Association between early changes in acid–base parameters within the first 24 h and mortality in critically ill patients with metabolic acidosis

Metabolic acidosis is a common and clinically important acid–base disorder in critically ill patients and is associated with poor outcomes. Although arterial blood gas analysis is routinely used to evaluate acid–base status, the prognostic value of early metabolic changes during the first 24 h of intensive care unit (ICU) admission remains unclear. This study aimed to evaluate the association between 24-hour arterial blood gas dynamics and mortality in critically ill patients with metabolic acidosis. This retrospective study included 357 adult patients with metabolic acidosis admitted to the tertiary care ICU. Patients were classified as survivors and non-survivors. Arterial blood gas measurements taken at admission and 24 h later were analysed. pH, PaCO₂, bicarbonate (HCO₃⁻), base excess, lactate were recorded and the corrected anion gap was calculated. The prognostic performance of these dynamic parameters in predicting mortality was assessed using receiver operating characteristic (ROC) analysis, and independent associations were examined using multivariate logistic regression. Overall mortality was 39.5% (141/357). Admission and 24-hour HCO₃⁻ and BE values were not significantly different between survivors and non-survivors. However, ΔHCO₃⁻ was significantly greater in survivors than in non-survivors [2.6 (1.0–6.3) vs. 2.2 (0.3–4.1), p = 0.011]. Similarly, ΔBE was significantly greater in survivors [3.5 (1.3–7.5) vs. 1.8 (-0.7–4.6), p < 0.001]. In ROC analysis, ΔBE showed the highest discriminatory performance among the significant dynamic parameters, with an AUC of 0.632 (95% CI: 0.574–0.691, p < 0.001), followed by ΔHCO₃⁻ and ΔCl⁻. In baseline-adjusted multivariable logistic regression, greater ΔHCO₃⁻ (OR: 0.927, 95% CI: 0.863–0.997, p = 0.040) and greater ΔBE (OR: 0.906, 95% CI: 0.850–0.966, p = 0.002) remained independently associated with lower mortality odds after adjustment for age, APACHE II score, mechanical ventilation, renal replacement therapy, and the corresponding admission acid–base value. In critically ill patients with metabolic acidosis who underwent serial arterial blood gas assessment, greater improvement in base excess and, to a lesser extent, HCO₃⁻ within the first 24 h was independently associated with survival. These dynamic parameters demonstrated modest discriminatory performance and may serve as complementary markers for early risk stratification rather than standalone predictors.