Association between systemic immune-inflammation index(SII) and all-cause and cardiovascular mortality in heart failure patients: a single-center retrospective analysis

Objective This study aimed to investigate the association between the systemic immune-inflammatory index (SII) and mortality in patients with heart failure (HF). Methods We conducted a retrospective cohort study of 1,084 HF patients. In this retrospective cohort study, we enrolled patients hospitalized for heart failure between January 2022 and June 2023. Follow-up was conducted via telephone and outpatient visits until death or July 22, 2025, with all-cause and cardiovascular mortality as primary endpoints. Patients were categorized by log-transformed SII (LnSII). Cox models assessed associations between LnSII and mortality, while restricted cubic splines evaluated nonlinearity. Subgroup, mediation (NT-proBNP, LVEF), and sensitivity analyses were performed. Results A higher LnSII was significantly associated with an increased risk of all-cause mortality (adjusted HR = 1.66, 95% CI: 1.08–2.55), but not with cardiovascular mortality. Subgroup analysis revealed a significant interaction with smoking status (P for interaction = 0.023), showing a stronger association between LnSII and all-cause mortality among smokers (HR = 2.41, 95% CI: 1.57–3.68). Mediation analysis indicated that NT-proBNP and LVEF mediated 35.8% and 15.0% of this association, respectively. Conclusion Elevated SII is independently associated with an increased risk of all-cause mortality in HF patients, particularly among smokers, and may serve as a useful prognostic biomarker.