Evaluation of Prognosis and Risk Factors of Fulminant Myocarditis Complicated with Malignant Arrhythmia

(1) Background: Malignant arrhythmia complicating fulminant myocarditis is associated with high in-hospital mortality, but evidence regarding its long-term prognosis and specific risk factors is limited. (2) Methods: This single-center retrospective cohort study (2016–2025) analyzed 241 consecutive fulminant myocarditis patients, stratified by malignant arrhythmia status (n = 58 vs. 183). The malignant arrhythmia group was further subclassified into malignant tachyarrhythmia (n = 22) and bradyarrhythmia (n = 36). Endpoints included major adverse cardiovascular events (MACE), cardiac dysfunction, and structural abnormalities. (3) Results: At 3-month follow-up, malignant arrhythmia patients had a significantly higher incidence of MACE compared to non-malignant arrhythmia patients (15.5% vs. 4.9%, p = 0.008), but no significant differences were found in cardiac dysfunction or structural abnormalities. Multivariate analysis identified low triglyceride level as an independent risk factor for in-hospital malignant tachyarrhythmia. For in-hospital malignant bradyarrhythmia, independent risk factors were delayed, such as intrinsicoid deflection, low diastolic blood pressure, bradycardia, and an elevated E/Em ratio, with the predictive model showing high discriminatory power. (4) Conclusions: Malignant arrhythmia is an independent predictor of adverse short-term, but not long-term, prognosis in fulminant myocarditis patients, with distinct risk factor profiles identified for malignant tachyarrhythmia and malignant bradyarrhythmia subtypes.