Re-entrant ventricular arrhythmias in the late myocardial infarction period. 3. Manifest and concealed extrasystolic grouping.

Re-entrant beats with regular extrasystolic grouping were seen in 44- of dogs 3--7 days following ligation of the anterior descending coronary artery. Utilizing direct recording of the re-entrant pathway (RP) from the epicardial surface of the infarction zone, we found extrasystolic grouping to be based on the infarction tachycardia-dependent conduction disorders in a potentially RP. Trigeminy and quadrigeminy were related, respectively, to a 3:2 and 4:3 Wenckebach-like conduction cycle in a RP. However, quadrigeminy could also be due to an underlying bigeminal rhythm with concealment of alternate re-entrant beats, i.e., concealed bigeminy. A bigeminal rhythm was the result of a 2:1 conduction pattern in a re-entrant pathway with a sufficient degree of conduction delay during the conducted beat of the 2:1 cycle to result in re-entry. A trigeminal or quadrigeminal rhythm could change to a bigeminal rhythm on critical shortening of the cardiac cycle. Fixed and variable coupling were related, respectively, to stable and changing conduction pattern in a re-entrant pathway. On the other hand, extrasystolic grouping could be concealed due to either block in the re-entrant pathway or entrapment in a small area of the closely bordering normal zone.