AU463520B2 - Atrial and ventricular pacer having independent rate and av delay controls - Google Patents

Description

49,0 5 2 n2

ventricular demand This invention relates to atrial and in which the rate and pacers, and more particularly to such pacers

AV delay controls are independent of each other. atrial and ventri- There are many patients who require an pacer. In cular pacer, as opposed to the more usual ventricular monitors the sponta- demand pacers of the latter type, a detector if too long a time interval neous ventricular beating of the heart; pulse is generated has elapsed since the last beat, a stimulating

to trigger the ventricular beat. In pacers of the former type, for United States Patent No. example, in that disclosed in Berkovitz additional circuit is pro- 3661158 filed on December 15, 1969, an pulses to compensate for vided for generating atrial stimulating synchronism between the irregular atrial activity. To maintain beat whether two pulsing circuits, every ventricular

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49,05 2172 spontaneous or stimulated causes the atrial timing

period to re-start together with the (longer) ventricular timing period. Following any ventricular beat, an atrial stimulating pulse is generated a short time after the next atrial beat should occur. The atrial stimulating pulse is generated even if a spontaneous atrial beat occurs. There-

after, a ventricular stimulating pulse is generated, but only if a spontaneous ventricular beat does not occur within a predetermined time interval subsequent to the previous ventricular beat. As explained in the Berkovits application, if an

atrial stimulating pulse is generated following an atrial contraction, that is, during the refractory interval of the atria, it has no effect on the beating action of the patient's heart. It is only the generation of a ventricular stimulating pulse during the refractory interval of the ventricles that can be dangerous. For this reason, the

detection of a spontaneous ventricular beat inhibits the generation of the ventricular stimulating pulse which would otherwise soon occur. But atrial stimulating pulses are not inhibited. In fact, even if the heart beats perfectly, in the conventional type atrial and ventricular pacer, an atrial stimulating pulse is generated during every heartbeat cycle. Of course, the timing of the atrial pulse generator is keyed to the ventricular beats so that at all times both pulsers and the natural heart activity remain in synchronism. The Berkovits pacer is provided with two timing cir- cuits. Typical escape intervals are 600 milliseconds for the atrial timing circuit and 800 milliseconds for the ventri- cular timing circuit. Each ventricular beat results in the 3 463,520

49,052/72 resetting of both timing circuits. After 600 milliseconds If have elapsed, an atrial stimulating pulse is generated. 200 milli- a ventricular beat is not detected within the next ventricular seconds, the ventricular timing circuit causes a ventri- stimulating pulse to be generated. If a spontaneous elapsed cular beat is detected before 800 milliseconds have circuits subsequent to the previous beat, then both timing are reset; the result is that a ventricular stimulating The pulse is not generated and a new cycle is initiated. stimulating time period between the atrial and ventricular illustrative ex- pulses is known as the AV delay, and in the ample is 200 milliseconds. thus Both escape intervals are usually adjustablel is the AV delay, which is the difference between them, both timing similarly adjustable. In the Berkovits pacer, Each capacitor periods are controlled by charging capacitors. ventri- is discharged upon the detection of a spontaneous cular beat or the generation of a ventricular stimulating a respect- pulse. Each capacitor then starts to charge toward ive firing level. The atrial capacitor reaches the atrial and the firing level after 600 milliseconds have elapsed charg- ventricular charging capacitor reaches the ventricular one ing level after 800 milliseconds have elapsed. Whenever level, the of the capacitors reaches its respective charging two escape respective stimulating pulse is generated. The resistors intervals are made adjustable by providing variable In the case of in the charging paths of the two capacitors. in which provision implantable pacers, there are numerous ways to the potentio- may be made for a physician to gain access For example, meters for adjusting the two escape intervals. 4 463,520

