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 on increasing heart rate (tachycardia-dependent block) and on slowing (bradycardia-dependent block), with normal intraventricular con- duction at intermediate heart rates.In some patients normal conduction disappeared within 2 to 12 hours with bundle-branch block present at all heart rates, to reappear again within 24 hours with block occurring at both higher and lower rates.Three patients were followedfor 3 to 7 months 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 fibres which may be due to differential localization of metabolites released in the ischaemic zone.Contrary to previous reports our findings suggest the presence of dissociation between enhanced automa- ticity of the pacemaker cells and bradycardia-dependent conduction disturbances.The various mechanisms that have been previously invoked to explain bradycardia-dependent conduction disturbances are critically reviewed with special reference to cases of bradycardia-dependent paroxysmal atrioventricular block.