U.S. patent number 3,759,266 [Application Number 05/078,659] was granted by the patent office on 1973-09-18 for cardiac-autopacer with variable refractory, non-linear feedback, latching reset and voltage increasing circuits.
This patent grant is currently assigned to Medcor, Inc.. Invention is credited to Jin W. Lee.
United States Patent |
3,759,266 |
Lee |
September 18, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
CARDIAC-AUTOPACER WITH VARIABLE REFRACTORY, NON-LINEAR FEEDBACK,
LATCHING RESET AND VOLTAGE INCREASING CIRCUITS
Abstract
A self-contained, implantable cardiac-autopacer providing ultra
long battery life and high reliability through parallel battery
redundancy and circuit techniques. The cardiac-autopacer operates
in a demand mode with a variable simulated refractory period which
is long during artificial stimulation and short during natural
ventricular depolarization. In this way the demand functioning of
the cardiac-autopacer is maintained from normal to tachycardia
rates. In the presence of noise or interfering signals with
frequencies beyond the tachycardia rate the autopacer will
continuously generate artificial heart stimulating pulses of a
predetermined interval. Both artificial pulse duration and interval
are maintained independent of load variation, but pulse interval
varies with battery voltage making it possible to externally
monitor battery condition by triggering the autopacer into a
continuous pulse mode.
Inventors: |
Lee; Jin W. (North Miami,
FL) |
Assignee: |
Medcor, Inc. (Hollywood,
FL)
|
Family
ID: |
22145461 |
Appl.
No.: |
05/078,659 |
Filed: |
October 7, 1970 |
Current U.S.
Class: |
607/27; 607/11;
607/29; 607/12 |
Current CPC
Class: |
A61N
1/365 (20130101) |
Current International
Class: |
A61N
1/365 (20060101); A61n 001/36 () |
Field of
Search: |
;128/419P,419R,421,422,2.6A |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Kamm; William E.
Claims
What is claimed is:
1. Electronic apparatus for producing artificial cardiac
stimulating pulses comprising:
a portable source of electrical energy;
resettable means operative in association with said source for
generating said artificial cardiac stimulating pulses a
predetermined interval after the last reset thereof;
said generating means including means operative to reset said
generating means upon the generation of an artificial heart
stimulating pulse;
means for detecting a signal representative of artificial and
natural heart stimulation;
first means responsive to a detected heart stimulation signal for
providing a reset signal for resetting said generating means;
means operative in association with a detected heart stimulation
signal for inhibiting resetting of said generating means by said
reset signal in response to a variable refractory period after the
detection of a natural and artificial heart stimulation signal;
and
means for providing said variable refractory period in response to
said detected stimulating signal with a duration which is
relatively longer at rates typical of artificial stimulation and
which prevents continuous pulse generation in response to detected
tachycardia pulse rates but enables continuous pulse generation at
more rapid detected pulse or noise rates.
2. The apparatus of claim 1 wherein said pulse generating means
includes means operative to maintain the predetermined interval and
the width of said pulse substantially constant despite variation in
the load on said generating means.
3. The apparatus of claim 1 wherein:
said means for providing a variable refractory period includes
means for producing a plurality of output pulses beginning with the
detection of each said heart stimulation pulse and continuing for a
delay period which increases with the level of said detected heart
stimulation pulses whereby natural heart stimulation pulses have a
negligible delay period while artificial stimulation pulses have a
substantial delay period to produce a relatively longer inhibition
for said artificial pulses; and
said inhibiting means includes means operative to provide a fixed
period of inhibition after the last pulse output of said output
pulse providing means.
4. The apparatus of claim 3 wherein:
said variable refractory period providing means comprises a high
gain amplifier having a saturation characteristic which produces
alternate state saturations of said amplifier from artificial heart
stimulation signals, said saturation states lasting for said delay
period with each change in said amplifier into and out of
saturation producing an output pulse from said detecting means.
5. The apparatus of claim 4 wherein said variable refractory period
providing means further comprises means for producing a unipolar
signal representative of the deviation of the output of said
amplifier from a noise threshold surrounding a quiescent state of
said amplifier with the output of said means for producing a
unipolar signal adapted to recommence the inhibiting period of said
inhibiting means.
6. The apparatus of claim 5 wherein said generating means is
further characterized by having:
a low current push-pull voltage doubler output circuit adapted to
produce said artificial heart stimulation pulse at a voltage
magnitude substantially higher than the magnitude provided by said
source; and
means for maintaining said predetermined interval and the width of
said artificial heart stimulation pulse at substantially constant
values with variation in the load for said artificial heart
stimulation pulse.
7. The apparatus of claim 5 wherein said inhibiting means further
comprises:
a capacitor;
means for charging said capacitor in response to each unipolar
signal;
means for discharging said capacitor over an interval which defines
said inhibiting period; and
means operative to inhibit resetting of said generating means
whenever the charge of said capacitor is above a selected
magnitude. 8The apparatus of claim 5 wherein said unipolar signal
producing means further includes a regenerative circuit to provide
consistent unipolar signals in response to
each deviation of the output of said amplifier. 9. The apparatus of
claim 1 wherein said means for producing the variable refractory
period is characterized by having:
a first RC timing circuit;
means for imparting a stable charge state to said timing
circuit;
means for applying to said timing circuit a variable change in
charge from said stable charge state upon the detection of each
heart stimulation pulse, the gradual return to said stable charge
state defining said variable period;
a second RC timing circuit with a longer time constant than said
first RC timing circuit;
means for imparting a stable charge state to said second timing
circuit;
means for applying to said second timing circuit a change in charge
from said stable charge state upon the detection of each heart
stimulation pulse; and
means for decreasing the change in charge applied to said first RC
circuit with each detected signal in correspondence with the charge
deviation of
said second RC circuit from its stable charge state. 10. The
apparatus of claim 1 wherein:
said source has an output voltage which decreases near exhaustion
of energy from said source;
said generating means includes means responsive to the voltage
magnitude of said source for controlling said predetermined
interval to partially represent said output voltage magnitude;
and
means responsive to external activation are provided to cause
continuous
pulse generation. 11. The apparatus of claim 1 wherein said supply
source is further characterized as having redundant batteries of
matched characteristics in parallel and a diode device connected
from each said
battery conducting to a point forming a terminal of said source.
