EFFECT OF CO2 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 CO2 and O2 in order to expose them to a steadily rising CO2 tension (PCO2). 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 PCO2. 2. Steady‐state exercise at 200 kg.m/min resulted in a small fall in the slope of the ventilation—CO2 response curve (SV) and a small, though not statistically significant, fall in the PCO2 at which ventilation would be zero by extrapolation (BV). There was a marked fall in the slope of the breath‐holding—CO2 response curve (SBH) and an increase in the PCO2 at which breath‐holding time became zero by extrapolation (BBH). 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 CO2 sensitivity on exercise, either during rebreathing or breath‐holding. 4. An increase in the resistance to breathing caused a marked reduction in SV and BV, but no change in the breath‐holding—CO2 response curve. These findings suggest that the flattening of the ventilation—CO2 response curve is mechanical in origin and acute airway obstruction produces no change in CO2 sensitivity. 5. On the basis of these results, we suggest that more information about CO2 sensitivity can be obtained by a combination of ventilation and breath‐holding—CO2 response curves. © 1969 The Physiological Society