Association of the systemic immune-inflammation index with in-hospital malignant ventricular arrhythmia in patients with acute myocardial infarction

Background Malignant ventricular arrhythmia (MVA) is a life-threatening in-hospital complication of acute myocardial infarction (AMI). The systemic immune-inflammation index (SII), derived from platelet, neutrophil, and lymphocyte counts, may reflect the inflammatory and thrombotic burden associated with arrhythmic vulnerability. This study investigated the association between SII and in-hospital MVA in patients with AMI. Methods This retrospective study included 13,465 patients hospitalized for AMI at the First Affiliated Hospital of Xi'an Jiaotong University from January 2019 to December 2024. SII was calculated as platelet count × neutrophil count/lymphocyte count. Patients were grouped according to SII quartiles, and SII was also analyzed as standardized ln(SII). Logistic regression and restricted cubic spline analyses were used to assess the adjusted association and dose-response pattern, with subgroup and sensitivity analyses used to examine robustness. Separately, the apparent incremental performance of adding SII to a clinical model was assessed using the area under the receiver operating characteristic curve (AUC), net reclassification improvement (NRI), and integrated discrimination improvement (IDI). Results During hospitalization, 440 patients (3.3%) developed MVA. MVA incidence increased from 1.37% in the lowest SII quartile (Q1) to 6.98% in the highest quartile (Q4). In the fully adjusted model, each 1-SD increase in ln(SII) was associated with higher odds of MVA (odds ratio [OR], 1.48; 95% confidence interval [CI], 1.34–1.64; P < 0.001). Compared with Q1, Q4 remained associated with MVA (OR, 2.91; 95% CI, 2.06–4.10; P < 0.001). The apparent AUC was 0.797 for the clinical model and 0.814 after adding standardized ln(SII) ( Δ AUC = 0.017). Conclusion Higher admission SII remained associated with greater odds of in-hospital MVA after multivariable adjustment. The incremental model performance was modest, and external validation is required before its predictive performance and clinical utility can be established.