Dose dependency of perceived breathlessness on hypoventilation during exercise in normal subjects

To determine whether a dose-dependent relationship exists between the subjective sensation of breathlessness and hypoventilation during steady-state exercise, we measured breathlessness at six levels of volitionally suppressed ventilation. To achieve this, subjects targeted their breathing at 0, 5, 10, 15, 20, and 25% below their spontaneous exercise level. All 12 subjects were successful in hypoventilating in a graded manner. However, in general, the degree of hypoventilation achieved was less than that of the target level set; this discrepancy was greatest at the higher target levels. Volitional hypoventilation at target levels of > or = 10% caused significant decreases in ventilation and significant increases in end-tidal PCO2. All levels of volitional hypoventilation caused increased ratings of breathlessness, reaching statistical significance at a set target level of 15%. Significant increases in breathlessness intensity were associated with increases in end-tidal PCO2 of 2-3 Torr. We conclude that, during steady-state exercise, there appears to be a dose-dependent relationship between breathlessness and volitionally induced inappropriately low ventilation. The need to minimize such subjective sensations of breathlessness may play a role in the increased ventilation observed during exercise.