Myocardial stunning in dogs: preconditioning effect and influence of coronary collateral flow

In open-chest dogs the left anterior descending (LAD) coronary artery diagonal branch was encircled with a pneumatic occluder. Pairs of ultrasonic crystals were inserted into LAD myocardium and remote normal muscle. The coronary artery was occluded for 5 minutes, followed by 10 minutes of reperfusion. This occlusion-reperfusion cycle was repeated 12 times, and after a final 90-minute reperfusion period the hearts were removed and stained with triphenyltetrazolium chloride. No heart had evidence of necrosis. Baseline shortening normalized for end-diastolic length averaged 10.4 +/- 1.0% in the LAD area and 7.4 +/- 0.8% in the remote normal myocardium. When analyzed as a percentage of baseline, segment shortening in the normal myocardium was not significantly altered by LAD occlusion and reperfusion. In contrast, during occlusions the LAD myocardium paradoxically lengthened. With the initial reperfusion, shortening was significantly depressed to 28.6 +/- 8.6% of baseline. Although with subsequent reperfusions the return of function progressively decreased, the rate of deterioration was markedly attenuated after the first occlusion. By the end of the protocol many LAD segments lengthened paradoxically even after reperfusion, but in five hearts in which active contraction was preserved there was no significant change in regional function after the third cycle, suggesting a protective or preconditioning effect of earlier ischemia. There was a moderately good correlation between collateral blood flow and the degree of dysfunction following the initial 10-minute reperfusion (r = -0.73). This correlation deteriorated during subsequent reperfusion periods, implying that collateral blood flow can be a predictor of the extent of myocardial stunning only after the initial one or two reperfusion cycles. Thereafter other as yet unidentified factors make baseline collateral flow unimportant.