The effect of pH on the hypothermic ventricular fibrillation threshold

During cardiac operations using hypothermia, the pH measured at 37°C (and corrected to the patient's body temperature) is generally kept at 7.40. However, ectotherms (cold-blooded animals) regulate pH alkaline of 7.40 as temperature falls, e.g., pH 7.58 at 25° C. Hypothermia and acidosis increase the propensity for ventricular fibrillation (VF). This study was undertaken to determine which scheme of pH regulation during hypothermia provided the highest level of cardiac electrical stability. Eleven dogs underwent median sternotomy and implantation of right ventricular and limb electrodes, aortic and central venous pressure catheters, and a probe to measure pulmonary artery blood temperature. To determine the VF threshold, a programmable stimulator was used to find the minimum current needed to produce VF by sweeping the vulnerable period of the cardiac cycle. The animals were divided into two groups for pH management: eight in the clinical scheme (pH 7.4) and seven in the ectothermic scheme, where pH varied with temperature. Control values were recorded prior to cooling and then repeated when core cooling had lowered the temperature to 25°C. In the clinical group, the VF threshold decreased from 23.1 ± 4.1 mA at 37° C to 17.0 ± 3.4 mA at 25° C (p = 0.002); in the ectothermic group, the VF threshold was unchanged by hypothermia (23.7 ± 3.2 to 22.8 ± 2.8 mA). Heart rate and arterial and central venous pressures were not different between the groups at either temperature. Corrected arterial pH and PCO₂ were unchanged from control in the clinical group at 25° C (pH 7.40 ± 0.01, arterial PCO₂ 34 ± 2 torr), whereas arterial PCO₂ was maintained at 20 ± 1 torr to achieve pH 7.59 ± 0.01 in the ectothermic group. Five of the eight dogs in the clinical group had spontaneous VF while cooling, as compared to one of the seven dogs in the ectothermic group. These studies demonstrate that allowing the corrected pH to rise with hypothermia and remain alkalotic relative to pH 7.4 improves the electrical stability of the heart during hypothermia, as evidenced by the VF threshold at 25° C. Since the ectothermic scheme increases the electrical stability of the heart, it could decrease the incidence of VF during hypothermia and decrease the temperature at which VF occurs during hypothermic cardiopulmonary bypass.