U.S. patent number 3,835,865 [Application Number 05/269,611] was granted by the patent office on 1974-09-17 for body organ stimulator.
This patent grant is currently assigned to General Electric Company. Invention is credited to David Lee Bowers.
United States Patent |
3,835,865 |
Bowers |
September 17, 1974 |
BODY ORGAN STIMULATOR
Abstract
The output circuit of an electric stimulator is connected to an
organ such as the heart through a coupling capacitor. A
semiconductor switch is triggered with a short pulse so that it
partly discharges the capacitor and stimulates the organ.
Immediately following the turn off of the first switch, a second
switch is activated to recharge the capacitor to its initial state
during which time reverse current is delivered momentarily to the
heart. In one embodiment, after the first switch turns off to
terminate the discharge of the capacitor which stimulates the
heart, the current through the heart is reversed immediately by
recharging the capacitor through a diode which is connected to a
supply line and is in series with the capacitor and the heart.
Turn-off of the first switch starts the diode conducting heavily. A
high input impedance detector determines whether there is a natural
electric signal on the organ and turns on a generator which
provides the trigger pulse if there is no natural signal. The
circuit is arranged so that the detector and output circuit present
a high impedance to the organ.
Inventors: |
Bowers; David Lee (Wauwatosa,
WI) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
26732305 |
Appl.
No.: |
05/269,611 |
Filed: |
July 7, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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53842 |
Jul 10, 1970 |
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Current U.S.
Class: |
607/13 |
Current CPC
Class: |
A61N
1/365 (20130101) |
Current International
Class: |
A61N
1/365 (20060101); A61n 001/36 () |
Field of
Search: |
;128/2.1R,419CD,419E,419P,419R,419S,421,422,423 ;307/268
;328/66,68,74 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Hohenfeldt; Ralph G. Wiviott;
Fred
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This is a continuation-in-part of copending application Ser. No.
53,842, filed July 10, 1970, now abandoned.
Claims
I claim:
1. An electric organ stimulator comprising:
a. terminals which are adapted to be connected to a d-c source,
b. a capacitor,
c. a first output terminal connected to one side of said capacitor
and adapted to be connected to an organ load,
d. a second output terminal connected to a source terminal and
adapted to be connected to an organ load,
e. first and second substantially unidirectionally conductive means
each having main conductive paths connected in series with each
other between said d-c source terminals, at least the first
conductive means including a control element for selectively
controlling conduction thereof in response to a signal on said
element, the other side of said capacitor being connected to a
point between said conductive means,
f. means for applying a control signal to said control element to
cause said first conductive means to conduct for a predetermined
interval at a time when stimulation of an organ is desired,
conduction by said first conductive means causing current flow
through said capacitor and between said organ load output terminals
to thereby alter the initial state of charge on the capacitor
rapidly, said second unidirectionally conductive means remaining
substantially nonconductive when said first means is
conducting,
g. said second conductive means responding to termination of
conduction of said first conductive means by conducting and causing
current flow through said capacitor and between said organ
terminals to thereby substantially reestablish the initial state of
charge on said capacitor rapidly,
h. a high value resistor means connected between one source
terminal and to said capacitor, said resistor means continuing to
conduct after said second conductive means stops conducting whereby
to complete reestablishing the charge on said capacitor to its
initial state.
2. The stimulator in accordance with claim 1 wherein:
a. said first unidirectionally conductive means is a semi-conductor
means having load terminals serially connected in said conductive
path and a control terminal to which said control signal is applied
to cause said first conductive means to conduct, and
b. said second unidirectionally conductive means is a diode.
3. The stimulator in accordance with claim 1 wherein:
a. said second unidirectional conductive means is a diode means and
said high value resistor means is also connected in parallel with
said diode means to complete reestablishment of said charge as
aforesaid.
4. An electric organ stimulator comprising:
a. terminals adapted to be connected to a d-c source,
b. a capacitor and a first output terminal connected to one side
thereof and adapted to be connected to an organ load,
c. a second output terminal connected to a d-c source terminal and
adapted to be connected to an organ load,
d. diode means and a controllable switch means having load
terminals and a control means, said diode means and said load
terminals being connected in series between said d-c source
terminals, said control means selectively controlling conduction
between said load terminals in response to a control signal on said
control means for a predetermined interval when stimulation of an
organ is desired,
e. means for applying a control signal to said control means,
f. the other side of said capacitor being connected to a point
between said diode means and controllable switch means, whereby to
enable said capacitor to charge to an initial state from said
source through said diode means when said switch means is
nonconductive,
g. a high value resistor means connected between the d-c source
terminal to which said diode means is connected and said other side
of said capacitor, said resistor thereby shunting said diode
means,
h. conduction of said controllable switch means in response to said
control signal causing current flow through said capacitor and
between said organ load output terminals to thereby alter the
initial state of charge on said capacitor rapidly, termination of
said signal being followed by altered current flow through said
diode, said capacitor and between said organ load terminals to
substantially reestablish the initial state of charge on said
capacitor rapidly, reestablishment of said charge being completed
by ensuing conduction through said high value resistor means.
