[Paradoxical atrioventricular block within the bundle of His].

We have been able to record an atrio-ventricular block dependent upon bradycardia, but also no tachycardia, in a patient of 76 who presented with syncopal attacks. The basal electrocardiogram showed no significant delay in atrio-ventricular or intraventricular conduction. Intracavitary electrophysiological investigation allowed us to localise the site of this block in the main trunk of the bundle of His; we also established, under basal conduction conditions, a first degree block with an exactly similar relationship to the duration of the preceding diastole.