The Respiratory Response to Inhaled Carbon Dioxide in Man after 3 Hours Exposure to 3% Carbon Dioxide

1. The respiratory response to inhaled 3% and 6% CO₂ was measured in 10 normal subjects after a 3 h acclimatization period to 3% CO₂ in an environmental chamber. Control studies were carried out after a 3 h period of breathing air in the chamber.

2. At the end of the acclimatization period studies were carried out during 20 min periods breathing 3% CO₂, 6% CO₂ and air.

3. At 2-min intervals during the studies measurements were made of tidal volume (V_t), breathing frequency (f_R), minute ventilation (V̇_e), viscous pulmonary rate of work (Ẇ_p) and total viscous rate of work across the lungs and apparatus (Ẇ_t). Blood gas tensions were measured at the end of this period.

4. After acclimatization to 3% CO₂ there was a significant shift in the response curves V̇_e/Pa,co₂ and Ẇ_t/Pa,co₂ such that subjects showed higher Pa,co₂ values for given values of V̇_e or Ẇ_t. There was no significant change in the slope of the response curves.

5. No correlation was found between the slope of the response curve after the control period breathing air and the degree of shift of the response curve.

6. There was no difference in respiratory pattern or in pulmonary resistance.

7. Similar results were found in two subjects studied after 24 h acclimatization to 3% CO₂ but one subject also showed a significant change in the slope of the V̇_e/Pa,co₂ curve.