Circadian variations of electrical properties of the heart.

Cardiac refractoriness and electrical inducibility of supraventricular tachycardia (SVT) were assessed at intervals of 1 to 2 hours over a period of 22 hours in 38 patients (25 with paroxysmal SVT). Daily variability of effective refractory period (ERP) had a mean range of 35 +/- 14 mseconds for the atria, 79 +/- 65 mseconds for the AV node, 24 +/- 10 m seconds for the ventricles, 51 +/- 27 mseconds for the retrograde Kent bundle, and 35 +/- 17 mseconds for the antegrade Kent bundle. Between 11 PM and 8 AM there was a significant prolongation of the ERP of the atria (ANOVA, p less than 0.001), AV node (p = 0.002), right ventricle (p less than 0.001), and retrograde Kent bundle (p = 0.005), and a reduction in electrical induction of SVT from 75% to 42% (p less than 0.05) with respect to the first electrophysiologic study. This nocturnal prolongation was preceded by ERP shortening of all explored cardiac sites and by increased tachycardia inducibility between 8 and 10 PM. Six patients with dual AV nodal pathways showed a prolongation of the fast pathway ERP at midnight, whereas conduction through the slow pathway followed an unpredictable daily variability. These data indicate a circadian influence on refractoriness of normal cardiac tissues and accessory pathways that exerted a midnight protection against electrical inducibility of reciprocating tachycardia but a transient arrhythmia facilitation at the evening.