Effects of gas density on pulmonary gas exchange of normal man at rest and during exercise

Changes in the physical properties of inspired gas might be expected to influence the distribution of ventilation in the lungs as well as the diffusive and convective (cardiogenic) mixing of inspired gas with lung residual gas, thus possibly affecting pulmonary gas exchange for O2 and CO2. The purpose of our work was to assess to what extent this occurs in practise in human subjects, who could compensate for the changes directly brought about by altering the physical characteristics of the inhaled gas by changing their breathing pattern. Six healthy, non-smoking men breathed, at rest and during moderate exercise, gas mixtures containing 21% oxygen completed either by 79% nitrogen (air), helium (O2-He) or sulphur hexafluoride (O2-SF6). We observed that the inhalation of these three different gas mixtures whilst at rest did not affect arterial partial pressures of O2 or CO2, the physiological dead space to tidal volume ratio, or the alveolo-aADCO2). During exercise, AaDO2 was slightly (2-3 mm Hg) but significantly higher with both O2-He and O2-SF6 than with air. Although minute ventilation did not change, breathing frequency was slightly but significantly affected by the type of gas mixture breathed, being lower with O2-SF6 and higher with O2-He. We conclude that, within the range studied, the physical properties of the inhaled gas do not affect pulmonary gas exchange in healthy man, either because the changes affected are minimal or because they compensate for each other.