5. The stimulator in accordance with claim 4 including:
a. a resistor means connected in series with said diode, said
resistor means having a substantially lower value than said high
value resistor means.
6. An electric organ stimulator comprising:
a. terminals which are adapted to be connected to a d-c source,
b. output terminals adapted to be connected to an organ,
c. a capacitor having one and another sides the one of which is
connected to one of said output terminals,
d. first and second unidirectionally conductive means, the first of
said conductive means being connected between one of said source
terminals and said other capacitor side and the second of said
conductive means being connected between said other capacitor side
and another of said source terminals, said conductive means being
connected in series with each other between said d-c source
terminals, the other of said output terminals being connected to
said one source terminal,
e. at least one of said unidirectionally conductive means having a
control means for selectively controlling said one conductive means
to conduct in response to a signal on said control means, means for
applying a control signal to said control means to cause said one
conductive means to conduct for a predetermined interval at a time
when an organ is to be stimulated, conduction by said one
conductive means causing current flow in one direction across said
output terminals and through said capacitor to thereby alter the
initial charge on said capacitor,
f. said other conductive means responding to said one conductive
means conducting by remaining substantially nonconducting, and said
other conductive means responding to said one conductive means
terminating conduction by immediately conducting to cause current
flow in an opposite direction across said output terminals and
through said capacitor to thereby reestablish the initial state of
charge on said capacitor,
g. means for sensing the appearance of natural organ signals on
said output terminals,
h. pulse signal generator means controlled by said sensing means to
produce said control signal in response to said sensing means
sensing no natural signal for a predetermined interval, and
i. high value resistor means connected between said one source
terminal and to said capacitor, said resistor means continuing to
conduct after said second conductive means stops conducting whereby
to complete charging said capacitor to its initial state.
7. The invention set forth in claim 6 wherein:
a. said first and second unidirectionally conductive means are
transistor means each having two load terminals and a control
terminal,
b. a diode means connected between said other side of said
capacitor means, one of said load terminals of said first
transistor means and said diode means also being jointly connected
between one of said load terminals of said second transistor means
and said control terminal of said second transistor means whereby
to establish said second transistor control terminal and its one
said load terminal at substantially the same potential when said
first transistor means conducts to thereby prevent said second
transistor means from conducting upon occurrence of said control
signal, the other of said load terminals of said second transistor
means being connected to one d-c source terminal and the other of
said load terminals of said first transistor means being connected
to another of said d-c source terminals.
8. The invention set forth in claim 7 wherein:
a. said transistor means each have an emitter and a collector
comprising said load terminals and a base terminal comprising said
control terminal,
b. said second transistor having its collector connected to said
another source terminal and its emitter connected to the anode of
the diode means,
c. said first transistor means having its collector connected to
the cathode of said diode means and its emitter connected to said
one of said source terminals,
d. the said other side of said capacitor being connected to said
anode of the diode means,
e. means for applying control signals to said control terminal of
said first transistor means said signals being applied in a
sequence corresponding with the desired responses of the organ, the
said first transistor responding to said signals by conducting and
thereby discharging said capacitor sufficiently to stimulate the
organ after which said second transistor means conducts in response
to termination of conduction by said first transistor means to
recharge said capacitor to its initial state.
Description
BACKGROUND OF THE INVENTION
Electric pulse generators have been used internally and externally
of the body for stimulating a body organ to function in the absence
of natural electric or natural nerve impulses. Stimulation of the
bladder, ureter and the atrium and ventricles of the heart are
common examples. The first generation of organ stimulators were
essentially fixed rate pulse generators which were connected to the
organ to furnish it with artificial pulses whether or not its
natural or intrinsic electric or nerve impulses had reoccurred.
In due course a stand-by type of organ stimulator was developed.
