CA1145406A - Demand cardiac pacemaker having alterable sensitivity - Google Patents

Abstract

ABSTRACT

A demand cardiac pacemaker of the type having circuitry for detecting the occurrence of natural heart activity. The sensitivity of the detecting circuitry is selectively and non-invasively altered via externally gen-erated signals. In a preferred embodiment, the detecting circuitry establishes a first level representative of sensed extraneous repetitive noise and a second level greater in absolute value than the reference level by an amount representative of sensed heart activity. Circuitry differentially responsive to the difference between the reference and second level detects the occurrence of nat-ural heart activity. The amount by which the reference and second level must differ to indicate natural heart activity is established by a bias circuit, the bias being alterable to establish one of a preselected plurality of sensitivities.

Description

~454~6 DEMAND CARDIAC PACEMAKER HAVING
ALTERABLE SENSITIVITY
I. DESCRIPTIO~
Background of Prior Art Body implantable cardiac stimulators or pacemakers are known to the prior art. An early pacemaker is disclosed by Greatbatch in U.S.
Patent No. 3,057,356, entitled '~edical Cardiac Pacemaker'~, which issued in 1962. This device included a relaxation oscillator that generated electrical pulses at a fixed rate. The pulses were applied to the heart to cause the heart to contract each time a pulse occurred.
Since 1962, the pacemaker has been continuously evolving. This evolution is outlined in co-pending Canadian application Serial No.
33S,143, filed November 5, 1979 in the name of David L. Thompson for Digital Cardiac Pacemaker, which is co-owned with the present invention.
As noted in the copending application, pacing technology has lagged behind conventional state of the art electronic technology in its utilization of digital electronic circuits. One reason for this has been the high energy required to operate digital electronic circuits. Energy requirements are a major concern in pacemaker design. However, with the continuing advances of electronic technology, digital electronic circuits are increasingly feasible within the context of commercial pacemaker units.
The accuracy and reliability of digital electronic circuits are factors that encourage their use within the pacemaker context. The facility with which they can be programmed and reprogrammed to alter one or more operating parameters further enhances their utility. For example, pace-makers have been disclosed which respond to magnetic and/or radio frequ-ency signals to alter an operating parameter. Pulse rate and pulse width may be programmed in this manner. In addition, pacemakers have been con-structed which are inhibited in the presence of certain signals. A more detailed outline of prior art programmable pacemakers is contained in the incorporated specification. It should be noted that, as indicated in the .

