Factors regulating ventricular rates during atrial flutter and fibrillation in pre-excitation (Wolff-Parkinson-White) syndrome.

Premature atrial stimulation and His bundle recordings were performed in 6 patients with pre-excitation (WPW) syndrome. The ventricular rates during bursts of atrialflutter and atrialfibrillation triggered by the earliest impulses depended on the functional properties of the accessory pathway. In 3 patients in whom the effective refractory period of the accessory pathway was longer than that of the normal AV pathway the ventricular rates did not exceed 2IO/min; the majority of the supraventricular impulses reached the ventricles through the normal AV pathway. In 3 other patients in whom the effective refractory period of the accessory pathway was very short (less than 220 msec) the ventricular rates were as high as 3IO/min; the majority of the impulses reached the ventricles through the accessory pathway. Concealed AV conduction occurred in both pathways producing RR pauses well in excess of the duration of the corresponding effective refractory periods. These tachyarrhythmias were due to stimulation during the atrial vulnerable period, not to a reciprocating mechanism involving two separate anatomical AV communications. Intracardiac recordings were necessary to evaluate the AV conduction patterns as well as the nature of the wide QRS complexes seen during the tachyarrhythmias.