Objective: While the ROX index has been extensively studied in acute hypoxemic respiratory failure, its prognostic value in patients with chronic obstructive pulmonary disease (COPD) remains unclear. This study aimed to investigate the association between the ROX index and mortality in critically ill COPD patients. Methods: We conducted a retrospective cohort study of 2,176 critically ill COPD patients admitted to the intensive care unit (ICU) using the MIMIC-IV database, a publicly available repository of critically ill patients admitted to intensive care units. The ROX index was categorized into quartiles. The association between the ROX index and all-cause mortality was analyzed using restricted cubic splines (RCS) and multivariable Cox proportional hazards models. Subgroup analyses were performed to validate the robustness of the findings. Results: RCS analysis revealed a nonlinear, L-shaped association between the ROX index and mortality, with an inflection point at 9.72. Below this threshold, each unit increase in the ROX index was associated with a significant 16% reduction in 28-day mortality (HR: 0.84, 95% CI: 0.78– 0.90, P< 0.001). Above 9.72, no significant association was observed (HR: 1.01, 95% CI: 0.96– 1.07, P = 0.628). Compared to the lowest quartile (Q1), patients in Q2 (HR=0.75, 95% CI: 0.59– 0.96, P=0.021), Q3 (HR=0.60, 95% CI: 0.46– 0.77, P< 0.001), and Q4 (HR=0.58, 95% CI: 0.45– 0.75, P< 0.001) had progressively lower 28-day mortality risks; Similar trends were observed for 3-month and 1-year mortality. The association remained statistically significant across all predefined subgroups (all P< 0.05). Conclusion: The ROX index is a practical bedside tool for risk stratification in critically ill COPD patients. Its predictive value follows a threshold-dependent pattern, with values below approximately 9.72 independently associated with increased mortality. This makes it particularly useful for the early identification of high-risk patients in the ICU. Keywords: ROX index, chronic obstructive pulmonary disease, MIMIC database, restricted cubic splines
Liu et al. (Sun,) studied this question.