U.S. patent number 3,757,792 [Application Number 05/197,895] was granted by the patent office on 1973-09-11 for automatic threshold compensating demand pacemaker.
This patent grant is currently assigned to Medtronic Inc.. Invention is credited to John R. Helland, Pieter M. J. Mulier.
United States Patent |
3,757,792 |
Mulier , et al. |
September 11, 1973 |
AUTOMATIC THRESHOLD COMPENSATING DEMAND PACEMAKER
Abstract
In a demand cardiac pacer having means for providing periodic
stimulating pulses to the heart, means for sensing natural
heartbeats, and means for inhibiting the stimulating pulses on the
occurrence of natural heartbeats, the improved apparatus including
means for sensing stimulated heartbeats, means for decreasing the
amplitude of a succeeding stimulating pulse on the occurrence of a
stimulated heartbeat and means for increasing the next succeeding
stimulating pulse by a predetermined amount on the failure to sense
a stimulated heartbeat.
Inventors: |
Mulier; Pieter M. J.
(Minneapolis, MN), Helland; John R. (Brooklyn Center,
MN) |
Assignee: |
Medtronic Inc. (Minneapolis,
MN)
|
Family
ID: |
22731180 |
Appl.
No.: |
05/197,895 |
Filed: |
November 11, 1971 |
Current U.S.
Class: |
607/11; 607/9;
331/183; 607/28 |
Current CPC
Class: |
A61N
1/371 (20130101); A61N 1/3712 (20130101) |
Current International
Class: |
A61N
1/362 (20060101); A61N 1/37 (20060101); A61n
001/36 () |
Field of
Search: |
;128/419P,421,422
;331/183 ;328/169 ;307/264 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Claims
We claim:
1. Cardiac pacer apparatus having first means for providing timed
cardiac stimulation pulses, second means for sensing natural
heartbeats, means connecting the second means to the first means
for inhibiting stimulation pulses when a natural heartbeat is
sensed, and electrode means connected to the first and second means
and adapted to be connected to a heart, the apparatus including:
third means connected to the electrode means for sensing stimulated
heartbeats; fourth means connected to the first and third means and
responsive to each sensing of a stimulated heartbeat for decreasing
the energy of the succeeding stimulation pulse by a predetermined
amount, the fourth means responsive to each failure to sense a
stimulated heartbeat for increasing the energy of the succeeding
stimulation pulse to an amount greater than the preceding
stimulation pulse but normally less than maximum energy; and energy
source means connected to all the means.
2. The apparatus of claim 1 including; further means connected to
the first and third means and responsive to each failure to sense a
stimulated heartbeat for providing an extra stimulation pulse prior
to the normal succeeding stimulation pulse.
3. The apparatus of claim 2 in which: the further means provides
the extra stimulation pulse at the maximum energy level.
4. The apparatus of claim 2 in which the electrode means comprises:
stimulating electrode means connected to the first means and
adapted to be connected to a heart; sensing electrode means
connected to the second means and adapted to be connected to a
heart; and further electrode means electrically common to the first
and second means and the stimulating and sensing electrode means,
and adapted to be connected to a portion of the body.
5. The apparatus of claim 4 in which: the further electrode means
includes dimensioned conductive surface means for minimizing
polarization effects of stimulation pulses.
6. The apparatus of claim 1 in which the second and third means
comprise: a single sense amplifier means having input means
connected to the electrode means and output means connected to the
first and fourth means.
7. The apparatus of claim 6 including: further means connected to
the first and third means and responsive to each failure to sense a
stimulated heartbeat for providing an extra stimulation pulse prior
to the normal succeeding stimulation pulse.
8. The apparatus of claim 7 in which: the further means provides
the extra stimulation pulse at the maximum energy level.
9. The apparatus of claim 1 including: fifth means connecting the
first means to the fourth means for inhibiting operation of the
fourth means in the absence of a stimulation pulse.
10. The apparatus of claim 9 including: further means connected to
the first and third means and responsive to each failure to sense a
stimulated heartbeat for providing an extra stimulation pulse prior
to the normal succeeding stimulation pulse.
11. The apparatus of claim 10 in which: the further means provides
the extra stimulation pulse at the maximum energy level.
12. The apparatus of claim 10 in which the fifth means includes:
means responsive to a stimulation pulse for enabling operation of
the fourth means for a limited time period following each
stimulation pulse.
13. The apparatus of claim 9 in which the fifth means includes:
means responsive to a stimulation pulse for enabling operation of
the fourth means for a limited time period following each
stimulation pulse.
14. The apparatus of claim 1 in which the electrode means
comprises: stimulating electrode means connected to the first means
and adapted to be connected to a heart; sensing electrode means
connected to the second means and adapted to be connected to a
heart; and further electrode means electrically common to the first
and second means and the stimulating and sensing electrode means,
and adapted to be connected to a portion of the body.