49,052/72 the potentiometer controls may be extended to the exterior of the pacer and access may be gained to them by using a conventional Keith needle. For maximum cardiac output, there is an optimum AV delay for each patient. The optimum AV delay is usually a fixed parameter for the patient. That is, even if the pacer rate is changed by the physician the AV delay should general- ly remain the same. However, there are situations in which the AV delay must be changed. It is apparent that by vary- ing the resistances in the two charging circuits, both the rate and the AV delay may be adjusted. For example, suppose that it is necessary to increase the pacer rate. This can be

achieved by decreasing the resistance in the ventricular charging circuit so that the ventricular escape interval is shortened. If the AV delay is to be maintained constant, the atrial charging resistance also should be decreased so that the atrial escape interval is similarly shortened; in this manner, an atrial stimulating pulse will still be gen- erated the same fixed time interval prior to the generation of a ventricular stimulating pulse. It is usually necessary for the physician to mani- pulate both potentiometers when making an adjustment. Sup-

pose that the initial escape intervals are 600 and 800 milli- seconds respectively, and that the ventricular escape inter- val is to be decreased by 50 milliseconds with no change in the AV delay. It is theoretically possible to gang the two potentiometurs to each other so that adjustment of the ventricular potentiometer results in the automatic adjustment of the atrial potentiometer. However, it is apparent that a reduction in the ventricular escape interval from 800 milli- 5 463,520

49,052/72 seconds to 750 milliseconds is a change of 6.25% while the

change in the atrial escape interval from 600 milliseconds

to 550 milliseconds is a change of 8.33%. It is thus

difficult to provide for proper tracking of the potentio-

meters, and even when a fixed AV delay is desired it is

usually necessary for the physician to adjust both potentio-

meters when he changes the pacer rate. Because the two potentiometer controls are not completely independent, two problems are presented. The

first is that it is difficult and time consuming for the physician to properly set both potentiometers. The second

problem is even more severe. It is possible for the physician to mistakenly set the atrial escape interval to so short a time period that two and even three atrial stimulating pulses occur during the overall ventricular escape interval (heart- beat cycle). Multiple atrial stimulating pulses of this type of course are not desirable.

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49,052/72 In one broad form the invention comprises a pacer

cormprising atrial terminal means, ventricular terminal means,

first timing means for applying a pulse to said atrial

terminal means at the end of a first time interval following

the resetting thereof, means for varying said first time

interval, second timing means for applying a pulse to said time ventricular terminal means at the end of a second interval following the application of a pulse to said atrial

terminal means, means for adjusting said second time interval,

means for detecting a ventricular beat of a patient's heart

and responsive thereto for inhibiting the application of a

pulse to said ventricular terminal means if said second time

interval has not expired subsequent to the application of the

previous pulse to said atrial terminal means and for resetting

said first timing means, and means for resetting said first

timing means at the end of a third time interval following

the application of a pulse to said atrial terminal means, said

third time interval being longer than the maximum second

time interval. In accordance with the various embodiments of this

invention,

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49,052/72 a first potentiometer, the basic rate control element, is in the atrial timing circuit, rather than in the ventricular timing circuit as in the prior art. Each atrial stimulating

pulse triggers two one-shot multivibrators. The first multi-

vibrator in the illustrative embodiment of the invention has

a variable pulse period of 50-250 milliseconds. The second

potentiometer control adjusts the period of this multi-

vibrator. The period of the multivibrator is equal to the

desired AV delay. At the trailing edge of the multivibrator pulse, a ventricular stimulating pulse is generated. How-

ever, if a spontaneous ventricular beat is detected, the ventricular stimulator is inhibited so that a ventricular stimulating pulse is not generated. It is thus apparent

that each atrial stimulating pulse results in a single ventricular stimulating pulse if one is necessary after

the pre-set AV delay. The second one-shot multivibrator has a period of

260 milliseconds. At the trailing edge of the multivibrator

pulse, the atrial timing circuit is reset. Consequently, even though the atrial charging capacitor starts to re-charge with the generation of an atrial stimulating pulse, the capacitor is discharged 260 milliseconds after the atrial stimulating pulse is generated. It is only when the capacitor

starts to charge once again at this time that it is allowed

to charge all the way up to the atrial firing level. Even

if a spontaneous ventricular beat occurs, since it occurs

after the atrial stimulating pulse by at most 250 milli-

seconds, the atrial charging capacitor is always discharged

260 milliseconds after an atrial stimulating pulse is gener-

ated and it is only at this time that it is allowed to start

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49,052/72 charging all the way up to the atrial firing level.

