Four brief periods of myocardial ischemia cause no cumulative ATP loss or necrosis.

The effect of repetitive periods of coronary occlusion on myocardial adenine nucleotides, lactate, and infarct size was studied. In one series of dogs, the circumflex artery was occluded for one, two, or four 10-min episodes, each separated by 20 min of reperfusion. Hearts were excised and sampled for metabolic assays after one or more periods of ischemia before or after reperfusion. One 10-min period of ischemia caused a 61% loss of ATP and 41% loss of adenine nucleotides from the most severely ischemic subendocardial zone. Reperfusion resulted in rapid restoration of the adenylate charge but in only slight repletion of the adenine nucleotide pool. However, two or even four 10-min periods of ischemia caused no further adenine nucleotide loss. In contrast, 40 min of continuous coronary occlusion caused an 87% depletion of ATP and 67% of the adenine nucleotide pool from the same subendocardial region. Collateral blood flow was similar during all occlusions, but lactate accumulation was less during later occlusions. In a second series of experiments, myocardial necrosis was quantitated 4 days after four 10-min periods of ischemia. Necrosis was observed in only one of six dogs and, in this dog, was only 1.5% of the anatomic area at risk. Thus intermittent reperfusion prevents cumulative metabolic deficits and myocardial ischemic cell death, perhaps by restoring the capacity for high-energy phosphate (HEP) production and/or washing out deleterious catabolites. A first episode of ischemia also slows HEP utilization in subsequent episodes.