15. The apparatus of claim 14 in which: the further electrode means
includes dimensioned conductive surface means for minimizing
polarization effects of stimulation pulses.
16. An implantable demand cardiac pacer comprising: sense means for
sensing electrical signals from a heart; first timed pulse
generator means; energy storage means; controllable output means
connected across the energy storage means; controllable energy
charge path means connected to the energy storage means; second
timed pulse generator means; stimulation electrode means connected
to the output means and adapted to be connected to a heart; sense
electrode means connected to the sense means and adapted to be
connected to a heart; means connecting the sense means to the first
and second timed pulse generator means for resetting the first and
second generator means on the sensing of an electrical signal from
the heart; means connecting the first generator means to the output
means for controlling stimulation output pulses; means connecting
the second generator means to the controllable energy charge path
means; the the charge path means including means responsive to the
second generator means for controlling charging of the energy
storage means such that the energy level to which the storage means
is charged is proportional to the energy in the storage means
following the last stimulation output pulse; and all the means
adapted to be connected to a source of energy.
17. The apparatus of claim 16 in which: the energy storage means
comprises a capacitor; and the charge path means includes time
constant means connected in series with the capacitor.
18. The apparatus of claim 17 in which the time constant means
includes inductance means.
19. The apparatus of claim 16 including: inhibit means connected
between the first generator means and the second generator means;
the inhibit means including means normally operative for inhibiting
operation of the second generator means and means responsive to a
pulse from the first generator means for allowing operation of the
second generator means for a predetermined time period.
20. The apparatus of claim 19 in which: the second timed pulse
generator has a pulse repetition rate less than the predetermined
time period.
21. The apparatus of claim 19 including; further means connecting
the first generator means to the second generator means for
resetting the second generator means in response to a pulse from
the first generator means.
22. The apparatus of claim 19 including: means connected between
the output means and the second generator means and responsive to a
pulse from the second generator means for connecting the
stimulation electrode means across the source of energy.
23. The apparatus of claim 16 in which: the sense means includes
amplifier means having a refractory period not greater than 20
milliseconds.
24. In an implantable cardiac demand pacer having a source of
energy, sensing means connected to the source of energy, periodic
pulse generator means connected to the source of energy, and means
connecting the sensing means to the generator means for resetting
the generator means, the improvement comprising: sense electrode
means connected to the sensing means and adapted to be connected to
a heart; capacitor means; time constant means; first switch means;
means connecting the time constant means, the first switch means
and the capacitor means in series across the source of energy;
output means including stimulation electrode means adapted to be
connected to a heart; second switch means; means connecting the
output means and the second switch means in series across the
capacitor means; means connecting the pulse generator means to the
second switch means; second periodic pulse generator means
connected to the source of energy; means connecting the sensing
means to the second pulse generator means for resetting the second
pulse generator means; and means connecting the second pulse
generator means to the first switch means for controlling
connection of the capacitor means and the time constant means to
the source of energy.
25. The apparatus of claim 24 in which: the time constant means
includes inductance means.
26. The apparatus of claIm 24 including: third switch means
connected between the second pulse generator means and the source
of energy for normally inhibiting operation of the second pulse
generator means; window timing means connected between the periodic
pulse generator and the third switch means for allowing operation
of the second pulse generator means for a limited window period of
time following a pulse output from the periodic pulse
generator.
27. The apparatus of claim 26 in which: the periodic pulses from
the second generator are at a repetition rate less than the window
period of time.
28. The apparatus of claim 26 including: means connected between
the first switch means and the output means for connecting the
output means across the source of energy when the first switch
means is actuated.
Description
BACKGROUND OF THE INVENTION
Demand cardiac pacers, usually R-wave responsive, are well-known in
the art of electronic cardiac treatment. Such devices sense natural
heartbeats to inhibit stimulating pulses to the heart in the
presence of natural beats, and provide periodic stimulating pulses
to the heart in the absence of natural beats. The sensing
amplifiers of such devices generally have a refractory period of
sufficient length to mask the initial responses of the heart to the
stimulation pulses, that is, the stimulated heartbeats. The
refractory period is necessary due to artifacts caused by
polarization of the electrodes which act both as stimulating and
sensing electrodes. A typical one of such demand type pacers is
Medtronic, Inc. No. 5842.
Such prior art demand devices inherently carry high safety margins.
