CA1098590A - Demand pacer having reduced recovery time - Google Patents

Abstract

Abstract Demand Pacer Having Reduced Recovery Time In a demand pacer, an input amplifier senses stimulat-ing pulses and natural heart beat signals, and responsively thereto establishes control of a subsequent stimulating pulse.
Charge accumulation on an output capacitor governs recovery time after each generated stimulating pulse. Respective first and second flip-flops, responsive to the input amplifier and the demand pacing logic, establishes a reduced recovery time, after each generated pulse, wherein charge accumulated on the output capacitor is dissipated.

Description

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Demand Pacer Havinq Reduced Recovery Time Technical Field This invention relates to implantable body function control apparatus and particularly, but not exclusively, to body tissue stimulating devic-es such as cardiac pacemakers.
Background Art Pacemakers for generating artificial stimulating pulses for the heart, and which may or may not be implanted in the ; body, are well-known. Pacemakers can be classified into demand and non-demand types. A demand pacemaker only issues an artificial pulse if the heart does not: produce its own satis-factory natural beat, whereas a non-d~3mand pacemakers issues artificial stimulating pulses without regard to the presence or absence of a natural beat.
A demand pacemaker normally includes an input amplifier for receiving and amplifying electrical signals from the heart ~which~signals migh~ result from either a natural beat or an artificial pulse which has just been generated by the pacemaker), ~a pacemaker control circuitry which receives the amplified signals and which causes a new artificial stimulatins pulse to be generated (for transmission to the heart) only if the amplified signals, or lack thereof, show that an artificial stimulating pulse is required~by the heart (i.e. on demand), and an output amplifier which receives and amplifies the artificial pulses generated by the control circuitry, for passage to the heart.
Many types of pacemak~er control circuitry as described above ~re available. Some ~unction on an analog basis to produce the accurately-timed artificial stimulating pulses, whereas $çveral recent designs employ~digital circuitry.
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. -Of necessi-ty, the input amplifier requires a high sensitivity and it has been found difficult to design an adequate amplifier that does not saturate for too long a period when an artificial pulse is transmitted to the heart by the output amplifier ~this pulse being detected by the input amplifier). However, this need not be a problem provided the saturation period can be kept sufficiently short so that the input amplifier recovers in time to detect the presence or absence of the next expected natural beat.
The load which is driven by the output amplifier (the electrodes and the heart tissue itself) has capacitive properties and these, coupled with the capacitive components normally present in the output amplifier, can act to extend the length of any artlficial pulse transmitted to the heart. Even if a sharp artificial pulse is generated by the pacemaker control circuitry, the capacitive effects at the output cause the trailing edge of the pulse to be extended so as to gïve a some-what exponential decay back to zero. This extension of the output pulse is reflected at the input amplifier by increasing the length of time for which the latter remains saturated.
Disclosure of Invention .
The present invention is concerned with alleviating this problem, so as to avoid these capacitive effects from increasing the saturation period of the input amplifier unnecessarily. This is accomplished, in this invention, by arranging for electrical energy to be fed into the pacemaker circuitry, at an appropriate moment after an artificial stimulating pulse is generated, in opposition to the energy stored by the capacitive components responsible for the exten-sion of the artificial pulse. This has the effect of shorting these capacitive components, thus providing a much sharper falling edge for the output pulse and hence reducing the period of time spent by the input amplifier in saturation.
Preferably, the capacitive effects are cancelled by including an additional transistor in the output amplifier which is turned on at a predetermined time after an artificial stimulating pulse has been senerated, which transistor then feeds current into the output circuit in opposition to the >. . ~

