Myocardial damage caused by keeping pH 7.40 during systemic deep hypothermia

Abstract

With rare exception, hypothermia is routine during cardiac operations and pH (measured at 37° C) is kept at 7.4. This clinical constraint does not occur in nature, for blood pH in ectotherms varies in concordance with a temperature-dependent neutrality point of water. This study tests the hypothesis that keeping pH 7.4 during hypothermia produces a degree of myocardial damage and limitation of effectiveness of cardioplegic protection which is avoidable by appropriate pH management. In 12 puppies, body temperature was lowered to 22° C with surface hypothermia, then to 17° C with extracorporeal circulation. During 60 minutes of circulatory arrest all hearts were protected with the same multidose K+ cardioplegic solution. In six dogs pH was kept at 7.4 and in six others pH was varied as in ectotherms (i.e., 7.9 at 17° C), principally by adjusting P co 2 during cooling and rewarming. During surface cooling, keeping pH at 7.4 caused inadequate cardiac output (hypotension, systemic lactic acidosis, 11% ± 5% production). Conversely, pH adjustment allowed 25%* higher cardiac output with normal systemic lactate metabolism. Keeping pH 7.4, P co 2 40 mm Hg at 22° C caused cerebral blood flow to fall 75%* (from 26 ± 6 to 10 ± 3 cc/100/min); raising pH to 7.75 by lowering P co 2 below 10 mm Hg allowed twice as much cerebral flow (20 ± 6 cc/100 gm/min*). Despite optimum myocardial protection with blood cardioplegia during circulatory arrest, postischemic myocardial performance was depressed 50% by keeping pH 7.4. In contrast, postischemic performance was normal when pH was varied appropriately during cooling and rewarming (stroke work index 0.62 versus 1.27 at a left atrial pressure of 25 mm Hg). Constraining pH to 7.4 during hypothermia causes a degree of myocardial damage and limitation of cardioplegic protection which is avoidable by adjusting pH to maintain relative alkalinity as in ectotherms. These findings have major implication in the routine management of hypothermia during all cardiac operations.