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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the invention.
This invention relates to improvements in apparatus for generating pulses
for heart stimulation, and more particularly to improvements in apparatus
which generates pulses for heart stimulation in the absence of naturally
occurring heart pulses.
2. Description of the prior art.
The living heart effects its pumping action with many complex muscle
contractions and relaxations. Typically, such contractions and relaxations
are accompanied by electro-chemical potential pulses which can be analyzed
and associated with particular heart muscle actions. For example, the
contraction of the ventricle heart muscle produces a positive pulse,
ordinarily of larger magnitude and time duration than the other
heart-produced pulses. Thus, the ventricle muscle contraction presents a
unique frequency spectrum which can be detected and distinguished from the
other heart-produced pulses.
As a result of disease, injury, or other cause, occasionally a heart fails
to function properly; for instance, one or more muscles may fail entirely
to properly contract and relax, or may intermittently fail to properly
function. In such cases, artificially produced electrical stimulation
pulses may be effective in restoring the proper function to the failing
muscle.
Many devices have been proposed for generating such pulses to stimulate a
naturally occurring heartbeat. The devices which operate to produce
stimulation pulses when the heart fails but which otherwise remain
quiescent or inactive, such as the device disclosed in U.S. Pat. No.
3,345,990, are commonly referred to as "demand" heart pacers, and
generally include one or more electrical conduction leads physically
implantable in the heart to detect the presence or absence of the
naturally generated electrochemical potential associated with a particular
muscular function and apply electrical stimulation pulses to simulate the
natural pulses in their absence. Demand pacers such as disclosed in U.S.
Pat. No. 3,253,595 have been advanced which are entirely implantable
within the body and include solid state circuitry potted in a unit having
a surrounding conductive portion to establish electrical contact with the
body at a remote point, such as in the abdomen. A single lead is provided
for connection to the heart at an appropriate location.
Of the demand heart pacers proposed heretofore, many employ circuits which
are continuously conducting, such as monostable multivibrators which
switch from a first conducting state, for instance through one transistor,
to a second conducting state, through a different transistor. Thus, the
multivibrator may draw current continuously from the voltage source, which
can shorten its useful lifetime and require frequent replacement and
attention.
Additionally, circuits of the prior art commonly employ relatively
complicated circuitry to distinguish the various heart pulses, heart
pacers commonly being triggered upon the presence or absence of the
so-called "QRS" complex heart pulses.
Finally, demand heart pacers have been proposed which employ multivibrator
apparatuses to generate periodic heart stimulation pulses at the
multivibrator frequency, but upon detection of the triggering naturally
occurring heart pulse, such as the QRS complex above mentioned, the output
from the multivibrator is switched to a substiture load, the multivibrator
being allowed to continue its periodic conduction as if it were still
supplying heart stimulation pulses to the heart itself. This also results
in current from the supply to flow through the transistors or current
regulating devices of the multivibrator and is another source of
unnecessary power drain from the supply voltage source.
Becoming of recently increasing interest, nuclear batteries are being
employed to supply power to heart pacer circuits. Nuclear batteries,
however, present problems in the circuit design not ordinarily encountered
in the use of ordinary chemical batteries, the voltage supply, for
example, being more dependent upon the impedance of the circuit to which
the voltage is applied, and, additionally, being of relatively lower
magnitude than a conventional chemical type battery of otherwise same
relative characteristics. The use of a nuclear battery, therefore,
requires particular circuitry design to obviate these problems.
BRIEF DESCRIPTION OF THE INVENTION
The invention, in its broad aspect, presents a demand heart pacer and
includes a pulse generating multivibrator, controlled by plural RC timing
circuits to free run at the desired heart stimulation rate. An amplifier
amplifies the multivibrator pulses, and, in a preferred embodiment usable
with a low voltage nuclear battery power supply or the like, an output
voltage doubler is provided to present an output current pulse of
sufficient magnitude for heart application.
A multi-stage frequency selective amplifier amplifies heart pulses
naturally occurring of a particular frequency spectrum, such as that of
"R" waves, to trigger a pulse generator, which, in turn, activates means
for discharging the capacitors of the timing circuits of the free running
multivibrator each time a natural heart pulse occurs so that no output
pulse is generated. The apparatus, therefore, generates a heart
stimulation pulse each time the heart fails to do so, but does not
generate a stimulation pulse if the heart pulses on its own.
