Exercise in acute and subacute hypoxic conditions: physiological responses affecting oxygen delivery and their applications to human health

Exercise at altitude is performed by a spectrum of individuals ranging from healthy to those with cardiovascular and pulmonary diseases. Reduced atmospheric oxygen pressure at increased altitude reduces oxygen availability and poses unique challenges during exercise. This review explores the physiological responses to exercise under acute and subacute exposure to hypoxia within the autonomic, pulmonary, cardiovascular and haematologic systems. In response to acutely reduced arterial pressure of oxygen, peripheral chemoreceptors stimulate centrally mediated reflexes including the hypoxic ventilatory response and increase sympathetic nerve-mediated activity to the peripheral vasculature and skeletal muscle. Additionally, reduced pressure of oxygen in the pulmonary alveoli and kidneys stimulate hypoxic pulmonary vasoconstriction and erythropoiesis, respectively. During acute hypoxic exercise, heart rate and minute ventilation are greater for any given workload, and maximal oxygen consumption is progressively reduced with increasing altitude. For individuals with cardiovascular and/or pulmonary diseases, exertional hypoxaemia and constraints related to the hypoxic ventilatory response and hypoxic pulmonary vasoconstriction further limit exercise at altitude and increase the risk of adverse events. Exercise in acute hypoxic conditions has been evaluated in patients with stable, mild-to-moderate cardiovascular and/or pulmonary diseases, where it is associated with anticipated decrements in exercise performance but appears to be largely tolerated. However, further study is required to counsel patients across a spectrum of disease severity. Finally, hypoxic exercise has been investigated in limited settings as a therapeutic intervention, including for overweight/obesity, right ventricular remodelling and ageing, but further research is needed to determine the potential roles for hypoxic exercise as therapy.