Effect of temperature during potassium arrest on myocardial metabolism and function

Past studies have not established the optimal myocardial temperature range for hyperkalemic arrest but have generated controversy regarding the safety of exposing the myocardium to more profound levels of hypothermia. We therefore used the isolated working rat heart model of ischemic arrest to study the metabolic and functional effects of cardioplegia at the full range of temperatures pertinent clinically. Experimental conditions were designed to reliably control and maintain myocardial temperature during the 60 minute arrest period. We found that nearly full recovery of function occurred when hearts were arrested at or below 16°C. High-energy phosphate levels measured immediately after arrest were better maintained at 4° and 8°C, despite evidence of decreased glycolysis. When measured after the recovery period, high-energy phosphate levels returned to somewhat less than control levels in all groups arrested at or below 24°C. Myocardial glucose utilization was best preserved in hearts arrested at or below 12°C. We found no evidence that greater myocardial edema resulted from arrest at colder temperatures. Severe and permanent damage was observed when hearts were arrested at or above 28° C. In this model, therefore, the best overall metabolic and functional protection occurred when hearts were maintained at 12°C or below during potassium-induced cardioplegia. Our results support the idea that cold injury to the heart does not occur and that colder temperatures provide better protection from ischemic myocardial injury.