In light of the above, it is an object of the invention to present an
apparatus for generating electrical pulses for heart stimulation in the
absence of natural heart pulses.
It is another object of the invention to present a demand pacer circuit.
It is still another object of the invention to present a demand pacer
circuit which can be utilized with a low voltage nuclear power source.
It is another object of the invention to present a demand pacer circuit
which distinguishes and amplifies a naturally occurring heart wave of a
particular frequency spectrum and attenuates other naturally occurring
heart pulses.
It is still another object of the invention to present a demand pacer
circuit which draws a minimum amount of current in operation and which has
a quiescent state between pulses.
It is another object of the invention to provide a demand pacer circuit
which incorporates a plural stage amplifier for amplifying only naturally
occurring "R" pulses.
It is still another object of the invention to present a demand pacer
circuit which incorporates a high impedance voltage multiplier output.
It is still another object of the invention to present a demand pacer
circuit which incorporates means for testing the circuit in operation
without removal from the patient.
It is yet another object of the invention to present a demand pacer circuit
which employs a disabling section which discharges the timing capacitors
of a free running multivibrator.
BRIEF DESCRIPTION OF THE DRAWING
The invention is illustrated in the accompanying drawing, wherein:
FIG. 1 is a curve showing typical heart generated pulses;
And FIG. 2 is a schematic diagram illustrating another preferred embodiment
of the pacer circuit, in accordance with the invention, responsive to
heart demand, with low voltage supply capability for use with a nuclear
battery or voltage source in which the voltage may vary with the load
thereupon.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As mentioned above, the normal actions of the pumping muscles of the heart
are ordinarily accompanied by identifiable electrical potentials or
signals. A typical composite waveform of the signals as a function of time
is shown in FIG. 1. Essentially, the contraction of the atrium is
accompanied by a "P" wave of positive potential, followed, after a delay
known as the "AV" or atrial ventricular delay, by a potential called the
"QRS" complex associated with the contraction and relaxation of the
ventricle muscle, followed by a "T" wave after a "refractory period". As
shown, the "P", "R" and "T" waves are of positive potential and the "Q"
and "S" are of negative potential. Depending on the location of the heart
connection, however, the respective polarities of the "P", "Q", "R", "S"
and "T" may be reversed, which, for example, could be displayed in the
manner of the waveform of FIG. 1 with the negative direction upwards. Each
has its own frequency spectrum, and, therefore, can be electronically
frequency distinguished from the others.
The demand pacer circuitry in accordance with the invention, as below
described in detail, is intended primarily to distinguish and detect the
existence of a particular naturally occurring heart wave, such as an "R"
wave, and to generate a heart stimulation pulse in its absence, but remain
quiescent or inactive, not generating a heart stimulating pulse, in its
presence. Thus, if the heart to which the stimulation pulses of the
circuitry are applied properly generates an "R" wave (or other wave to
which the circuit is tuned), the stimulation pulse generating portion of
the circuitry will remain quiescent. On the other hand, in the absence of
the particular naturally occurring heart pulse, the pacer circuitry will
generate the appropriate stimulation pulse.
A preferred embodiment of the pacer circuit, in accordance with the
invention, for primary utilization with a nuclear potential source or
power supply is illustrated in FIG. 2. The circuit includes five main
sections, each outlined by dotted lines. A free running multivibrator 120
generates output pulses at the desired application rate to a high
impedance voltage multiplier output amplifier 121, which produces an
output current pulse for application to the heart. Naturally occurring
heart generated pulses are conducted to a frequency filtering amplifier
including a four-stage a-c coupled frequency selective amplifier and pulse
shaping section 122, where the "R" wave is selectively amplified and
shaped and the other pulses of different frequency spectra are attenuated.
Each amplified pulse is directed to a normally nonconducting monostable
multivibrator (hereinafter referred to merely as a monostable) 123 which
is triggered into conduction by the amplified "R" wave to produce an
output pulse. The monostable output pulse is then directed to a disabling
circuit 124 which discharges the capacitors of the free running
multivibrator 120 for one pulse period each time a heart generated "R"
wave occurs. The disabling circuit 124 additionally includes a magnetic
switch to bypass the transistor control voltage to ground to enable the
disabling circuit, free running multivibrator and power source to be
tested.
In the circuit embodiment of FIG. 2, terminals 130 and 131 are connected to
negative and positive terminals, respectively, of a voltage source or
source of electrical potential.
