The purpose of the trial is to compare the effects of high intensity interval training (HIIT) with usual care - moderate intensity steady state training (MISS) - in UK cardiac rehabilitation (CR) programmes.

Current cardiac rehabilitation (CR) guidelines recommend moderate intensity, steady state exercise training (MISS). However, this guideline predates significant advances in interventional and medical treatment for coronary heart disease (CHD) and may not, therefore, be the most effective strategy for the 'modern' patient with CHD. Recent research has indicated that greater benefit may be attained by participating in high intensity interval training (HIIT) which involves repeated short bursts of harder exercise interspersed with short recovery periods. Studies suggest that this can be performed without any compromise in exercise adherence or patient safety. The benefit of this approach in 'real world' CR programmes in the UK cannot be confirmed, as studies have been predominantly conducted in the laboratory and exclusively outside the UK.

In patients attending CR programmes in the UK, the investigators hypothesise that HIIT will improve VO2 peak to a greater extent than MISS. The investigators also hypothesise that HIIT will: 1) be more palatable than MISS and demonstrate greater patient compliance and adherence; 2) improve cardiovascular health to a greater extent than MISS; 3) improve HR-QoL to a greater extent than MISS; 4) lead to more positive motivation and attitudes to exercise than MISS; 5) increase short and medium-term participation in lifestyle physical activity to a greater extent than MISS; 6) be a cost effective alternative to MISS; 7) be as safe as MISS.

The HIIT or MISS study is a pragmatic single-blind, multi-centre, longitudinal, randomised controlled trial. Participants will be randomly allocated to 8 weeks of HIIT or MISS (usual care). Outcomes will be assessed at baseline, 8 weeks and 12 months by assessors blinded to group allocation. Study interventions will be delivered by clinical (not research) staff.

General inclusion criteria:

1. Successfully revascularised following percutaneous coronary intervention (PCI) or coronary artery bypass graft (CABG)
2. Angiographically documented non-obstructive coronary artery disease (CAD)
3. Left ventricular ejection fraction (LVEF) > 40%
4. Clinically stable (symptoms and medication) for > 2 weeks 5.18 - 75 yrs of age

General exclusion criteria:

1. Symptoms of ischemia
2. Significant left main stem stenosis
3. NYHA class III-IV symptoms
4. Compromising cardiac ventricular arrhythmia
5. Significant valvular heart disease
6. Inability to comply with guidelines for participation in exercise testing and training
7. Significant limiting comorbidities that would prevent full participation

Additional exclusion criteria:

Further to the analysis of cardiopulmonary exercise test and echocardiography at baseline, and prior to randomisation, patients will be prevented from continuing their involvement in the study if there is indication of:

1. Exercise induced ischemia or significant hemodynamic compromise
2. LVEF < 40%
3. Clinical instability in accordance with CR guidelines
4. Inability to comply with guidelines for participation in exercise testing and training