Purpose: Chronic obstructive pulmonary disease (COPD) concurrent with respiratory failure (RF) is devastating, and may result in death and disability. Systemic immune-inflammation index (SII) is a new prognostic biomarker linked to unfavorable outcomes of acute coronary syndrome, ischemic stroke, and heart failure. Nonetheless, its role in COPD is rarely investigated. Consequently, this study intends to assess the accuracy of SII in predicting the prognosis of COPD.
Patients and Methods: The clinical information was retrospectively acquired from the Medical Information Mart for Intensive Care-IV database. The outcomes encompassed the incidence of RF and mortality. The relationship between different SII and outcomes was examined utilizing the Cox proportional-hazards model and restricted cubic splines. Kaplan-Meier analysis was employed for all-cause mortality.
Results: The present study incorporated 1653 patients. During hospitalization, 697 patients (42.2%) developed RF, and 169 patients (10.2%) died. And 637 patients (38.5%) died during long-term follow-up. Higher SII increased the risk of RF (RF: HR: 1.19, 95% CI 1.12– 1.28, P< 0.001), in-hospital mortality (HR: 1.22, 95% CI 1.07– 1.39, P=0.003), and long-term follow-up mortality (HR: 1.12, 95% CI 1.05– 1.19, P< 0.001). Kaplan-Meier analysis suggested a significantly elevated risk of all-cause death (log-rank P< 0.001) in patients with higher SII, especially during the short-term follow-up period of 21 days.
Conclusion: SII is closely linked to an elevated risk of RF and death in COPD patients. It appears to be a potential predictor of the prognosis of COPD patients, which is helpful for the risk stratification of this population. However, more prospective studies are warranted to consolidate our conclusion.

Keywords: systemic immune-inflammation index, chronic obstructive pulmonary disease, respiratory failure, mortality, MIMIC-IV database