The stand-by type of heart pacer, commonly called a Pacemaker,
includes a pulse generator which is under the control of a heart
signal detector. If an intrinsic heart signal is detected, the
detector turns off the pulse generator or otherwise inhibits
delivery of an artificial stimulating pulse to the heart. If no
intrinsic signal is detected on the heart within a period that
corresponds with the desired beating rate of the heart, the pulse
generator becomes inhibited and delivers one or more artificial
stimulating pulses to the heart as required.
A rather sophisticated detector is required for intrinsic heart
signals. These signals are composed of various frequency components
and there is often a broad spectrum of noise frequencies present.
The amplitude of the signals is low and must be detected down as
low as 2 millivolts ordinarily. There are some unpredictable
phenomena of electrical and chemical nature occurring at the
interface of the stimulating electrodes and the heart tissue that
result in signals being produced which confuse detection of the
natural signals that must necessarily be detected for proper
control of the pulse generator. When the heart coupling capacitor
is slowly recharged through the heart as it is in prior art devices
following the short duration pulse which stimulates the heart
artificially, the noise modulates the recharging current thus
accentuating the noise which the detector must distinguish from
other frequency components of the intrinsic heart signals.
Another problem encountered with heart pacers is the migration of
metal ions from the heart attachment electrodes into the tissue of
the heart. Besides the deleterious effect this may have on heart
tissue it is also suspected of being a contributor to electrical
instability of the heart immediately after stimulation and during
the detection time. It has been proposed in the prior art to
minimize metal ion migration by stimulating the heart with
alternately positive and negative pulses which are spaced from each
other by the desired beating period of the heart. This was expected
to attract the ions back to the electrodes on every alternate pulse
but experience has shown that it is not fully effective, probably
because there is too much time between pulses. Moreover, it did not
contribute to the electrical stability of the heart during the
critical detection time.
An object of the present invention is to mitigate the above
disadvantages by providing a stimulator with a new type of fast
recovery output circuit.
Another object is to provide an output circuit which puts the heart
in a substantially stable electric state during the detection
period, or in other words, to stabilize the electrode surfaces, the
interface between the electrodes and tissue and to minimize the
effect of polarization barriers.
A further object is to provide a fast recovery output circuit which
minimizes low frequency noise components and eliminates them from
the detector input so that improper detection due to capacitor
charging or electrode polarization is vitiated.
A still further object is to provide means for charging the heart
coupling output capacitor during the refractory period of the
detector so that erroneous detection is prevented.
Another object is to provide an output circuit and a stimulator
which results in a high impedance being presented to the heart to
facilitate detecting the low energy signals which are produced by a
depolarization of heart cells.
Another object is to provide for fast recovery of the electric
stability of the heart regardless of variations in the heart load
impedance or in the intensity or energy of the stimulating
pulse.
Briefly, the stimulator output circuit comprises two conductive
devices which may be transistor switches each of which has
collector and emitter terminals serving as load circuit terminals
or one may be a diode and the other may be a switch. One of the
devices may be triggered by a pulse which occurs at the instant
when it is desired to stimulate the organ. Conduction by this
device discharges a coupling capacitor or charges the same,
depending on the embodiment, through the organ to provide the
stimulus. The other device is prevented from conducting or is
substantially nonconducting when the first one is conducting. When
the first one terminates conduction, the second one immediately
becomes conductive and recharges or discharges, respectively, the
capacitor to substantially the same voltage that it had initially.
The recharging circuit also includes the organ so the recharge
current is substantially equal to the discharge current plus losses
regardless of the fact that the organ impedance is variable. When
the second device has stopped conducting, the remainder of the
charge deficiency is made up on the capacitor through a high
resistance which is supplied from the d-c power source. Thus, the
second device is inactive when the capacitor recharge is
substantially completed and the organ signal detector and the organ
will only see the high valued charging resistance which is in
parallel with the second device.
As illustrated, the output circuit is incorporated in a stand-by
known type of heart pacer or stimulator. The pacer includes a
detector of any electric signals appearing on the organ. If the
desired intrinsic signal does not appear when it should, the
detector turns on a pulse generator which controls the output
circuit and causes an artificial stimulus to be applied. The output
circuit is, however, usable in and functions in the same way in
other organ stimulators.
How the above mentioned objects and other more specific objects are
achieved will appear from time to time through the course of the
ensuing more detailed description of a preferred embodiment of the
invention taken in conjunction with the drawing.
DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic diagram of a heart stimulator which
incorporates the new output circuit;
FIGS. 2-4 are schematic diagrams of alternate forms of the new
output circuit;
FIGS. 5-6 depict wave forms which are applicable to the invention;
and
FIG. 7 is a schematic diagram of an alternative embodiment of the
output circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1, an organ which is to be stimulated is symbolized by a
resistive load marked R.sub.L. The load is connected to a
stimulator output terminal 10 by means of a lead 11. In the case of
a heart pacer, all the other circuit elements shown in FIG. 1 are
encapsulated in a resin and covered with a body compatible coating
so that the whole device may be implanted in the body and connected
to the organ with a conductive catheter or other leads.
Alternatively, the device may be located outside the body and leads
11 may constitute a conductive catheter which runs through a blood
vessel and is attached to or in contact with the organ which is to
be stimulated.
It is apparent that stimulating signals from the stimulator will
appear on output terminal 10 and that signals which are intrinsic
to the organ will also appear thereon.
A conductor 12 is connected to output terminal 10 for picking up
any signals that appear on the organ. This conductor leads to a
detector at the far left of the figure and has a reference numeral
13. The detector 13 is basically a preamplifier and a filter which
produces a ringing signal at its output when it is shocked, the
output signal being exemplified by the oscillatory wave form 14. A
similar signal is produced regardless of whether the incoming
signal to detector 13 is positive or negative. The detector 13 and
its associated circuitry are so designed that the system is
refractory for a short period of time following receipt of an
incoming signal. In other words, there is a lapse of time before an
output signal can be produced following a preceding signal.
Detector output signals 14 are furnished to a gated threshold
trigger 15 through a capacitor 16. The structural details of the
trigger 15 may be accomplished by a skilled electronic designer so
they will not be discussed here. For present purposes it is
sufficient to observe that input to trigger 15 of a positively
swinging portion of the oscillatory wave 14 results in the output
of a pulse 17 which appears on the top of a resistor 18.
Also associated with gated threshold trigger 15 is an inhibiting
transistor 19. The collector resistor of this transistor is marked
20. Connected across the collector and emitter of the transistor is
a resistor 21 across which the oscillatory signal voltage 14 is
developed and applied to trigger 15 ordinarily. However, under
certain circumstances it is necessary to inhibit the output of
pulses 17 by trigger 15 in which case an inhibiting signal is
applied to the base of transistor 19 by means of a conductor 22.
How this inhibiting signal is developed will be discussed later. At
the present, it is sufficient to note that when transistor 19 is
rendered conductive the oscillatory signal 14 will be shunted to
ground and will not operate trigger 15.
Assume for the sake of explanation that an intrinsic organ signal
has been detected on output terminal 10 and that a corresponding
pulse 17 has been produced by trigger 15. The pulse, or consecutive
pulses, are furnished to an interference rejector 23. Rejector 23
is essentially a selective filter which produces output signals
such as 24 only if signals 17 are coming at a predetermined rate.
The details of rejector 23 are not pertinent to the present
invention.
Each pulse signal 24 is applied to the base of a transistor 25
which has its collector resistor 26 connected to positive line.
Transistor 25 controls a timing pulse generator 27 which is
enclosed in broken lines and will now be described.
Timing pulse generator 27 has a timing network including series
connected resistors 28 and 29 and a timing capacitor 30 which are
connected between positive line and ground. The capacitor 30
charges and will cause the pulse generator to turn on when it
reaches a certain voltage level unless the capacitor is prematurely
discharged in whole or in part. There is a voltage divider
including resistors 31 and 32 for controlling the bias on the
timing pulse generator. Point A in the divider has a specific
positive voltage value with respect to ground. A capacitor 33
parallels resistor 32 and stabilizes the d-c voltage at point A.
When timing capacitor 30 charges to above a certain value, it
causes a transistor 34 to become forward biased through its
emitter-Base circuit and a resistor 35 leading to point A. As the
timing capacitor 30 charges, its voltage is increasing toward the
point where transistor 34 will be forward biased for producing a
pulse 36 on its collector. This pulse will be produced unless
capacitor 30 is prematurely discharged. When capacitor 30 is
charged sufficiently, it will supply the necessary current to
forward bias the emitter to base circuit of transistor 34 through a
path which includes resistor 35 at which time it also renders the
emitter to collector circuit of transistor 34 conductive and causes
the pulse 36 to appear on its collector. The pulse duration is
about two milliseconds which is desirable for stimulating the
heart. Pulses of other duration may be chosen for stimulating other
organs.
