Smaller infarct after preconditioning does not predict extent of early functional improvement of reperfused heart

We evaluated the ability of ischemic preconditioning to restore function to salvaged myocardium in rabbits. Although ischemic preconditioning reduces infarct size, few investigators studying recovery of function after coronar y occlusions lasting greater than or equal to 30 min have reported any mech anical benefit in preconditioned hearts. However, because myocardial functi on was seldom evaluated beyond 5 h after reperfusion stunning may have mask ed the benefit. Accordingly, rabbits were chronically instrumented with a p neumatic occluder around a branch of the left coronary artery, a pair of l- mm ultrasonic crystals in the myocardial territory destined to become ische mic, and electrocardiogram (ECG) leads. One week after surgery the ECG and segment length tracing were recorded at rest, during 30-min occlusion and 1 h of reflow and again at 24, 48, and 72 h. In ischemically preconditioned rabbits, 5-min coronary occlusion and 10-min reperfusion preceded the long occlusion. The beginning and end of systole were determined by recording th e first and second heart sounds with a hand-held precordial microphone. Pos tmortem infarct size was measured with triphenyltetrazolium chloride. Durin g the 30-min coronary occlusion all segments became nearly akinetic or bulg ed during systole. After 60 min of reflow there was little return of functi on in either group. Between 24 and 72 h there was minimal recovery in the c ontrol group (segment shortening equals 13.3 +/- 4.1% of baseline), whereas function was much better in preconditioned hearts (44.2 +/- 7.4% of baseli ne, P < 0.02). Infarct size as a percentage of risk zone was much smaller i n preconditioned hearts (10.2 +/- 1.4 vs. 29.7 +/- 1.8%, P < 0.001). Thus t here is a gradual recovery of systolic function of reperfused myocardium af ter a coronary occlusion. Although early mechanical recovery is significant ly better after preconditioning, it is much less than would be predicted by the reduction of infarct size.