With specific reference to the various sections of the pacer circuit of
FIG. 2, the free running multivibrator 120 for the stimulation pulse
initiation includes a p-n-p transistor 132 and an n-p-n transistor 133
connected to present a current flow or conduction path through their
series connected emitters and collectors, as shown. Means for presenting a
time varying voltage level, such as at least one and preferably, as shown,
a pair of RC timing circuits control the rate at which the multivibrator
pulses, the first timing circuit. The first timing circuit includes series
connected resistors 135 and 136, in turn, in series with capacitor 137,
the base of transistor 132 being connected between resistors 135 and 136.
Likewise, the second RC timing circuit for transistor 133 includes series
connected resistors 140 and 141 in series with capacitor 146, the base of
transistor 133 being connected between resistors 140 and 141. Thus, in
operation, the potential from the source applied at terminals 130 and 131
builds up upon capacitors 137 and 146 until the transistors 132 and 133
become forward biased into conduction. At that point, the voltage upon the
capacitors discharges, primarily through the base circuits of their
respective transistors until the reduced voltage upon the respective bases
of transistors 132 and 133 cause the transistors to fall out of
conduction, starting the cycle again.
The values of resistors 135, 136, 140 and 141 and of capacitors 137 and 146
are chosen to cause the multivibrator to generate pulses of sufficient
width at a normal heartbeat rate, such as 71 beats per minute.
Additionally, the values (exemplars listed below) of the corresponding
components, the capacitors 137 and 146, the resistors 135 and 140, and the
resistors 136 and 141 of each RC timing circuit are the same. Thus,
redundant RC timing networks are presented, which minimize the effects of
component value changes in the operation of the circuit. For instance, if
the value of the capacitor 137 in one of the RC timing networks were to
change, its effects would not drastically change the pulse rate of the
multivibrator because of the remaining proper valued capacitor 146 in the
other RC network.
In the multivibrator circuit illustrated, the pulse rate is primarily
determined by the charging time of the capacitors 137 and 146 as
controlled by the values of the capacitor 146 and resistor 140, and by the
values of the capacitor 137 and resistor 135 (the values of resistors 136
and 141 being small with respect to the values of the resistors 135 and
140, therefore having little effect on the charge rate of the capacitors
137 and 146). The width of the pulses, on the other hand, is primarily
determined by the values of resistors 136 and 141 in conjunction with
their respective capacitors 137 and 146. With components as listed below,
for example, a pulse repetition rate of approximately 70.+-.5 beats/minute
and pulse width of approximately 0.8-1.0 milliseconds can be achieved.
The conduction path defined by conducting transistors 132 and 133 permits
current flow through voltage dividing resistors 144 and 145 to present a
voltage upon the emitter-base junction of transistor 148 of the output
stage 121. The multiplier stage 121, illustrated, acts as a voltage
doubler and presents an output current pulse to the output terminal 149.
In operation, a voltage in the quiescent state is built up upon capacitor
150 through resistors 151 and 152. When the multivibrator stage 120
permits conduction through resistor 145 forward biasing the base-emitter
junction of transistor 148, the voltage upon capacitor 150 is added to the
supply voltage from terminal 130 through the collector and emitter of
transistor 148 and the emitter resistor 153.
At the time, the base-emitter junction of transistor 155 is also forward
biased into conduction, the base voltage of transistor 155 being
maintained by the resistor 158, and the collector bias being established
through resistor 160 to the positive common terminal 131. The transistor
155 thus presents a current output path through its emitter and collector
and d-c isolating capacitor 159 to the output terminal 149 for conduction
to the heart through an appropriate heart lead (not shown). The output
transistor 155 additionally presents a high output impedance upon the
output terminal 149.
If a naturally generated "R" heart wave occurs, it is conducted through the
heart lead (not shown) to terminal 149, thence along conductor 162 to the
frequency selective amplifier 122. The heart pulses initially are filtered
through a resistor 164 connected in series with a capacitor 165 before
application to the first amplifier stage. The values of resistor 164 and
capacitor 165 are chosen to present a low frequency roll off
characteristic to present a high impedance to and partially attenuate low
frequency heart waves below the frequency spectrum of the "R" wave.
The capacitor 165 also differentiates the incoming signal. Thus, if a
negative pulse is applied to the resistor 164 and capacitor 165, it is
differentiated to produce first a negative voltage output corresponding to
the rate of the negative voltage increase, followed by a positive pulse,
corresponding to the subsequent rate of negative voltage decrease.
