Ventilatory response to carbon monoxide during exercise in hypoxia and hypercapnia

We tested if the addition of CO to inspired gases with different inspired O₂ and CO₂ fractions (F_IO₂ and F_ICO₂) stimulates ventilation at rest or during submaximal exercise. We measured minute ventilation (V_E) in goats breathing combinations of F_IO₂ ranging from 0.21 to 0.06 and F_ICO₂ from 0 to 0.05, both with and without inspired CO resulting in carboxyhemoglobin fractions (F_HbCO) of 0.02 (no CO added), 0.15, or 0.45. We did this while they stood on a treadmill, walked at 1.4, or trotted at 2.5 m s⁻¹. Hypoxia, hypercapnia, and exercise, alone and in combination, increased ventilation compared to breathing air at rest. Both elevated F_HbCO increased V_E compared with ambient F_HbCO during exercise (increases of 1.50 and 5.53 ml s⁻¹ kg⁻¹ for F_HbCO 0.15 and 0.45, respectively; P = 0.035), but not at rest (P = 0.958), when the ventilatory effects of F_IO₂ and F_ICO₂ are factored out. Additionally, F_HbCO 0.45 (but not 0.15) increased V_E compared to F_HbCO 0.02 for all F_IO₂ and F_ICO₂ when the ventilatory effects of exercise are factored out. Taken together, these data suggest that exercise intensity and F_HbCO dose interact to stimulate ventilation during exercise.