CASE REPORTS Arrival of Excitation at the Right Ventricular Apical Endocardium ill Wolff-Parkinson-White Syndrome Type

His bundle and bipolar right ventricular apex (RVA) and right ventricular outflow tract (RVOT) catheter electrograms taken from sites 1 mm apart were recorded simultaneously with various surface leads in two patients with Wolff-Parkinson-White syndrome (WPW) type B. In "fusion " beats the presence of a normal H-RVA interval indicated that the apex of the right ventricle was activated by the impulse emerging from the right branch. On the other hand, a shorter-than-normal H-RVA interval implied that the RVA was depolarized by the activation front propagating from the pre-excited site. When this occurred, the V-RVA intervals gave a rough estimate of conduction time from pre-excited area to RVA. The values obtained (40 and 50 msec, respectively) were shorter than in two other patients with WPW type A. The arrival of excitation patterns in the right ventricular endocardium were similar in WPW type B and in beats produced by RVA stimulation but differed markedly from that of left an-terior hemiblock even when the surface electrocardiographic leads showed abnormal left axis deviation in all instances. This resemblance between ventricular complexes attributed to WPW type B and those resulting from stimulation of an inferior (apical) site suggests, but does not prove, that the impulses propagated from an equivalent region of the right ventricle. These si