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Editor’s note: Altitude exposure is associated with a reduction in exercise capacity, which is mainly explained by a reduced level of arterial oxygenation. However, reductions in maximal cardiac output, leg blood flow, and impaired pulmonary gas exchange may also contribute to reduced exercise capacity. In addition, impaired right ventricular function could be involved, as inhibition of hypoxic pulmonary vasoconstriction by the phosphodiesterase-5 (PDE-5) inhibitor sildenafil has been reported to improve maximum workload in hypoxia.