Differentiation between parasystole and extrasystoles. Influence of vagal stimulation on parasystolic impulse formation.

Recently, it has been shown that when a sinus impulse falls late in the parasystolic cycle, it usually hastens the next ectopic discharge. Thus, in many cases, the classic criteria for the diagnosis of parasystole (ie, varying coupling intervals and constant shortest interectopic intervals) cannot be used. To differentiate between parasystole and extrasystoles in such cases, the influence of vagal stimulation on parasystolic impulse formation was investigated in seven cases of "true" parasystole in which one or more "pure" ectopic cycles without any intervening nonectopic QRS complexes were found spontaneously. In all cases pure ectopic cycles were found during sinus arrest caused by vagal stimulation; namely, none of the cases showed extreme prolongation of the parasystolic cycle. These results strongly suggest that instead of the classic criteria, vagal stimulation causing temporary sinus arrest is the optimal method for differentiation between parasystole and extrasystoles in cases without spontaneous pure ectopic cycles.