Plasma Bicarbonate as a Determinant of Fluid-Induced Acid–Base Changes in Postoperative Critically Ill Patients: A Retrospective Analysis

Background: Intravenous fluids modify acid–base balance by changing plasma strong ion difference (SIDPL) and total non-volatile weak acids. Experimental data suggest that pre-infusion plasma bicarbonate (HCO3−) may further modulate these effects. We tested this hypothesis in a large cohort of postoperative ICU patients receiving intravenous fluids. Methods: We retrospectively analyzed all-consecutive post-operative ICU admissions over a 21-month period who received fluid therapy. Fluid inputs/outputs, plasma electrolytes, and arterial blood gases were collected from admission to 9:00 A.M. of postoperative day one. Average SID of infused fluids (SIDINF) was calculated, and SIDPL and standard base excess variations (ΔSBE) were assessed. Patients were stratified by SIDINF tertiles (low, 2500 mL). Conclusions: In postoperative ICU patients, fluid-induced acid–base changes are largely driven by SIDINF-HCO3− difference, supporting individualized fluid selection based on baseline HCO3−.