4~36 incorporated specification, no known prior art pacemaker is capable of having more than two parameters, features or tests programmed on command.
The implementation of digita:L electronic circuitry within the pacemaker context provides the opportunity to program or reprogram one or several operating parameters, on command, via externally generated signals.
For example, pulse rate, pulse width and pulse amplitude can be externally established at one of any number of combinations. In addition, the re-fractory period may be established and altered. Further, digital circuitry can be programmed on a temporary or permanent basis, as desired. Of course, other operating parameters or characteristics can also be externally programmable.
Clearly then, a pacemaker utilizing digital electronic cir-cuitry would have a more universal application by allowing the pacemaker to be programmed to fit the needs of a particular application as opposed to being manufactured for limited applications. In addition, such a unit c~n be instructed to give an external indication of its program status, particularly in instances where that status is not directly observable.
~owever, even with the implementation of digital circuitry, certain analog circuitry is necessary to generate and/or transmit various control signals ~0 and to respond to the digital circuitry to effect its programming.
Brief Summary of the Invention The present invention provides analog circuitry intended for cooperation with the digital circuitry disclosed in the copending appli-cation to assist in the performance of the pacemaking function. Among the analog circuit functions necessary within the context of the digital circuitry of the copending application, are the demodulation of the pro-gramming signal, a detection of heart activity during operation in a demand mode and provision of clock pulses. Additionally, analog circuitry is employed to give an indication of battery status and to impose an upper rate limit on the stimulation initiating signals generated by the digital circuit. The digital circuitry of the copending application provides a ~S~Q6 signal to control the sensitivity of the sense amplifier and a signal to establish a refractory period within the sense amplifier. The output analog circuit is controlled by the digital circuit to speed up the re-charging of a capacitor in the output circuit, to establish the magnitude of the output pulses and to impose an upper rate limit on the output stimulation pulses. As detailed in the copending application, one of the clock pulse generators is enabled by a signal from the digital circuit.
Within the context of cooperating analog and digital circuitry for the generation and application of stimulating pulses, the present invention is directed to a sense amplifier having an output indicative of the detection of heart activity, the sense amplifier being responsive to input signals to establish its sensitivity and a refractory period. The sense amplifier includes a preamplifier having a degree of response essentially independent of the polarity of signals applied to its input.
An absolute value circuit responds to the output of the preamplifier to provide a single polarity output signal representative of signals of either polarity appearing at the input of the preamplifier. Thus, the polarity disparity attending prior art sense amplifiers is largely overcome as a result of the essentially polarity independent response of the preamplifier and the single polarity output of the absolute value circuit. A reversion circuit detects a signal resulting from sensed heart activity in the out-put of the absolute value circuit, that detection resulting in the pro~
vision of an output signal from an output circuit. A sensitivity control is provided to cooperate with the reversion circuit to alter its sensitiv-ity to signals appearing at the output of the absolute value circuit while a buffer is provided between the absolute value circuit and the reversion circuit and is controlled to regulate the transmission of signals between the absolute value circuit and the reversion circuit. The period during which signals are blocked from the reversion circuit is referred to herein as a refractory period. The output circuit includes a system for altering the sensitivity of the reversion circuit during an output signal to enhance ~54~

the response of the reversion circuit to a signal resulting from natural heart activity.
Thus, in accordance with a broad aspect of the invention, there is provided, in a demand cardiac pacemaker of the type having means for sensing heart activity and means for establishing a reference level representative of sensed extraneous noise and a second level greater in absolute value than said reference level by an amount representative of sensed heart activity, the improvement which comprises differential ampli-fier means responsive to the difference between said reference and second levels for detecting heart activity, means for establishing the level difference to which said differential amplifier means responds including single resistance means for establishing a differential amplifier means biasing voltage in accordance with the current flow therethrough and means connected to said resistance means including terminal means and further means responsive to the polarity of a single signal and the lack of a sig-nal at said terminal means for selectively establishing the current flow through said resistance means.
In a preferred embodiment, the preamplifier has a differential input and a differential output, the output signals being of like absolute value but of opposite polarity. The absolute value circuit responds to the positive output signal from the preamplifier to provide an output of a single plurality without regard to the polarity of the signals appearing at the input of the preamplifier. Thus, there is provided a versatile sense amplifier which greatly reduces polarity disparity and which has a programmable sensit:ivity and an externally established refractory period.
Brief Description oE the Drawings Figure 1 shows the interconnection and cooperation between the digital circuit of the incorporated specification and a cooperating analog circuit of which the present invention is a part.

Figure 2 illustrates the operation of a sense amplifier forming a part of the analog circuitry cf Figure 1.

