|Table of Contents: ||Coronary artery disease (CAD) is the progressive narrowing of the arteries supplying the heart muscle from oxygen and other nutrients. It is the main cause of death in Europe and worldwide, accounting for 20% of all deaths. This progressive narrowing can lead to an acute myocardial infarction. Coronary artery bypass grafting (CABG) or percutaneous coronary intervention (PCI) are frequently used procedures in severe CAD. Patients with CAD receive a multidisciplinary treatment including medication, education, psychological support, dietary changes and smoking cessation. However, the core component in their treatment is structured exercise training, since exercise-based cardiac rehabilitation decreases all-cause (20%) and cardiovascular mortality (26%) substantially compared to conventional treatment. These improved survival rates are mediated through training-related increases in maximal exercise capacity (peak oxygen uptake; peak VO2). An increase of 1 ml/kg/min in peak VO2 is associated with a 15% decrease in mortality. The main purpose of exercise-based cardiac rehabilitation is therefore to optimally increase the peak VO2.
Even though current programs seem to be effective, the search for the most optimal exercise characteristics (duration, frequency, intensity, mode) is still ongoing. Current practice includes continuous endurance training (20-45 minutes) at moderate intensity (40-80% of peak VO2). However, there’s evidence that higher intensities (80-90% of peak VO2) are more effective to increase peak VO2. Yet, these high intensities cannot be sustained for a long duration and thus require an interval structure in which high intensity bouts (30 seconds – 4 minutes) are alternated by periods of relative rest (30 seconds – 3 minutes). In 2004, aerobic interval training at high intensities (AIT) was introduced in cardiac rehabilitation for CAD patients. Small single-centre studies comparing this AIT with continuous training at moderate intensity (MCT) showed more favourable results after AIT, or equal effects after both interventions.
Therefore, the main goal of this doctoral research was to investigate the short- and long-term effects of AIT and MCT in patients with CAD, using meta-analytic statistics (summarizing existing literature) and by designing a large multicentre interventional study (SAINTEX-CAD = Study on Aerobic INTerval EXercise in Coronary Artery Disease patients).
From our meta-analysis including 9 studies, we could conclude that AIT was more effective compared to MCT in increasing peak VO2; a mean difference of 1.60 ml/kg/min was found, which is clinically relevant. Mean increases were 20.5% after AIT (100 patients) and 12.8% after MCT (106 patients), the latter being low compared to existing literature. In contrast, our intervention study, in which 200 patients performed 3 weekly AIT or MCT cycling sessions during 12 weeks, resulted in similar improvements after both training interventions (AIT +22.7%, MCT +20.3%). To clarify this inconvenience, a final study was set up in which we objectively measured the energy expenditure of frequently used AIT and MCT protocols, including those of the SAINTEX-CAD study. We could conclude that AIT is more efficient than MCT to increase peak VO2, since a lower energy expenditure and a shorter duration are needed for similar improvements in peak VO2. On the other hand, energy expenditure is not a main goal of exercise-based cardiac rehabilitation, making it worth saying that MCT is equally effective when intensity is sufficiently high and duration is prolonged (intensity x duration = energy expenditure). Patients’ preferences should be met, in order to increase the intrinsic motivation to ensure a lifelong physical activity behaviour. In the SAINTEX-CAD study we observed that patients preserved a satisfactory physical activity level, with more than 90% of all patients meeting the guidelines of the World Health Organization to be physically active for at least 150 minutes per week at moderate intensity. This resulted in a sustained peak VO2 nine months after finishing the centre-based AIT or ACT intervention, which is good for the prognosis of the CAD patients.