Adding lactate to the prime solution during hypothermic cardiopulmonary bypass: a quantitative acid-base analysis

Background. The effect of adding lactate to the cardiopulmonary bypass (CPB) prime was investigated using Stewart's quantitative acid-base approach. According to this quantitative model, serum pH and bicarbonate are determined by three independent factors: the partial pressure of carbon dioxide (P-CO2), the total concentration of weak acids (e.g. albumin), and the strong ion difference. the apparent strong ion difference is calculated as the sum of sodium, potassium, magnesium and calcium minus chloride concentrations. The pH decreases with a smaller strong ion difference and vice versa. Methods. Twenty patients scheduled for coronary surgery were studied prospectively. All patients were treated identically, except for the prime, which either contained lactate or was lactate free. just before bypass and before coming off bypass, haemoglobin, glucose, plasma osmolality and colloid osmotic pressure were determined; albumin, lactate, sodium, potassium, ionized calcium, magnesium, phosphate, arterial pH, P-CO2, bicarbonate, and base excess were measured for use in Stewart's analysis. Results. Metabolic acidosis had resolved by the end of bypass with the lactated prime. Although the strong ion gap (apparent minus effective strong ion difference) increased significantly in both groups, its composition differed significantly between the groups. The Stewart technique detected polyanionic gelatin as a weak acid component contributing to the unidentified anion fraction. Colloid osmotic pressure was maintained in both groups. Conclusion. Exogenous lactate attenuates acidosis related to CPB. The oncotic and weak acid deficits produced by hypoalbuminaemia may be compensated for temporarily during CPB by polyanionic synthetic colloids such as succinylated gelatin.