Analogy of Electronic Pacemaker and Ventricular Parasystole with Observations on Refractory Period, Supernormal Phase, and Synchronization

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Abstract
The artificial pacemaker has been compared to a physiologic ventricular parasystolic focus. In hearts being driven by a pacemaker, in the absence of atrioventricular (AV) block, capture (interference) phenomena are constantly seen. While variations in QRS complexes have been observed when the external pacemaker stimulus occurs early in the T-wave period of a sinus conducted beat, no sequence of aberrant beats has been observed. One patient showed occasional sinus beats interpolated between ventricular beats arising from the artificial pacemaker, and these occurred at a very narrow time band suggesting that AV conduction was permitted by phenomenon of a supernormal phase. At a later date this patient showed also retrograde conduction believed related to a supernormal phase during recovery of junctional tissue penetrated by an impulse entering from above, though the impulse itself was blocked. Records of two patients are used to illustrate grossly aberrant QRS complexes produced by the stimulus of the electric pacemaker when this fell in the semirefractory period. Noteworthy was the presence of a latent period before potentials of the propagated impulse were recorded, there being evident stimulus-to-QRS delays. The situation wherein the rhythm is most chaotic is that where AV block is absent, the sinus rate is fast, and the electric pacemaker rate is relatively slow. In two patients, retrograde activation of the atria occurred as a stable mechanism. In one patient, nodal rhythm of the RP type occurred, and sequences of the records suggested that the AV node might have become synchronized with the extrinsic pacemaker cycle. In this last instance three independent centers of effective impulse formation coexisted, the sinus node, AV node, and stimulating electrodes of the electronic pacemaker. A marked increase in the refractory period of the ventricle, as measured by its response to the set stimulus of the electric pacemaker delivered at varying instants with respect to the sinus mechanism, occurred in one case within a few days. The possibility that reserpine therapy contributed to this change is possible but has not been established, for the effective refractory period, as measured, later remained constant over a period of many weeks.