Inhibition of a ventricular synchronous pacemaker

Inhibition of a ventricular synchronous pacemaker Seymour Furman, M.D. Bronx, N. Y.

Noncompetitive ventricular synchronous pacemakers (VVT) 1 have been available since 1965. 2 T h e characteristics of their operation have been well delineated and understood. 3' 4 T he recent appearance of a new variety of ventricular synchronous pacer, of different mode of operation than those previously used, may lead to misinterpretation of the ECG and a diagnosis of pacer malfunction. Early and most presently available VVT pulse generators (such as Cordis Ectocor)* have had a single, fixed pulse-to-pulse interval (interval in milliseconds is the mathematical reciprocal of rate) divided into two portions, a total refractory period of 350 to 400 msec. following sensing of a QRS complex or emission of a stimulus, followed by a sensitive period, the duration of which is dependent on the pulse interval. If the pacemaker automatic rate is 70 per minute, the pulse-topulse interval will be 857 msec., the refractory period 400 msec. and the sensitive period 460 msec. Slower or more rapid rates will have correspondingly longer or shorter sensitive periods as the refractory period is fixed. Any QRS complex or other electrical signal during the sensitive period produces a pacemaker stimulus. The generator cannot be inhibited under any circumstances. The Cordis Omni-Ectocor* operates quite differently. It has a refractory and two sensitive periods, one during which the generator is inhibFrom the Cardiothoracic Service, Division of Surgery, Montefiore Hospital and Medical Center, Bronx, N. Y. Supported in part by United States Public Health Service Grant No. H-04666-15. Received for publication Jan. 19, 1976. Reprint requests: Seymour Furman, M.D., 111 E, 210 St., Bronx, N. Y. 10467. *Cordis Corporation, Miami, Fla.

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ited2 The pulse-to-pulse interval is noninvasively variable to produce six different rates, 60, 65, 70, 80, 90, and 100 per minute, though for reasons of the possibility of competition of a stimulus and a T wave in the presence of a short refractory period, use of the 90 and 100 per minute rates is discouraged by the manufacturer. Those two rates can be used as they have been by the author, but with caution and then largely for atrial pacing where ventricular T wave competition does not occur (Fig. 1). T he refractory and sensitive periods of the Omni-Ectocor pulse-to-pulse interval are further subdivided-the refractory into an absolute refractory and a noise sampling period, the sensitive into a short sensitive inhibited (S-I) and a longer sensitive synchronous (S-S) portion. The pulse-to-pulse interval and its four subdivisions have fixed proportions but none is of fixed duration. As all generator output parameters vary with rate, the impulse duration too varies as a function of the pulse-to-pulse interval 6 (Table

I). T he pulse-to-pulse interval is subdivided into 512 bits: the refractory period is 5/16 of 512 or 160 counts, the refractory noise sampling period 1/16 of 512 or 32 counts, the sensitive inhibited period 1/8 of 512 or 64 counts, and the sensitive synchronous period 1/2 of 512 or 256 counts. Programming the pulse generator simply changes the duration of each bit, changing the duration of each interval but not the relative proportions. Clinical consequences

All earlier ventricular synchronous pacemakers could not be inhibited or slowed by an electrical stimuli. 7 Spontaneous QRS complexes, in any combination (a QRS within the refractory period

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Fig. 1. The pulse generator cycleis represented by the diagram, no matter what the stimulation rate. The duration of each function, the total interval or any part varies with the pulse generator rate. The relative proportions remain constant.

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Fig. 2. Inhibition of an Omni-Ectocor is produced by a spontaneous contraction within the sensitive-inhibited period (above). Transcutaneous stimulation at the appropriate rate inhibits the generator (below). Note also the pseudofusion beats and paced beats of customary ventricular synchronous (VVT) pacer function. of 400 msec. would be unsensed) could n o t inhibit the pulse generator, ~ though the generator might be unaffected, go to an asynchronous mode, be driven to the m a x i m u m synchronous rate, or develop an erratic rate. The Omni-Ectocor offers two additional circumstances, t h a t of inhibition and slowing below the predetermined automatic rate. B o t h are based on discrete stimuli falling into the "sensitive inhibited" (S-I) period, recycling the unit and once again falling into the "S-I" period. For a generator set at a rate of 70 per m i n u t e the "S-I" period extends for 107 msec. from 322 msec.

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after a sensed QRS or the emission of a stimulus to 429 msec. after. A premature ventricular contraction, i.e., coupled at 322 to 429 msec. will be sensed, inhibit and recycle the generator as if it were ventricular inhibited (Fig. 2). Similarly a ventricular tachycardia of rate 140 to 186 will inhibit the generator, as will discrete electrical stimuli at a similar rate. At other rates, other intervals exist which will inhibit for a single cycle or continuously inhibit the generator (Table I). Electromagnetic interference detected during the "noise sampling" period drives the unit to asynchronous operation; during the S-I period inhibits it and during the "S-S" period triggers the unit. Consequently, a variety of responses are possible and m a y exist consecutively as the frequency of the E M I varies or fluctuates (Fig. 3). Conclusions

