Exercise MRI to Evaluate Cardiorespiratory Fitness in Children With Heart Disease

Inclusion Criteria: * 10 - 18 year olds followed at the Stollery Children's Hospital * Heart transplant recipients ≥6 months post-transplant * Have a moderate-complex congenital heart disease diagnosis Exclusion Criteria: * Non-English speaking * Exercise restricted by the patient's clinical cardiologist * Clinical antibody- or cellular-mediated rejection within 3-months of the assessment or during the study period (for HTRs) * Previous involvement in a CR or exercise intervention program * Previous exercise stress test demonstrating sustained arrhythmias, ST segment elevation or depression greater than 3mm, an inappropriate rise in blood pressure (BP) (\<20 mmHg) or a systolic BP \>200 mmHg, or symptoms of chest pain or syncope * Resting arterial saturation \<85% or oxygen requirements * Moderate ventricular systolic dysfunction (or worse) at the most recent echocardiogram * History of chest pain on exertion; unrepaired/unpalliated CHD * Arrhythmias in the last year (including supraventricular tachycardia, ventricular tachycardia, atrioventricular block or history of (Mobitz II or worse)) * New York Heart Association class II or worse symptoms * Active medical inter-current illness limiting ability to participate * Cognitive impairment limiting the communication needed for the exercise MRI * Research MRI contraindications (e.g. any type of pacemaker), or any orthopedic limitation preventing exercise testing * Extracardiac or congenital abnormality limiting the participant's functional ability to exercise * Pregnant