12. An electronic heart pacer including the electronic apparatus of
claim 1 wherein:
electrodes are provided to apply said artificial pulses to said
heart and receive heart stimulation signals; and
said variable refractory period providing means includes means for
providing a relatively longer refractory period in response to
detected artificial heart stimulation and a relatively shorter
period in response to detected natural heart stimulation whereby
signals resulting from recovery of said electrodes from artificial
heart stimulation are inhibited from resetting said generating
means and tachycardia natural
heart beat signals are effective to reset said generating means.
13. The electronic apparatus of claim 1 wherein:
said source has a voltage output substantially below the necessary
voltage magnitude for artificial heart stimulation; and
said generating means further includes a high efficiency saturating
switch means for augmenting the voltage magnitude of said
stimulating pulses above the output magnitude of said source to a
magnitude compatible with
artificial heart stimulation. 14. Electronic apparatus for
producing artificial demand cardiac pulsing for application to
heart terminals, said apparatus comprising:
means for generating artificial heart stimulating electrical pulses
for application to said terminals;
said generating means including non-linear feedback means for
regulating said pulses to a preselected period and width
substantially unaffected by variations in the load across said
terminals;
means responsive to signals on said terminals for sensing a natural
heart beat QRS electrical signal and an artificial heart
stimulating electrical signal;
means for recommencing the pulse period between each said
electrical pulses without generation of said electrical pulse when
each said electrical
signal is sensed. 15. The apparatus of claim 14 characterized by
said generating means including:
an astable multivibrator operating at said preselected period;
a one-shot multivibrator producing a pulse of said preselected
pulse width upon a change of state of said astable
multivibrator;
said one-shot multivibrator having said non-linear feedback means
and including said astable multivibrator with said feedback means
adapted to maintain said pulse width substantially constant with
variations in the load for said pulse; and
said one-shot multivibrator including means operative to isolate
the preselected period of said astable multivibrator from loading
effects.
The electronic apparatus of claim 14 further including:
a portable source of operating power;
said generating means having means to cause the timing of said
artificial pulses to represent remaining energy in said source
whereby the timing of said artificial pulses is relatively
responsive to source energy condition; and
said non-linear feedback means including means for regulating the
timing to be relatively unresponsive to variations in the load
across said terminals
to provide an accurate measure of source energy. 17. Electrical
apparatus for generating a train of artificial body muscle
stimulating electrical pulses having a potential magnitude greater
than the potential of its energy source comprising:
a source of electrical energy and providing an electrical potential
between first and second terminals thereof;
a first capacitor having an associated means for supplying energy
from said source to said first capacitor;
means for producing a reference signal of potential intermediate
the potentials of the source terminals;
means for discharging said capacitor in response to the signal on
said first capacitor achieving a set relationship to said reference
signal, said discharging means including;
a rectifying device and a capacitor in parallel therewith, said
device conducting only during discharge of said first
capacitor;
a first transistor of one conductivity type;
a second transistor of the opposite conductivity type with the
emitter, base, and collector of said first transistor connected
respectively to said first capacitor at a first terminal thereof,
to said rectifying device for conduction from the emitter of said
first transistor through said controlled conduction device, and to
the base of said second transistor; and with the emitter and
collector of said second transistor respectively connected to a
first terminal of said source including a connection to a second
terminal of said first capacitor and to the opposite terminal of
said rectifying device including a connection to a first
junction;
said discharging means having said first junction connected to said
first terminal of said source during discharging of said first
capacitor;
a second capacitor connected from said first junction to a first
terminal adapted for connection to a body muscle;
means for charging said second capacitor to the potential of said
source during charging of said first capacitor with said first
junction at the potential of said second terminal of said source;
and
means for connecting said second terminal of said source during
discharging
of said first capacitor to said load at a second terminal thereof.