Suppose, for example, that it is desired that the pacer rate be 75 beats per minute corresponding to an inter-

beat interval of 800 milliseconds. In such a case, the rate- control potentiometer should be set such that the atrial

charging capacitor reaches the atrial firing level 540 milli- seconds after its initial discharge. Each atrial stimulating pulse triggers the 260-millisecond one-shot multivibrator.

At the same time, the atrial charging capacitor is discharged

and starts to charge towards the firing level. But 260

milliseconds later (by which time it has not reached the

firing level) the capacitor is discharged and it starts to

charge once again. After another 540 milliseconds have

elapsed, the firing level is reached and another atrial

stimulating pulse is generated. Consequently, atrial stimul-

ating pulses are generated at 800-millisecond intervals.

Each atrial stimulating pulse triggers the variable one-shot multivibrator and consequently a ventricular stimulating

pulse is generated following the pre-set AV delay after each

atrial stimulating pulse provided that a spontaneous ventricular beat is not detected and the ventricular stimul-

ator is not inhibited. Although the physician, as he adjusts

the atrial timing circuit potentiometer while looking at the

ECG signal on an oscilloscope, is in reality adjusting the

atrial escape interval, what he observes are the large

ventricular stimulating pulses; as far as the physician is

concerned, he is adjusting the ventricular pulse rate. Of

course, adjusting one of the atrial and ventricular pulse

rates is equivalent to adjusting the other since for each

atrial pulse there should be one ventricular pulse (in the

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49,052n2 with the two absence of a spontaneous ventricular beat) Since physicians pulses separated by a fixed time interval. the ventricular pulse are more likely to think in terms of atrial charging capacitor rate, they can be informed that the rate control. It potentiometer control is the ventricular are completely is apparent, however, that the two controls independent of each other. The rate control potentiometer and ventricular pulses controls the rate at which both atrial the time which elapses occur. The AV delay control determines ventricular pulse. between each atrial pulse and the next of the It is important to understand the function illustrative embodiment 260-millisecond multivibrator in the when reference is made of the invention. In the pacer art, to think in terms of to rate adjustment it is conventional But if the ventri- varying the ventricular escape interval. only way to keep the AV cular escape interval is varied, the adjusting the atrial delay constant would appear to involve between the two escape escape interval so that the difference working "backwards in intervals is constant. This requires occurs, the time" whenever a ventricular stimulating pulse occurred a fixed time atrial stimulating pulse should have a difficult thing to interval earlier. This is obviously pacer rate to the atrial accomplish. However, by keying the delay to control the charging circuit, and using a variable rate and AV delay AV delay, it is possible to make the The difficulty with controls independent of each other. the detection of a spon- this approach, however, is that if of a ventricular taneous ventricular beat, or the generation the atrial charging stimulating pulse, is used to synchronize would necessarily be a circuit, then the basic pacer rate 463,520

49,0 5 2n2 function of the AV delay or the time when a spontaneous

ventricular beat occurs. For example, with a short AV delay

the period between successive atrial pulses would be short-

ened. However, by providing the 260-millisecond delay, the

atrial charging capacitor is always discharged (synchronized)

at a fixed time after any possible ventricular stimulating

pulse or spontaneous ventricular beat. Consequently, the

basic pacer rate can be made independent of spontaneous

ventricular beats and ventricular stimulating pulses simply by controlling the atrial escape interval to be equal to the

difference between the desired inter-beat interval and the

delay of the 260-millisecond one-shot multivibrator. It is a feature of our invention to provide an

atrial timing circuit which is always re-started a fixed

time interval after each atrial stimulating pulse and which

includes a variable element for determining the basic pacer

rate, the atrial timing circuit being re-started in all

normal cases after the detection of a spontaneous ventricular

beat or the generation of a ventricular stimulating pulse. It is another feature of our invention to provide

