Daily variability of electrically induced reciprocating tachycardia in patients with atrioventricular accessory pathways.

Sequential bedside electrophysiologic testing was performed over 22 hours at intervals of 1 to 2 hours in 13 patients with left-sided Kent bundles to assess possible daily variations in the capacity to electrically induce reciprocating tachycardia. In all patients the tachycardia involved the accessory pathway in retrograde atrial activation. Between midnight and early morning the more relevant findings with respect to the first testing performed at midday were a significant prolongation of the effective refractory period of the atrial (from 212 +/- 22 msec to 229 +/- 22 msec; p less than 0.01), atrioventricular node (from 235 +/- 22 msec to 285 +/- 15 msec; p less than 0.005), right ventricle (from 209 +/- 15 msec to 221 +/- 12 msec; p less than 0.001), and retrograde Kent bundle (from 278 +/- 34 msec to 294 +/- 24 msec; p less than 0.01) and a reduction of the inducibility of tachycardia from both the coronary sinus from 90% to 50%; p less than 0.001) and the right ventricle from 80% to 15%; p less than 0.001). Thus our results indicate that there exists a nocturnal protection against electrical induction of reciprocating tachycardia that is associated with a prolongation of the atrial, atrioventricular nodal, ventricular, and Kent bundle refractoriness.