THE EFFECTS OF LOW-LEVELS OF CO2 ON VENTILATION DURING REST AND EXERCISE

Background:Measurements of pulmonary gas exchange are especially sensi tive to low levels of CO2 in the environment; this is an important con sideration in measurements in enclosed spaces. Methods: In order to de termine the responses to these low levels, subjects were exposed in fi ve 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 1 5 mmHg during a progressive exercise to (V) over dot O(2)max on a cycl e ergometer. Results: In the two resting studies. total pulmonary vent ilation 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 c lear evidence of CO2 retention in both studies. During exercise at 15 mmHg the (V) over dot O(2)max was reduced significantly by 13%, compar ed with air at about the same maximal ventilation, but (V) over dot O( 2)max was not reduced at 9.4 mmHg. A 6% decrease in (V) over dot O(2)m ax 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 th ree exercise studies was always lower with CO2 than corresponding air measurements, indicating CO2 storage. Evaluation of submaximal mea surements provided an equation for predicting ventilation as a functio n of PICO2 and (V) over dot O-2/(V) over dot O(2)max and demonstrated that ventilation during submaximal exercise is increased significantly by the lowest CO2 level. BP and heart rate responses during submaxima l and maximal work were not predictably altered by CO2 at these levels . Conclusion: These studies demonstrate that minimal CO2 levels have s ignificant influences on pulmonary ventilation during rest and exercis e and must be considered in acute studies in confined spaces such as s pace cabins. The inspired CO2 should be stated when ventilation measur ements are reported under these conditions.