slowly decaying output pulse, thus returning the latter to zero at a faster rate.
Preferably, the pacemaker control circuitry includes a pulse generator for providing an artificial stimulating pulse, and means for resetting the pulse generator control-led either by an artificial pulse just generated or by a signal representative of a natural heartbeat, so that the next artificial pulse is generated in timed relationship with the previous artificial pulse and only on demand. With such circuitry, the preferred additional transistor in the output amplifier can be arranged to be controlled by the -reset provided to the pulse generator. In such a circum-stance the reset determines tha pulse width of the artificial pulse and by causing the additional transistor to compensate for the capacitive effects once the reset is applied, the sharp trailing edge of the artificial pulse is substantially maintained. A slow decay after the reset is applied is thus avoided, as therefore is an extension of the input amplifier saturation time.
According to a broad aspect of the present invention there is provided a demand-type cardiac stimulating apparatus comprising electrode means for coupling stimulating pulses to the heart. An output amplifier stage is provided ~or generating stimulating pulse~ for the heart. The output stage has an output capacitor connected to the electrode means. The output stage has an output impedance recovery interval established, after each stimulating pulse generation, in substantial part by the accumulation and subsequent dissi-pation of charge on the output capacitor. Input amplifier means, responsive to signals from the output stage and to naturally occurring heartbeat signals, is also provided for producing a control signal relative to the generation of a next subsequent stimulating pulse. Logic means, responsive to the control signal, is provided for selectively energizing the output amplifier to generate a stimulating pulse based ~'' . .

on predetermined demand pacing criteria. The improvement comprises means for shortening the recovery interval and including a first bistable means, conditioned to a first enablin~ output state by the lo~ic means at a predetermined time after each of the selective energizing. The means for shorting the recovery also comprises second bistable means, clo~ked to an enabling output state by the enabling state of the first bistable means. The enabling output state of the second bistable means is terminated by the lo~ic means after a second predetermined duration. The second duration defines a reduced recovery interval of the output sta~e. The means for shortening the recovery interval further includes tran-sistor means, responsive to the second bistable means and energized during the second duration, for dissipating charge on the output capacitor and thereby establishing a shortened recovery interval for the output stage.
Brief Descri~tion of Drawinqs Preferred features of the invention will now be described with reference to the accompanying drawings in 2G which:
Figure l illustrates schematically the electrical circuitry for a demand cardiac pacemaker, and Figure 2 represents a timing diagram for use with Figure l.
~ 25 sest Mode of Carryin~ Out the Invention - Referring to the drawings, parts of the pacemaker are shown in three sections within separate dotted lines.
The input amplifier is represented by section l, the pace-maker control circuitry which generates artificial stimulat-ing pulses on demand is represented by section 2, and the output amplifier is represented by section 3. The pacemaker load, i.e. the electrodes and the body tissue therebetween, is illustrated by a resistive/capacitive combination within a further section, section 4.
Many input amplifier, pacemaker control circuitry, and output amplifier combinations can be selected for use with the invention and therefore, to a lar~e extent, many of the components of the illustrated pacemaker are shown .~,-............................................................ .

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functionally in block form. The particular selection of components for each block will be apparent to those skilled in the art.
Sections l, 2, and 3 can be considered as representing a basic demand pacemaker. Oscillator 5 free runs and t~e particular artificial stimulating pulse rate appropriate to the patient is selected by counter 6 (the Qx output stage) for transmission to the output amplifier of section 3~ If a natural heart beat is detected by the input amplifier of -10 section l, a xeset circuit 7 for counter 6 (consisting of an OR gate follo~ed by a delay D~ is activated so that the artificial pulse count is not reached and no artificial pulse is generated. If no such natural beat is detected, the -artificial pulse count is reached, and an artificial pulse is transmitted to the heart (section 4) by means of the output amplifier (,section 3~. In such a circumstance, the pulse width is determined by the delay D generated in the reset ' circuit 7 - the CQunter 6 being reset at the termination of this delay.
Although the output pulse generated by counter 6 has a fast rise and fall ((a) in Figure 2), the capaci~ve effects '' in sections 3 and 4, particularly of capacitors 8 and 9, retard the fast fall o~ the artificial stimulating pulse at the heart ((b) in Figure 2~ and this, as explained above, increases the -time spent by the input amplifier in saturation.
To compensate for these capacitive effects, the pace-maker circuitry additionally includes a D flip-flop 10 which receives, at its cLock input via an inverter ll, the reset pulse for counter 6. The reset input for flip-flop l~ is supplied by the counter 6 output, its D input is tied to the positive supply rail and its Q output clocks a second D flip-flop 12. Flip-flop 12 is reset by a system cloc]c (derived from an appropriate stage Qy of counter 6~ and has its D
input tied to the positive supply rail. The Q output of flip-flop 12 controls the gate of a field effect transistor 13. The transistor 13 drain and source terminals are connect-ed between the positive supply rail, via a resistor 14, to the output amplifier, adjacent output capacitor 8.

