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.