~S~

Figure 3 illustrates in more detail the sense amplifier of Figure 2.
Detailed Description of the Invéntion Referring now to Figure 1 there is shown a block diagram illustrating the interconnections between Digital Circuitry 10 ~as dis-closed in the copending application) and Analog Circuitry 11 (of which the present invention is a part). Both the Digital Circuit 10 and Analog Circuit 11 are connected between a source of positive potential +V and a reference potential, such as ground. The source of positive potential may be a battery such as the conventional lithium diode battery which generates approximately 2.8 volts.
The Analog Circuit 11 consists of various distinct electrical systems which may be referred to functionally as an RF Demodulator, a Sense Amplifier, an Output Circuit, a Battery Monitor and Status Indicator, a Crystal Clock and a Voltage Controlled Oscillator Clock. The Digital Circuit 10 includes all of the digital logic necessary to cause a program-ming change, memory to store the digital code manifesting the desired values for the program parameters and digital timing means for causing a stimulation pulse to be generated in the programmed manner. The signals applied between the Digital Circuit 10 and Analog Circuit 11 are REED, DATA, SENSE, SENSITIVITY, BLANK, SINGLE, DOUBLE, RATE LrMIT, RECHARGE, BATTERY, XTAL, VCO and VCO ENABLE.
A magnetically actuated reed relay switch 12 is connected be-tween the source of positive potential +V and both the Digital Circuit 10 and the RF Demodulator of Analog Circuit 11. Reed switch 12 is normally open and is closed as by placing a magnet in close proximity thereto. When closed, a +V, or logic "1", REED signal is applied to both the Digital Circuit 10 and Analog Circuit 11. On removal of the magnet, the reed switch 12 opens and a ground, or logic "0", signal is applied to the Digital Circuit 10 and Analog Circuit 11. The RF Demodulator is enabled by a +V REED signal produced by a closing of the reed swi~ch 12 to provide .45~)6 a DATA signal to the digital circuit 10. The DATA signal (the Digital Circuit 10 programming signal) is a puls`e signal going from logic "0" to logic "1", as described in the copending application, which is represent-ative of pulse bursts generated externally.
The Sènse Amplifier portion of the analog circuit 11 provides a SENSE signal each time natural heart activity is detected to restart the timing cycle of the Digital Circuit 10, when operating in a demand mode. A SENSITIVITY signal is provided by the Digital Circuit 10 in accordance with its programming to establish the detection level of the Sense Amplifier. A BLANK signal is generated by the Digital Circuit 10 and applied to the Sense Amplifier portion of the Analog Circuit 11 -o establish the refractory period of the Sense Amplifier and to allow the components within the Sense Amplifier to reset themselves.
The Output Circuit of analog circuit 11 includes output ter-minals 13 and 14 which are adapted for connection to a conventional lead, in a known manner. The output terminal 14 may be connected to a metal casing housing the pacemaker unit or a plate forming a part of the casing in a unipolar lead system or it may be connected to a second lead in a bipolar lead system, depending on the type of lead system employed. Out-put terminal 13 is coupled through a capacitor 14 to the analog Output Circuit and to the heart (not shown). In addition, a pair of Zener diodes 15 and 16 have the:ir anodes coupled together and their cathodes coupled to output terminals 13 and 14, respectively. Diodes 15 and 16 function in a conventional manner to prevent damage to the pacemaker circuitry in the presence of large extraneous signals such as are caused by electrocautery.
The Output Circuit of Analog Circuit 11 includes elements responsive to a SINGLE or DOUBLE signal from Digital Circuit 10 to control the amplitude of output signals applied across output terminals 13 and 14. A RECHARGE sig-~59~6 nal from Digital Circuit 10 speeds up the recharging of output capacitor 14 while the Output Circuit of Analog Circuit 11 provldes a RAT~ LIMIT signal to Digltal Circuit 10 to provide an upper limit to the rate at wh~ch stimula-tion initiating signals are generated. Digital circuit 10also provides a R~T~ LIM~T signal to the Output Circuit of Analog Circuit ll to pro~ide an upper limit to the rate at which stimula~ion pulses may be applied by the Output Circuit.
In addition to the above, Analog Circuit 11 in~
10 cludes circuitry which monitors the status of the battery to provide an indication of that status in the form of the signal BATTERY. Also, clock pulses are pro~ided to the Digital Circuit 10 in the form of signals XTAL and VCO.
Within the context of the Digital Circuit of the incor-15 porated specification, the XTAL signal is a generallysquare wave pulse signal occuring at a frequency of 32,768 Hz and the VCO signal is a square wave pulse signal having a preset frequency-~ whenever ~V is equal to 2.8 volts.
As +V decreases with time, as the battery depletes, the 20 frequency of the VCO signal will also decrease, in known manner. The VCO signal is used in the timing circuitry of Digital Circuit 10 to establish the exact width of stim-ulating pulse. In order to maintain a constant energy of this pulse, it is necessary that the pulse increase in 25 width as ~V decreases. The VCO clock pulse generator is enabled only during the time the stimulating pulse is to be provided and is ena~led by the signal VCO
EN~3LE.
Referring now to Figure 2, there is shown a 30 block diagram of a Sense Amplifier forming a portion of the Analog Circuit 11 of Figure 1. The Sense Amplifier includes a preamplifier section indicated generally at 20, an Absolute Value Circuit 21, a Reversion Circuit 22, an Output Circuit 23 and a Sensitivity Control 24. A Buffer 35 25 is provided intermediate the Absolute Value Circuit 21 and Reversion Circui~ 22 to prevent loading on the Absolute Value circuit ~rom the Reversion Circuit. Buffer 25 is responsive to the BLANK signal from Digital Circuit 10 to turn off and block signals from the Reversion Circuit 22.

