THE METABOLIC EFFECTS OF SURFACE COOLING NEONATES PRIOR TO CARDIAC-SURGERY

Neonates undergoing cardiac surgery at The Children's Hospital of Phil adelphia frequently developed metabolic acidemia as they passively coo led prior to the start of cardiopulmonary bypass. This study was perfo rmed in an attempt to identify the mechanism for this acidemia. After receiving an initial dose of fentanyl (10 mu g/kg) and pancuronium, 22 neonates were randomly assigned to maintain normothermia by active wa rming (Group I), or to permit passive cooling (Group II) before surger y. Arterial blood samples were obtained prior to, and at 10 and 45 min after entering the operating room for the analysis of pH, gas tension s, lactate, pyruvate, plasma free fatty acids, acetoacetate, beta-hydr oxybutyrate, total CO2, and glucose concentrations. In the last 11 pat ients studied, the observation period was extended to 75 min at which time another arterial blood sample was obtained. There was a steady de crease in heart rate as the Group II patients cooled; however, arteria l blood pressure did not change in either group. There were no changes in blood values measured in Group I neonates. In the Group II patient s, there was a progressive decline in calculated base excess, total CO 2, and an increase in serum lactate as the patients cooled. The metabo lic acidemia that develops in neonates represents lactate accumulation as a consequence of surface cooling prior to surgery and the institut ion of cardiopulmonary bypass. Whether lactate accumulates as a result of anaerobic metabolism in underperfused tissue beds or reduced hepat ic clearance could not be distinguished in this study. Since neither c linically significant hemodynamic changes nor differences in outcome w ere found between the two groups, the authors believe this mild lactic acidemia is inconsequential and does not require therapy.