This study is a single center randomized controlled trial evaluating the effect of an exercise training intervention (interval training or moderate continuous training) compared to usual care (no training) on exercise capacity in patients with congenital heart disease (CHD).

Our primary aim is to determine the impact of interval training (IT) compared to moderate continuous exercise training (MCT) versus controls (no exercise) on exercise capacity as measured by peak VO2. Secondary aims are to: 1) assess the safety of exercise training in CHD, 2) evaluate the effect of exercise training on cardiac structure and function and 3) assess the effects of exercise training on quality of life and long-term physical activity.

Adults with congenital heart disease (ACHD) have reduced exercise capacity which is an independent predictor of adverse outcome including hospitalization and mortality in this patient population. When aerobic exercise capacity is measured by cardiopulmonary exercise testing (CPET), peak oxygen consumption (peak VO2) averages 22ml/kg/min in ACHD, less than half of age matched controls, and similar to patients with heart failure. Factors contributing to low exercise capacity include central factors related to abnormal cardiac anatomy, ventricular systolic and diastolic dysfunction, valvular abnormalities, pulmonary hypertension, chronotropic incompetence and impaired pulmonary function as well as peripheral factors such as skeletal muscle conditioning. Clinicians may further contribute to reluctance around exercise by failing to encourage physical activity or giving inappropriate exercise restrictions to ACHD.

The aim of our study was to explore the efficacy of HIIT compared to MCT on aerobic exercise capacity as measured by peak VO2 in ACHD patients. Secondary aims were to determine the effect of exercise on various physiologic variables, to assess the safety of HIIT in ACHD, to evaluate the effect of exercise training on cardiac structure and function as assessed by cardiac MRI and to measure the impact of exercise training on quality of life (QoL).

This was a single center randomized controlled trial designed in accordance with CONSORT guidelines. Participants were randomized with a 1:1:1 ratio into three groups: HIIT, MCT or control group. Randomization occurred by clusters defined by week of attendance in the clinic. Permuted blocks of 3, 6, and 9 were used to ensure a similar number of weeks randomized to the three groups, with randomization sequence generated by one author (AM). Participants were enrolled by a study coordinator who assigned participants to one of the three groups. Participants underwent testing at baseline and after 12 weeks of the intervention.

Inclusion Criteria:

• Medically stable patients aged 16 or great with congenital heart disease.

Exclusion Criteria:

• Uncorrected cyanotic congential heart disease (SpO2 </= 85%)
• Severe outflow tract obstruction (peak Doppler gradient >50 mmHg)
• Sustained arrhythmia

Individual participant data will not be shared. Data will be analyzed and presented per treatment group.