Survival of Subendocardial Purkinje Fibers after Extensive Myocardial Infarction in Dogs

Alterations in cardiac electrophysiology that accompany myocardial infarction were studied in dogs subjected to a two-stage ligation of the anterior descending coronary artery. A multipolar transmural needle electrode was used to record electrical activity from the in situ infarcted heart 24 hours after coronary occlusion. Bipolar electrograms recorded from subendocardial regions of infarcted myocardium demonstrated the persistence of early, rapid deflections suggesting Purkinje fiber activity; evidence of ventricular muscle activity in the infarct was absent in both subendocardial and intramural electrograms. The infarcted myocardium and the adjacent non-infarcted tissue were then excised and studied with intracellular microelectrodes in vitro. Transmembrane action potentials could be recorded from one or two cell layers of subendocardial Purkinje fibers at all sites within the infarcted region, but no ventricular muscle action potentials were found. Subendocardial Purkinje fibers which survived in the infarct had reduced maximum diastolic potentials, action potential amplitudes, and maximum depolarization velocities compared with normal subendocardial Purkinje fibers; also, action potential durations in these surviving fibers were extraordinarily prolonged. Spontaneous diastolic depolarization was evident in some surviving fibers. Since subendocardial Purkinje fibers that generate abnormal action potentials survive in an infarct, these fibers may participate in the genesis of ventricular arrhythmias that accompany infarction.