Optimizing Outcomes in Cardiac Rehabilitation: The Importance of Exercise Intensity

Abstract

Exercise based cardiac rehabilitation (CR) is recognized internationally as a class 1 clinical practice recommendation for patients with select cardiovascular diseases and heart failure with reduced ejection fraction. Over the past decade, several meta-analyses have generated debate regarding the effectiveness of exercise-based CR for reducing all-cause and cardiovascular mortality. A common theme highlighted in these meta-analyses is the heterogeneity and/or lack of detail regarding exercise prescription methodology within CR programs. Currently there is no international consensus on exercise prescription for CR, and exercise intensity recommendations vary considerably between countries from light-moderate intensity to moderate intensity to moderate-vigorous intensity. As cardiorespiratory fitness [peak oxygen uptake (VO₂peak)] is a strong predictor of mortality in patients with coronary heart disease and heart failure, exercise prescription that optimizes improvement in cardiorespiratory fitness and exercise capacity is a critical consideration for the efficacy of CR programming. This review will examine the evidence for prescribing higher-intensity aerobic exercise in CR, including the role of high-intensity interval training. This discussion will highlight the beneficial physiological adaptations to pulmonary, cardiac, vascular, and skeletal muscle systems associated with moderate-vigorous exercise training in patients with coronary heart disease and heart failure. Moreover, this review will propose how varying interval exercise protocols (such as short-duration or long-duration interval training) and exercise progression models may influence central and peripheral physiological adaptations. Importantly, a key focus of this review is to provide clinically-relevant recommendations and strategies to optimize prescription of exercise intensity while maximizing safety in patients attending CR programs.

Keywords: cardiorespiratory fitness; cardiovascular disease; coronary artery disease; exercise prescription; heart failure; interval training; peak oxygen consumption; progression.

Figure 1

Physiological adaptations from exercise training that may contribute to improvement in VO₂peak in patients with cardiovascular disease or heart failure. Cardiac adaptations contribute to VO₂peak improvements primarily through increases in stroke volume. Pulmonary adaptations contribute to VO₂peak by optimizing arterial oxygen content and therefore oxygen delivery. Muscle adaptations contribute to VO₂peak improvements through increased exercise tolerance and enhanced oxygen extraction. Vascular adaptations contribute to oxygen delivery by reducing arterial resistance, increasing capillary density, and enhancing blood flow distribution, which in turn enhances function of the cardiac, pulmonary, and muscle systems. Increases in plasma volume can occur within days of commencing exercise training. Adaptations within skeletal muscle and to the vasculature can occur within weeks of training. Current evidence supports greater adaptations in mitochondrial content, vascular vasodilatory function, and stroke volume following HIIT compared with MICT, although the precise cardiac mechanisms that lead to increased stroke volume remain unclear. VO₂peak, peak oxygen uptake; SV, stroke volume; HR, heart rate; a–vO₂ difference, oxygen content difference between arterial and venous blood; LV, left ventricle.

Figure 2

Example of a HIIT progression model within a cardiac rehabilitation program. Exercise intensity remains constant for each HIIT protocol with high intensity intervals eliciting 85-95 %HRpeak and RPE 15-18, and the low intensity intervals involving recovery at 50-75 %HRpeak or RPE 11-14. CR, cardiac rehabilitation; HIIT, high intensity interval training; HRpeak, peak heart rate; MICT, moderate intensity continuous training; RPE, rating of perceived exertion on 6-20 Borg scale; VO₂peak, peak oxygen consumption. This figure has been adapted from the previously published work of (158); with permission of Mayo Foundation for Medical Education and Research, all rights reserved.