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Fluid-induced acid-base variations in postoperative critically ill patients: physiological determinants and renal
Francesco Zadek1, Luca Zazzeron2, Michele Ferrari3
1Department of Medicine and Surgery, University of Milan-Bicocca, Monza, Italy.
Frontiers in Medicine
|July 16, 2026
Summary
The difference between infused fluid strong ion difference (SIDINF) and plasma bicarbonate (HCO3-) influences acid-base changes in critically ill patients. Unmeasured anions also play a role, while renal effects are minimal in the short term.
Area of Science:
- Critical Care Medicine
- Renal Physiology
- Acid-Base Balance
Background:
- Intravenous fluid administration alters acid-base balance primarily through changes in plasma strong ion difference (SIDPL) and nonvolatile weak acids.
- The interplay between infused fluid SID (SIDINF) and pre-infusion plasma bicarbonate (HCO3-) is a key factor in fluid-induced acid-base shifts.
- The roles of unmeasured anions and renal electrolyte handling in these alterations require further elucidation.
Purpose of the Study:
- To investigate the influence of the SIDINF-HCO3- difference on acid-base changes in postoperative critically ill adults.
- To assess the contribution of unmeasured anions to acid-base variations during fluid replacement therapy.
- To evaluate the impact of renal electrolyte handling on plasma acid-base status in the early postoperative period.
Main Methods:
- A prospective observational study was conducted on postoperative critically ill adults receiving early intravenous fluids.
- Fluid composition, volume, acid-base variables, and plasma/urinary electrolytes were recorded at ICU admission and on postoperative day one.
- Patients were analyzed based on crystalloid type and SIDINF-HCO3- tertiles, examining associations with changes in SIDPL, standard base excess (SBE), and strong ion gap (SIG).
Main Results:
- Patients received significant fluid volumes, resulting in a slightly positive fluid balance.
- Plasma strong ion difference (SIDPL) and standard base excess (SBE) increased, while the strong ion gap (SIG) decreased, indicating reduced unmeasured anions.
- Changes in SIDPL and SBE correlated with crystalloid type and SIDINF-HCO3- tertiles; SBE variation was minimal when SIDINF-HCO3- approached zero.
- Urinary electrolyte excretion did not quantitatively correlate with SIDPL or SBE changes, though urinary chloride decreased with increasing SIDINF-HCO3-.
Conclusions:
- The SIDINF-HCO3- difference significantly influences fluid-induced acid-base alterations in postoperative critically ill patients.
- Changes in unmeasured anions contribute to variations in standard base excess (SBE) during fluid replacement.
- Renal effects on plasma acid-base status appear limited in quantitative impact over short postoperative time frames.
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