a variable delay for controlling the generation of a ventri-

cular stimulating pulse at a selected time after the gener-

ation of an atrial stimulating pulse, the detection of a

spontaneous ventriculating beat inhibiting the generation of

a ventricular stimulating pulse. Further objects, features and advantages of our

invention will become apparent upon consideration of the

following detailed description in conjunction with the draw-

ing, in which: Figure 1 depicts schematically the atrial and 11 463,520

49,0 522 ventricular demand pacer disclosed in the above-identified Berkovits application; and Figure 2 depicts schematically the atrial and of the ventricular demand pacer of our invention, the design individual circuits shown as blocks in the drawing being view of apparent to those skilled in the art especially in the detailed circuitry disclosed in the Berkovits application. Figure 1 depicts three circuit blocks, the details Ventri- of which are disclosed in the Berkovits application. cular demand stimulator 12 applies a ventricular stimulating at a time after pulse, on ventricular electrodes El and E2, or stimul- the previous ventricular beat, whether spontaneous 18. ated, which is determined by the setting of potentiometer might be 800 milli- Typically, the ventricular escape interval pacer seconds. Since potentiometer 18 determines the basic rate by varying the interval between ventricular stimulating the R waves pulses, it is labelled "R-R rate control" because A ventri- in an ECG signal correspond to ventricular beats. on the cular beat also results in the appearance of a pulse to ventri- electrodes and conductors 22, 24 which is extended cular beat detector 14. The ventricular beat detector ex- demand tends a signal over conductor 16 to the ventricular stimulator, in response to the detection of a ventricular beat, to inhibit the generation of the next ventricular to control-the pulse which otherwise would be generated and demand start of a new ventricular timing interval. Atrial stimulator 10 is provided to extend an atrial stimulating pulse to electrodes E3 and E4. The atrial stimulator timing timing period starts together with the ventricular stimulator extended period, the ventricular beat detector output being 12, 463.520

49,052/72 to both pulse generators. However, the timing period of the

atrial stimulator is, for example, 600 milliseconds rather

than 800 milliseconds. The atrial escape interval is determ-

ined by the setting of potentiometer 20. Since atrial beats

correspond to P waves in an ECG signal and potentiometer

determines the interval between a ventricular beat and an

atrial beat, it is labelled "R-P control".

In the circuit of Figure 1, as shown in detail in

the Berkovits application, two capacitors are provided to

control the atrial timing period and the ventricular timing

period. Each capacitor charges from an initial value toward

a respective firing level. If potentiometer 20 is set to

provide an atrial escape interval of 600 milliseconds, then

it takes 600 milliseconds for the atrial capacitor to reach

the atrial firing level, at which time an atrial stimulating

pulse is generated, the capacitor is discharged and a new timing interval begins; if potentiometer 18 is set to provide

a ventricular escape interval of 800 milliseconds, it requires

800 milliseconds for the ventricular timing capacitor to

reach the ventricular firing level, at which time a ventri- cular stimulating pulse is generated, the capacitor is dis-

charged and a new ventricular timing period begins. Both

.capacitors are discharged and new timing periods begin when

either a spontaneous ventricular beat is detected or a

ventricular stimulating pulse is generated.

Atrial demand stimulator 10 in Figure 2 is similar

to atrial demand stimulator 10 in Figure 1. However, potentio-

meter 20 controls the basic pacer rate and accordingly is

labelled "P-P rate control". The atrial timing period begins

when the reset input of atrial demand stimulator 10 is pulsed.

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49,0 52n2 An atrial charging capacitor (as disclosed in the Berkovits application) starts to charge and %'hen it reaches the atrial firing level a stimulating-pulse is generated on atrial electrodes E3 and E4. At the same time, a pulse is applied on conductor 40 to the trigger input of each of 260-milli- one- second one-shot multivibrator 30 and 50-250 millisecond shot multivibrator 32. imultivibrator 30 applies a short

pulse at its output 260 milliseconds after it is triggered. The pulse is extended through OR gate 38 to the reset input of atrial demand stimulator 10. it requires longer than 260

milliseconds for the charging capacitor in atrial demand follow- stimulator 10 to reach the firing level. Accordingly, ing the generation of each atrial stimulating pulse, the it is capacitor starts to rise toward the firing level, but always discharged 260 milliseconds into the timing period to before it has reached the charging level. it then starts charge once again. Potentiometer 34 is adjusted in accordance with the AV desired AV delay. For example, suppose that the desired delay is 200 milliseconds. in such a case, when conductor