Therefore, the pacer circuit, responsive to positive "R" signals, as will
be apparent below, will respond to a negative input pulse from the heart,
since a positive pulse resulting from the capacitor differentiation will
nevertheless be applied to the first transistor stage including transistor
166, biased for Class A operation. Thus, the precise location of the
electrode implanted in the heart is not critical insofar as it is not
required to conduct heart pulses of any particular polarity to the pacer
circuit.
The first transistor 166 includes a high frequency roll off collector load
impedance, including resistor 168 in parallel with capacitor 169. The
emitter-base bias on the first stage transistor 166 is maintained by a
resistor 167 to the common terminal 131 and by a resistor 176 to the
emitter of the second stage transistor 171, for stabilization.
In operation, capacitor 169 presents a low impedance in the collector lead
of transistor 166 at high frequencies, thereby diminishing or reducing the
high frequency gain of the first amplifier stage, including transistor
166. On the other hand, at low frequencies, the gain of the first
amplifier stage is determined by the resistance of resistor 168, capacitor
169 presenting, essentially, an infinite or very high impedance.
The output from transistor 166 developed across the load resistor 168 is
applied to the base of the second stage transistor 171 also biased for
Class A operation. In the emitter circuit of transistor 171, a first
resistor 172 is connected in parallel with the series connection of a
second resistor 173 and a capacitor 174. A load resistor 175 is connected
in the collector lead of transistor 171 to the common terminal 131. In
operation, at low frequencies, the capacitor 174 presents a high
impedance, whereby the resistors 172 and 175 determine the voltage
developed across resistor 175 and hence the gain of the second amplifier
stage including transistor 171. On the other hand, at high frequencies,
the capacitor 174 presents little impedance, thereby connecting resistor
173 in parallel with the resistor 172 to decrease the gain of the second
amplifier stage. Thus, the second amplifier stage presents a low frequency
roll off to amplify only signals at and above the "R" wave frequency
spectrum.
The output of the transistor 171 is developed across a load resistor 175
and is a-c coupled by a capacitor 178 to the base of the high gain third
amplifier stage transistor 179. Since capacitor 178 blocks any d-c
potential which may exist on the collector of transistor 171, a d-c bias
is established upon the base of transistor 179 by a resistor 181 to the
negative terminal 130. The third stage transistor 179 in conjunction with
an emitter resistor 183 to the common terminal 131 and a collector load
resistor 184 to the positive terminal 130 amplifies the a-c signal and
applies it through a coupling resistor 186 and capacitor 187, in parallel,
to the base of a transistor 190 biased to exhibit high gain for shaping
the input pulses. The base-emitter bias of pulse shaping transistor 190 is
maintained by resistor 191 between the base and the common terminal 131.
The positive portion of the output signal developed across the collector
resistor 193 to the positive terminal 130 is coupled by a capacitor 195
and a resistor 196 connecting the low side of the capacitor 195 to the
negative terminal 130 to the monostable 123 through a coupling diode 198.
The monostable 123 includes two transistors 200 and 201. A positive pulse
from the fourth transistor stage of the amplifier section 122 is applied
to the base of transistor 200 to forward bias its emitter-base junction
into conduction. When the transistor 200 conducts, the voltage upon series
connected voltage divider resistors 206 and 207 drops, presenting a more
negative voltage upon the base of transistor 201 to forward bias that
transistor into conduction through its collector resistor 211, thereby
charging capacitor 204 through resistor 210 and further forward biasing
the transistor 200. When the charge on capacitor 204 discharges, the bias
upon the base of transistor 200 established by resistor 212 is reduced,
causing the transistor 200 to fall back into nonconduction, which, in
turn, raises the voltage developed across resistor 206, causing the
transistor 201 also to lapse into nonconduction.
The output voltage from the monostable 123 developed across resistors 206
and 207 is coupled by a capacitor 215 into the disabling circuit 124 to be
applied through series connected voltage dividing resistors 217 and 218 to
the base of transistor 220, causing it to conduct. At the same time, the
conduction current through transistor 220 is coupled by resistor 222 to
the base of the second transistor 224 causing it also to conduct. The
respective collectors and emitters of transistors 220 and 224 are
connected in parallel with the timing capacitors 137 and 146 of the free
running multivibrator stage 120, to provide a discharge path for the
capacitors when the transistors 220 and 224 are biased into conduction
upon the arrival of a pulse from the monostable 123.
