Nonlinear association between systemic immune-inflammation index and in-hospital mortality in critically ill patients with chronic obstructive pulmonary disease and atrial fibrillation: a cross-sectional study.

Background: Chronic obstructive pulmonary disease (COPD) and atrial fibrillation (AF) frequently coexist, and their concurrence is associated with worse clinical outcomes than either condition alone. Inflammation plays a central role in the pathogenesis of both diseases. The systemic immune-inflammation index (SII), derived from neutrophil, platelet, and lymphocyte counts, has emerged as a promising marker reflecting systemic inflammation. However, its prognostic value in critically ill patients with concurrent COPD and AF remains unclear. This study aimed to investigate the association between SII and in-hospital mortality in intensive care unit (ICU) patients with both COPD and AF. Methods: We identified ICU patients from Medical Information Mart for Intensive Care meeting the following criteria: patients with first ICU admission, concurrent diagnoses of COPD and AF; exclusion criteria included patients aged <18 years, patients without COPD, patients without AF, length of admission to ICU less than <24 hours, lymphocyte, neutrophil, or platelet counts missing or zero. Baseline patient characteristics included vital signs, laboratory profiles, medications, and critical illness severity scores. Baseline patient characteristics included vital signs, laboratory profiles, medications, and critical illness severity scores. The highest peripheral blood cell count recorded during the first 24 hours of ICU admission was used to calculate SII, and log transformation was applied. The study endpoint was in-hospital mortality, defined as death from any cause occurring during the hospitalization period. Logistic regression analysis, restricted cubic spline (RCS) regression, two-piecewise logistic regression modeling with smoothing, and subgroup analyses were performed to assess the relationship between SII and in-hospital mortality using data. Results: The cohort (mean age 74.1±9.4 years, 60.1% male) had an in-hospital mortality rate of 20.4%. After adjustment for sex, age, vital signs, medications, comorbidities, each unit increase in log transferred SII conferred an odds ratio (OR) of 1.72 [95% confidence interval (CI): 1.00-2.94, P=0.048]. The high log transferred SII group (≥2.9) showed 2.78-fold higher mortality (OR =2.78, 95% CI: 1.37-5.62, P=0.005) compared to the low log transferred SII group. RCSs demonstrated a nonlinear association between log transferred SII and in-hospital mortality (P for non-linearity =0.019). Subgroup analyses confirmed the robustness of this association. Conclusions: Our findings position SII as a potentially valuable biomarker for risk stratification in patients with COPD and AF, with the identified threshold potentially serving as a clinical decision point for intensifying monitoring or considering immunomodulatory therapies. Future prospective studies should validate these findings and explore whether SII guided management improves outcomes in this high-risk population.