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

<b>Background</b>: Intravenous fluids modify acid-base balance by changing plasma strong ion difference (SID_PL) 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. <b>Methods</b>: 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 (SID_INF) was calculated, and SID_PL and standard base excess variations (ΔSBE) were assessed. Patients were stratified by SID_INF tertiles (low, <41.0 mEq/L; medium, 41.2-54.6 mEq/L; high, ≥55.0 mEq/L), median pre-infusion HCO₃^- (24.3 [22.4-26.3] mmol/L), and tertiles of SID_INF-HCO₃^- difference. <b>Results</b>: Among 650 admissions, 641 were included (83% elective surgery). Pre-infusion acid-base was, as average, within normal ranges. Total infused volume averaged 2327 ± 1111 mL. Across SID_INF tertiles, ΔSBE increased from 1.2 ± 3.4 to 3.0 ± 3.0 and 3.4 ± 3.0 mmol/L (<i>p</i> < 0.001), paralleled by ΔSID_PL rise (0.6 ± 2.3, 1.3 ± 2.4 and 1.4 ± 2.3 mEq/L, respectively; <i>p</i> = 0.004). For any given SID_INF, patients with lower pre-infusion HCO₃^- showed greater ΔSBE and ΔSID_PL (<i>p</i> < 0.001). When analyzed by tertiles of SID_INF-HCO₃^- difference, ΔSBE rose from 1.0 ± 3.2 to 2.7 ± 2.9 and 4.0 ± 3.0 mmol/L (<i>p</i> < 0.001), with amplified effects at higher infused volume (>2500 mL). <b>Conclusions</b>: In postoperative ICU patients, fluid-induced acid-base changes are largely driven by SID_INF-HCO₃^- difference, supporting individualized fluid selection based on baseline HCO₃^-.