Association between the serum sodium-to-chloride ratio and 30-day mortality following coronary artery bypass grafting: insights from the MIMIC-IV database

Abnormalities in electrolyte balance are common following coronary artery bypass grafting (CABG) and are associated with adverse outcomes. The serum sodium-to-chloride ratio has emerged as a potential prognostic marker in critical illness, but its role in predicting short-term mortality after CABG remains unclear. This study explored the relationship between the serum sodium-to-chloride ratio and 30-day mortality following CABG, aiming to assess the predictive value of the sodium-to-chloride ratio for short-term mortality and provide insights for optimizing postoperative electrolyte management. A retrospective cohort study was carried out using the Medical Information Mart for Intensive Care IV (MIMIC-IV) database, covering intensive care unit (ICU) admissions from 2008 to 2022 at the Beth Israel Deaconess Medical Center (BIDMC). The sodium-to-chloride ratio, measured within 24 h and divided into tertiles, was the exposure variable, while 30-day mortality was the outcome. Covariates included demographics, comorbid conditions, physiological scores, vital signs, and laboratory results. Multivariable Cox regression, restricted cubic splines (RCS), subgroup analyses, and doubly robust estimation (IPTW combined with Cox regression) were used to evaluate the independent association, dose–response relationship, and the robustness of the results. Patients in the highest tertile of sodium-to-chloride ratio had a significantly higher risk of 30-day mortality than those in the lowest tertile. In the fully adjusted model, each 0.01-unit increase in the sodium-to-chloride ratio was associated with a 6% higher mortality risk (HR = 1.06, 95% CI: 1.02–1.11). Kaplan–Meier analysis showed lower survival rates in the highest tertile (p = 0.038). RCS demonstrated a significant association between the sodium-to-chloride ratio and 30-day mortality (P for overall = 0.011), with no evidence of non-linearity (P for non-linearity = 0.565), suggesting a linear relationship. Subgroup analysis showed a significant interaction in the MBP subgroup (p for interaction < 0.05), but not in other subgroups. Sensitivity analysis using doubly robust estimation yielded consistent results. An elevated serum sodium-to-chloride ratio within 24 h is independently associated with increased 30-day mortality after CABG, showing a linear dose–response relationship. These findings can guide clinicians in optimizing electrolyte management strategies to improve the short-term outcomes.