Supernormal myocardial excitability and conduction (author's transl)

The phenomenon of supernormality of myocardial excitability and conduction has been recognized for many years but somewhat neglected in recent literature. Supernormal excitability can be demonstrated occasionally in patients with pacemakers and high myocardial thresholds at an interval of 400 to 520 ms after the R-wave. Supernormal excitability is a prerequisite for supernormal conduction which may be observed to follow conducted sinus beats as well as beats of ectopic origin. In the latter case, retrograde concealed conduction must be assumed to be operative. The basis for the detection of supernormality in only occasional, but not all, similarly predisposed patients remains unestablished. The differential diagnosis of supernormality may pose a difficult electrocardiographic problem, the analysis of which should include consideration of the following: simultaneous occurrence of both premature atrial and ventricular beats, AV-dissociation with capture beats, longitudinal dissociation giving rise to varying AV-intervals and bilateral bundle branch block.     

Supernormal conduction in the accessory pathway of patients with overt or concealed ventricular pre-excitation

It was recently shown that supernormal conduction in the diseased His-Purkinje system is more common than previously thought, and is always associated with prolongation of refractoriness. To assess whether supernormal conduction could also occur in the accessory pathway of patients with ventricular pre-excitation, 21 patients with manifestly prolonged refractoriness in the accessory pathway were studied. Under these conditions, programmed atrial stimulation revealed a phase of supernormal conduction in 16 (76%) of the 21. Therefore, what was believed to be a nonexistent or exceptional physiologic event was shown to be a rather common finding, at least under certain circumstances. Supernormal conduction occurred in all 7 patients with an anterograde refractory period of 480 to 980 ms, and in 5 of 10 patients with a refractory period greater than 1.0 second or with no anterograde conduction. Supernormal conduction could not be demonstrated in four patients with a refractory period less than or equal to 440 ms, but appeared in all four patients after the refractory period was prolonged by a rapid rate of stimulation or administration of ajmaline. The electrophysiologic changes underlying the occurrence of supernormal conduction in the accessory pathway are similar to those previously reported for the bundle branch system. The demonstration of supernormal conduction in the accessory pathway may uncover the presence of concealed ventricular pre-excitation. Supernormal conduction over the accessory pathway may facilitate a rapid ventricular response during atrial fibrillation, even if the refractory period is prolonged.     

Double ventricular parasystole. Supernormal phase of conduction as a mechanism of intermittent parasystole. Report of a case.

A rare case of spontaneous double ventricular parasystole was studied in depth, together with a critical review of similar cases in the literature. The discussion was focused on 1) the variation of the shortest interectopic interval (SIEI), 2) entrance block and its failure, 3) supernormality as a mechanism of intermittence, and 4) effects of lidocaine and atropine on such an arrhythmia. In double ventricular parasystole a greater than usual variation in the SIEI tended to occur in one of the two parasystolic groups. If, however, such variations were too great in the face of otherwise parasystolic rhythm, presence of intermittence was confirmed. A temporary loss of the entrance block was deemed primarily responsible for the intermittency. That is to say, invasion, discharge, and resetting of one parasystolic focus by another parasystolic impulse during the supernormal phase of the ventricle was considered the cause of an intermittence. In a strict sense, this is the first report in the literature in which the supernormality was clearly indicated as one mechanism of intermittent ventricular parasystole. The advantage of the concept of double ventricular parasystole as compared to single parasystole in defining such a mechanism is stressed.