The stimulation energy is set at an amount known to be several
times that of the maximum required threshold energy in most
patients. As the threshold energy of the patient is not constant,
and may vary during a day, and as the threshold information is not
known to the device, maximum energy is always provided for
stimulation. Obviously, this results in maximum drain from the
power sources, and the result is shortened lifetime for the cardiac
demand device. It is to overcome this problem that the present
invention has been made, and the present invention has been found
to operate with significantly less battery drain than the prior art
demand devices.
It has been long known in the art that information related to the
patient's cardiac stimulus threshold is useful for many purposes,
such as detecting "end of life" for a pacer, and various prior art
schemes have existed for sensing threshold levels. One such prior
art threshold device is that described in "The Proceeding of the
Nordic Meeting on Medical and Biological Engineering," An
Experimental Pacemaker: A Method For Automatic Determination of
Heart Stimulation Threshold, by J. Meibom and L.S. Nielsen, Jan.
15, - 18, 1970, Finland. In this article there is described a pacer
which in the pacing mode provides that each stimulating impulse is
decreased by a given amount after each detection of a stimulated
heartbeat. Sensing of the stimulated heartbeat occurs for 100
milliseconds following each pacing impulse, and the pacing impulses
are continually reduced until no stimulating heartbeat is sensed
during the 100 millisecond period. When no stimulating heartbeat is
sensed, a new pacing impulse of full amplitude is generated
immediately after the 100 millisecond period, and the reduction
process commences again. Because the pacing impulses are reduced in
amplitude by a predetermined amount commencing with full value, the
threshold level can be determined by noting at which pulse heart
capture is lost. To avoid the problem of electrode polarization and
the resulting refractory period, this device uses separate pairs of
electrodes for stimulating and sensing.
Though the prior art cardiac device described in the preceding
paragraph teaches detection of threshold levels, it does not teach
and has little effect on battery drain as the stimulating pulse
returns to full amplitude following each loss of capture. The
apparatus of this invention provides for greatly decreased battery
drain and thus greatly increased device lifetime by sensing each
stimulated heartbeat and providing that each succeeding stimulating
pulse be decreased in energy until such time as no stimulated
heartbeat occurs. When loss of capture is sensed, the next
succeeding stimulating pulse is increased in energy, but is
increased by an amount only enough to be safely over the threshold
hysteresis level and not to maximum level. In one embodiment of the
present invention, an early extra stimulating pulse of maximum
energy is provided following loss of capture, and the following
normal pulse is reduced to a level only sufficiently above the
threshold hysteresis level to be safe, and it is from this reduced
pulse that further stimulating pulse energy reductions occur until
capture is once again lost.
SUMMARY OF THE INVENTION
Briefly described, the apparatus of this invention comprises a
demand cardiac pacer, R-wave responsive, with no built-in
refractory period. Sensing of natural and stimulated heartbeats are
accomplished with a single sensing amplifier. Separate sensing and
stimulating electrodes are preferable, each working with a third
common electrode, preferably with a large enough conductive area to
avoid significant polarization problems. The output stimulation
energy is provided from a capacitor which is connected across a
battery by time constant means and a switch. Each output pulse will
decrease the charge on the capacitor so that the succeeding output
pulse will be of less energy. Should an output pulse occur and no
stimulated heartbeat be sensed by the amplifier, a second timed
pulse circuit is provided, in addition to the output pulse circuit,
which will activate the switch for a predetermined period of time
to allow charging of the capacitor prior to the next output
stimulation pulse. Thus more energy will be available for the
output pulse following an output pulse which results in loss of
capture. The period of time for which the switch is on, and the
time constant means connected to the capacitor, are predetermined
such that the energy level of the capacitor will increase an amount
sufficient to provide a stimulation pulse safely above the
threshold level, taking into account threshold hysteresis, but not
maximum level unless it is needed. In one embodiment of the
invention, to avoid loss of a heartbeat due to loss of capture, an
immediate stimulation pulse of maximum energy level is provided,
and the next output pulse comes at the level below maximum value
and just safely above threshold level. In that embodiment, the
maximum level stimulation pulse is provided following a short
sensing period of approximately 100 milliseconds.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a combination block diagram and schematic of the first
embodiment of the apparatus of this invention;
FIG. 2 is a schematic diagram of a portion of FIG. 1, showing
components added to make a second embodiment of the apparatus of
this invention;
FIG. 3 is a graph of a generalized stimulated heartbeat response
using the apparatus of this invention;
FIG. 4 is a stylized graph of the output pulses during the pacing
mode of the apparatus of FIG. 1;
FIG. 5 is a stylized graph of the output pulses of the apparatus of
FIG. 2; and
FIG. 6 is a graph of output voltage verses time for a prior art
demand pacer, and for the apparatus of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to the figures of the drawings, it should be
understood that the graphs of FIGS. 3 - 6 are intended to be
representative of the plots they disclose, and are not necessarily
drawn to scale. It should also be understood that the term
"stimulating pulse" as used herein means an output pulse from the
apparatus of this invention, whether or not the heart responds to
the pulse. Also, as used herein, both the natural occurring and
artificially stimulated cardiac depolarizations are described and
shown as QRS signals.
