and having
further circuitry to cause asynchronous operation at a third rate
in the presence of external electrical noise of a periodic
nature.
* * * * *