Myocardial preservation by topical hypothermia
Cardiac operations are facilitated by a still and bloodless field provided by arrest of both mechanical cardiac activity and coronary perfusion. The magnitude of ischemic damage to the heart during periods of aortic cross-clamping without coronary perfusion is reduced substantially by lowering myocardial temperature and by sustained membrane depolarization with potassium ion. Topical myocardial hypothermia alone, using 0.9% saline at 3 to 4 C, is capable of consistently providing protective levels of left ventricular midwall hypothermia in the range of 15 to 20 C. Meticulous attention to details of local cooling, however, is necessary to avoid large swings or regional gradients in myocardial temperature. Cooling is initiated immediately after cross-clamping by lavage of the pericardial reservoir with 2 liters of cold saline and is continued by infusion of saline into the pericardial sac at 100 to 150 ml/min. Immersion of both ventricles in cold saline is enhanced by orientation of the operating table, and additional myocardial cooling is achieved by endocardial lavage during procedures involving cardiotomy. Low flow cardiopulmonary bypass (40 to 50 ml/kg/min) and systemic hypothermia in the range of 31 to 33 C minimize the warming effect of collateral coronary circulation via mediastinal and bronchial vessels. Complete right heart emptying is assured by double venous cannulation. During a single cross-clamp interval, valve replacement(s), ventricular resection, and all distal coronary anastomoses are carried out.
Left ventricular intramyocardial temperatures decrease gradually over a 5- to 10-minute period after initiation of cooling, and in the anterolateral left ventricular midwall temperatures in . . .