Abstract Body (Do not enter title and authors here): Background: Arrhythmogenic right ventricular cardiomyopathy (ARVC) increases the risk of ventricular arrhythmias and heart failure. Cardiac MRI (CMR) offers a comprehensive assessment of myocardial dysfunction, but its prognostic value for adverse outcomes in ARVC is underexplored.

Purpose:
To evaluate the prognostic value of CMR-derived atrial strain for adverse outcomes in patients with definite ARVC.

Materials and Methods:
This retrospective study included consecutive patients with definite ARVC who underwent CMR between December 2010 and December 2020. Biatrial and biventricular strain parameters were quantified using CMR-based feature-tracking (FT) technology. The primary endpoint was a composite of sudden cardiac death (SCD), aborted SCD, heart failure-related death and heart transplantation. Associations were evaluated by multivariable Cox regression. Incremental prognostic value was assessed via C-indices and likelihood ratio tests.

Results:
A total of 316 participants (mean age 38 ± 15 years; 209 men) were analyzed. Over a median follow-up of 68 months, 63 patients (19.9%) reached the primary endpoints, and 113 patients (35.8%) reached the secondary endpoints. Multivariate analysis showed that decreases in right atrial (RA) reservoir strain were significantly associated with both primary (hazard ratio [HR] 0.94, 95% CI: 0.91, 0.97; P < .001). Spline analysis showed a nonlinear relationship between RA reservoir strain and HRs for the primary endpoint. The HR decreased significantly when RA reservoir strain was below 15.6% but plateaued above 15.6%. A combined model using RA reservoir strain plus clinical and imaging factors (C index=0.84, likelihood ratio=92.8) showed better discrimination and calibration for primary endpoints than models with clinical variables alone (C index=0.73, likelihood ratio=37.6) or clinical-imaging model (C index=0.78, likelihood ratio=69.0, all P < .001).

Conclusion: CMR-derived RA reservoir strain is an independent predictor of adverse outcomes in ARVC and provides incremental prognostic value beyond traditional markers, enhancing early risk stratification and guiding treatment decisions.