To add a time delay to the pulse traversing the amplifier 122, monostable
123, and disabling circuit 124, a capacitor 230 is provided between the
base of transistor 220 and the common terminal 131. Thus, the output pulse
from the outpulse stage 121, as fed back to the input of the frequency
selective amplifier section 122 will not prematurely disable the free
running multivibrator 120.
A zener diode 232 is provided between the output terminal 149 to the common
terminal 131 to permit application of a fillibration or other external
voltage to the patient without overloading the devices of the pacer
circuitry. Thus, if a voltage of positive polarity is applied to the
output terminal 149, it is immediately bypassed to the common terminal
131. On the other hand, if a negative voltage above the breakdown voltage
of the zener diode 232 is applied, it will be also bypassed to the common
terminal 131.
As above mentioned, the pacer circuit of FIG. 2 is intended for use with a
nuclear battery (not shown). A suitable nuclear battery may be such as
that described by Steve A. Kolenik and Thomas F. Hursen, Nuclear Energy
Sources, 167 Annals of the New York Academy of Sciences, October 30, 1969,
pp. 661 et seq.; and by S. A. Kolenik, T. F. Hursen, and G. W. Maurer,
Radioisotope Powered Pacemaker Development Program, Annual Winter Meeting
of the American Nuclear Society, Oct. 18, 1971; and in U.S. Pat. No.
3,649,367. Because the nuclear battery may have a relatively high
resistance, on the order of 5K-11K ohms to be applied to terminal 130, a
relatively large capacitor 234 is connected between the battery terminals
130 and 131 to reduce IR losses in the battery during high instantaneous
current demands in the circuit, for example, when the free running
multivibrator stage 120 conducts. Thereafter, upon demand of the circuit
upon the voltage source, the current may be obtained from both the battery
itself and from the discharge current from the capacitor 234. It should
also be noted that nuclear batteries typically have a voltage
characteristic in which an increasing load upon the battery results in
decreased output voltage. Consequently, the capacitor 234 assures proper
circuit operating voltage regardless of the instantaneous load applied to
the battery terminals 130 and 131.
Finally, to permit the free running multivibrator and output amplifier
sections to be tested, a magnetic switch 236 is provided between the base
of transistor 220 and the common terminal 131. To test the operation of
the circuit, therefore, a magnetic field can be applied to the patient
wearing the heart pacer circuit to close the magnetic switch 236 and
disable the transistors of the disabling section 124. The free running
multivibrator, therefore, will independently pulse without being disabled
and can be detected to assure proper multivibrator, voltage source, and
output amplifier operation.
In the circuit of FIG. 2, by way of example, it has been found that the
following component types and values result in proper circuit operation.
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Resistors (ohms)
135,140 1.8M
136,141 680
144 22K
145,151, 47K
152,172
153 120
158 27K
160,164, 10K
173
168,175, 100K
193,222
167,210 470K
176,186,
191,196 1.5M
212
181 6.8M
183 39K
184,206 150K
207 390K
211 560K
217 210K
218 120K
Capacitors (microfarads)
137,146,
165,169, 0.47
174,215
150,159 39
178,187, 0.047
195,204
230 0.02
234 120
Diodes
198 1N3010
232 (zener) IN756A
Transistors
131,166,
179,190, 2N2907A
201,220
133,148,
155,171, 2N222A
200
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It is apparent from the circuit of FIG. 2, above described, that the
transistors of different conductivity type (n-p-n, p-n-p, etc.) may be
substituted into the circuit with appropriate modification in the supply
voltage polarity and associated component values. It should also be
appreciated that although the circuitry has been illustrated and described
with respect to transistors, that any solid state device, such as
integrated circuits, single circuit semi-conductor chips, and the like,
can be advantageously employed with appropriate circuit value
modifications, as will be apparent to those skilled in the art. It should
also be pointed out that although the components listed above described
particular types of transistors, that any transistor having the same
functional characteristics can be employed, again with appropriate
associated component adjustment for proper biasing and operation.
Although the circuitry of the invention has been described and illustrated
with a certain degree of particularity, it is understood that the present
disclosure is made by way of example only and that numerous changes and
modifications will become apparent to those skilled in the art without
departing from the spirit and scope of the invention as hereinafter
claimed.
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