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

We measured the effects of adding CO 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 (FIO 0.125), inhaled CO (FICO 0.045), and combination of hypoxia and inhaled CO. The highest ventilation (VE) and VE/CO output were recorded in CO inhalation and combined treatments. Arterial O 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 O content was higher in combined treatment than in hypoxia at rest, but no more at maximal exercise. Cardiac output was higher and O extraction lower when breathing O-poor gas mixtures than under the two other treatments. For a given oxygen consumption, hypoxia and combined treatment showed similar cardiac output and O extraction.