Significance of discordant ST alternans in ventricular fibrillation. With the use of epicardial mapping, we investigated the electrical alternans of the ST segment during acute myocardial ischemia and studied the difference in ST alternans between dogs with resultant ventricular fibrillation and those without it. During the 7-minute occlusion of the left anterior descending coronary artery below its first diagonal branch, 60 epicardial unipolar electrograms were recorded simultaneously at 1-minute intervals by a computerized mapping system. ST alternans was found in the eight dogs we observed. The amplitude of ST alternans (difference in the ST segment elevation of two consecutive electrograms) was greater in dogs with ventricular fibrillation (n = 4) than in those without it (n = 4) (3.92 +/- 1.24 versus 0.58 +/- 0.49 mV, p less than 0.05). Three of the four dogs with ventricular fibrillation demonstrated discordant ST alternans (i.e., adjacent leads were out of phase). Results from the present study indicate that an increased amplitude and discordance of ST alternans during acute myocardial ischemia are related to ventricular fibrillation and act as indicators of time and spatial unevenness of ventricular repolarization. 