Calcium and cardioplegic protection of the ischemic immature heart: impact of hypoxemia from birth.

Because many infants who require cardiac operation have cyanotic heart disease, we determined whether the existing calcium content of St. Thomas' II solution (1.2 mmol/L) is optimal to protect the immature rabbit heart hypoxemic from birth during subsequent ischemia. Modified hypothermic St. Thomas' II solutions (calcium content, 0 to 2.4 mmol/L) were compared with hypothermic Krebs bicarbonate buffer in protecting chronically hypoxemic (PaO2 = 34 +/- 11 mmHg, SaO2 = 63% +/- 3%) versus normoxemic (PaO2 = 76 +/- 11 mmHg, SaO2 = 92% +/- 3%) immature hearts (7 to 12 days old) during ischemia. Hearts (n = 6 per group) underwent aerobic 'working' perfusion with Krebs bicarbonate buffer and cardiac function was measured. The hearts were then arrested with a 3 minute infusion of either cold (14 degrees C) Krebs buffer (1.8 mmol calcium/L) as hypothermia alone or modified St. Thomas' II solution before 6 hours of hypothermic (14 degrees C) global ischemia. Hearts were reperfused and postischemic enzyme leakage and recovery of function were measured. A bell-shaped dose-response profile was observed for recovery of postischemic aortic flow but not for postischemic creatine kinase leakage, with improved protection occurring at lower calcium concentrations. Optimal myocardial protection occurred at a calcium content of 0.4 mmol/L, which was significantly better than with hypothermia alone or standard St. Thomas' II solution. We conclude that the existing calcium concentration of St. Thomas' II solution is responsible, in part, for its inadequate protection of immature myocardium hypoxemic from birth during ischemia.