Abstract Body (Do not enter title and authors here): Introduction: Double Outlet Right Ventricle (DORV) encompasses a heterogenous range of congenital heart diseases (CHD). While survival to adulthood has improved, the risk of long-term sequelae such as life-threatening arrhythmias and sudden cardiac death (SCD) remains significant. In this study, we present the most comprehensive follow-up of adults with DORV to date.

Methods: This multi-centre, retrospective cohort study enrolled adults with CHD under follow-up at two tertiary cardiac centres in the United Kingdom from 2000-2023. DORV patients were eligible irrespective of surgical outcomes. Patients were grouped according to the following classification: group 1 (VSD-type), group 2 (Fallot-type), group 3 (transposition of the great arteries with pulmonary stenosis), group 4 (transposition of the great arteries without pulmonary stenosis), group 5 (B1-remote VSD; B2-univentricular physiology, unrepaired cases). The primary composite outcome for the study was all-cause mortality, heart transplantation, aborted SCD, or appropriate ICD shock.

Results: Among 254 patients (median age 20 years; 52.8% male), 62.2% underwent biventricular repair. All DORV anatomical groups were present: Group 1 (n=34, 13.4%); Group 2 (n=76, 29.9%); Group 3 (n= 23, 9.1%); Group 4 (n=30, 11.8%); Group 5-B1 (n=5, 2%) and Group 5-B2 (n=86, 33.9%). Over a median follow-up of 12.4[7.4-17.3] years, 47 patients died, with heart failure (38.3%) and SCD (27.7%) as the leading causes. There were 4(1.6%) cases of aborted SCD; 4(1.6%) patients received a heart transplant and appropriate ICD shocks were delivered in 10(4.0%) patients. Arrhythmias were common, with 13% of patients developing ventricular tachycardia and 29.5% developing supraventricular tachycardia (SVT) during follow-up. Age, pulmonary hypertension, baseline QRS >180ms, non-sinus rhythm, SVT history, Rastelli operation and number of reparative surgeries were univariate predictors of poor outcome in patients with biventricular repair. On multivariate analysis, age, pulmonary hypertension and baseline QRS >180ms remained as negative prognostic indicators. (Table 1)

Conclusion: In adult survivors of DORV, late mortality is driven by SCD and heart failure, and transplant was rarely achieved in our cohort. Strong predictors of adverse outcome include failure to achieve a repair, advancing age and pulmonary hypertension. Life-long, specialist surveillance of these patients is essential for risk stratification and timely intervention.