Referring first to FIG. 1, there is disclosed a sense amplifier 11
shown in block diagram. Amplifier 11 is of the type generally used
in demand pacers and well-known to those familiar with the art, and
includes such circuits as extraneous noise or interference signal
filters, but excludes a designed long refractory period and
preferably has a design to prevent saturation.
There is also shown in FIG. 1 a stimulation pulse generator 15,
again shown in block diagram as such generators are well-known to
those skilled in the art. Generator 15 includes timing means for
providing periodic signal pulses, which timing means are reset when
the signal pulse is provided or when a signal from sense amplifier
11 indicates that an R-wave has been sensed from the heart.
Sense amplifier 11 has an input connected to a sensing electrode 12
adapted to be connected to a portion of the heart for sensing
heartbeats. The output of sense amplifier 11 is connected to the
input of generator 15 and to a reset bus 14. The output of
generator 15 is connected through a serial combination of a diode
17 and resistor 18 to the base of an output transistor 20. The
collector of transistor 20 is connected through a resistor 21 to a
positive bus 22 adapted to be connected to the positive terminal of
a source of energy such as batteries. The collector of transistor
20 is also connected through an output capacitor 23 to a
stimulating electrode 25 adapted to be connected to a portion of
the heart for providing stimulating pulses during the pacing mode
of the apparatus of this invention. An electrode 26 is connected to
positive bus 22. Electrode 26 acts as a common electrode for both
of electrodes 12 and 25 and is preferably of comparatively large
dimension to avoid polarization effects.
The emitter of transistor 20 is connected to a junction 27. The
base of transistor 20 is connected through a resistor 28 to
junction 27. Junction 27 is connected to bus 22 through a capacitor
29. Junction 27 is connected through an inductor 30 to the
collector of a transistor 31. The emitter of transistor 31 is
connected to a negative bus 32, adapted to be connected to the
negative terminal of the source of energy. The base of transistor
31 is connected through a resistor 33 to bus 32 and through a
resistor 39 to a junction 34. Junction 34 is connected through a
resistor 35 to the collector of a transistor 36. The emitter of
transistor 36 is connected to positive bus 22. The collector of
transistor 36 is connected through a serial combination of diode 37
and a resistor 38 to reset bus 14. The base of transistor 36 is
connected through a resistor 41 to bus 22, and is directly
connected to the collector of a transistor 42. The emitter of
transistor 42 is connected through a resistor 43 to negative bus
32. The base of transistor 42 is connected through a capacitor 45
to junction 34. The base of transistor 42 is also connected through
a resistor 46 to a junction 47. Junction 47 is connected through a
capacitor 48 to bus 32.
A transistor 50 has its collector connected to junction 47, and its
emitter is connected through a resistor 51 to bus 32. The collector
of transistor 50 is also connected through a resistor 52 to the
collector of a transistor 55. Transistor 55 has its emitter
connected to bus 22. The base of transistor 55 is connected through
a resistor 56 to bus 22, and is connected through a diode 57 to bus
22. The base of transistor 55 is also connected through a capacitor
58 to a junction 61. Junction 61 is connected through a resistor 62
to bus 32. Junction 61 is connected through a serial combination of
a diode 63 and a resistor 64 to the output of generator 15. The
base of transistor 50 is connected to reset bus 14, and is
connected through a serial combination of a diode 67 and a resistor
66 to the output of generator 15.
The modes of operation of the apparatus of this invention as shown
in the preferred embodiment of FIG. 1 are similar to those of prior
art demand cardiac pacers. The apparatus has a standby mode when
the heart is beating naturally, thus causing the reset of generator
15 following each sensed natural heartbeat to prevent an output
pulse for stimulating the heart. There is also a pacing mode which
is initiated when no natural heartbeat is sensed and no reset of
generator 15 occurs. The output signal pulse from generator 15 will
then cause stimulation pulses to be felt between electrodes 25 and
26, in a unique manner to be more fully described below. Finally,
the apparatus of FIG. 1 is constructed in a manner known to those
skilled in the art such that on failure of the sensing mechanism no
reset pulses will reach generator 15 and the entire apparatus will
operate as an asynchronous pacer with maximum energy
stimulation.