~s~

Preampli~ier 20 is a dif~erential input, differ-ential output device having a dual feedback active filter formed of resistors 26-31 and capacitors 32-3~, It is designed to have an open loop gain of approximately 60,000 with a unity gain crossover point of approximately 2kHz.
The dual negative feedback method minimizes the number of external components used. As wil~ be described more fully below, the differential input of the preamplifier 20 allows an essentially polarity independent degree of re-sponse to signals appearing at the input terminals 36 and37. That is, preamplifier 20 is essentially identically responsive to signals appearing at the terminals 36 and 37 without regard to polarity. The differential output of preamplifier 20 provides two signals of opposite polarity but of essentially ihe same absolute value, each being representative of signals appearing at the inputs 36 and 37.
The Absolute Value Circuit 21 responds to the differential output signals of preamplifier 20 to provide a single polarity signal representative of the signal sensed at the inputs 36 and 37. Thus, preamplifier 20 and Absolute Value Circuit 21 combine to provide signals of a single polarity representative of signals appearing at the terminals 36 and 37, but without regard to the 25 polarity of the signals at the terminals 36 and 37. In this way, the detecting circuitry contained within Rever-sion Circuit 22 need be responsive to signals of but a single polarity without thereby creating a polarity dispar-ity within the Sense Amplifier illustrated in Figure 2.
30 The sensitivity of the Reversion Circuit 22 to the output of the Absolute 'Value Circuit 21 is controlled by Sensi-tivity Control 24, the sensitivity being established by the SENSITIVITY signal from the Digital Circuitry of the incorporated speci~ication causing its timing cycle to 35 be restarted in 'a known manner, The line between Output Circuit 23 and sensitivity control represents a sensitiv-ity hysteresis function which will be explained more fully below.

