Tachycardia and bradycardia dependent bundle branch block after acute myocardial ischaemia

Critical observation of the onset and course of transient bundle branch block after acute myocardial ischaemia in 5 patients showed that the block was initially demonstrated both by increasing heart rate (tachycardia dependent block) and slowing (bradycardia dependent block), with normal intraventricular conduction at intermediate heart rates. In some patients normal conduction disappeared within 2 to 12 hr with bundle branch block present at all heart rates, and reappeared again within 24 hr with block occurring at both higher and lower rates. Three patients were followed for 3 to 7 mth and showed that the critical rates for both tachycardia and bradycardia dependent block altered in such a way that the range of heart rates associated with normal conduction gradually increased until the block could no longer be elicited at the heart rates studied. It is suggested that tachycardia and bradycardia dependent bundle branch block after acute myocardial ischaemia can be explained by characteristic changes in the electrophysiological properties of two groups of Purkinje fibers which may be due to a differential localization of metabolites released in the ischaemic zone. Contrary to previous reports, these findings suggest the presence of dissociation between enhanced automaticity of the pacemaker cells and bradycardia dependent conduction disturbances. The various mechanisms previously invoked to explain bradycardia dependent conduction disturbances are critically reviewed with special reference to cases of bradycardia dependent paroxysmal atrioventricular block.