In the apparatus of this invention as shown in the embodiment of
FIG. 1, and the same sense amplifier 11 which is used in prior art
demand pacers is also used to control a second pulse generating
circuit comprising, in brief, transistors 36 and 42 along with
timing capacitor 48 and reset transistor 50. In a manner more fully
described below, if a stimulated heartbeat is not sensed between
electrodes 12 and 26 for a predetermined period of time following a
stimulation signal to the heart, the second pulse generator will
provide a signal to transistor 31 causing output voltage capacitor
29 to be charged for a period of time determined by capacitor 45.
Thus, the next stimulation pulse will be at a greater energy
magnitude than the preceeding stimulation pulse. Should a
stimulated heartbeat be detected by electrodes 12 and 26 during the
predetermined time, then a signal on reset bus 14 will turn on
transistor 50 to discharge capacitor 48. There will be no output
from the second pulse generator, and transistor 31 will remain off.
Thus, capacitor 29 will not be charged and the next stimulation
pulse will be of a decreased energy magnitude by an amount equal to
the energy expended from capacitor 29 in the previous stimulation
signal.
To best understand the operation of the apparatus of FIG. 1, assume
first that the device has been newly connected to the heart and
that capacitor 29 is at maximum charge. Assume now that natural
heartbeat fails, putting the apparatus of FIG. 1 into a pacing
mode. Stimulation pulse generator 15 will then provide an output
signal pulse approximately 1 millisecond in duration. This pulse
will have the several effects described below.
First, the stimulation pulse generator 15 output signal pulse will
pass through diode 17 and resistor 18 to turn on transistor 20. The
full charge across capacitor 29 will be felt across resistor 21 and
thus across electrodes 25 and 26 to stimulate the heart to which
electrode 25 is connected. This stimulation signal will be at
maximum energy due to the maximum charge on capacitor 29.
The output signal pulse from generator 15 will also be felt through
resistor 66 and diode 67 to turn on reset transistor 50, thus
discharging capacitor 48, the timing capacitor for the second pulse
generator. At the same time, the output signal pulse from generator
15 will cause the discharge of timing capacitor 58. This discharge
is through diode 57 and bus 22 through a portion of generator 15
(not shown) and back through resistor 64 and diode 63.
After the 1-millisecond output signal pulse from generator 15,
capacitor 58 will commence to recharge through resistors 56 and 62.
The recharge time for capacitor 58 is selected to provide the
"window" during which the apparatus of FIG. 1 will sense the
heart's response to the stimulation pulse between the electrodes 25
and 26. In the preferred embodiments of FIG. 1, this time is
selected to be 100 milliseconds. During the charge time of
capacitor 58, transistor 55 will be turned on, and at the end of
100 milliseconds transistor 55 will turn off.
The turn-on of transistor 55 completes a charge path for capacitor
48, comprising bus 22, transistor 55, resistor 52, capacitor 48 and
bus 32. This charge time is the period for the second pulse
generator and is selected to be less than the window time provided
by capacitor 58, which in this preferred embodiment it is selected
to be 80 milliseconds. The reasons for the timing selections will
be more fully described below with reference to FIG. 3 of the
drawings.
If the charge of capacitor 48 is allowed to continue for 80
milliseconds, the resulting voltage level will turn on transistor
42, which will in turn switch on transistor 36 to provide an output
pulse of a width determined by capacitor 45. This output pulse,
felt at the collector of transistor 36 will pass through resistors
35, 39 and 33 to turn on transistor 31. The turn-on of transistor
31 enables the charging of capacitor 29 through the timing constant
path comprising bus 22, capacitor 29, inductor 30, transistor 31
and bus 32. The timing constant for the charge of capacitor 29 is
selected so that the recharge of capacitor 29 provides more energy
than was expended in providing the last stimulation pulse. Of
course, when the last stimulation pulse has been at maximum energy,
capacitor 29 will only recharge back to maximum energy.
The output pulse on the collector transistor 36 will also pass
through diode 37 and resistor 38 to turn on reset transistor 50 and
discharge capacitor 48 thus turning off transistors 42 and 36 after
the output pulse width determined by capacitor 45.
It is to be noted that the above operation of the apparatus of FIG.
1 describes the method of recharging capacitor 29, which recharge
occurs only when capacitor 48 is allowed to charge for a full
predetermined period of time, in this case 80 milliseconds during a
window of 100 milliseconds. The pulse from transistor 36 which
actuates the time constant charging path for capacitor 29 will
hereinafter be referred to as the "escape" pulse. It is desired to
have the escape pulse provided only when the heart has not
responded, during the window time, to the stimulation pulse
provided between electrodes 25 and 26, that is, only when the
energy provided by the stimulation pulse is below the stimulation
threshold of the patient to cause cardiac response.
