Contribution of His bundle recordings to analysis of abnormal beats with right bundle-branch block--superior axis pattern.

His bundle electrograms were recorded in four patients with ectopic rhythms showing a right bundle-branch block superior axis pattern. In one case what appeared to be a slow idioventricular rhythm was identified as a His bundle rhythm with transient right bundle-branch block and left anterior hemiblock. The second patient had an idioventricular tachycardia. H deflections were not seen in front of the corresponding QRS complexes. Impulse formation probably occurred in .the posteroinferior wall of the left ventricle. To test this assumption regarding the pacemaker site, the latter area was stimulated through a catheter introduced into the middle cardiac vein. Extrasystoles, thus induced, showed a superior axis deviation with predominant R waves in the right chest leads and a VH interval of 50 msec. The ectopic beats observed in the third patient