Ischemic Preconditioning Reduces Infarct Size in Swine Myocardium

We evaluated the hypothesis that stunning swine myocardium with brief ischemia reduces oxygen demand in the stunned region and increases tolerance of myocardium to longer periods of ischemia. Wall function was quantified with ultrasonic crystals aligned to measure wall thickening, and stunning was achieved with two cycles of left anterior descending coronary artery (LAD) occlusion (10 minutes) and reperfusion (30 minutes), after which the LAD was occluded for 60 minutes and reperfused for 90 minutes. Infarct size (as a percent of risk region) was then determined by incubating myocardium with para-nitro blue tetrazolium. Regional oxygen demand was measured as myocardial oxygen consumption before the 60-minute LAD occlusion in the stunned region; tracer microspheres were used to determine blood flow, and blood from the anterior interventricular vein and left atrium was used to calculate oxygen saturations. After the second reperfusion period, wall thickening in the stunned region was reduced to 1.4±2.4% compared with 36.7±2.5% (mean±SEM) before ischemia (p < 0.001). Regional myocardial oxygen consumption after stunning (3.1±0.7 ml O2/min/100 g) was no different from regional myocardial oxygen consumption before stunning (3.7±0.6 ml O2/min/100 g). In the nine pigs “preconditioned” by stunning, infarct size was 10.4±6.3% of the risk region compared with 48.0+12.7% in the six control pigs subjected to 60 minutes of ischemia without prior stunning (p < 0.005). The risk regions were similar (14.4±1.5% vs. 14.6±1.9% of the left ventricle, preconditioned vs. control pigs, respectively). We conclude that stunning swine myocardium with two cycles of a 10-minute LAD occlusion followed by reperfusion increases ischemic tolerance but that changes in regional demand in stunned myocardium do not predict the marked reduction in infarct size that follows a subsequent 60-minute period of ischemia.