A latching voltage keeping this multivibrator circuit in conduction
appears across the resistor 35 and is additive to the bias voltage
across resistor 31. Another transistor 37 in the timing pulse
generator also conducts and the potential appearing on its emitter
is applied to a resistor 38 and conductor 22 to the base of the
inhibiting transistor 19 at the far left of the drawing. A filter
network including capacitor 39 and a parallel resistor 40 prevents
the emitter of transistor 37 from floating above ground potential
and provides emitter impedance during the conductive state of
transistor 37. The purpose of applying a signal to transistor 29 is
to make it conductive and thereby inhibit threshold trigger 25 so
that it does not sense or respond to artificial heart pacing
signals. As explained earlier, when transistor 19 conducts, the top
of resistor 21 is placed at ground potential and bias is removed
from threshold trigger 15 so it produces no output pulse 17.
Whenever detector 13 senses the presence of an intrinsic or natural
signal on the heart, timing capacitor 30 is discharged prematurely
in this example so that it will not forward bias transistor 34 nor
produce a timing pulse 36. Capacitor 30 is prematurely discharged
through a circuit which includes a diode 41, a resistor 42 and a
collector to emitter path of transistor 25. Recall that transistor
25 is rendered conductive whenever a natural signal is detected so
it causes capacitor 30 to be discharged to restart the timing
cycle.
When one or more pulses 36 are produced by timing pulse generator
27 they produce a signal voltage across a resistor 43. This voltage
is delivered through a resistor 44 to the base of a transistor 45.
Pulses 36 turn on transistor 45 and cause it to produce output
pulses such as 46 on its collector. There is a collector resistor
47 for transistor 45 and connected to it is a filter circuit
including resistors 48 and 49 and a capacitor 50. The filter shapes
pulses 36 in square form for being applied to the base of a
transistor 51.
Transistor 51 amplifies and with its associated circuitry permits
setting the current level of the new output circuit and,
accordingly, the current which is furnished to the organ for
stimulating it. Connected between the emitter of transistor 51 and
positive line is a resistor 52 in series with a parallel
combination comprising a fixed resistor 53 and a variable resistor
54. These resistors determine the bias current of transistor 51 and
its load current which flows through an adjustable resistor 55 and
a fixed resistor 56. The voltage appearing on the wiper 57 of
variable resistor 55 is preferably a square wave of two millisecond
duration in heart stimulators. Wiper 57 can be set for zero output
voltage when variable resistor 54 and its calibrated current
indicating dial, not shown, is set for zero stimulating current.
These pulses 62 are delivered through a resistor 58 to one of the
terminals 59 of the new fast recovery output circuit.
The FIG. 1 embodiment of the new stimulator output circuit
comprises a first controllable conductive device which may be any
suitable switch but is here shown as transistor 60 whose load
terminals comprise a collector and emitter of the transistor. The
transistor 60 has a resistor 61 in its emitter circuit and
functions as an emitter follower. The value of resistor 61 in one
embodiment is 47 ohms and transistor 60 is a silicon type 2N 930.
Current through resistor 61 and, as it will appear, the current
delivered to the organ, is equal to the voltage on the base of
transistor 60 minus its base-to-emitter drop divided by the value
of resistor 61. Thus, the amplitude of the input pulse to the base
of transistor 60 determines the current through resistor 61 and the
voltage on the latter follows the base.
The fast recovery output circuit includes another controllable
conductive device or switch which is in the form of a transistor 63
which may be of the same type as transistor 60. Transistor 63 has a
collector and emitter as load terminals and is adapted in the
embodiment to charge an organ coupling capacitor 67 rapidly and to
restore the capacitor to its initial state of charge if it has been
partially discharged for the purpose of delivering a stimulating
pulse to the organ. From a terminal 70 which is connected to
positive line, transistor 63 is supplied with base-emitter bias
current through a resistor 65. The collector resistor of the
transistor is marked 64 which, in this example, has a value of
1,000 ohms to limit current although its value may be reduced to
zero under certain circumstances. Connected between the emitter of
transistor 63 and positive line terminal 70 is a resistor 66 of
preferably very high value such as one megohm. The anode of a diode
68 is connected to the emitter of transistor 63 and the cathode of
the diode is connected to the collector of transistor 60. The base
of transistor 63 is connected to the cathode of the diode by means
of a conductor 71.