18. Electronic apparatus for producing variable simulated
refractory period demand cardiac pulses comprising:
resettable means for generating artificial heart stimulation
electrical pulses a predetermined interval after the last occurring
artificial pulse or reset thereof;
means for detecting artrificial and natural heart stimulation
electrical signals;
said detection means including means operative to reset said
generating means upon the detection of stimulation signals;
said detecting means further including means operative to inhibit
the resetting of said generating means for a variable refractory
interval after the occurrence of each detected heart signal;
and
means for producing said variable interval with a duration varying
in response to predetermined characteristics of said detected
natural and artificial heart signals to prevent continuous
artificial pulse generation
during normal and tachycardia heart rates. 19. The apparatus for
producing variable simulated refractory period demand cardiac
pulses of claim 18 wherein said means for producing said variable
refractory interval comprises:
means for developing a plurality of signals from the detection of
each said heart stimulation signal, said plurality of signals
spanning a period which increases with the level of the detected
heart stimulation signals, said spanned period being longer for
artificial signals, and at least substantially shorter for natural
signals;
means for producing a further portion of said interval upon the
occurrence
of each of said plurality of signals. 20. The apparatus for
producing variable simulated refractory period demand cardiac
pulses of claim 19 wherein:
said means for developing a plurality of signals includes an
amplifier having a prolonged saturation response to detected
artificial stimulation signals and substantially less saturation
response to detected natural stimulation signals; and
said means for producing a further portion includes an RC timing
circuit excited by each of said plurality of signals to leave a
stable level of charge to which said timing circuit tends to
gradually return;
said inhibiting means including means responsive to a range of
levels of charge during the gradual return to said stable level for
producing a reset inhibition, and to said stable level of charge
for producing no
reset inhibition. 21. The apparatus of claim 18 wherein said
variable interval producing means comprises:
a first RC timing circuit;
means for imparting a stable charge state to said first timing
circuit;
means for applying to said timing circuit a variable change in
charge from said stable charge state upon the detection of each
heart stimulation pulse, the gradual return to said stable charge
state defining said variable interval;
a second RC timing circuit with a longer time constant than said
first RC timing circuit;
means for imparting a stable charge state to said second timing
circuit;
means for applying to said second timing circuit a change in charge
from said stable charge state upon the detection of each heart
stimulation pulse; and
means for decreasing the change in charge applied to said first RC
circuit with each detected signal in correspondence with the charge
deviation of
said second RC circuit from its stable charge state. 22. The
electronic apparatus of claim 18 wherein said variable interval
producing means includes means for producing a relatively longer
interval in response to detected artificial heart stimulation
electrical signals and a relatively shorter interval in response to
detected natural heart stimulation
signals. 23. Electronic apparatus for producing artificial cardiac
stimulating pulses comprising:
a portable, exhaustible energy source for powering said
apparatus;
said energy source having a power output voltage magnitude
representative of remaining source energy;
means for generating said artificial cardiac stimulating
pulses;
said generating means including non-linear feedback for regulating
the timing of said artificial cardiac stimulating pulses to isolate
pulse timing variations from effects other than said energy
source;
means for producing a substantial variation in pulse timing with
source voltage, said variation in pulse timing having a unique
polarity of change in pulse rate for a corresponding polarity of
change in source voltage;
means for sensing natural and artificial cardiac stimulation;
means responsive to sensed cardiac stimulation for inhibiting the
generation of a pulse and for reinitiating the interval between
pulses; and
means actuable by an externally applied field for blocking the
action of said inhibiting means thereby to permit continuous pulse
generation with timing uniquely representative of the energy
condition of said energy
source. 24. Electronic apparatus for producing artificial cardiac
stimulating pulses comprising:
a portable, exhaustible energy source for powering said
apparatus;
said energy source having a power output voltage magnitude
representative of remaining source energy;
means for generating said artificial cardiac stimulating
pulses;
means for producing a substantial variation in pulse timing with
source voltage, said variation in pulse timing having a unique
polarity of change in pulse rate for a corresponding polarity of
change in source voltage;
means for sensing natural and artificial cardiac stimulation;
means responsive to sensed cardiac stimulation for inhibiting the
generation of a pulse and for reinitiating the interval between
pulses; and
means actuable by an externally applied field for blocking the
action of said inhibiting means thereby to permit continuous pulse
generation with timing uniquely representative of the energy
condition of said energy source;
said pulse generator including:
a source of electrical energy;
a first transistor of one type;
a second transistor of a second type;
a capacitor;
means for charging said capacitor from said source;
means for providing a reference level which is a function of said
source voltage;
means for interconnecting said first and second transistors and
said capacitor to provide regenerative activation of said reference
level less the forward base conduction voltage of one of said
transistors;
means for interconnecting said capacitor and said transistors to
provide discharge of said capacitor during activation of said
transistors; and
means interconnecting said capacitor and said transistors to
provide regenerative deactivation of said transistors in response
to discharge of said capacitor to a predetermined level;
the charge and discharge intervals of said capacitor being a
function of source voltage whereby variation in both provides a
confirmed indiciation of source energy depletion;
said inhibiting means including regeneratively operative latching
means for discharging said capacitor in response to sensed cardiac
stimulation thereby to permit complete discharge of said capacitor
regardless of the frequency at which said cardiac stimulation is
sensed.
Description
FIELD OF THE INVENTION
The present invention relates to cardiac pacers, and in particular
electronic, long life implantable demand cardiac pacers.
BACKGROUND OF THE INVENTION
Today the lives of thousands of cardiac patients are being
maintained and extended by the cardiac pacer which produces
artifical heart stimulation pulses to the heart's ventricle upon
failure of the heart to produce normal rate ventricle
contractions.
Cardiac pacers can be classified as continuous or demand pacers.
The first provides a continuous sequence of artificial heart
stimulation pulses at a normal rate of 60 to 85 pulses per minute.
Demand pacers produce an artifical pulse only in the absence of
normal or in the presence of irregular ventricular depolarization
causing ventricular contraction.
Early examples of demand cardiac pacing systems are shown in
patents to Davies, No. 826,766 (British) and Berkovits, U.S. Pat.
No. 3,345,990. These early systems, however, were not basically
implantable, lacking light weight, small size, and long life
without servicing. The transistor and modern batteries have made
the implantable cardiac pacer possible. Examples of continuous
implantable cardiac pacers includes patents to Greatbatch, U.S.
Pat. No. 3,057,356; Tischler, U.S. Pat. No. 3,109,340; Keller, U.S.
Pat. No. 3,474,353; and Sessions, U.S. Pat. No. 3,518,997. Modern
demand cardiac pacers are shown in three patents to Keller, U.S.
Pat. Nos. 3,253,596; 3,431,912; and 3,433,228 and one patent to
Greatbatch, U.S. Pat. No. 3,478,746.
