US3548831A - Cardiac pacer circuit - Google Patents

Description

United States Patent [56] References Cited UNITED STATES PATENTS 3,045,191 7/1962 Blanchard 331/108 3,421,512 l/1969 Frasier 128/419 OTHER REFERENCES Butler, Wireless World" Feb. 1965, pp. 58- 61 copy in 331-- 108 Primary ExaminerWil1iam E. Kamm Attorney-Raymond A. Robic ABSTRACT: An improved circuitry of a transistorized, implantable and battery powered cardiac pacer, essentially com prising a sine wave phase shift oscillating stage, a nonlinear amplifying stage for amplifying the sine wave and to produce a square wave, a RC-difierentiating stage for differentiating the square wave and to produce a pacing pulse which, as the last step, is amplified in a source follower amplifying stage and from thereon led to the heart via electric conductors.

I CARDIAC PACER cnicurr This invention relates to an improved design of an implantable cardiac pacer.

Implantable cardiac pacers are known in the art and the use of totally implanted and battery powered artificial stimulators for the treatment of patients who have been or are subject to Stokes-Adams seizures, is now a conventional medical procedure. With increased medical confidence in the use of these devices, many are implanted in patients who need them only for brief periods at infrequent intervals, or in patients whose condition has improved since implantation. It is with particular reference to these classes of patients that increasing concern is being felt by physicians about the effects of certain types of component failure within the pacer. An electrical component may be said to fail when its characteristics have changed sufficiently so that the circuit of which it forms a part cannot perform its intended function. In the case of cardiac pacers, three types of functional circuit failure are possible, and are experienced. They are:

1. Output pacing pulses weak or absent 2. Output pulse rate slower than normal 3. Output pulse rate faster than normal Failure modes 1 and 2 are the more frequent, and may be fatal to the patient, but the majority of patients will survive, without discomfort until a pacer replacement is possible. Sudden large increases in pacing rate, due to mode 3-type failures are, however, serious for all patients, in that they promote inevitable tachycardia, probable fibrillation and death. These latter patients are the victims of the failure of a prosthetic device which they may no longer even need.

It therefore is a primary object of the invention to prevent these unnecessary deaths by totally avoiding their cause; this is realized by utilizing pacer circuits which incorporate the principles and features according to the present invention and in connection with which potential component failures which, normally, would result in circuit failure type 3, now cause circuit failure type 1 and, unlike existing conventional designs, failure of any part of the present pacer imposes no additional hazards on the patient other than that involved with the cessation of pacing.

The cardiac pacer circuit in accordance with the invention, comprises a phase shift oscillator'for generating a sine wave signal, a nonlinear amplifier connected to said oscillator for converting said sine wave signal into a square wave signal, a RC-differentiating circuit connected to said nonlinear amplifier for producing a pulse signal, a capacitor connected between the output of said nonlinear amplifier and the input of said oscillator for feeding back a portion of said square wave signal to said oscillator to shorten the rise and fall times of said square wave signal, and electrodes connected to the output of said differentiating circuit for applying said pulse signal to the hearth of a patient.

Further advantages, principles and features will be apparent from the following detailed description of a pacer circuit arrangement according to the invention and as illustrated in the drawings, in which:

FIG. 1 shows a timing oscillator circuit incorporated in a conventional pacer;

FIG. 2 illustrates a timing circuit of the sine wave phase shift oscillator type used in the invention;

FIG. 3, in detail, shows one embodiment of a complete pacer circuit according to the invention, incorporating the timing circuit of FIG. 2. I

For a better understanding of the features, principles and design of the present invention, reference .will first be made to the circuitry of a typical and conventional pacer as shown in FIG. 1.

This FIG. is seen to comprise a circuit for producing pulses, shown generally at la, an insulated lead 2a and electrodes 3a. The typical circuit la is simplified and'omits parts not contributing to the possibility of failure mode 3 and not germane to the discussion of its incidence, which follows.

To produce pacing pulses at a regular and proper rate, pacers do require a timing oscillator circuit. In existing pacers, the timing circuit is of the type known as a relaxation oscillator. In essence, the operation of this circuit is as follows: The transistor 4a, when conducting, draws current from the battery 7a, thus inducing voltages into the windings of the feedback transformer 5a.

