Serum phosphate variability and mortality in critically ill patients with atrial fibrillation

Background: atrial fibrillation (AF) is common in critically ill patients and associated with high mortality. Phosphate plays a key role in energy metabolism and myocardial function. Although abnormal phosphate levels predict poor outcomes, the prognostic impact of phosphate variability during intensive care unit (ICU) stay in patients with AF remains unclear. Methods: this retrospective cohort study used data from the MIMIC-IV database (2008-2019). Adult ICU patients with AF and at least two phosphate measurements were included. Phosphate variability (ΔPhos) was defined as the absolute difference between maximum and minimum serum phosphate levels during ICU stay. Directional changes in phosphate were also analyzed. Primary outcomes were 28-day, 90-day, and 180-day mortality. Associations between ΔPhos and mortality were examined using multivariable Cox regression and restricted cubic spline analyses. Results: among 7,237 patients, each 1 mg/dl increase in ΔPhos was independently associated with higher 28-day (HR = 1.04; 95 % CI 1.02-1.07), 90-day (HR = 1.06; 95 % CI 1.03-1.09), and 180-day (HR = 1.06; 95 % CI 1.03-1.09) mortality. Restricted cubic spline analysis revealed a significant U-shaped association (p < 0.001), with both increases and decreases in phosphate linked to elevated mortality, and upward changes showing stronger effects. Conclusions: greater phosphate variability, especially rising trends during ICU stay, independently predicts mortality in critically ill patients with AF. Monitoring phosphate dynamics may enhance prognostic assessment and support timely management in the ICU.