With the use of the implantable cardiac pacer it was additionally
required that they be of low current drain and ultra-reliable over
a period of years in both circuit operation and durability of the
portable energy source. It is furthermore necessary to provide a
simple and accurate means for detecting energy depletion in the
portable energy source or battery. Additionally, it is desirable in
cardiac pacers to regulate the interval between artifically
produced pulses to be independent of varying heart load. Prior art
designs have been unable to develop circuitry which provides for
all of these requirements and desirable features in a single low
voltage, redundant battery pacer while maintaining the unit size,
complexity and weight of the cardiac pacer at a level compatible
with body implantation.
In past designs a simulated electronic refractory period providing
a long refractory period to precluded resetting of the timing
generator during electrical stimualtion and a short period to
accommodate premature ventricular contraction has not been
available. With only a short simulated refractory period it is
possible to alter the timing of the artificial heart stimulating
pulse. With only a long simulated refractory period, on the other
hand, continuous artificial stimulation can occur in the presence
of tachycardia heart beats.
Accordingly, it is a general object of the present invention to
provide a demand cardiac pacer of very long implant life and
reliability in combination with many sophisticated cardiac pacing
capabilities.
It is a specific object of the present invention to provide a
cardiac pacer having a low voltage redundant portable power supply
for long life and reliability.
It is a further specific object of the present invention to provide
a cardiac pacer having a low current consumption.
It is a further specific object of the present invention to provide
a demand cardiac pacer having a variable refractory period which
provides for continuous artificial cardiac pulsing in the presence
of high frequency noise but inhibits artificial pulsing at normal
heart rates up to and including tachycardia rates.
It is a further specific object of the present invention to provide
a cardiac pacer having a low current drain pulse generating means
which maintains the pulse interval and duration substantially
constant with variations in the pulse load.
It is a further specific object of the present invention to provide
an implantable cardiac pacer having an inherent indication of
battery condition with simple readout means.
BRIEF SUMMARY OF THE INVENTION
According to exemplary preferred embodiments of the present
invention, a light weight, high reliability, implantable cardiac
pacer is shown adapted to operate from a low voltage, redundant,
portable energy source and to draw a very low current therefrom
while providing sophisticated demand cardiac pacing features
including a variable refractory period, continuous pacing in the
presence of high frequency noise and beat rates, means to
externally read out energy source condition from inherent circuit
operation, and a regulated pulse width and interval independent of
load.
Pulse generation and interval timing is achieved by capacitor
charge and discharge cycles in conjunction with a highly
regenerative switch coupled into the feedback path of a pulse width
regulator. Means is provided for sensing each artificial heat
stimulating pulse and each natural heart beat ventricular signal;
and for resetting the timing of the pulse generating means through
capacitor discharge upon the occurrence of each sensed pulse and
signal.
During a simulated refractory period the sensing means prevents
resetting of the pulse generator timing interval. The simulated
refractory period is composed of two parts, a longer refractory
period during artificial stimulation and a shorter refractory
period during normal heart beat including tachycardia. In this way
natural heart pulses from the normal to the tachycardia rates
maintain the cardiac pacer in a standby condition. Only when the
natural beating is absent, or its rate drops below a preset rate,
or when a higher frequency beat or noise condition is sensed, does
the cardiac pacer provide artificial heart stimulating pulses.
The pulse generating means includes push-pull voltage doubling
output means allowing a redundant low voltage energy source. This
low voltage makes the pulse interval inherently dependent on source
voltage which is detected by externally triggering the pulse
generator into a continuous pulsing mode using an external magnet
and internal flux sensitive switch. Specific circuit design
regulates the pulse interval and duration.
Other circuit techniques result in very low current
consumption.
BRIEF DESCRIPTION OF THE DRAWINGS
The structure and the operation of the present invention will be
more fully understood by referring to the following description of
a preferred embodiment presented for purposes of illustration and
to the accompanying drawings, of which:
FIG. 1 is an electrocardiograph waveform of a normal heart
beat;
FIG. 2 is a schematic diagram with block function indications of a
complete demand cardiac pacer;
FIG. 3 is a schematic diagram of a modified pulse generating
portion for a cardiac pacer; and
FIG. 4 is a schematic diagram of a further modified pulse
generating portion of the cardiac pacer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 indiciates the electrocardiograph waveform of a normal heart
beat of 0.86 second period comprising a P wave produced immediately
preceding the contraction of the atrium, a QRS complex signal
causing the ventricle to contract followed by an S-T wave. A
refractory period extends after the QRS complex for approximately
0.3 second during which the heart will not respond to artificial
stimulus. A vulnerable period of very short duration follows the
refractory period and indiciates a time when artificial stimulation
could cause heart damage.
Referring to FIG. 2, there is shown a schematic diagram with broken
lines indicating functional blocks of a complete demand cardiac
pacer exemplary of the present invention.
In reference to the functional blocks of the FIG. 1 circuitry, a
low voltage redundant energy source 12 provides electrical
operating power to the pacer over high and low (or ground) lines 14
and 16 respectively.
Pulse lines 18 and 20 conduct artificial heart stimulation
electrical pulses from a pulse generator 22 to the ventricle of a
heart load 24 and natural QRS signals from the heart 24 through
series capacitances within the generator 22 to a high gain, low
current, feedback amplifier 26. Normally the pulse line 20 is
attached to a plate embedded within the body or on an external
surface of an implanted pacer while the line 18 actually contacts
the ventricle muscle of a heart 24 via a catheter electrode not
shown.
The pulse generator 22 generates pulses of 1 millisecond duration
with a preselected time interval (approximately 860 millisecond)
elapsing between pulses unless the generator is reset to recommence
the time interval. The resetting function is achieved by sensing,
amplifying, and processing the signal which results on lines 18 and
20 whenever a natural QRS signal or artificial stimulating pulse is
produced.
The amplifier 26 receives signals representing the natural QRS
signals or artificial heart stimulating electrical pulses at an
input 28 and presents them highly amplified at an output 30.
