Association between serum bicarbonate and 30-day mortality in critically Ill patients with influenza: A retrospective...

Influenza remains a major public health challenge worldwide, particularly among patients admitted to the intensive care unit (ICU). While serum bicarbonate levels have been associated with mortality in various critical conditions, their relationship with influenza-related critical illness remains unclear. We conducted a retrospective cohort study using data from the MIMIC-IV v2.2 database with influenza patients. Serum bicarbonate was measured within the first 24 hours of ICU admission, and the maximum value was used for analysis. The primary outcome was 30-day mortality. Associations between serum bicarbonate and mortality were assessed using univariable and multivariable Cox regression analyses, as well as restricted cubic spline regression, with subgroup and sensitivity analyses performed to validate the robustness of the findings. The study included 276 critically ill influenza patients. The 30-day mortality rate was 15.5%. The cohort had a median age of 65 years, and females accounted for 51% of the population. After adjusting for confounding factors, higher serum bicarbonate was associated with lower mortality (adjusted hazard ratio: 0.9; 95% confidence interval: 0.83-0.98; P = .011). Each unit increment in serum bicarbonate was associated with 10% lower decrease in 30-day mortality. When serum bicarbonate was categorized, patients with serum bicarbonate ≥26 mEq/L (T3) had a lower hazard ratio of 0.38 (95% confidence interval, 0.16-0.89; P = .026) compared to those with <22 mEq/L (T1). Consistent associations were observed in subgroup and sensitivity analyses. These findings indicate that higher serum bicarbonate levels on ICU admission are independently associated with reduced 30-day mortality in critically ill influenza patients, warranting further large-scale prospective studies to validate and explore underlying mechanisms.