is pulsed together with the generation of an atrial multi- stimulating pulse, multivibrator 32 is triggered. The vibrator pulses its output 200 milliseconds after it is triggered, Ventricular pulser 36 simply serves to amplify to the pulse and to apply a ventricular stimulating pulse electrodes El and E2, provided it is not inhibited from oper-. ini ating by a reset pulse on conductor 16. Consequently, the absence of the detection of a spontaneCous ventricular beat, a ventricular stimulating pulse is applied on electrodes is El and E2 -at a time after the atrial stimulating pulse 1 4s 14463,5

49,0 5 2n2 generated in accordance with the AV delay setting of potentiometer 34. If during the AV delay interval a spontaneous ventricular beat occurs, the electrical signal on conductors 22, 24 is detected by ventricular beat detector 14. Con- ductor 16 is pulsed and the reset input of ventricular stimulator 36 is energized. The stimulator is reset so that a ventricular stimulating pulse is not generated at the end of the pulse generated by one-shot multivibrator 32. Con- ductor 16 is also connected to an input of R0gate 38 so that the detection of a ventricular beat re-starts the atrial timing period. The 260-millisecond period of one-shot multi- vibrator 30 is made longer than the maximum AV delay inter- val. In the event a spontaneous ventricular beat occurs before multivibrator 32 completes its timing period, ventri- cular stimulator 36 is inhibited so that a ventricular stimulating pulse is not generated, OR gate 38 operates and the charging capacitor in atrial demand stimulator 10 is discharged. The capacitor starts to charge once again, but it is again discharged at the trailing edge of the 260-milli- second pulse output of multivibrator 30. On the other hand, if a spontaneous ventricular beat does not occur, then with the expiration of the pulse period of multivibrator 32, a ventricular stimulating pulse is generated. This similarly results in the operation of ventricular beat detector 14 and the resetting of atrial demand stimulator 10. The atrial charging capacitor starts to charge but is discharged in this case also at the end of the 260-millisecond timing interval. No matter what the ventricular activity under normal

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49,0 52/72 controls the resetting circumstances, multivibrator 30 always 260 milliseconds after the of the atrial demand stimulator Consequently, the effect- previous atrial stimulating pulse. the charging cycle of the ive atrial timing period, that is, in the reaching of the capacitor which ultimately results milliseconds after a previous firing level, always begins 260 any desired basic pacer atrial stimulating pulse. To achieve to adjust potentiometer 20 so rate, all that is necessary is demand stimulator 10 is that the escape interval of atrial 260 milliseconds. the desired inter-beat interval minus the basic pacer rate because potentiometer 20 thus controls one is needed, always a ventricular stimulating pulse, if pulse in accordance with occurs after an atrial stimulating and the time separation the setting of potentiometer 34, is necessarily the between ventricular stimulating pulses atrial stimulating pulses. same as the time spacing between potentiometer 20 con- As far as the physician is concerned, rate, although in reality trols the basic (ventricular) pacer the time interval between potentiometer 20 directly affects successive atrial stimulating pulses. 30 is as The reason for one-shot multivibrator beat takes place with- follows: No matter when a ventricular the generation of an atrial in the 260 milliseconds following that ventricular beat results stimulating pulse, even though demand stimulator, it does in the resetting of the atrial pulse timing. Instead, not affect the atrial stimulating is fixed in accordance the atrial stimulating pulse timing 20 because muitivibrator with the setting of potentiometer escape interval no matter when always re-starts the atrial its output between the ventricular beat detector 14 pulses 16, 4635

