Ventricular activity during atrial fibrillation. Studies with His bundle electrography.

His bundle electrograms were recorded in 12 patients with atrial fibrillation. In all cases but one, the dominant rhythm was atrial fibrillation. In 1 case the dominant rhythm was fascicular tachycardia due probably to digitalis overdosage. However ventricular, fascicular and junctional beats could be often recognized on His bundle tracings, even when QRS configuration was similar to supraventricular conducted beats due to atrial fibrillation. In the patients in whom sinus rhythm occurred, the H-V interval was unchanged. Although it is admitted that during atrial fibrillation the zone of concealment of A-V conduction is in the A-V nodal area, concealed penetration of atrial impulses in the fascicular system may be encountered in patients not treated with digitalis.