Metabolic acidosis is a common complication following cardiac surgery with cardiopulmonary bypass (CPB). Recent evidence suggests that unmeasured anions (UAs), assessed via the strong ion gap (SIG), may play a significant role. This study aimed to explore acid-base changes using the Stewart physicochemical model to better characterize the contribution of UAs in the early postoperative period.

Single-center, prospective observational study.

A tertiary university hospital in Italy.

Fifty adult patients undergoing elective cardiac surgery with CPB. The inclusion criteria were: elective cardiac surgery with CPB and written informed consent. The exclusion criteria were emergency surgery, heart transplant or ventricular assist device implantation, pregnancy, and lack of informed consent.

Blood samples were collected at predefined perioperative time points for comprehensive acid-base analysis, including traditional parameters and Stewart-derived variables: apparent and effective strong ion difference, total weak acids, SIG, and alactic base excess. Statistical comparisons were performed using paired tests and repeated-measures analysis of variance.

At ICU arrival, patients exhibited significant metabolic acidosis (mean pH 7.33 ± 0.05, BE –3.87 ± 3.3 mmol/L), elevated lactate (2.52 mmol/L), and a markedly increased SIG (median 9.8 mmol/L v 6.0 at baseline, p < 0.001). Despite normalization of pH by postoperative day 1, the presence of elevated SIG indicated that UAs significantly contributed to early acidosis, independent of hyperlactatemia. Alactic base excess also revealed persistent non-lactic metabolic derangements.

The Stewart approach reveals a substantial role for unmeasured anions in postoperative metabolic acidosis after cardiac surgery. These findings challenge the reliance on lactate and base excess alone and suggest a potential role for advanced acid-base analysis in guiding postoperative care.