Association between remnant cholesterol inflammation Index and in-hospital New-onset atrial fibrillation in patients with ST-segment elevation myocardial infarction

Background New-onset atrial fibrillation (NOAF) is a common in-hospital complication among patients with acute ST-segment elevation myocardial infarction (STEMI) and is closely associated with adverse outcomes. The remnant cholesterol inflammatory index (RCII), a novel composite marker integrating lipid metabolism abnormalities and systemic inflammation, has not been well studied in relation to NOAF in STEMI patients. This study aimed to investigate the association between RCII and the occurrence of in-hospital NOAF in patients with STEMI. Methods This single-center retrospective observational study consecutively enrolled patients with first-time acute STEMI undergoing emergency percutaneous coronary intervention (PCI) from January 2022 and December 2025. RCII was defined as the product of remnant cholesterol (RC) and high-sensitivity C-reactive protein (hs-CRP). Results A total of 945 STEMI patients were included, of whom 79 (8.36%) developed NOAF. Patients with NOAF had significantly higher levels of RCII, RC, and hs-CRP compared with those without NOAF. Multivariable logistic regression analysis demonstrated that hs-CRP (OR = 2.96, 95% CI: 1.72–5.08, P < 0.001), RCII (OR = 4.71, 95% CI: 2.86–7.78, P < 0.001), and RC (OR = 2.78, 95% CI: 1.88–4.10, P < 0.001) were each independently associated with the occurrence of NOAF in separate multivariable models. Restricted cubic spline analysis indicated a significant nonlinear dose–response relationship between RCII and NOAF risk ( P for overall < 0.001, P for nonlinear < 0.05). ROC curve analysis showed that RCII identified NOAF with an area under the curve of 0.756 (95% CI: 0.698–0.814), outperforming RC alone ( Z = 3.105, P = 0.002) or hs-CRP alone ( Z = 2.805, P = 0.005). Conclusions Higher RCII was independently associated with an increased risk of in-hospital NOAF and showed moderate discriminatory ability. RCII may serve as a simple, cost-effective, and efficient early risk stratification tool for identifying patients at high risk of NOAF.