Alterations in anion gap following cardiopulmonary bypass

Objectives: To evaluate the changes in the anion gap and their relation to hyperlactatemia and alterations in plasma proteins after cardiopulmonary bypass. Design: Prospective study. Setting: Cardiothoracic intensive therapy unit. Patients: One hundred eleven consecutive patients after cardiopulmonary bypass. Measurements and Main Results: Data were collected before cardiopulmonary bypass and every 6 hrs for 24 hrs after cardiopulmonary bypass. Results were analyzed for the entire cohort and for hyperlactatemic subgroups. The major finding of this study was that the anion gap decreased significantly at all sampling periods relative to precardiopulmonary bypass values, despite the presence of clinically important hyperlactatemia. No correlation between the decrease in plasma protein concentrations and the decrease in anion gap could be demonstrated. Conclusions: The decrease in anion gap after cardiopulmonary bypass appears to represent a balance between the influences of increased serum chloride and lactate concentrations and reduced plasma protein concentrations. This analysis demonstrates the limitations of the anion gap in the evaluation of a metabolic acidosis after cardiopulmonary bypass.