Temporal relation of epicardial electrographic, contractile and biochemical changes after acute coronary occlusion...

The increasing use of changes in the S-T segment of local epicardial electrograms to quantitate myocardial infarct size has led to the need for a better understanding of this method. Accordingly, we studied the local electrographic, tension and biochemical changes that occurred after coronary occlusion and subsequent reperfusion in 44 dogs using epicardial electrograms from 10 to 12 sites, Walton-Brodie strain gauge arches and myocardial ratios of potassium ion to sodium ion (K+/Na+). After coronary occlusion for 1 hour, total S-T segment elevation increased from 10.2 +/- 2.4 to 78.3 +/- 13.7 mv (P less than 0.001) and tension development decreased to 63.6 +/- 7.0% of control value (P less than 0.001); occlusion for 3 hours resulted in a total S-T segment elevation increase from 5.8 +/- 3.4 to 56.7 +/- 8.7 mv (P less than 0.001) and a tension decrease to 61.4 +/- 5.3% (P less than 0.001) of control value. After reperfusion two types of response were observed. In nine experiments new local pathologic Q waves appeared in an average of 5.3 of 8.2 ischemic electrode sites within 5 to 10 minutes of reperfusion concomitant with a marked further decrease in total tension from 67.3 +/- 5.5% to 42.4 +/- 6.0% of control value (P less than 0.001). Simultaneously, total S-T elevation decreased from 66.1 +/- 8.2 to 25.3 +/- 3.4 mv (P less than 0.001). In seven experiments no Q waves appeared after reperfusion and there was no significant change in tension. Total S-T elevation again decreased from 58.3 +/- 12.7 to 27.1 +/- 5.7 mv (P less than 0.025). When normal saline solution was perfused distal to the coronary arterial occlusion total S-T elevation decreased from 68.0 +/- 3.6 to 36.3 +/- 5.2 mv (P less than 0.001). After 3 hours of coronary occlusion, myocardial K+ decreased and Na+ increased in the ischemic zone, resulting in a significant decrease in the K+/Na+ ratio (P less than 0.005). Reperfusion for 2 hours resulted in a further depletion of K+ and an increase in Na+ with a resultant complete reversal of the K+/Na+ ratio (P less than 0.001). In summary, after reperfusion the S-T segment abnormalities rapidly decreased in all experiments despite the appearance of new Q waves in more than half of these studies concomitant with either a decrease or no change in contractile ability and continuing myocardial K+ loss and Na+ accumulation. S-T segment mapping therefore appears to be of limited value in assessing the effect of reperfusion on infarct size. The decrease in S-T segments that occurred with perfusion of either blood or saline solution suggests a "washout" phenomenon.