Group-Based Trajectory Modeling of Serum Sodium and Mortality in Critically Ill ICU Patients with Community-Acquired Pneumonia: An Analysis of the MIMIC-IV Database

Purpose Community-acquired pneumonia (CAP) is a major cause of prolonged intensive care unit (ICU) admissions and is associated with substantial mortality. Abnormalities in serum sodium are common in critically ill patients and may carry prognostic significance. This study examined early serum sodium trajectories in ICU patients with CAP and their associations with 28-day, 90-day, and 365-day mortality. Methods Using the MIMIC-IV database, we retrospectively analyzed adult patients with a primary diagnosis of CAP, an ICU stay of more than 7 days, and at least four sodium measurements within the first 7 days. Group-based trajectory modeling was applied to identify distinct patterns of sodium change. Associations between trajectory groups and mortality were evaluated using multivariable Cox proportional hazards models, adjusting for demographics, comorbidities, illness severity, and laboratory parameters. Results A total of 3,146 patients were classified into five distinct trajectories: normal level with slow increase (trajectory 1), rapid increase followed by slow decrease (trajectory 2), low level with slow increase (trajectory 3), low level with rapid increase (trajectory 4), and normal stable (trajectory 5). Using trajectory 2 as the reference group, trajectory 5 was associated with significantly lower mortality at 28 days (adjusted HR 0.50, 95% CI 0.38–0.65, p < 0.001), 90 days (HR 0.63, 95% CI 0.50–0.79, p < 0.001), and 365 days (HR 0.68, 95% CI 0.55–0.83, p < 0.001). Similarly, trajectories 1 (28-day HR 0.58, 95% CI 0.45–0.76, p < 0.001) and 3 (28-day HR 0.51, 95% CI 0.36–0.73, p < 0.001) were associated with lower risk. Trajectory 4 showed no significant difference in 28-day mortality compared to trajectory 2 (HR 0.80, 95% CI 0.55–1.18, p = 0.263). Trajectory 2 was consistently associated with the highest mortality risk. Kaplan-Meier analysis demonstrated significant survival differences among the trajectory groups at 28 days (global log-rank test p = 0.019). Conclusions Distinct early sodium trajectories are independently associated with mortality in ICU patients with CAP and prolonged stays. The rapid increase followed by slow decrease pattern (trajectory 2), indicative of impaired physiological resilience, was linked to the poorest outcomes, whereas the normal stable pattern (trajectory 5) predicted optimal survival. Trajectory-based sodium monitoring may have relevance for early risk stratification and for consideration of individualized electrolyte management.