Fluid-induced acid–base variations in postoperative critically ill patients: physiological determinants and renal response

Background: Alterations of acid–base balance induced by intravenous fluids are primarily related to changes in plasma strong ion difference (SIDPL) and nonvolatile weak acids. Previous studies suggest that the relationship between the SID of infused fluids (SIDINF) and pre-infusion plasma bicarbonate (HCO3−) may act as an integrative factor influencing acid–base changes during fluid replacement. However, the contribution of unmeasured anions and renal electrolyte handling remains incompletely characterized.Methods: We conducted a prospective observational study in postoperative critically ill adults receiving intravenous fluids early after admission to intensive care unit (ICU). Fluid composition and volume were recorded. Acid–base variables, plasma and urinary electrolytes were assessed at ICU admission and study end (postoperative-day one). The average SIDINF and SIDINF-HCO3− difference were calculated. Patients were grouped by crystalloid type and SIDINF-HCO3− tertiles. Associations with changes in SIDPL, standard base excess (SBE), strong ion gap (SIG), and urinary anion gap (uAG) were analyzed.Results: Fifty-seven consecutive patients were included. Patients received 3,152 ± 1,027 mL of fluids, with a slightly positive fluid balance (+802 ± 1,212 mL) over 19 [18–20] hours. SIDPL (38.7 ± 2.4 to 39.8 ± 2.7 mEq/L) and SBE (−1.5 ± 2.4 to 0.9 ± 2.9 mEq/L) increased (p < 0.001 for both), whereas SIG decreased (5.0 ± 2.8 to 3.9 ± 2.5 mEq/L; p < 0.001), indicating reduced unmeasured anions. Changes in SIDPL and SBE increased across crystalloid groups and SIDINF-HCO3− tertiles, with minimal SBE variation when SIDINF-HCO3− approximated zero. SIG changes were similar across groups. Urinary electrolyte excretion showed no quantitative association with SIDPL or SBE changes; however, urinary Cl− decreased with increasing SIDINF-HCO3−, consistent with renal Cl− modulation.Conclusion: In postoperative critically ill patients, SIDINF-HCO₃− appears to influence fluid-induced acid–base changes, particularly during fluid replacement. Changes in unmeasured anions also contribute to SBE variations. In contrast, renal effects appear temporally limited and do not quantitatively influence plasma acid–base status over short time frames.