If a stimulated cardiac response is sensed by amplifier 11 across
electrodes 12 and 26, amplifier 11 will provide an output pulse,
which, in addition to resetting the timing of generator 15, will be
felt on reset bus 14 to turn on transistor 50. The turn-on of
transistor 50 will discharge capacitor 48 thus restarting the 80
millisecond charge time for the second pulse generator. If the
signal on bus 14 has occurred during the first 20 milliseconds of
the window time, capacitor 48 will still have sufficient time to
charge through transistor 55 to a level sufficient to turn on the
second pulse generator to provide the escape pulse. However, should
the sensing of the stimulated heartbeat occur after the first 20
milliseconds of the window time, capacitor 58 will reach a full
charge and turn off transistor 55 to prevent the charge on
capacitor 48 from reaching a turn-on level for the escape pulse.
Failure to produce the escape pulse will result in no recharge of
capacitor 29, and the next stimulating output pulse will be
provided at an energy level below that of the preceding pulse.
Further, the next output pulse signal from generator 15 will reset
capacitor 48, as described above, so that the watch time for the
escape pulse generation will commence with capacitor 48 starting
uncharged.
The cycle of operation described in the above paragraph, where no
escape pulse is provided and the stimulation energy provided from
capacitor 29 decreases with each succeeding stimulating signal by
an amount equal to the energy expended in the preceding stimulating
signal, will continue causing a train of cardiac stimulation pulses
of decreasing energy until such time as the stimulation energy
falls below the stimulus threshold of the patient's heart. When
this occurs and capture is lost, no sensing of stimulated heartbeat
is felt at amplifier 11 and capacitor 48 is not reset during the
window period. Then, as described above, an escape pulse will be
provided from transistor 36 and transistor 31 will turn on to
charge capacitor 29. However, the charge on capacitor 29 will not
return to maximum level in all cases, but only to an amount greater
than that last stimulation pulse which did provide capture. The
amount of charge of capacitor 29, when an escape pulse is provided,
is determined by the on time of the escape pulse which is
determined by capacitor 45 as described above, and by the timing
constant means including inductor 30. Capacitor 45 and inductor 30
are selected to assure that there is a sufficient recharge of
capacitor 29 to provide a safety margin, recognizing that there is
a stimulus threshold hysteresis effect in most patients, and that
the stimulation pulse following the pulse which loses capture must
be sufficiently high to provide capture on the next stimulation
pulse.
When capture has been lost and an escape pulse has provided a
recharge of capacitor 29, as described above, the cycling of the
apparatus will continue and if the first stimulation pulse
following recharge of capacitor 29 is sufficient to cause a
stimulated heartbeat, the next and succeeding stimulation pulses
will each be reduced in energy until capture is again lost and
another escape pulse is provided. Thus, the apparatus of this
invention as shown in the preferred embodiment of FIG. 1 will
"seek" the patient threshold level and will operate with
stimulation energy sufficient only to provide the minimum necessary
safety margins. By providing that the charge on capacitor 29 has
not returned to maximum energy level upon loss of capture, the
apparatus of this invention achieves great savings in power supply
drain, and thus adds significant time to the life of the automatic
threshold compensating demand pacer of this invention.
The apparatus of this invention is intended to be used primarily as
a fully implantable device. Therefore, the entire apparatus as
shown in FIG. 1, along with batteries used as a source of energy,
is intended to be encased in a substance substantially inert to
body fluids and tissue and placed subcutaneously in the body.
Electrodes 12 and 25 are preferrably connected to the heart to
perform their respective sensing and stimulating functions. Common
electrode 26 may, for example, be an external metal enclosure of
the implanted device, but is in any case preferrably of a large
conductive surface to avoid depolarization problems.
Reference to FIG. 3 will enable a better understanding of the
selection of the window timing for the above described apparatus of
FIG. 1. The cardiac depolarization response, or the QRS complex, of
the heartbeat is known to those familiar with the art. FIG. 3 shows
a 1-millisecond stimulating pulse from the apparatus of FIG. 1,
followed by the QRS response from the heart. It can be seen that
the QRS response occurs within approximately 15 milliseconds after
the stimulating pulse, and that the R-wave occurs within
approximately 75 milliseconds after the stimulating pulse. The peak
of the R-wave occurs approximately 50 milliseconds after the
stimulating pulse. It can also be seen that the T-wave does not
occur until approximately 210 milliseconds after the stimulating
pulse and lasts for approximately 100 milliseconds. It is the QRS
complex that the apparatus of FIG. 1 wishes to sense to determine
whether or not to provide an escape pulse. As is known to those
familiar with the art, the timing of the waves shown in FIG. 3 will
vary from patient to patient, and may vary within the patient
himself. It has been experimentally shown, and is generally
accepted by those familiar with the art, that the R-wave will be
completed in virtually all patients within 100 milliseconds after
the stimulating pulse. It is for this reason that the window has
been chosen at that length for the preferred embodiment. It is also
generally known from experiments that stimulation artifacts may
occur during the first 10 to 20 milliseconds after the stimulating
pulse. Because such an artifact may cause a reset pulse to appear
from amplifier 11 on reset bus 14 to reset capacitor 48, the timing
of the escape pulse of the apparatus of FIG. 1 has been set for 80
milliseconds. Thus, should a reset due to an artifact occur during
the first 20 milliseconds and the actual R-wave not be stimulated,
the apparatus of FIG. 1 will still be able to provide an escape
pulse to recharge capacitor 29. Should the artifact cause resetting
during the first portion of the window, and the R-wave in fact
occur during the window, then the escape pulse will be blocked as
described above.
