Abstract Body (Do not enter title and authors here): Objectives:
Fontan patients have impaired exercise capacity. Exercise training has been shown to improve exercise performance. We aim to determine the effect of exercise training on SVH function, and its correlation with exercise capacity, body composition and self-reported QOL.

Methods:
31 paediatric Fontan patients were prospectively recruited to complete 3x30 minute homebased HIIT sessions per week for 20 weeks. Baseline physical activity levels, including structured sports participation, daily step-counts and active-zone-minutes (AZM) were recorded by accelerometer (Fitbit inspire3). Patients were considered active if they met 2 of the 3 criteria:
1/Structured sports training ≥ 2 per week (moderate-vigorous intensity >30 minutes)
2/Daily steps ≥ 10k
3/AZM ≥60 minutes per day

Bio-impedance-analysis, cardiopulmonary exercise tests, echocardiography including deformation imaging and PedQL questionnaires were performed at baseline and upon completion.

Results:
28 participants completed the programme. 86% had good compliance (completed >45/60 sessions). 17 male (60.7%), mean age 14.3±2.03 years, 14 LV-dominant, 14 non-LV dominant (9RV, 5BiV). No serious adverse events reported.

The exercise duration improved from 7.56±2.6 to 9.16±2.5 minutes, p<0.001 and peak workload from 110±43.6 to 129±45.8 watts, p<0.001. Indexed oxygen consumption (VO₂/kg peak) improved from 30.9±7.1 to 32.2±7ml/min/kg, p=0.003 and oxygen pulse, O₂pulse_peak from 9.1±2.6 to 9.7±3 mL/beat, p<0.001. Submaximal analysis showed normal %predicted VO2 (>40%) and improvement of O₂pulse, p=0.024.

LV systolic function showed improved MAPSE from 1.65±0.2 to 1.86±0.2, p=0.017, and improved apical rotation from 2.3±3.8 to 5.9±3.1, p=0.043. RV systolic function showed improved TAPSE of 1.4±0.2 to 1.6±0.3, p=0.032 and FAC of 44±5.6% to 49±10.5%, p=0.170.

Self-reported QOL scores were higher in Fontan children who were physically active at baseline, with a downward trend noted when poor compliance was evident (Child’s QOL V3.0 91.21±0.7 to 86.1±5.2, p=0.018 (Table 2).

Although no significant change in body composition, a higher appendicular skeletal muscle mass, SMMa% and lower body fat% were noted pre and post exercise training in the active Fontan group (Table 3),

Conclusions:
We observed significant improvements in both the submaximal and peak exercise performance in paediatric Exercise training associated with improvement of single ventricle longitudinal systolic function and ventricular mechanics.