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

Alterations of acid-base balance induced by intravenous fluids are primarily related to changes in plasma strong ion difference (SID_PL) and nonvolatile weak acids. Previous studies suggest that the relationship between the SID of infused fluids (SID_INF) 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.

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 SID_INF and SID_INF-HCO₃ ^- difference were calculated. Patients were grouped by crystalloid type and SID_INF-HCO₃ ^- tertiles. Associations with changes in SID_PL, standard base excess (SBE), strong ion gap (SIG), and urinary anion gap (uAG) were analyzed.

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. SID_PL (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 SID_PL and SBE increased across crystalloid groups and SID_INF-HCO₃ ^- tertiles, with minimal SBE variation when SID_INF-HCO₃ ^- approximated zero. SIG changes were similar across groups. Urinary electrolyte excretion showed no quantitative association with SID_PL or SBE changes; however, urinary Cl^- decreased with increasing SID_INF-HCO₃ ^-, consistent with renal Cl^- modulation.

In postoperative critically ill patients, SID_INF-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.