The effects of low levels of CO2 on ventilation during rest and exercise

Measurements of pulmonary gas exchange are especially sensitive to low levels of CO2 in the environment; this is an important consideration in measurements in enclosed spaces.

In order to determine the responses to these low levels, subjects were exposed in five studies to partial pressures of inspired CO2 (PICO2) of 5.7 and 7.5 mmHg for 30 min during basal conditions at rest and to 5.4, 9.4 and 15 mmHg during a progressive exercise to VO2max on a cycle ergometer.

In the two resting studies, total pulmonary ventilation and alveolar ventilation were increased by 19% at 7.5 mmHg (1.1% sea level equivalent) and 10% at 5.4 mmHg (0.8% equivalent), with clear evidence of CO2 retention in both studies. During exercise at 15 mmHg the VO2max was reduced significantly by 13%, compared with air at about the same maximal ventilation, but VO2max was not reduced at 9.4 mmHg. A 6% decrease in VO2max at a PICO2 of 5.4 mmHg may have resulted from these subjects being less fit. The maximal CO2 output and respiratory exchange ratio in the three exercise studies was always lower with CO2 than corresponding air measurements, indicating CO2 storage. Evaluation of submaximal measurements provided an equation for predicting ventilation as a function of PICO2 and VO2/VO2max and demonstrated that ventilation during submaximal exercise is increased significantly by the lowest CO2 level. BP and heart rate responses during submaximal and maximal work were not predictably altered by CO2 at these levels.

These studies demonstrate that minimal CO2 levels have significant influences on pulmonary ventilation during rest and exercise and must be considered in acute studies in confined spaces such as space cabins. The inspired CO2 should be stated when ventilation measurements are reported under these conditions.