Referring now to Figure 3 there is illustrated in separate Figures 3A and 3B a preferred embodiment of the Sense Ampllfler o;~ guxe 2 wtth functional elements 20-25 being set out i~n boxes o~ like reference nume~al 5 formed of broken lines. . ~51emelits forming the dual feedback filter illustrated in Figure 2 are illustrated with like reference numeral in Figuxe 3. Boxes 41A and 41B contain elements which set. up supply~independent voltage references and bias currents as is described more fully below.
10 Connecting lines A to .I in Figure 3A are intended for con-nection to the line of like reference character in Figure B.
Signals appearing at the input terminals 36 and 37 are applied through the active filter, to the bases of transistors ~5 and 46. Transistors 45 and 46 15 form the input differential pair having their collectors connected to ~V through diode 47. The emitter of trans-istor 45 is connected to.a current sink formed of trans-istor 48 and resis.tor. 49 .and.to the emitter of a trans-istor 50. Similarly, the emitter of transistor 46 is con-20 nected to a current sink formed of transistor 51 and re-sistor 52 and to the emitter of transistor 53. The bases of transistors 50 and 53 and the collector of transistor 50 are connected to a current source formed of transistor 54 and 55. The collector Gf transistor 53 is connected to 25 a junction 56 via capacitor 57 and, via a diode 61, to the base of transistor 58 and a current source formed of transistor 59 and resistor 60. A capacitor 62 connects a junction 63 to ground..
The emitter of transistor 58 is connected to the 30 base of a transistor 64 and to a current sink formed of transistor 65 and resistor 66. The bases of transistors 48, 51 and 65 are connected to a junction 67, the junc-tion 67 being connected to ~round via diode 68 and resis-tor 69 and to a junction 77 ~ria resistor 165. A transis-35 tor 70 has its emitteX connected to the junction 77, itscollectox connected to ~V w~le its base is connected to t~V via xesistoX 71 and to thé collector o~ a transistor 72.
The emittex of. transistoX 72 is connected to ground via re-sistor 73. The ~ase o~ transistor 59 is connected to the 40 base of transistor 54, the collector and base of a trans-S~)6 istor 74 and the collector of a transistox 75. The emitterof transistor 75 is connected to ground via a resistor 76 and the base of transistor ~5, and the base of transistor 72, are connected to the junction 77. Junction 77 is con-nected to the junction 67 via resistor 165 and the base ofa transistor 78, transistor 78 having its emitter connected to the emitter of transistor 64 ana to ground via resistor 79. The collectors of transistors 64 and 78 are connected to junctions 56 and 63, respectively. Junction 63 is con-nected to ~V via resistor 80 and to the base of a transis-tor Bl. Similarly, junction 56 is connected to ~V via re-sistor 82 and to the base of a transistor 83. The collect-ors of transistors 81 and 83 are connected to +V while their emitters are connected to junctions 84 and 85, re-spectivly. Junctions 84 and 85 serve as the output term-inals for preamplifier 20 with junction 84 being connected to ground via resistor 86 and diode 87 and junction 85 Deing connected to ground via resistor 88 and diode 89.
As stated above, transistors 45 and 46 consti-tute the input differential pair. Diode 47 prevents base-collector current to ~V in transistor 46 during a stimu-lation pulse while diode 61 increases the dynamic range of the input differential pair during supply voltage de-pletion. Transistors 70 and 72, in conjunction with re-sistors 71 and 73, set up a one-half ~V stable reference at the junction 77. Diode 68 and resistors 69 and 165 set up a tail current for transistors 45 and 46 via the current sinks ~ormed in part by transistors 48 and 51 as well as for transistor 58 via the current sink formed in part by transistor 65.
Transistor 58 is an emitter follower which cou-ples the signal from the input differentail pair to the transistors 64 and ?8 which form the second g~in stage in the amplifier. The signals appearing at the junctions 56 and 63 are o~E opposite polarity having an absolute am-plitude value representative of signals appearing across the terminals 3ID and 37. The transistors 81 and 83 are emitter followers which drive the Absolute Value Circuit 21.

