Effectiveness of high-intensity interval training in patients with coronary heart disease: a systematic review protocol

Review question/objective What is the effectiveness of high-intensity interval training for the improvement of cardiorespiratory fitness and other measures of cardiovascular function in patients with coronary heart disease? Background Approximately one third of deaths globally, equivalent to over 16 million deaths, can be attributed to cardiovascular disease.1 Of these, over seven million deaths are due to coronary heart disease (CHD), which is now the leading cause of mortality worldwide. Coronary heart disease is classified as an ischemic heart disease2 characterized by disease of the blood vessels supplying the heart, with atherosclerotic narrowing resulting in reduced blood supply and insufficient oxygenation of the cardiac tissues. Coronary heart disease is associated with a number of risk factors, including alcohol and tobacco use, obesity, abnormal blood lipids, high blood pressure and physical inactivity. Coronary heart conditions also encompass patients with ischemic heart disease, including those who have suffered a myocardial infarction (MI) and those with angina pectoris. High physical activity levels and cardiorespiratory fitness have been shown to be inversely related to cardiovascular disease morbidity and mortality.3,4 Exercise is a key element in secondary prevention and rehabilitation in patients with existing CHD. Cardiac rehabilitation refers to a coordinated, multifaceted intervention designed to improve a patient's physical, psychological and social functioning, as well as slowing, or even reversing, the underlying atherosclerotic processes. A core component of cardiac rehabilitation is exercise training.5 A previous systematic review and meta-analysis that included over 10,000 patients showed significant reductions in both cardiovascular and all cause mortality, as well as reduced hospital admissions, following exercise-based cardiac rehabilitation.6 Additionally, there was also a trend for an improvement in quality of life scores in this population. Despite the recommendation for exercise as a core element of cardiac rehabilitation and the documented benefits of exercise-based cardiac rehabilitation, enrolment into such programs is typically poor7 with participation rates reaching only 15-30% of all eligible patients. Barriers to uptake of cardiac rehabilitation commonly include patients’ lack of knowledge of services, patient identity, perceptions of heart disease, and financial or occupational constraints.8 In this context, elucidating the optimal exercise prescription for achieving maximal physiological and clinical benefit within a given exercise program is highly desirable. The most commonly recommended form of exercise for both clinical and healthy populations is continuous aerobic exercise (CONT) of at least 30 minutes per day, prescribed five to seven days per week.9 However, alternative training methods that reduce the time commitment required to meet exercise guidelines have shown significant advantages in both healthy and diseased individuals.10 Considerable evidence is available to support the role of high-intensity interval training (INT) programs as an effective and time-efficient method of stimulating both central and peripheral adaptations that can be linked to improved health outcomes11,12, including improvements in peak oxygen uptake (VO2peak), which is associated with reduced morbidity and mortality in patients with cardiovascular disease.13,14 In clinical settings, INT commonly refers to multiple, short bouts of exercise (up to four minutes) at a high workload (85-95% of peak heart rate; HRpeak), above the intensity at which exercise can be sustained for greater periods of time. Each interval is separated by active pauses exercising at a lower workload. Specifically, a number of randomized controlled trials (RCTs) have demonstrated the efficacy of INT compared to CONT in patients with heart failure,15 coronary heart disease16,17 and following MI.18,19 Interventions including INT typically range from four to 12 weeks in duration, with exercise sessions attended two to three times per week. Whilst high-intensity exercise is commonly avoided due to perceived risks in a cardiac population, a recent analysis revealed the adverse event rate associated with this mode of training to be low, and thus suitable for patients with CHD.20 Recent meta-analysis21 of seven studies comparing INT versus CONT aerobic exercise training in patients with heart failure revealed greater improvements in cardiorespiratory fitness with INT compared to CONT, but no difference in left ventricular ejection fraction, a measure of cardiac function that is commonly reduced in heart failure patients. Despite the availability of RCTs examining INT versus CONT exercise in patients with CHD, to date there are no systematic reviews available in this population.