Separate paths of DC negative feedback and AC negative feedback
within the amplifier 26 provide stabilization and allow a very high
gain in amplifier 26 despite changes in characteristics of the
active elements and of the source 12.
The high gain characteristics of the amplifier 26 force it into
heavy saturation when the input 28 receives large signals which
occur during artificial stimulation. The amplifier 26 is held in
saturation for a period substantially longer than the duration of
the artificial heart stimulating electrical pulse inputted to it,
thereby delaying the last output from the amplifier 26 beyond the
termination of each artificial pulse. Because the signals received
at input 28 from natural QRS signals are of a lower level than the
signals from artificial pulses, saturation of amplifier 26 is
minimal and signal delay negligible during natural heart
stimulation. This variable saturation delay of amplifier 26 forms a
substantial portion of a simulated refractory period and gives this
simulated refractory period a variable characteristic.
The output 30 of the amplifier 26 is fed to a comparator 32 which
produces at an output 34 a unipolar signal representative of the
amount by which the electrical magnitude of the output 30 of the
amplifier 26 exceeds a threshold surrounding a quiescent level at
the output 30.
The unipolar pulse output 34 of the comparator 32 begins the
running of a fixed portion of the simulated refractory period. This
fixed period is triggered by an inhibiter circuit 36 a short
interval after it receives as input the unipolar pulses out of
comparator 32. The fixed portion of the simulated refractory period
is defined by an inhibit condition of inhibiter 36 during which an
output 38 of the inhibiter 36 is disabled. In the short interval
between input of a unipolar pulse to inhibiter 36 and the
establishment of the inhibit condition, the unipolar pulse appears
at the output of inhibiter 36 where it is effective to reset
generator 22 and recommence the time interval of generator 22 via a
reset circuit 40 between generator 22 and inhibiter 36.
The pulse generator 22 has an internal capacitor charge timing
circuit 42, the time constant of which determines the time interval
between pulses. A capacitive discharge circuit 43 discharges the
capacitor resulting in the generation of a pulse and the
recommencement of the time interval. The reset circuit 40 on
receiving a unipolar signal also effects a discharge of the
capacitor. The discharge path 43 used in generating a pulse is
included within the feedback loop of a monostable pulse width
regulator 44 which establishes and regulates the duration of the
artificial heart stimulating pulse from the generator 22 and which
further isolates the time interval between pulses from loading
effects due to variations in the load 24 from patient to patient or
in the same patient across lines 18 and 20. Finally a push-pull
voltage doubler circuit 46 receives and regulated pulse from the
monostable circuit 44 and doubles the magnitude of the voltage
output so that the pulse present across the lines 18 and 20 is
nearly twice the magnitude of the voltage supplied by the source
12.
Thus, the pulse generator 22, in the absence of any natural heart
beat signals, generates continuously pulses of a preset interval
and width. When a natural beat signal is received, however, the
pulse generator is immediately reset to the beginning of a pulse
time interval without the generation of an artificial pulse, unless
that natural pulse occurs within a simulated variable refractory
period after the last natural or artificial stimulating pulse, in
which case the pulse generator is not reset. The fixed plus
saturation portions of the refractory period totalling
approximately 350 milliseconds insure that no signals detected by
the amplifier are effective to reset the pulse generator if they
occurred during a long refractory period after an artificial
stimulation pulse. The pulse time interval thus runs from the last
artificial heart stimulation pulse until at least after this long
refractory period.
In tachycardia conditions the heart beats up to 220 times per
minute as contrasted to a normal rate of between 60 and 85. It is
desirable to preclude any artificial stimulation during this
condition and yet to produce continuous stimulation with noise,
interference, or beat rate signals above tachycardia present at the
input of amplifier 26. The shorter fixed refractory period,
approximately 250 milliseconds, allows resetting of the pulse
generator 22 up through 220 beats per minute of natural tachycardia
contractions. High frequencies at the input to amplifier 26
produced by noise, interference, or faster beat rates such as from
fibrillation keep the inhibiter 36 in an inhibit condition
continuously, prevent reset circuit 40 from receiving any unipolar
pulse signals and allow pulse generator 22 to generate artificial
pulses continuously
Further features of this invention can be understood by describing
in detail the operation of each block of the preferred embodiment
of FIG. 2.
Turning to this more detailed description, the source 12 further
comprises batteries 50 and 52 (typically two series connected
mercury cells each). Negative terminals of batteries 50 and 52 are
connected to line 16 and positive terminals are connected
respectively to emitters of transistors 54 and 56. The bases of
these transistors are circuit grounded to line 16 via a resistance
58, and their collectors are joined at line 14. The transistors
provide a low saturation voltage drop between their emitter and
collector so that little voltage or energy is lost in them between
the batteries 50 and 52 and the lines 16 and 14. If either battery
short circuits or open circuits, however, the transistors 54, 56
operate as an OR gate so that the lines 14 and 16 continue to be
fed by the one remaining good battery. A capacitor 59 across lines
14 and 16 provides transient isolation.
The amplifier 26 receives at input 28 both the QRS complex signals
from line 18 as well as the signals resulting from artificial heart
stimulating electrical pulses produced by the pulse generator 22.
Line 28 feeds a two stage DC coupled amplifier composed of first
stage NPN transistor 60 and collector coupled second stage PNP
transistor 62. The collector and emitter of transistor 60 are
connected to the lines 14 and 16 through high resistances 64 and 66
respectively. The collector and emitter of transistor 62 are
connected to lines 16 and 14 respectively through high resistance
68 and high resistance 70 in series with high resistance 72. The
base of transistor 60 is biased by a resistance 74 from the emitter
of transistor 62 which also DC stabilizes the amplifier in addition
to biasing transistor 60. The emitter of transistor 60 is connected
via a capacitance 76 to the junction between resistances 70 and 72
providing AC feedback for the amplifier 26. The emitter of
transistor 62 is circuit grounded through a capacitor 71 and
normally closed switch 73. In its normally open position, switch 73
adds a series resistor 75 between ground and capacitor 71.
