Effects of interval hypoxia on exercise tolerance: special focus on patients with CAD or COPD.

INTRODUCTION: Repeated short-term hypoxia (interval hypoxia) has been suggested to increase exercise tolerance by enhancing stress resistance and/or improving oxygen delivery. As low exercise tolerance contributes to mortality in patients with coronary artery disease (CAD) and chronic obstructive pulmonary disease (COPD), interval hypoxia might be a valuable preventive and therapeutic tool for these patients. Yet, mechanisms responsible for the improvement of exercise tolerance are still largely unknown. Therefore, this review intends to present an overview for better understanding of such mechanisms and to stimulate further research work on this important topic. DATA SOURCE: Articles were selected from a search of the PubMed database up to 2009 using the search terms hypoxia, intermittent, interval in various combinations with exercise, capacity, tolerance, CAD, COPD, and various haematological and cardio-respiratory parameters.
RESULTS: Generally, the effects of 2-4 weeks of interval hypoxia on exercise tolerance are contrasting. Whereas aerobic exercise performance improved or remained unchanged, anaerobic performance tended even to worsen. Benefits on exercise tolerance seem to be greater in patients with CAD or COPD when compared to healthy subjects. DISCUSSION: The mechanisms responsible for these benefits are the increases in total haemoglobin mass, lung diffusion capacity, more efficient ventilation, and a decrease in the responsiveness of the adrenergic system to stimulation and/or an increase in parasympathetic activity. If confirmed in further studies, interval hypoxia might become an attractive strategy to complement the known beneficial effects of exercise training, especially in patients with CAD or COPD.