Transmembrane potential changes and ventricular fibrillation during repetitive myocardial ischaemia in the dog.

Electrophysiological changes occurring soon after experimental coronary occlusion have been examined in 13 dogs by combined recording of epicardial action potential, endocardial-epicardial conduction time, and epicardial ST segment elevation during successive 5-minute occlusions of the left anterior descending coronary artery. Mter initial coronary occlusion, action potential duration shortened from 182 ± 9 ms to 113 ± 9 ms at 2 minutes and conduction time was prolonged from 24 i 1 ms to 71 ± 17 ms associated with progressive ST segment elevation. These changes were followed by the appearance of potentials of 'slow response' type morphology, alternans of action potential duration and amplitude, alternans of epicardial and endocardial ST segment elevation, and intermittent conduction block. Ventricular fibrillation was preceded by action potential alternans in 95 per cent and by conduction block in 65 per cent of instances and occurred at a time of maximum conduction delay. These data strongly support the concept of re-entry in the pathogenesis of early ventricular fibrillation. Changes in action potential duration and conduction time were significantly greater during the first occlusion compared with subsequent occlusion periods. There was no significant difference however between data taken from occlusions 2, 3, 4, or 5. This questions the validity of experimental studies of myocardial ischaemia based on comparisons of sequential coronary occlusions using the first as the control.