The ventilatory response to carbon dioxide in hyperoxic exercise.

The sensitivity of the ventilatory response to carbon dioxide in hyperoxia during light (25 W) exercise was compared to that at rest in 14 volunteers. The method used was that of rebreathing. Two factors were found to produce artefactual changes in the slope of the response during exercise. First, breath-by-breath response lines showed that the maximum limit of ventilation was reached in 3 volunteers before the end of rebreathing, despite the low exercise load. The inclusion of such breaths in the calculation of the slope of the response could produce an artefactual decrease in slope. Second, most of the response lines showed an increase in their slope during exercise. However, a model of rebreathing in exercise showed that an increase in sensitivity could be the result of variation in the difference between end-tidal and central chemoreceptor carbon dioxide levels during exercise. A criterion derived from the model, proportional to the variation in this difference, was found to be correlated with the increase in sensitivity from rest to exercise. It was therefore concluded that the sensitivity of the ventilatory response to carbon dioxide during light exercise is unchanged from that at rest.