His bundle recordings in paroxysmal atrioventricular block produced by carotid sinus massage.

Electrograms from the bundle of His were recorded in two patients with acute myocardial infarction during paroxysmal atrioventricular block induced by carotid sinus massage. From the viewpoint of the surface electrocardiogram this arrhythmia was considered as a form of Type II (Mobitz) Ulo:k, the onset of which was related to a decrease in atrial rate. In the first patient, the conduction disturbance appeared at the AH, and in the second patient at the HV, level. Paroxysmal atrioventricular block could be explained by assuming the existence of: (a) vagal effects at the atrioventricular node or upper His bundle (Case I) and at 'lower' His bundle (Case 2); or (b) phase 4 block related to diastolic depolarization in 'upper' (Case i) and 'lower' (Case 2) parts of the His bundle; or (c) vagal effects at the atrioventricular node (Case I) and phase 4 block due to diastolic depolarization within the bundle of His (Case 2).