The window feature described in the discussion of FIG. 1 is a
desirable but not a mandatory feature. The apparatus of FIG. 1
could work reliably simply by providing a refractory period of
approximately 20 milliseconds in amplifier 11 to eliminate
artifacts, and by directly sensing the R-wave and providing an
escape pulse should the R-wave not occur in the timing period of an
escape pulse generator. However, in a second embodiment shown in
FIG. 2 and to be fully described below, the window becomes
mandatory to avoid providing a stimulating pulse during the heart's
vulnerable period.
The second preferred embodiment of the apparatus of this invention
is best understood with reference to FIG. 2. The second embodiment
of this invention includes all of the apparatus of the embodiment
of FIG. 1 with the addition of one electrical component and one
electrical interconnection. In FIG. 2 that portion of the circuitry
of FIG. 1 including these additional connections is shown, and it
will be understood that the apparatus shown in schematic form in
FIG. 2 is connected to the circuit the same as that of FIG. 1. The
same descriptive numbers are used for like members of the circuit
with new numbers indicating only the additions.
Referring to FIG. 2, there is again shown output energy capacitor
29, inductor 30, and transistor 31, serially connected between
buses 22 and 32. A diode 80 is added in serial connection between
inductor 30 and transistor 31. There is also shown electrodes 25
and 26 connected across resistor 21 by circuitry including output
capacitor 23. Transistor 20 is again shown connected between
resistor 21 and junction 27. An added electrical conductor 81 is
shown connecting the collector of transistor 20 to the collector of
transistor 31.
The purpose of the second embodiment is to provide an early maximum
energy stimulating pulse to the heart when the preceding
stimulating pulse has resulted in loss of capture. It will be
recognized that in the embodiment of FIG. 1, as the stimulating
pulses step down to an energy level below the patient's threshold,
one heartbeat will be missed by the patient before the next
increased energy stimulating pulse due to the charging of capacitor
29 caused by an escape pulse. It is to avoid this occasional missed
heartbeat that the second embodiment may be used.
The operation of the circuitry of FIG. 2 is precisely the same as
that of FIG. 1 with the single exception that when an escape pulse
does result from the second pulse generator, the turn-on of
transistor 31 will not only commence charging of capacitor 29, but
will also, through conductor 81, connect electrode 25 through
capacitor 23 and transistor 31 to bus 32. Thus electrodes 25 and 26
will be directly across buses 22 and 32 and the maximum energy
available from the power supply will be applied directly to the
heart. This application, due to the window feature of the escape
pulse, will occur within 100 milliseconds after loss of capture is
sensed. Therefore, the stimulating output pulse which fell below
the patient's threshold level will be followed very quickly by a
maximum level output pulse to provide the missed stimulation. Also,
because the escape pulse performs its normal function of causing
the charging of capacitor 29, the next normal stimulating pulse
following the maximum energy pulse will be less than maximum but at
a level above threshold in exactly the same manner of operation as
that described above with regard to the operation of FIG. 1.
In reviewing the operation of the embodiment of FIG. 2, it will
become apparent why, as mentioned above, the window is a mandatory
feature. As previously described, it is known that the R-wave it is
desired to sense will occur within 100 milliseconds after the
stimulating pulse. As sensing of the R-wave is directly followed by
a maximum energy stimulating pulse in the second embodiment, it is
important to assure that that maximum energy stimulating pulse does
not occur during the patient's vulnerable time period following the
QRS complex. This vulnerable period is well-known to those familiar
with the art, and could result in the patient entering
fibrillation. Therefore, the window is provided in the embodiment
of FIG. 2 to assure that the sensing of artifacts or, for example,
the T-wave, will not cause the output of a maximum energy
stimulating pulse following the 100 millisecond period in which the
R-wave normally occurs.