The output circuit operates in the following manner. Assume that
organ coupling capacitor 67 is initially uncharged when battery 72
is connected to power input terminal 70. Coupling capacitor 67 is
then charged through a series circuit starting at positive terminal
70 and including bias resistor 65, the base-emitter of transistor
63, capacitor 67, output terminal 10, lead 11 and then the organ
load R.sub.L which is returned to the negative side of battery 72.
With the flow of bias current, of course, there is an amplified
current flow through the collector circuit of transistor 63 which
causes capacitor 67 to be charged very rapidly. When capacitor 67
is near full charge, transistor 63 turns off because the difference
between the voltage on its base and the capacitor voltage is too
small to forward bias the transistor. Full charge of capacitor 67
to line voltage results from conduction through high resistor 66.
When capacitor 67 is fully charged it is ready for delivering a
stimulating pulse to the organ load R.sub.L on demand.
A stimulating pulse will be delivered when first transistor 60 is
pulsed on. When this occurs, capacitor 67 discharges through a
series circuit which includes diode 68, the collector-emitter path
of transistor 60, resistor 61, and the organ load R.sub.L from
which there is a ground return to the capacitor 67. Note that when
transistor 60 conducts, the base of transistor 63 is pulled down to
substantially ground potential in which case the latter transistor
is not forward biased and is non-conducting. Upon termination of
the trigger pulse which turned on transistor 60 the latter turns
off and transistor 63 then becomes forward biased again to recharge
capacitor 67 very rapidly. The recharge is initiated simultaneously
with termination of discharge of the capacitor. When the coupling
capacitor 67 is almost fully recharged, the difference between the
voltage on the capacitor and on the base of transistor 63 will be
inadequate to sustain conduction of the transistor and it turns
off. Then, due to continued conduction through high valued resistor
66, the voltage on capacitor 67 will approach line voltage.
The amount of current delivered to the organ during the discharge
interval of capacitor 67 depends upon how hard transistor 60 is
driven by its trigger pulse. The voltage amplitude of trigger pulse
62 and, hence, stimulating current amplitude depends on where the
wiper of variable resistor 54 is set as is explained earlier.
The wave form for a discharge and recharge cycle of capacitor 67
for one current intensity setting of the output circuit shown in
FIG. 1 is shown in FIG. 5. Before transistor 60 conducts the
voltage on terminal 10 and the load R.sub.L is negative by an
amplitude indicated by the numeral 73. When the two millisecond
pulse terminates, discharge of the capacitor 67 also terminates and
the potential applied to the organ is less negative and has an
amplitude indicated by the numeral 74. The shaded area under the
curve is indicative of the stimulus energy delivered to the organ.
Immediately after transistor 60 is off, transistor 63 turns on to
recharge capacitor 67 by the amount of charge that it has lost. The
recharging curve is marked 75 and the shaded area under it
corresponds with the energy dissipated in the organ plus losses.
Net current flow through the organ is substantially zero and the
fact that flow is quickly reversed tends to minimize ion migration
from the metal electrodes which connect to the organ.
It is immaterial that recharge current will tend to stimulate an
organ, such as the heart, in the same manner as the discharge
current from capacitor 67 because the heart is already undergoing
contraction and is refractory to further stimulation at that
time.
FIG. 6 shows the capacitor 67 charging and discharging curves which
occur when the impedance of the organ is much lower than in the
previous case. In this case most of the energy which is stored in
capacitor 67 is dissipated in the organ during discharge of the
capacitor. Nevertheless, a balancing current is quickly driven
through the heart over a short interval immediately following the
stimulation of the heart as indicated by the charging current curve
76.
In a commercial embodiment, the capacitor 67 has a value of 22
microfarads. In general terms, it charging time constant is the sum
of the saturation resistance of transistor 63, resistor 64 and
R.sub.L times the capacitance in farads of capacitor 67. This is a
non-linear impedance dependent on the base drive through resistor
65 and the voltage differential between the source voltage and the
voltage on capacitor 67 at the time when the discharge pulse is
completed.
In the current mode of operation the discharge time constant is a
function of the constant source current through cathode resistor 61
assuming that transistor 60 is in a nonsaturating mode.
If resistor 61 is zero, the circuit will be operating in the
voltage mode and the time constant will be the sum of the load
resistance R.sub.L, the saturation resistance of transistor 60 and
the forward resistance of diode 68 multiplied by the capacitance of
capacitor 67.
