Cardio-respiratory responses to hypoxia combined with CO₂ inhalation during maximal exercise

We measured the effects of adding CO₂ to an inhaled hypoxic gas mixture on cardio-respiratory parameters during maximal exercise. Eight young males performed four incremental maximal exercise tests on cycle under ambient air, hypoxia (FIO₂ 0.125), inhaled CO₂ (FICO₂ 0.045), and combination of hypoxia and inhaled CO₂. The highest ventilation (VE) and VE/CO₂ output were recorded in CO₂ inhalation and combined treatments. Arterial O₂ partial pressure was higher in combined than in hypoxia treatment, but the difference between the treatments narrowed from rest to end-exercise, at least partly because the magnitude of the increase in VE (%) at exercise was smaller in combined treatment than in hypoxia. Arterial O₂ content was higher in combined treatment than in hypoxia at rest, but no more at maximal exercise. Cardiac output was higher and O₂ extraction lower when breathing O₂-poor gas mixtures than under the two other treatments. For a given oxygen consumption, hypoxia and combined treatment showed similar cardiac output and O₂ extraction.