Upon receipt of a signal at amplifier 26 resulting from an
artificial electrical pulse, the amplifier 26 is heavily driven
into one saturation state and remains there for a substantial time
after the disappearance of the pulse signal at its input. Upon
coming out of this saturation state, the amplifier overshoots to
saturation in the opposite state and remains there for a
substantial period. It produces a pulse at output 30 when coming
out of saturation, altogether approximately 130 milliseconds after
the input to amplifier 26. This saturation characteristic is
imparted to amplifier 26 by the AC and DC feedback in conjunction
with a phasing change produced by capacitors 71 and 76 as is
understood by those skilled in the art to produce step response
overshoot.
Because of the amplifier's limited low frequency response only the
QRS complex of the natural heart signal is recognized, and it will
not saturate on receiving natural QRS signals because their
magnitude is substantially less than the magnitude of signals from
artificial stimulation pulses. No amplifier delay is thus
produced.
The pulses at output 30, through a differentiating capacitance 78
from the collector of transistor 62, are of both polarities and the
QRS complex signals can be of either polarity. The comparator 32 is
required to full-wave rectify the pulses at output 30 since reset
circuit 40 can respond to only one polarity.
Accordingly, the output 30 is fed to the base of an NPN transistor
80 and to the emitter of an NPN transistor 82 and has a path to
ground through resistance 84. The collectors of transistors 80 and
82 are joined and fed through a resistance 86, a PNP transistor 88,
and the high voltage line 14 through a resistance 90. The emitter
of transistor 88 contacts the high line 14 through a normally
closed switch 85 while the collector is connected to output 34,
and, via serially connected capacitance 92 and resistance 94, to
the base of transistor 82 and to a resistance 96 to ground.
Parallel capacitor 89, resistor 87 and zener diode 91 connects line
14 to the emitter of transistor 88 in the normally open position of
switch 85.
When a pulse of either polarity appears on output 30 one of the
transistors 80 and 82 is turned on, thereby turning on transistor
88 and regeneratively holding transistors 82 and 80 on for a period
specified by the time constant of capacitance 92 and resistances 94
and 96.
Transistors 80, 82 and 88 are in the off condition except during a
pulse output from amplifier 26 and no bias current is drawn by the
comparator 32 except during an output from amplifier 26, thereby
minimizing the current drain of comparator 32. Forward conduction
voltages of transistors 80 and 82 establish a partial noise
discriminating threshold.
The output 34 of comparator 32 is fed to the inhibiter 36
consisting of a conduction path from the output 34 through a series
diode 98 and resistance 100 to both a capacitance 102 and a
resistance 104 which in turn leads into a grounded emitter NPN
transistor 106. A variable resistor 107 from ground to the base to
transistor 106 is provided to discharge capacitor 102 through the
cutoff point of transistor 106 and to adjust the refractory period.
Similarly, from the output 34 a high value resistance 108 conducts
to the collector of transistor 106 from which the output 38 of the
inhibiter 36 is taken.
As the designed, the inhibiter 36 operates to pass pulses from the
output 34 of the comparator 32 to the reset circuit 40 via the
output 38 until the transistor 106 has been turned on, at which
time the output 38 becomes grounded. Thus the first pulse from the
comparator 32 both passes to the reset circuit 40 and charges the
capacitor 102 to the point where the transistor 106 turns on and
inhibits further pulses from the comparator 32 passing to the reset
circuit 40 until the capacitance 102 has discharged through
resistance 104, resistor 107 and transistor 106, thereby defining
the fixed portion of the refractory period. Each pulse from
comparator 32, in the meantime, though not passing to the reset
circuit 40, recharges the capacitor 102 and effectively recommences
the timing of the fixed portion.
In perspective then, the amplifier 26 receives signals from both
natural QRS ventricular stimulation and artificially generated
pulse stimulations. The former are amplified without delay and
trigger the fixed portion of the refractory period as well as
acting via reset circuit 40 to reset the pulse generator if the
inhibiter 36 was not in an inhibit condition when the QRS signal
occurred. When an artificial stimulating pulse is generated, a
pulse signal passes through amplifier 26 with the same results as
for the QRS signal except that saturation behavior of the amplifier
26 produces a series of pulses at its output occurring
substantially after the passage of the artificial stimulating pulse
and retriggering the fixed portion of the simulated refractory
period to cause a longer overall duration to the inhibit
condition.
A variation in the characteristics of the simulated refractory
period is achieved by placing both the switches 73 and 85 in the
normally open condition which places resistance 75 between ground
and capacitor 71 and places the parallel combination of resistance
87, capacitance 89 and zener 91 between line 14 and the emitter of
transistor 88. The addition of resistor 75 in the circuit of
amplifier 26 alters the phasing and damping of the closed loop
amplifier to prevent pulse step response overshoot which causes
saturation with signals from artificially stimulated beats. The
capacitance 89 charges each time that the capacitor 102 is charged
and discharges through resistor 87 at a slower rate than the
discharge of capacitor 102. With each pulse out of amplifier 26,
the capacitor 102 will be charged to a variable level depending
inversely upon the charge remaining in capacitor 89. With more
rapidly occurring signals at the input to amplifier 26, capacitor
89 discharges less between pulses to comparator 32 from amplifier
26 resulting in a lower charge and voltage on capacitor 102. In
turn, the period of inhibition produced by inhibiter 36 is shorter
with more frequently received heart signals.
