The effect of CO 2 on ventilation and breath‐holding during exercise and while breathing through an added resistance

1. Ventilation was measured while subjects were made to rebreathe from a bag containing CO 2 and O 2 in order to expose them to a steadily rising CO 2 tension ( P CO 2 ). The object of the experiments was to determine the effect of a variety of stimuli upon the increase in ventilation and fall in breath‐holding time which occurs in response to the rising P CO 2 . 2. Steady‐state exercise at 200 kg.m/min resulted in a small fall in the slope of the ventilation—CO 2 response curve ( S V ) and a small, though not statistically significant, fall in the P CO 2 at which ventilation would be zero by extrapolation ( B V ). There was a marked fall in the slope of the breath‐holding—CO 2 response curve ( S BH ) and an increase in the P CO 2 at which breath‐holding time became zero by extrapolation ( B BH ). 3. These results have been interpreted with the aid of a model of the control of breath‐holding and it is suggested that there is no change in CO 2 sensitivity on exercise, either during rebreathing or breath‐holding. 4. An increase in the resistance to breathing caused a marked reduction in S V and B V , but no change in the breath‐holding—CO 2 response curve. These findings suggest that the flattening of the ventilation—CO 2 response curve is mechanical in origin and acute airway obstruction produces no change in CO 2 sensitivity. 5. On the basis of these results, we suggest that more information about CO 2 sensitivity can be obtained by a combination of ventilation and breath‐holding—CO 2 response curves.