49,05 272 generation of an atrial stimulating pulse and the completion of the timing interval of one-shot multivibrator 30. As for

the AV delay, it is keyed to the generation of the atrial stimulating pulses, with a ventricular stimulating pulse, if one is necessary, being generated a pre-set time after the generation of an atrial stimulating pulse. The two controls are completely independent of each other. Varying the set-

ting of potentiometer 34 does not affect the basic pacer rate and instead only results in a change in the AV delay. Similarly, a change in the setting of potentiometer 20 changes the basic pacer rate; a ventricular stimulating pulse, if one is necessary is always generated at a fixed time after the generation of an atrial stimulating pulse in accordance with the setting of pctentiometer 34. The output of ventricular beat detector 14 resets the atrial demand stimulator during normal operation even though such resetting has no effect because the atrial demand stimulator is always reset once again when multivibrator completes its timing interval. The reason for allowing the ventricular beat detector to reset the atrial demand stimul- ator is to take into account the possibility of an abnormal ventricular beat which occurs more than 260 milliseconds after the generation of an atrial stimulating pulse. For example, it is possible for one-shot multivibrator 32 to stimulate a ventricular beat, and that some time shortly thereafter a spontaneous ventricular beat occurs before

the next atrial stimulating pulse. In order to key the pacer operation to the last ventricular beat, ventricular beat detector 14 causes the atrial timing period to start all over again. 17, 463.520

49,0 52/72 the pacers of It should be noted that although most part functionally equivalent, Figures 1 and 2 are for the the functional sequences of there is a situation in which the Suppose, for example, that the two pacers are different. for 800 milliseconds, (that inter-beat interval is adjusted in the pacer of Figure 2 is is, the atrial escape interval the AV delay is set to 200 540 milliseconds), and that that a spontaneous ventri- milliseconds. Suppose further after the generation of cular beat occurs 100 milliseconds In the pacer of Figure 1, the an atrial stimulating pulse. results in the resetting of spontaneous ventricular beat and the ventricular demand both the atrial demand stimulator next atrial stimulating pulse stimulator. Consequently, the the spontaneous ventri- will occur 600 milliseconds after stimulating pulse, if cular beat, and the next ventricular 800 milliseconds after the one is necessary, will occur However, in the pacer of spontaneous ventricular beat. beat the spontaneous ventricular Figure 2, the detection of is atrial demand stimulator 10 has no effect. Even though the generation of the previous reset 100 milliseconds after 30 causes the atrial atrial stimulating pulse, multivibrator once again 160 milliseconds demand stimulator to be reset the atrial timing is concerned, later. In effect, as far as that a spontaneous ventricular the pacer ignores the fact before a ventricular stimul- beat occurred 100 milliseconds been generated. The extra ating pulse would have otherwise delay before the next atrial approximately 100-millisecond provides for a built-in com- stimulating pulse is generated in this delay, it is more pensatory pause. By building beat will occur, followed likely that a spontaneous atrial 18. 463.520

49,052/72 by a spontaneous ventricular beat, than in the case of the

pacer of Figure 1 in which the normal timing of the pacer

ensues without any compensation being made for the fact that

a premature ventricular beat occurred. Of course, if a spontaneous ventricular beat does occur before the atrial

demand stimulator completes its timing interval, then battery

life is extended because even an atrial stimulating pulse is

not generated during the next cycle. It is emphasized, how- ever, that the major advantage of the pacer of Figure 3 is

believed to be the independent control of the basic pacer

rate and the AV delay, and also the prevention of the

possibility of the controls being adjusted such that more

than one atrial stimulating pulse occurs between successive

ventricular stimulating pulses.

Ventricular stimulator 36 operates to generate a ventricular stimulating pulse at the trailing edge of the

pulse output of multivibrator 32, but as described above a

stimulating pulse is not generated if the reset input of the

ventricular stimulator is energized. Many different cir- cuits can be used for this purpose. A preferred circuit is one which is verysimilar to the basic timing circuit in-.

cluded in a demand stimulator. In this circuit, the output of one-shot multivibrator 32 is capacitor coupled to the

reference potential terminal. The reference potential is normally high. The output of the multivibrator is a posi- tive pulse, and consequently at its trailing edge a negative

spike is coupled through the capacitor to the reference

potential terminal. Only at this time is the reference

potential to which the timing capacitor voltage is compared

lowered sufficiently such that the.timing capacitor voltage

19 46 3,520

49,0 5 2n2 exceeds it and causes a stimulating pulse to be generated.