~5~6 The source current for the input differential pair are set up by transistors 54, 57, 74 and 75 and resistors 55, 60, 76 and 90. Capacitors 57 and 62 set the high frequency rolloff of the preamplifier while diode 87 and resistor 86 and diode 89 and resistor 88 set up a tail current from the emitter followers formed of transistors 81 and 83, re-spectively.
Preamplifier 20 has a degree of response essen-tially independent the polarity of the signals appearing at the terminals 36 and 37. It has a differential input and a differential output, the signals appearing at the output being of opposite polarity with the absolute value of the signals appearing at the terminals 36 and 37. In this manner, the polarity disparity attending prior art sense amplifiers is greatly reduced and the Sense Ampli-fier can reliably respond to sensed signals representative of heart activity of either polarity.
The junction 84 is connected to the base of a transistor 91 while the junction 85 is connected to the base of a transistor 92. The bases of transistors 91 and 92 are also connected to resistors 30 and 31r respectively, resistors 30 and 31 being the negative feedback resistors associated with the active filter. The collectors of trans-istors 91 and 92 are connected to +V while their emitters are connected to a jur.ction 93. The junction 93 comprises the output terminal o~ the Absolute Value Circuit 21 and is connected to the collector of a transistor 94. The emitter of transistor 94 is connected to ground via re-sistor 95 and its base is connected to a junction 96. Re-sistors 97 and 98 and transistor 99 in box 41A set up areference voltage which is applied to the base of trans-istor 94 as well as to transistors forming a portion of the Sensitivity Control 24, as will be described below.
Transistors 91 and 92 are emitter followers.
Accordingly, the signal appearing at the junction 93 will approximate the most positive signal appearing at the bases of transistors 91 and 92~ Inasmuch as the signals appearing at the junctions 84 and 85 are of opposite pol-~s~

arity the positive one of those signals will result in a positive signal at the junction 93. Accordingly, a posi-tive signal appears at the junction 93 which is represent-ative of signals appearing at the.terminals 36 and 37 with-out regard to polarity. That is, signals of either polar-ity appearing at terminals 36 and 37 will result in a sing-le polarity signal (in this case positive) at junction 93, that signal being representative o:E the signal at the term-inals 36 and 37.
The signal appearing at junction 93 is applied as an input to a unity gain amplifier 25. The amplifier 25 functions as a buffer between the Absolute Value Cir-cuit 21 and the Reversion Circuit 22. The junction 93 is connected to the emitter of a transistor 110. The base of transistor 110 is connected to a junction 111 while the collector of transistor 110 is connected to a junction 112 via diode 113. The junction 112 is connected to a transistor 114 and to a current source formed of trans-istor 115 and resistor 116. As described to this point, the devices 110, 113, and 11~ establish a unity gain Buffer be~ween the absolute value circuit 21 and the reversion circuit 22.
A blanking circuit in the form of a terminal 117, resistor 118, resistor 119 and diode 120 are interconnected with the Buffer 25 to provide a refractory period during which the Reversion Circuit 22 resets itself and to prevent a charging of the.Reversion Circuit capacitor during and after stimulation pulses. When a BLANK signal, a logic 'IOn signal, is applied to the terminal 117, the unity gain Buffer circuitry is turned off for the duration of the BLANK signal, 100 milliseconds, for example.
Junction 111 serves as the nput to Reversion Circuit 22, the Reversion Circuit 22 including the capac-itor 25 as well as the components within the box 22. As is well known in the art, thexe is an offset between junctions 93 and 111 such that the signal appearing at the junction 111 is slightly less than the signal appear-ing at the junction 93 but~is otherwise identical to it.

