Second-degree atrioventricular block in the His-Purkinje system following acute myocardial infarction. Clinical observations on its evolution.

The evolution of second-degree atrioventricular block in the His-Purkinje system was studied in four patients with acute myocardial infarction, utilizing continuous electrocardiographic monitoring and serial His bundle recordings. Three patients had anterior wall infarction; all developed acute right bundle-branch block and two also showed a left anterior hemiblock one to four hours prior to onset of second-degree atrioventricular block. The fourth patient had an old anteroseptal infarction and chronic right bundle-branch block and left anterior hemiblock; he developed second-degree atrioventricular block following an acute lateroposterior infarction. His bundle electrograms were obtained during the insertion of a temporary ventricular pacemaker within the first 24 hours of onset of chest pain. A second His bundle electrogram was obtained during removal of the temporary paceemaker 7 to 14 days after the first recording. In all cases the block was localized in the His-Purkinje system. In three patients the first His bundle recording showed second-degree atrioventricular block with minimal increment of delay in conduction (3 to 10 msec) prior to the blocked beat (equivalent of Mobitz type 2 block). The second His bundle electrogram showed the development of significant increment of delay in conduction (35 to 210 msec) prior to block (equivalent of Wenckebach conduction). The fourth patient illustrated the reverse order of the observations seen in the other three. Three patients showed resumption of 1:1 atrioventricular conduction; however, when one patient was challenged with rapid atrial pacing, he developed repetitive block in the His-Purkinje system.