Note that in this output circuit the charge on capacitor 67 is
restored to its initial state in a short interval immediately after
the organ is artificially stimulated. This balanced recharge
current, in the case of the heart restores its electrical stability
very quickly so that later the detector 13 does not receive the
kind of noise that it does when the capacitor is recharged slowly
over most of the interval between artificially stimulated or
natural heart beats as is the case in prior art devices. Noise due
to capacitor charging or electrode polarization at the tissue
interface is especially harmful in prior art devices because the
noise is rich in low frequency components which interfere with
proper operation of detector 13 since it is tuned to respond to
frequencies in range of 20 to 40 Hz which prevail in the natural
R-wave on the heart. This frequency range is the predominant one in
the intrinsic electrocardiograph signal. Thus, with the new output
circuit, the effect of noise is removed and the heart is
electrically stable when detector 13 is called upon to sense
whether the next natural or intrinsic heart signal will be produced
or whether it will be called upon to dictate production of stimuli
if the intrinsic signal does not occur within the desired time
interval.
Note that when transistor 63 is turned off, the impedance seen by
the heart through output terminal 10 is constituted by high
impedance 66 in combination with the high impedance of detector 13.
Thus, the heart is not loaded by the stimulator and detection
sensitivity is enhanced. In this invention, resistor 66 is about
one megohm whereas in prior art stimulators the coupling capacitor
such as 67 was charged through a resistor about one-thirtieth as
large or only about 33 kilohms. This is a low impedance for the
heart to encounter and makes signal detection on the heart unstable
and uncertain.
In the present circuit, coupling capacitor 67 is recharged in about
100 to 150 milliseconds. The detector 13 is designed, as indicated
earlier, so that it is refractory for a little longer time and does
not sense the signal coincident with the recharging of capacitor
67.
FIG. 2 shows an alternative form of the new stimulator output
circuit. In this figure parts which are the same as in FIG. 1 are
given the same reference numerals. In a sense, this embodiment is
the converse of the preceding one because the coupling capacitor 67
is now charged to stimulate the heart upon receipt of a trigger
pulse on terminal 59. When there is a trigger pulse, transistor 60
turns on for about two milliseconds and capacitor 67 charges from
positive line terminal 70, through load R.sub.L, diode 68,
transistor 60 and resistor 61. The right side of the capacitor 67
is then positive but it is not fully charged because of the load
resistance R.sub.L.
When transistor 60 turns off with expiration of the trigger pulse,
it effectively disconnects the base of transistor 63 from ground
and the base-emitter of transistor 63 becomes foward biased because
the voltage on the left side of the capacitor 67 is more negative
than the line. This causes initial heavy conduction through the
collector-emitter path of transistor 63 into capacitor 67 and
finally lesser conduction through high resistor 66. Current for
this charge equalization flows from positive line, through
paralleled transistor 63 and resistor 66 and back to positive line
through heart load impedance R.sub.L. Thus, the stimulating and
recharging waveform is the inverse of that shown in FIG. 5 but
there are the same balanced currents through the heart.
FIG. 3 is another alternative form of the output circuit which uses
PNP transistors. As in FIG. 1, this version stimulates by
discharging capacitor 67 when a trigger pulse is applied to the
base of PNP transistor 80 and resistor 81. The trigger pulse in
this case is negative. Capacitor 67 is first charged positive on
its right side. A trigger pulse on the base of transistor 80 then
permits discharge beginning at the positive right side of the
capacitor, through R.sub.L, the emitter-base of transistor 80, the
diode 68 and back to the left side of the capacitor. Immediate
recharge occurs through a circuit starting with positive line and
including R.sub.L, capacitor 67, the emitter-base of transistor 82
which is then turned on. Final charge is again applied through high
resistor 66.
The FIG. 4 alternative uses PNP transistors. Here stimulation
occurs when the capacitor 67 is being charged. To charge, a
negative pulse is applied to the base of transistor 80 turning it
on and charging capacitor 67 through a circuit including transistor
80, diode 68, capacitor 67 and R.sub.L. This keeps the other
transistor turned off. The left side of capacitor 67 is positive
but below line potential because of the current limiting effect of
R.sub.L. When transistor 80 turns off, transistor 82 turns on and
the charge on capacitor 67 is equalized by conduction of transistor
82 through R.sub.L. Thus, a balanced reverse current again flows
through the heart immediately after it is stimulated.
FIG. 7 is another embodiment of the fast recovery output circuit
concept. This embodiment is ideal for replacing the slow recovery
output circuits in existing implantable stimulator designs since
the new circuit employs fewer components than the previously
described embodiments or the piror art slow recovery circuits. In
FIG. 7 components which have the same characteristics as in the
previous embodiments are given the same reference numerals.