The operating principles of the embodiment of FIGS. 1 and 2 can be
more fully understood with references to FIGS. 4 and 5. FIG. 4 is a
stylized graph of volts versus time in which the vertical lines
represent the voltage levels of the stimulating pulse outputs from
the embodiment of FIG. 1, assuming that the output energy capacitor
29 is charged to maximum at the beginning of operation, as was
assumed in the above discussion of the operation of FIG. 1. The top
horizontal line of FIG. 4 indicates the maximum output voltage
available, the level at which the first stimulating pulse occurs.
The lower horizontal line in the graph of FIG. 4 indicates the
patient threshold level. It will be recognized to those familiar
with the art, that though respresented as a single line on the
graph, the patient's threshold level is instead a wider band of
voltage range, due to a hysteresis effect found in many patients
whereby once stimulation has dropped below the stimulus threshold
level of the patient, to regain capture it is necessary to raise
the stimulus voltage to a point above that level at which capture
was lost.
From FIG. 4 it is apparent that each succeeding output stimulating
pulse is decreased in amplitude, until the level of the stimulating
pulse falls below the level of the patient's threshold. At that
time capture is lost and an escape pulse causes the charging of
output capacitor 29. The next succeeding pulse is therefore raised
to an energy level greater than the last pulse which resulted in
capture, but less than maximum pulse level. Thereafter the process
is repeated until the stimulating pulse level falls below threshold
again, whereafter the next stimulation pulse is again increased and
the process repeated yet again. It is to be carefully noted that
the stimulus threshold of cardiac response in a patient may vary
considerably, and may in fact vary in a single patient during the
day. The apparatus of this invention will always seek and follow
the threshold level whether it rises or falls. The graph of FIG. 4
indicates how the stimulating pulses "seek" the threshold level and
then "ride" that level to provide a safe margin of stimulus at a
minimum energy expenditure. Finally, though the graph of FIG. 4 is
in terms of volts versus time, it will be recognized that the
apparatus of this invention could work equally as well by varying
other parameters of the output stimulating pulse, with the
important factor being the amount of stimulating energy supplied to
the heart.
With reference to FIG. 5, there is shown exactly the same graph as
in FIG. 4, but as applied to the second embodiment of this
invention of FIG. 2. It will be noted that the operation of the
embodiment of FIG. 2, as shown in FIG. 5, is exactly the same as
that of the first embodiment, as shown in FIG. 4, with the
exception that shortly following each loss of capture, one maximum
energy pulse is provided to the heart to avoid loss of a heartbeat.
Thereafter, the succeeding stimulating pulse from the apparatus of
FIG. 2 is the same battery drain-saving, low level as that of the
first embodiment, from which the cycle will repeat again.
As stated above, the advantage of the second embodiment of this
invention over the first embodiment is the avoidance of the loss of
a heartbeat in the patient. However, the second embodiment has a
disadvantage with respect to the first embodiment in that it does
use somewhat more energy. When the apparatus of the first and
second embodiments are in storage and not connected to a patient,
as they are completely selfcontained battery operated units adapted
for implantation, each will be outputing maximum energy pulses,
however, the second embodiment will be outputing extra pulses of
maximum energy, that is, twice as many pulses as the first
embodiment and thus causing significantly greater battery drain
during storage. This same double pulse effect will occur when
embodiments are implanted, should sensing be lost.
FIG. 6 is a graph on a scale of output voltage versus time, of both
the prior art demand pacer output voltage level, and the output
voltage level of the apparatus of this invention as shown in the
embodiment of FIG. 1. From FIG. 6 it becomes clear that the output
voltage required by the apparatus of FIG. 1 is significantly less
than the prior art output voltage throughout the entire operation
of the device. Though the threshold level of the patient may vary,
seen from FIG. 6, the prior art demand cardiac pacers always
provides maximum energy, while the apparatus of the present
invention seeks the threshold level and thereafter provides the
minimum safe energy necessary to achieve cardiac stimulation.
A further feature of the apparatus of this invention, found in both
the first and second preferred embodiments, is the ability to
remember the energy level at which the last stimulation pulse
occurred. That is, should the automatic threshold compensating
demand pacer of this invention leave the pacing mode and enter the
standby mode due to the sensing of natural heartbeats, capacitor 29
will hold the charge remaining after the cycle following the last
stimulated heartbeat. When the apparatus of this invention again
enters the pacing mode due to failure to sense a natural heartbeat,
the next normal stimulating pulse will be at the energy level held
by capacitor 29, as though the pacing mode had not been
interrupted. This feature of the apparatus of this invention
obviously adds significantly to the decrease in battery drain. If
the heart of the patient should respond intermittently with natural
heartbeats, the battery drain would be greatly increased if the
pacer returned to maximum stimulation following each natural
heartbeat.
It will be apparent that the principles embodied in the above
described preferred embodiments of this invention can be
encompassed in structures other than those specifically shown,
without departing from the scope of the invention.
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