Effects of changes in ventilation on respiratory discomfort during isocapnic exercise

We studied the effects of changes in the level of ventilation on respiratory discomfort during isocapnic exercise. Six subjects exercised at 60 W on a cycle ergometer. They initially breathed 2.5% CO 2 in oxygen for 75 breaths. Ventilatory parameters of the last 10 breaths were used as controls. In 7 subsequent test sessions each lasting 2 min alternating with 2 min free breathing periods, subjects tried to achieve a target ventilation ranging from 0.7 to 1.3 times the control with a visual feedback system. End-tidal CO 2 was regulated automatically at the control level by changing the inspired CO 2. Breathing discomfort was measured at the end of each session using a visual analog scale. Isocapnic constraining of ventilation (0.7 times the control) during exercise significantly increased respiratory discomfort ( p < 0.05). Increases in ventilation had no significant effect. These results indicate that respiratory discomfort during exercise is exaggerated when the level of ventilation falls below that spontaneously adopted even when chemical drives are held constant.