Around 8-12 in every 1000 babies are born with an abnormal heart, known as a congenital heart defect (CHD). Thanks to major advances in medical and surgical care, most babies born with CHD now live into adulthood.

As people with CHD get older, many need repeated heart operations. We now know from other areas of medicine that "prehabilitation" - a short, structured exercise programme completed before surgery - can improve fitness and help patients cope better with major operations. However, this approach has never been tested in adults with CHD.

We are running a randomised controlled trial to find out whether a prehabilitation programme can improve physical fitness before heart surgery in adults with CHD. Participants will be randomly assigned to usual care or a prehabilitation group. Those in the prehabilitation group will follow a tailored, supervised exercise plan for several weeks before surgery. All patients undergoing cardiac surgery routinely complete a cardiopulmonary exercise test (CPET) as part of their standard pre-operative assessment. CPET is a well-established test that shows how the heart and lungs respond to increasing levels of exercise. During the test, participants exercise on a bike while their breathing, heart rate, and overall physiological response are continuously monitored. The results of each participant's routine CPET will be used to guide the individualised exercise programme in the intervention group. Exercise levels prescribed in the study will not exceed the intensity already safely achieved during the routine CPET, ensuring that activity remains within clinically assessed limits.

Participants in the trial will complete an additional second CPET following the trial period.

Our key question is whether a tailored exercise programme can improve fitness in adults with CHD before heart surgery, and whether this approach is feasible, acceptable and practical to deliver, including successful recruitment and measurable physiological improvements.

Inclusion Criteria:

• Adults with congenital heart disease requiring cardiac surgery via sternotomy or thoracotomy.
• Capacity to provide informed consent.
• Ability and willingness to engage in exercise-based interventions.
• Peak VO2 <30ml/kg/min at preoperative exercise assessment.
• Participant reached peak VO2 in preoperative exercise assessment (RER >1.0).
• Ability to communicate in English.

Exclusion Criteria:

• Surgery is scheduled for a date that precludes participation in the programme - surgical date is <9 weeks away.
• Pregnancy of <6 months postpartum.
• History of pneumothorax within the last 3 months.
• Resting oxygen saturations <94%.
• Uncorrected right-to-left shunt with exercise.
• Fontan physiology.
• Pulmonary hypertension.
• Critical valvular obstruction.
• Sustained ventricular arrhythmia.
• Severe aortic dilatation (>45mm in Marfan syndrome or hereditary aortopathy, >50mm in bicuspid or tricuspid aortic valves).

The oldest person operated upon by the ACHD service was 78 years old. Upper age is not an exclusion criteria.

Pooled data will be published.

The study will be outcome-blinded. Only the study team will have access to randomised allocation.