The effects of hypoxia and hypercapnia on perceived breathlessness during exercise in humans

1. The sensation of breathlessness increases when ventilation is reflexly stimulated but it is not clear whether different stimuli have specific effects in the genesis of this sensation. 2. Our aim was to compare subjective assessments of the intensity of breathlessness at the same levels of ventilation induced by different combinations of reflex ventilatory stimuli. 3. Against a background of progressive exercise (maximum workload 170 W) in 'blinded' normal naive subjects, normoxic hypercapnia (maximum end-tidal CO2, PET, CO2, 56 mmHg) or isocapnic hypoxia (minimum O2 saturation 88%) was induced to achieve levels of ventilation (maximum 60 l min-1) 'matched' with those resulting from a higher intensity of exercise alone. Subjective breathlessness was rated with a visual analogue scale. 4. For a given ventilation, compared with exercise alone, breathlessness scores were similar during hypercapnia and were lower during hypoxia. 5. These results do not support the idea that during exercise, hypercapnia or hypoxia has a specific role in the genesis of the sensation of breathlessness. 6. The findings are consistent with the hypothesis that the degree of reflex ventilatory activation, however achieved, is an important determinant of the intensity of perceived breathlessness in healthy humans.