Components which have the same function are given the same numerals
with a prime mark. Different components are differently marked. In
FIG. 7 the coupling capacitor 67' may be considered charged
initially. When organ stimulation is demanded, a pulse 62 is
applied to terminal 59 of a selectively conductive device which in
this case is the base of a transistor 60. A resistor 61 may be
optionally connected between battery return and the emitter of
transistor 60. When transistor 60 conducts, due to application of a
trigger pulse 62 from the stimulator, capacitor 67' which may be
considered to be charged positively on its left side in this
example, discharges through a series circuit including the load
terminals of transistor 60, resistor 61, the ground path, and the
organ load R.sub.L. In a heart stimulator, the duration of
conduction and stimulation is about 1 millisecond. When pulse 62
terminates transistor 60 turns off and discharge of capacitor 67'
terminates.
Simultaneously with termination of the capacitor discharge,
restoration of the capacitor to substantially its initial charge
state begins. The capacitor 67' is recharged rapidly from source
terminal 70 through a resistor 91 and a diode 90 in series with it.
The series recharge circuit begins at source terminal 70 and
includes resistor 91, diode 90, capacitor 67', the organ load
R.sub.L and returns through what is shown as ground circuit to
battery 72. Diode 90 conducts until the difference between the
source voltage at terminal 70 and the voltage on the capacitor is
less than the drop across the diode 90. Recharging or restoration
of the capacitor to its initial charge state is then completed by
conduction through high resistor 66'. Generally, capacitor 67' will
be restored completely to its initial charge state within 100 to
200 milliseconds of the time at which organ stimulation is
terminated. Thus, the circuit has high input impedance in about 200
milliseconds or less after the stimulus pulse occurs and the input
impedance at the terminals of the stimulator approaches the value
of resistor 66' which is in parallel with the impedance of the
sensing amplifier 13.
Some additional current is drawn by this output circuit as a result
of diode 90 conducting when transistor 60 is on. It is possible to
select a resistor 91 which is high enough in value to limit diode
current to about 2 or 3 percent of the organ load current which
resistor will still be low enough in value to provide sufficient
recharging current during the fast recovery phase.
The FIG. 7 circuit used in an implantable stimulator embodiment
employs a resistor 91 having a value of 10 kilohms, resistor 66' of
100 kilohms, and the capacitor 67' of 3.3 microfarads. These values
may vary, of course, depending on other parameters of the circuit.
Other values may be used as long as the high input impedance is
maintained during the period when intrinsic organ signals are being
sensed. Those skilled in the art will appreciate that comparable
results can be obtained with a circuit that reverses the polarity
of the d-c source and the polarities of diode 90 and transistor
60.
The fast recovery output circuit of FIG. 7, like those previously
described, impresses reverse current on the organ shortly after it
is stimulated so that there are stable potentials at the interface
of the organ and its electrodes during the sensing period. The
reflected input impedance of the stimulator is maintained high so
that the generated signals from the organ are not attenuated
significantly. The effect of noise is removed and the organ is
electrically stable when detector 13 is called upon to sense if an
intrinsic organ signal has been produced or if it will be called
upon to dictate production of an artificial stimulus if the
intrinsic signal does not occur within the desired time
interval.
In summary, several versions of the new output circuit for organ
stimulators have been described. Each provides a balanced current
through the heart or other organ which is opposite in sense to the
stimulating pulse current. In all cases the reverse current is
applied immediately after the stimulating current in a short period
of time which is much shorter than the period between natural or
artificially induced stimuli. Detection of the electrical state of
the heart after an artificial stimulus is made easier because the
heart is electrically stable soon after stimulation, in view of no
recharging current flowing during the critical detection period.
The natural depolarization potentials of the heart are more easily
detectable because of the high impedance of the new output circuit
as compared with any known stimulator. In addition, the balanced
reverse currents used are believed to minimize metallic ion
migration from the electrodes into the tissue to a greater extent
than any heart pulsing sequence practiced heretofore.
Although the new output circuit has been described in connection
with a stand-by or demand type of cardiac Pacemaker, it will be
understood by those versed in the art that the circuit may be
advantageously employed in fixed rate Pacemakers and in stimulators
for other organs too. Moreover, even though specific embodiments
have been described, such description is to be considered
illustrative rather than limiting for the inventive concepts
presented herein may be variously embodied and are to be limited
only by construing the claims which follow.
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