Altered chemosensitivity to CO₂ during exercise

The effect of exercise on chemosensitivity to carbon dioxide (CO₂ ) has been controversial. Most studies have been based on rebreathing to alter inspired CO₂ which is poorly tolerated in exercise. Instead, inhaling a fixed 3% CO₂ from rest to moderate exercise was found to be well tolerated by seven normal subjects enabling CO₂ chemosensitivity to be studied with minimal negative reaction. Results showed that chemosensitivity to CO₂ following 5-6 min of stimulation was significantly enhanced during mild exercise (p < 0.01). This motivated exploring how much of the dynamic ventilatory response to mild exercise breathing air could be predicted by a model with central and peripheral chemosensitivity. Chemoreceptor stimulation combined with hypercapnia has been associated with long-term facilitation of ventilation (LTF). 3% CO₂ inhalation during moderate exercise led to ventilation augmentation consistent with LTF following 6 min of exercise in seven normal human subjects (p < 0.01). Increased ventilation could not be attributed to hypercapnia or metabolic changes. Moderate exercise breathing air resulted in significantly less augmentation. In conclusion, both peripheral and central chemosensitivity to CO₂ increased in exercise with the peripheral chemoreceptors playing a dominant role. This separation of central and peripheral contributions was not previously reported. This chemoreceptor stimulation can lead to augmented ventilation consistent with LTF.