5~6 The signal appearing at junction lll is applied to a junc-tion 126 between the resistor 127 and a capacitor 128.
Capacitor 128 is connected to grou~d while resistor 127 is connected to the base o a txansistor 129. Junction 111 is also connected to the base of a transistor 130 and to capacitor 125 via resistor 131. Capacitor 125 is connected to ground.
The emitters of transistors 129 and 130 are connected to the collector of a transistor 132. Transis-tor 115 and transistors 132-141, together with their assoc-icated resistors within box 41B, set up a supply-independent voltage reference for a Battery Monitor Circuit (not shown) forming a part of the Analog Circuit 11 of Figure 1 and set up bias currents for the unity gain Buf~er 25 and Reversion Circuit 22. For example, transistors 134-136 are 2.72 area scaled transistors which, in conjunction with transistor 140 and a resistor 142 set up current in their respective collectors. Transistor 141 together with resistors 143, 144 provide a current for startup when power is first applied. Transistors 133 and 137 minimize effects on currents at low battery conditions.
The collector of transistor 129 is connected to the collector and base of a transistor 145 while the col-lector of transistor 130 is connected to the collector of a transistor 146 and to the base of a transistor 147.
The emitter of transistor 147 is connected to ground via resistor 148 and to the base of a transistor 162 while its collector is connected to +V via resistor 149. Tran-sistors 145 and 146 form current sinks in conjunction 30 with resistors 150 and 151, respectively.
Transistor 147 and resistors 148 and 149 provide a sensitivity hysteresis function in a manner to be described more fully below. In addition, Reversion Circuit 22 works in conjunction with Sensitivity Control Circuit ~4 and the 35 Output Circuit Z!3. Transistors 152~155 of ~ensitivity Control Circuit Z!4 have their collectors connected to the base of transistor 129 and their bases connected to junc-tion 96. The emitter of transistor 152 is connected to a 54~36 terminal 156 via resistor 157 and to the emitter of transistor 153 via diode 158 and resistor 159. The termina:L 156 receives the SENSITIVITY
signal from the Digital Circuit of the copending application. The emitter of transistor 153 is connected to ground via resistor 160 while a resistor 161 connects the emitter of transistor 154 to ground. The emitter of transistor 155 is connected to resistor ]L48, to the emitter of transistor 147 and to the base of transistor 162. Transistor 162 controls the output signal and has its collector connected to a terminal 163 and to +V via resistor 164 while its emitter is connected to ground. The SENSE signal is provided to the Digital Circuitry of the copending application at terminal 163.
As will become apparent from the following discussion, when no signal is applied to terminal 156, an intermediate sensitivity is selected for the Reversion Circuit 22. When the terminal 156 is connected to a positive potential via the Digital Circuitry 10, the Reversion Circuit 22 is in its most sensitive state. Conversely, when the terminal 156 is connected to ground via the Digital Circuitry 10, Reversion Circuit 22 is in its least sensitive state. Assuming for the moment, that Sensitivity Control Circuit 24 is in the intermediate setting (i.e. no signal appearing at terminal 156) each of the current sinks including transistors 153-155 are operative and establish a voltage drop across resistor 127 resulting in a turnoff of transistor 130 and a turnon of transistor 129. This is the quiescent Reversion Circuit condition. When a signal is applied to the junction 111 the base of transistor 129 rises more rapidly than the base of transistor 130 because of the time constant associated with resistor 131 and capacitor 125. If the inpu~ is of sufficient amplitude to overcome the bias on transistors 129 and 130 established by the voltage drop across resistor 127, trans:istor 130 turns on and transistor 129 turns off. The turn on of transistor 130 turns on transistor 147 and thus transistor 162.
The turn on of transistor 162 results in a signal at terminal 163 of ground potential--a logic "0" SENSE signal in the context of digital circuit 10 of ~S~6 Figure l--indicating the detection of heart activity.
I~hen the terminal 156 is connected to a positive potential, the current associated with the current sink including transistor 153 is dis-abled and thus a lesser current is flowing through resistor 127 establish-ing less of a bias on the base of transistor 129. Thus, Reversion Circuit 22 has a greater sensitivity. Conversely, when terminal 156 is connected to ground, the current sink including transistor 152 is enabled increasing the current flow through resistor 127 and the bias on transistor 129. In this condition, the Reversion Circuit 22 is in its least sensitive setting.
Reversion Circuit 22 functions as an interference discriminator in a manner similar to the circuit disclosed in United States Patent No.