The simulated refractory period thus varies with the rate at which
amplifier 26 receives signals from either natural or artificially
stimulated beats. This allows a relatively long simulated
refractory period for normal beating rates, either natural or
artificially induced, but allows a short simulated refractory
period to accompany tachycardia heart beats to prevent a continuous
inhibit condition. The zener diode 91 limits the charge on
capacitor 89 so that a minimum simulated refractory period is
established and so that higher frequency signal or noise received
by amplifier 26 will cause a continuous inhibit condition and
continuous artifical pulse generation as before.
Of course in any pacer according to this invention, the switches 73
and 85 may be deleted and the pacer constructed with or without the
resistors 75 and 87, capacitor 89 and zener 91 to operate with
whichever simulated refractory period is desired.
It is also clear that capacitor 89 can be connected to line 16
instead of line 14, or that the entire three elements in parallel
may be inserted at any point in the charge path of capacitor 102
other than between it and line 16.
A reed relay 110 forms a normally open contact between the output
38 and ground, but may be activated by an external magnet to close
the contact grounding the output 38, and temporarily grounding the
input of the reset circuit 40.
The reset circuit 40 is composed of NPN transistor 112 fed at its
base by the output 38. The emitter and collector of the transistor
112 are connected respectively to the collector and base of a PNP
transistor 114 which has its collector grounded and emitter
connected to its base through a resistance 116. Resultingly, when
the input of the reset circuit 40 is grounded or lacks a pulse
signal, both transistors 112 and 114 are held off and have no
effect upon a voltage at the emitter of transistor 114. When a
pulse comes through the inhibiter 36, both transistors 112 and 114
are turned on effectively grounding the emitter of transistor
114.
Transistors 112 and 114 are off except when resetting the pulse
generator and thus contribute to maintaining a low current drain
for the circuit.
The emitter of transistor 114 is connected to the ungrounded side
of a grounded low value timing capacitance 118. That same side of
capacitance 118 is fed from the high line 14 through a variable
high value resistance 120 and a fixed high value resistance 122. A
combination of resistance 120 and 122 and capacitance 118 forms a
low current drain RC charging circuit which can be rapidly
discharged by the reset circuit 40 whenever a pulse is present at
its input to ground the emitter of transistor 114.
The junction between the capacitance 118 and resistance 122 is
connected to the emitter of a PNP transistor 124. The base and
collector of transistor 124 are respectively connected to the
collector and base of a grounded emitter NPN transistor 126. A low
current drain reference voltage is established by a high resistance
128 from the high line 14 to the collector of transistor 126 and by
a high resistance 130 between the collector and emitter of
transistor 126. When the voltage across the capacitance is
increased to equal the voltage at the collector of transistor 126
plus the forward conduction voltage of the emitter-base diode of
transistor 124, the capacitance 118 is discharged rapidly through
transistors 126 and 124 and a collector resistance 132 to ground
until the capacitance has been thoroughly discharged. During this
discharge interval the collector of transistor 126 is grounded
producing a negative pulse at the collector of transistor 126
during the discharge. This pulse is conducted from the collector of
transistor 126 through a capacitance 134 and through a resistance
136 to the base of a PNP transistor 138 having its emitter tied to
the high line 14. A biasing resistance 140 connects the high line
14 to the junction between capacitance 134 and resistance 136. The
collector of the transistor 138 is connected through a feedback
circuit comprising a parallel combination of a capacitance 142 and
serially connected resistance 144 and diode 146 to the base of
transistor 126.
Resistances 120 and 122 and capacitance 118 in conjunction with a
discharge path for the capacitance 118 through the transistors 124
and 126 form a periodic pulse generator with a period specified by
the charging time of the capacitance 118 through the resistances
120 and 122. Pulses can be inhibited indefinitely by discharging,
or keeping discharged, the capacitance 118 through the reset
circuit 40. On the other hand, when the pulse generator is
operating continuously to provide a train of pulses, as for
instance when the reed relay 110 is activated to ground the base of
transistor 112, the period between pulses will be constant set at
about 0.8 second for a constant voltage from the source 12. The
period between pulses will increase at a known rate, however, with
decreasing voltage from the source 12 because the emitter-base
diode forward conduction voltage of the transistor 124 is a
significant portion of the voltage from the source 12, thereby
increasing the percentage of supply voltage which must be across
the capacitance 118 to produce conduction of transistor 124. This
inherent dependence to pulse rate on voltage allows accurate and
easy monitoring of the pulse rate at low voltage with the single
addition of relay 110.
The pulse which is generated at the collector of transistor 138 and
passes to the base of transistor 126 is regulated in duration to
about 1 millisecond because of timing circuit 134, 136 and 140 and
the feedback path from the collector of transistor 138 to the base
of transistor 126 utilizing circuitry already a part of the pulse
generator. This particular feedback configuration reinforces the on
condition of transistors 124 and 126 and contributes to maintaining
the width of the pulse at the collector of transistor 138
substantially constant in the face of varying loads upon the
collector of transistor 138. Regulation of the pulse interval is
achieved by isolation of the timing capacitance 118 from the
output. Pulse duration decreases with source voltage, a feature
which provides an additional indiciation of source condition and
distinguishes it from other pacer problems.
Collector of transistor 138 feeds a push-pull voltage doubler
circuit 46 through a voltage divider composed of resistors 148 and
150 to ground. The connection between these resistances feeds the
base of a grounded emitter transistor 152 with its collector
connected to one side of a voltage doubling capacitor 154 which has
its other side connected to pulse line 18. The collector of
transistor 152 is also connected through serially connected
capacitance 156 and resistance 158 to the input 28 of amplifier 26,
thereby providing a path between line 18 and the input of amplifier
26. Finally the collector of transistor 152 is connected through
resistances 160 and 162 to a high line 164 fed and decoupled for
short circuit protection from line 14 by serial resistance 166 and
shunt capacitance 168. The junction of resistances 160 and 162 is
connected through a capacitance 170 to the base of a PNP transistor
172 having its emitter connected to the line 164 and its collector
connected to the pulse output line 20 and further having its base
joined to the line 164 through resistance 174. The pulse lines 18
and 20 are connected to ground through respective resistances 176
and 178, and are connected to each other by zener diode 180 to
prevent excessive voltage across them.