The secondary voltage causes current to flow through the capacitor 6a, thus providing base current for the conduction of transistor 4a. Conduction continues for a brief period until the base current of transistor 4a has charged up capacitor 6a sufficiently to oppose the secondary voltage of transformer 5a. Conduction of current in 4a then ceases, until such time as the charge in capacitor 6a has been discharged by current from battery 7a through resistor 8a. This resultant time delay is proportional to this charge, and inversely proportional to the current through the resistor 8a and such other unintended additional current which may tend to discharge capacitor 6a. Such additional currents leading to failure mode 3 include, but are not limited to:

A. Leakage between base and emitter of transistor 4a B. Leakage between base and collector of transistor 4a C. Leakage through capacitor 6a D. Leakage from battery 74 through ingress of body fluid.

Thus, since these unidirectional currents cause an increase in the capacitor 6a discharge current, they cause an increase in pacing rate. Many embodiments of relaxation circuits are possible, but since they all control the pacing rate by current flow in a capacitor, and possess one or more of the above leakage current sources A through D, all are liable to failure mode 3.

The timing circuit used in the invention as shown in FIG. 2, is of the sine wave phase shift oscillator type. The rate of oscillation depends on a mathematical relationship between the component values of the timing components shown generally at 4, and since no unidirectional voltage is necessary or exists across the resistors 5, 6, 7, then the aforementioned leakage current listed A D either cannot flow or does not affect the rate of oscillation.

' FIG. 2 is seen to comprise an enhancement mode metal oxide-silicon fieldeffect transistor 1, which is used as a linear amplifier and is equipped with a load resistor 2, and is powered from a multicell battery or other suitable unidirectional source 3. The source 3 is connected to the reference ground and the timing components, generally indicated at 4, are connected between the drain 11 and the gate 12 of transistor 1. These timing components comprise resistors 5, 6 and 7 in series from drain 11 to gate 12 with capacitors l0, 9 and 8 connected from the ground to the gate and the resistor junctions respectively. If the loaded voltage gain exceeds 28 and the timing resistor and capacitor values are approximately equal between themselves, then the circuit will oscillate with a period proportional to the resistancecapacitance product. Furthermore the necessary voltage gain may be achieved with this type of transistor with indefinitely low power drain, by increase in the values of the resistors 2, 5, 6 and 7.

The circuit of FIG. 3 shows one possible way of providing the necessary short pacing pulse from the sine wave oscillator. It includes a sine wave oscillator with transistor 1 and timing components 4, exactly as described in FIG. 2, except that load resistor 2 of FIG. 2 becomes coupling resistor 2 in FIG. 3, and that a small capacitor 29, is connected to the gate 12 of transistor 1. Furthermore, an intermediate common emitter amplifier transistor 14 amplifies the sine wave in a nonlinear manner to produce a square wave at its collector 16, which square wave is fed back in small proportion through capacitor 29 to the gate 12 of transistor 1 in order to shorten the rise and fall times of the square wave. The aforementioned square wave is differentiated by a capacitor 22 and resistor 26 to produce a pulse which is further amplified by a source follower transistor 20, whose drain 23 is connected to ground and whose source 24 is connected to the output leads 28,

3 ,5 48,83 1 3 4 through a storage capacitor 27. Other necessary components metal-oxide-silicon, field effect transistor for generating a are resistors l8, l9 and 25 which provide for the necessary sine wave signal; continuance of current from the electrodes they serve to the b. a phase shift network including three capacitors and battery or other unidirectional source connections. three resistors forming the timing components of said We claim: 5 oscillator circuit and connected to said field effect 1. A cardiac pacer circuit comprising: transistor; P shlfi oscillator for generating f sine W Signal; 0. a nonlinear amplifier including a common emitter a f f p w sald oscluatof for transistor connected to said field effect transistor for con- Vemng {531d i i mm a Square ii verting said sine wave signal into a square wave signal; a RQ-dlfferwmns connected to Sam nonlmear 10 d. a capacitor connected between the collector of said compmducmgapulse sgnali mon emitter amplifier and the gate of said field effect capacno: connected ,between h of Sam transistor for feeding back a portion of said square wave linear amplifier and the input of said oscillator for feeding Signal to Said field efi-ect transistor to shorten the rise and back a portion of said square wave signal to said oscillator fall times of Said Square wave signal, g gszfgzg the use and of Sald Square wave e. a RC-differentiating circuit connected to said nonlinear lifier for producing a pulse signal e. electrodes connected to the output of said RC-difamp ad m the shit;,izrztzznzizfiiifizaziifi 1; a? hearth of a patient. g and p p y g p Fardlac Pacer circuit as d-efined m claim g. electrodes connected to said source follower amplifier for compnsmga mince-follower gm-phfierconnect-ed to sald RC- 21 l in the out ut of said source follower am lifier to differentiating circuit for amplifying said pulse signal. K i f p 3. A cardiac pacer circuit comprising: t e e O a a. an oscillator circuit including an enhancement mode,

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