3,927,677, issued December 23, 1975 for DE~D CARDIAC PACER, which is co-owned with the present invention. Within the context of the present invention, a continuous wave signal results in a charging of capacitor 125 to a reference level (an average value determined by the time constant of capacitor 125 and resistor 131 and the repetition rate of the incoming signal). If a non-repetitive signal ~such as an R wave) occurs during this continuous wave signal, it will result in a charge of capacitor 125 to a second level (a peak riding above the average DC level) allowing transistor 129 to respond to it and turn off (dependent, of course, on the magnitude of the signal and sensitivity of the Reversion Circuit 22). Thus, the Reversion Circuit 22 responds differentially to signals representative of extraneous repetltive noise and sensed heart activity to result in a digital circuit compatible output signal (SENSE), even in the presence of repeti-tive noise, ~hile its sensitivity may be programmed in accordance with the state of the digital circuitry. During the BLANK signal, the Reversion Circuit 22 resets itself by assuming the quiescent condi-;4~6 tion--transistor 129 "on" and transistor 130 "off".
Transistox 1~7 and resistors 148 and 149 per-form a sensitivity hysteresis function to enhance the pro-vision of an output signal. As an output pulse is ini-tiated, the junction between the emitter of transistor 1~7and resistor 148 rises in voltage which turns off transis-tor 155. This decreases the current flow through the re-sistor 127 Thus, a positive feedback is initiated in that as transistor 129 is turned off by a positive signal 10 coming from the unity gain Buffer 25, an output is ini-tiated and the current through the resistor 127 is lowered resulting in an increase in the base voltage of transis-tor 129 turning it off even faster. When the output tran-sistor 162 begins to turn off, the voltage at the ~unction 15 of the emitter of transistor 147 ana resistor 148 drops toward ground initiating the flow of current through tran-sistor 155 and increasing the current flow through resis-tor 127 speeding up the turn on of transistor 129 and, thus, the turnoff of transistor 130.
The essentially polarity independent response of the preamplifier 20 and Absolute Value Circuit 21 for the purpose of reducing polarity disparity are enhanced by the fact that the current sources and sinks have their emitters "degenerated" by resistors which stabilizes them 25 to make them more nearly matching for polarity disparity.
However~ other techniques may be employed without depart-ing from the scope of the invention. In addition, at least the input differential pair formed of transistors 45 and 46 and the Absolute Value Circuit transistors 91 30 and 92 are preferably "matched" by placing them in close proximity and providing them with the same geometry and connections, again for polarity disparity. This is pref-erable with the feedback resistors 30 and 31 as well. In a preferred embodiment, the transistor 129 can be an area 35 scaled device which, together with the selection of the resistances of resistors 150 and 151 can provide an offset corresponding to the offset between the junctions 93 and 111. The use of current sources or sinks stabilizes the currents over the voltage ranges of interest. However, the present invention may be implementea in manners other than those stated as preferable. Accordingly, it is to be understood that, within the scope of the appended claims, the invention may be practicea otherwise than as specifically described.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a demand cardiac pacemaker of the type having means for sens-ing heart activity and means for establishing a reference level represent-ative of sensed extraneous noise and a second level greater in absolute value than said reference level by an amount representative of sensed heart activity, the improvement which comprises differential amplifier means responsive to the difference between said reference and second levels for detecting heart activity, means for establishing the level difference to which said differential amplifier means responds including single resist-ance means for establishing a differential amplifier means biasing voltage in accordance with the current flow therethrough and means connected to said resistance means including terminal means and further means responsive to the polarity of a single signal and the lack of a signal at said ter-minal means for selectively establishing the current flow through said resistance means.

2. The demand cardiac pacemaker of Claim 1 wherein said current flow establishing means comprises a network of resistance means and select-ively conductive switch means.

3. The demand cardiac pacemaker of Claim 1 wherein said current flow establishing means comprises a plurality of selectively conductive current sink means.

4. The demand cardiac pacemaker of Claim 1 wherein said current flow establishing means comprises means serially connected with said resist-ance means.

5. The demand cardiac pacemaker of Claim 4 wherein said current flow establishing means comprises a network of resistance means and select-ively conductive switch means.

6. The demand cardiac pacemaker of Claim 5 wherein said select-ively conductive switch means comprises a plurality of selectively con-ductive electronic switches.