Unlike earlier ventricular synchronous pacemakers, t h a t of the Omnicor series is capable of inhibition by single or multiple ventricular or interfering signals and of inhibition, slowing or increase of the stimulus formation rate as a function Of the timing of a premature ventricular contraction, a run of ventricular tachycardia or of electromagnetic interference. The rate of interference producing various effects is a function of the automatic rate in which the generator operates. A synchronous response m a y be converted to inhibited (as well as the reverse) by programming another automatic rate. Each of these factors must be considered to allow proper interpretation of the ECG. Two implications also exist: One t h a t electromagnetic interference between 2 and 2.7 times the a u t o m a t i c rate will inhibit the generator, eliminating the absolute safety of the conven-

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Inhibition of ventricular synchronous pacemaker

Fig. 3. Variation in pacer automatic rate affects its response to a tachycardia. Left, Pacer automatic rate is 60 per minute and is inhibited by a tachycardia at a rate of 160 per minute. Center, Pacer programmed to automatic rate of 80 per minute, is triggered by the same tachycardia. Right, Reprogrammed to 60 per minute the pacer is inhibited. (X is the time of the programmer artifact.)

Table I. O m n i - E c t o c o r i n t e r v a l s Not recommended for ventricular pacing Rate per minute... Pulse duration (msec.) Total interval (msec.) Refractory (msec.) Noise sampling (msec.) Total sensitive (msec.) Sensitive inh. (msec.) Sensitive synch. (msec.) Inhibition interval (msec,) for single stimulus Inhibition rate for tachy~ardia per minute (upper and lower limits) (msec.)

60 1.95 1,000 375 63 625 125 500 375 500 120 160

65 1.80 923 346 58 577 115 462 346 461 130 173

tional ventricular synchronous unit and that i n t e r f e r e n c e p u r p o s e f u l l y p e r f o r m e d will a l l o w demonstration of the underlying ventricular c a r d i a c r h y t h m , a s is p o s s i b l e w i t h v e n t r i c u l a r inhibited units and unlike other ventricuiar synchronous units.

Summary Noncompetitive ventricular synchronous pacem a k e r s ( V V T ) h a v e b e e n a v a i l a b l e s i n c e 1965. Most presently available and earlier models have a single, fixed p u l s e - t o - p u l s e i n t e r v a l d i v i d e d i n t o a t o t a l r e f r a c t o r y p e r i o d of 350 to 400 msec., followed by a sensitive period. During the refract o r y p e r i o d t h e p a c e m a k e r will n o t r e s p o n d t o QRS complexes or electrical signals, but a pacer s t i m u l u s will b e p r o d u c e d d u r i n g t h e s e n s i t i v e period. T h e g e n e r a t o r c a n n o t b e i n h i b i t e d u n d e r any circumstances. The Cordis Omni-Ectocor has a refractory period and two sensitive periods, d u r i n g o n e of w h i c h t h e g e n e r a t o r is i n h i b i t e d .

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70 1.67 857 322 54 535 107 428 322 429 140 186

80 1.46 750 281 47 469 94 375 281 375 160 213

90 1.30 667 250 42 421 84 333 250 334 180 240

100 1.17 600 225 37.5 375 75 300 225 300 200 267

Unlike other ventricular synchronous pacers, the O m n i - E c t o c o r is c a p a b l e o f i n h i b i t i o n b y single o r multiple ventricular or other signals, of slowing or increase of the stimulus formation rate which depends on the timing of the premature ventricular contractions, and of inhibition by a run of ventricular tachycardia or by electromagnetic i n t e r f e r e n c e . T h e r a t e of i n t e r f e r e n c e p r o d u c i n g t h e s e effects is a f u n c t i o n of t h e a u t o m a t i c r a t e t o w h i c h t h e g e n e r a t o r is p r o g r a m m e d . A l l of t h e s e f a c t o r s m u s t b e c o n s i d e r e d for p r o p e r i n t e r p r e t a tion of the ECG.

REFERENCES 1. Parsonnet, V., Furman, S., and Smyth, N. P. D.: Implantable cardiac pacemakers status. Report and resources guideline, Circulation 50:A-21, 1974. 2. Parsonnet, V., Zucker, I. R., Gilbert, L., and Myers, G. H.: Clinical use of an implantable standby pacemaker, J. A. M. A. 196:704, 1966. 3. Furman, S., and Escher, D. J. W.: Ventricular synchronous and demand pacing, AM. HEART J. 76:445, 1968. 4 . Sowton, E.: Ventricular-triggered pacemakers: clinical experience. Br. Heart J. 30:363, 1968.

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5. Omni-Ectocor programmable cardiac pacer (Manual), Cordis Corp., Miami, Fla., 1973. 6. Parsonnet, V., Cuddy, T. E., Escher, D. J. W.I Furman, S., Morse, P., Gilbert, L., and Zucker, I. R.: A permanent pacemaker capable of external non-invasive programming, Trans. Am. Soc. Artif. Intern. Organs 29:224, 1973. 7. Furman, S., Parker, B., Krauthamer, M., and Escher,

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D. J. W.: The influence of the electromagnetic environmerit on the performance of artificial cardiac pacemakers, Ann. Thorac. Surg. 6:90, 1968. 8. Smyth, N. P. D., Keshishian, J. M., Hood, O. C., Hoffman, A. A., Baker, N. R., and Podolak, E.: Effect of an active magnetometer on permanently implanted pacemakers, J. A. M. A. 221:162, 1972.

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