Relation between monophasic action potential duration, ST segment elevation, and regional myocardial blood flow after coronary occlusion in the pig

There is mounting interest in the use of the monophasic action potential for electrophysiological investigation of regional myocardial ischaemia. There is, however, no systematic study of the relation between the changes in monophasic action potential and regional myocardial blood flow. In this study monophasic action potential and, for comparison, epicardial electrograms were recorded by suction electrode at 14 selected sites on the anterior surface of the normal left ventricle of the intact pig heart in situ (n = 16). The monophasic action potential duration varied across the left ventricle; it was shortest close to the left anterior descending coronary artery, increased laterally across the left ventricle, and was longer at the apex than at the base. This variation was independent of regional myocardial blood flow (measured with radiolabelled microspheres). After coronary occlusion, in nine of these pigs monophasic action potential duration significantly decreased in proportion to the reduction in regional myocardial blood flow. The ST segment of the epicardial electrogram was elevated in relation to the reduction in regional myocardial blood flow but correlated less well. The reduction in regional myocardial blood flow of approximately 50% produced maximal ST elevation. The monophasic action potential could be recorded from some sites within the border zone for up to 5 h but could not be recorded from the centre of an ischaemic zone. With ischaemic episodes of this duration there was, however, a return to baseline of the elevated ST segment for all sites within the compromised region. Monophasic action potential duration, unlike epicardial electrogram ST elevation, discriminates between central ischaemic and non-ischaemic and ischaemic sites on either side of the zone with a reduced blood flow. The results suggest that monophasic action potential duration measured by suction electrode may be a simple and reliable index of transmural-epicardial myocardial ischaemia in experimental studies and in surgery.