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The operation of the input amplifier saturation-reducing circuit components will now be described.
When an artificial stimulating pulse is generated by counter 6 (see (a) in Figure 2), this is not only transmitted to the output amplifier but it also resets flip-flop 10, whose Q output thus drops to low (see (d) in Figure 2). After a delay generated by reset 7 which is appropriate to the artificial stimulating pulse width desired (see (c) in Fisure

2), counter 6 is reset and, at the termination of the reset pulse, flip-~lop 10 is clocked via inverter 11. Clocking of flip-flop 10 causes its Q output to revert high and this clocks flip-flop 12. Clocking of flip flop 12 causes its Q
output to drop low (see (e) in Figure 2) and this causes transistor 13 to conduct.
Current is then fed into the output amplifier by transistor 13 in a direction which increases the current flow-ing as a result o~ the slow decay o~ the capacitive components, and this acts to speed the decay, providins a faster return to the steady state con~ition, reducing the saturation time of the input amplifier.
Current continues to be fed by transistor 13 until ~lip-flop 12 is reset b~ an appropria-tely timed system clock ~ulse derived from counter 6. This reset causes the Q output of flip-flop 12 to revert high, thus switching transistor 13 --off.
It will .be observed from t~e above description that there is a delay between transistor 13 conducting and the end of the generated artificial pulse ("t" in (f), Figure 2).
This is to prevent a short circuit appearing across the voltage supply line at the output in the event of the counter 6 generat-ing an output pulse simultaneously with transistor 13 conducting.

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Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed axe defined as follows:

1. Demand-type cardiac stimulating apparatus compris-ing:
(a) electrode means for coupling stimulating pulses to the heart;
(b) an output amplifier stage for generating stimulating pulses for the heart, said output stage having an output capacitor connected to said electrode means, said output stage having an output impedance recovery interval established, after each stimulating pulse generation, in substantial part by the accumulation and subsequent dissipation of charge on said output capacitor;
(c) input amplifier means, responsive to signals from said output stage and to naturally occurring heart beat signals, for producing a control signal relative to the generation of a next subsequent stimulating pulse;
(d) logic means, responsive to said control signal, for selectively energizing said output amplifier to generate a stimulating pulse based on predetermined demand pacing criteria;
(e) and the improvement comprising means for shorten-ing said recovery interval including (i.) first bistable means, conditioned to a first enabling output state by said logic means at a pre-determined time after each said selective energizing;
(ii) second bistable means, clocked to an enabling output state by said enabling state of said first bistable means, said enabling output state of or said second bistable means being terminated by said logic means after a second predetermined duration, said second duration defining a reduced recovery interval of said output stage, and (iii) transistor means, responsive to said second bistable means and energized during said second duration for dissipating charge on said output capacitor, and thereby establishing a shortened recovery interval for said output stage.

2. Apparatus as described in claim 1 wherein said logic means comprises an oscillator, counter means for counting pulses from said oscillator, and delay means, energized by said control signal or by a first predetermined count at said counter, for producing an output pulse a predetermined delay time after being energized, wherein said first bistable means is reset by said first predetermined count of said counter, and is clocked to its said first enabling output state by said output pulse from said delay means, and wherein said second bistable means is reset by a second predetermined count, later than said first count, at said counter means.

3. Apparatus as described in claim 2 wherein said counter means is reset by each output pulse from said delay means.

4. Apparatus as described in claim 3 wherein said bistable means each are D-type flip-flops, having their respective D-inputs connected to a positive voltage supply, said output of said first flip-flop being its Q output, and said output of said second flip-flop beings its ? output.