Increased steady-state VO2 and larger O2 deficit with CO2 inhalation during exercise

Aim: To examine whether inhalation of CO₂‐enriched gas would increase steady‐state during exercise and enlarge O₂ deficit. Methods: Ten physically active men ( 53.7 ± 3.6 mL min^?1 kg^?1; ± SD) performed transitions from low‐load cycling (baseline; 40 W) to work rates representing light (≈ 45%; 122 ± 15 W) and heavy (≈ 80%; 253 ± 29 W) exercise while inhaling normal air (air) or a CO₂ mixture (4.2% CO₂, 21% O₂, balance N₂). Gas exchange was measured with Douglas bag technique at baseline and at min 0–2, 2–3 and 5–6. Results: Inhalation of CO₂‐enriched air consistently induced respiratory acidosis with increases in PCO₂ and decreases in capillary blood pH (P < 0.01). Hypercapnic steady‐state was on average about 6% greater (P < 0.01) than with air in both light and heavy exercise, presumably because of increased cost of breathing (ΔVE 40–50 L min^?1; P < 0.01), and a substrate shift towards increased lipid oxidation (decline in R 0.12; P < 0.01). during the first 2 min of exercise were not significantly different whereas the increase in from min 2–3 to min 5–6 in heavy exercise was larger with CO₂ than with air suggesting a greater slow component. As a result, O₂ deficit was greater with hypercapnia in heavy exercise (2.24 ± 0.51 L vs. 1.91 ± 0.45 L; P < 0.05) but not in light (0.64 ± 0.21 L vs. 0.54 ± 0.20 L; ns). Conclusion: Inhalation of CO₂‐enriched air and the ensuing respiratory acidosis increase steady‐state in both light and heavy exercise and enlarges O₂ deficit in heavy exercise.