During the time that no pulses are generated or received,
capacitance 154 charges up to the full voltage between the lines
164 and 16. Then during the pulse interval when capacitance 118 is
being discharged, the transistors 152 and 172 connect this
capacitance 154 in series with the voltage between lines 164 and 16
across the pulse lines 18 and 20 producing there a pulse of voltage
twice the voltage across the source 12.
In addition to the current required to produce a heart stimulation
pulse, the pulse generator 22 requires very little ambient current
due to the high impedances of reference resistances 128 and 130 and
charge resistances 120 and 122 along with capacitance 118, and due
to the off condition of transistors 124, 126, 138, 152 and 172
except during a pulse when they are saturated and dissipate little
energy.
It can now be appreciated how the voltage doubling of the pulse
generator's output contributes to an increase in battery life,
reliability and an inherent indication of battery condition. The
voltage doubling feature allows for a lower supply voltage and
resulting lower current drain. Fewer batteries are needed or
redundant batteries may be provided. The lower supply voltage also
greatly increases the dependence of pulse interval on supply
voltage allowing an electrocardiograph to monitor battery condition
reflected in battery voltage. The relay 110 can be activated by an
external magnet to make this measurement possible. Also because the
voltage doubling is achieved without a transformer, an inherently
unreliable device, length of expected life is increased.
In FIG. 3 there is shown an alternative embodiment of the pulse
generator of FIG. 1. The pulse generator of FIG. 3 differs from the
generator 22 of FIG. 2 beyond the capacitance 134 where it joins a
resistance 182 connected to the high line 14. The capacitance 134
is also joined to the collector of a grounded emitter NPN
transistor 184 and to one terminal of a voltage doubling
capacitance 186 with the other terminal of capacitance 186
connected to the pulse line 18. The input to the amplifier 28 is
also taken from the capacitance 134 through the capacitance 156.
Capacitance 188 connects the capacitance 134 through a resistance
190 to the base of a PNP transistor 192 which has its emitter
connected to the line 164. The collector of transistor 192 is
connected to the pulse line 20 and via resistance 194 to ground. A
resistance 196 connects the other output pulse terminal 18, to
ground and a zener diode 198 connected between the lines 18 and 20
provides over voltage protection. The collector of transistor 192
is also connected to the feedback network composed of resistance
144 and diode 146 preferably without capacitor 142 and also is
connected through resistance 200 to the base of transistor 184.
The operation of the circuitry of FIG. 3 is similar to that of FIG.
2 but the same function is achieved more reliably at lower weight
and power consumption by having one fewer transistor. During the
standby state when no pulses are generated or received the
capacitance 186 charges up to the full voltage of source 12. Then
during a pulse the transistors 184 and 192 switch this capacitance
in series with the source 12 voltage across the output pulse
terminals 18 and 20 providing there a voltage pulse of twice the
voltage from source 12.
FIG. 4 shows a further modified embodiment of FIG. 2. The charge
and discharge circuits for the capacitance 118 are basically the
same as shown in FIG. 1 with the exception that the collector of
transistor 126 is connected to the base of transistor 124 through a
parallel combination of a low forward voltage diode 202 and
capacitance 204 with the diode 202 oriented for conduction from
base to collector. A resistance 206 connects the high line 14 to
the collector of transistor 126 and a pulse conducting capacitance
208 conducts from the collector of transistor 126 through a
resistance 210 to the base of a PNP transistor 212 having its
emitter connected to the line 14, and its collector connected
through a resistance 214 to ground. The pulse line 20 is connected
to the collector of transistor 212 and the pulse line 18 is
connected through a voltage doubling capacitance 216 to the
collector of transistor 126. The feedback network of components 144
and 146 is connected from the collector of transistor 212 to the
base of transistor 126. A capacitance 218 may be provided for
connection from the collector of transistor 126 to the input 28 of
the amplifier 26. A resistor 220 connects line 14 to the junction
between resistor 210 and capacitor 208.
During standby conditions of no pulse generation or reception the
capacitance 216 charges up to the full voltage between lines 14 and
16 through the load across terminals 18 and 20. Then during the
pulse state, when capacitance 118 is being discharged, the
transistors 126 and 212 connect capacitance 216 in series with the
source 12 across the load 24. The diode 202 prevents the resistance
206 from effecting the very high impedance reference impedances 128
and 130 during standby condition. The diode can be replaced by an
FET, or be any low resistance device conducting only during
discharge of capacitor 118.
The circuitry of FIG. 4 resettably generates pulses at ultra-low
current drain with a minimum of components. By itself it can be
used as a continuous generator of a train of pulses for use as a
continuous cardiac pacer where demand pacing capability is not
required. To achieve this efficiency of minimal components some
regulation in the pulse width and interval is sacrificed for
varying load.
Having described a preferred embodiment according to the invention,
it is clear how the cardiac patient can now be benefitted by the
provision in one simple light weight pacer of long implant life,
high reliability, inherent battery condition monitoring, regulated
pulse output, automatic change in the simulated refractory period,
continuous pacing in the presence of noise, and other features. It
will also be clear to those skilled in this art how modifications
can be made to this pacer without departing from the spirit and
scope of the invention as defined in the following claims.
* * * * *