However, a reset pulse generated by ventricular beat detect-

or 14 causes the timing capacitor to discharge (just as it

is discharged in the circuit of Figure The time con-

stant of the charging circuit is adjusted such that it

requires more than 250 milliseconds for the capacitor volt-

age to rise to a level which exceeds even the low reference

potential which results at the trailing edge of the output

pulse of multivibrator 32. Consequently, once a reset pulse

has discharged the charging capacitor in the ventricular

stimulator, a stimulating pulse cannot be generated, even at

the trailing edge of the multivibrator pulse.

Although the invention has been described with

reference to a particvlar embodiment, it is to be understood

that this embodiment is merely illustrative of the application of the principles of the invention. Numerous modifications

may be made therein and other arrangements may be devised

without departing from the spirit and scope of the invention.

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Claims (3)

1. 49,0 5 2n2 The claims defining the invention are as follows: 1. A pacer comprising atri1. teri-'~l ventricular terminal means, first timing means for applying a pulse to said atrial terminal means at the end of a first time interval following the resetting thereof, means for varying said first time interval, second timing means for applying a pulse to said ventricular terminal means at the end of a second time interval following the application of a pulse to said atrial terminal means, means for adjusting said second time interval, means for detecting a ventricular beat of a patient's heart and responsive thereto for inhibiting the application of a pulse to said ventricular terminal means if said second time interval has not expired subsequent to the application of the previous pulse to said atrial terminal means and for resetting said first timing means, and means for resetting said first timing means at the end of a third time interval following the application of a pulse to said atrial terminal means, said third time interval being longer than the maximum second time interval.
2. 2. A pacer in accordance with claim 1 wherein said second time interval is adjustable within minimum and maximum time intervals defining a normal range of AV delays.
3. 3. A pacer comprising atrial terminal means, ventric- ular terminal means, means for generating atrial stimulating pulses on said atrial terminal means at spaced time intervals, means for generating a ventricular stimulating pulse on said ventricular terminal means at the end of a selected time interval following the generation of each stimulating pulse on said atrial terminal means, means for establishing the maximum value of said selected time interval to be less than

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   49.0 52/12 the generation of said atrial a predetermined interval between of the next said spaced .i:-;lti.g nulse and the start of a spontaneous ventricular beat interval, means for detecting selected time interval following a patient's heart within said pulse on said atrial terminal the generation of a stimulating of a corresponding stimulat- means but prior to the generation terminal means for inhibiting ing pulse on said ventricular pulse on said ventricular the generation of said stimulating controlling the time interval terminal means, and means for beat and the generation of between a spontaneous ventricular be said atrial terminal means to the next stimulating pulse on of a between the generation greater than the time interval terminal means and the stimulating pulse on said ventricular pulse on said atrial termin- generation of the next stimulating al means. claim 3 wherein the 4. A pacer in accordance with interval separating a time difference between the time on and the next stimulating pulse spontaneous ventricular beat a the time interval separating said atrial terminal means and terminal means and the stimulating pulse on said ventricular atrial terminal means corres- next stimulating pulse on said which follows a premature beat ponds to the compensatory pause

   of said patient's heart. 3 wherein said A pacer in accordance with claim means includes another atrial stimulating pulse generating of stimulating pulse at the end means for generating an atrial a following the detection of another selected time interval beat or following the lapse spontaneous premature ventricular after the generation of a of a predetermined time interval later. pulse, whichever occurred previous atrial stimulating

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   49,0 5 2n2 6. A pacer in accordance with claim 5 further

   in.cluidinq a pair of means each for adjusting one and only one

   of a respective one of said selected time interval and said

   other selected time interval.

   7. A pacer in accordance with claim 6 including

   means for establishing said predetermined time interval longer

   than the maximum value of said selected time interval.

   8. A pacer as hereinbefore described with reference

   to Figure 2 of the accompanying drawings.

   DATED this TWENTY SIXTH day of JUNE, 1975

   AMERICAN OPTICAL CORPORATION

   Patent Attorneys for the Applicant SPRUSON FERGUSON

   O 23 463,520

   49,0 52,r2

   2 S. 25. 3,5 ~462 0