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 A V 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 A V 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 A V conduction patterns as well as the nature of the wide QRS complexes seen during the tachyarrhythmias.Factors regulating the ventricular rates during atrial flutter and fibrillation in patients with pre-excitation (WPW) syndrome are not well known. Previous studies from our department suggested that the AV conduction patterns occurring during these tachyarrhythmias depended on the duration of the effective refractory period of the accessory pathway and on its relation to that of the normal pathway (Castellanos et al., 1973).The present communication corroborates these assumptions.

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