Effect of increased gas density on pulmonary gas exchange in man

Abstract

Pulmonary gas exchange was measured in 7 resting supine subjects breathing air or a dense gas mixture containing 21% O2 in SF6. The mean value of the alveolar-arterial oxygen difference (AaDO2) decreased from 12.4 on air to 7.0 on SF6 (P < 0.01), and increased again to 13.4 when air breathing resumed (P < 0.01). No differences occurred between gas mixtures for O2 consumption, respiratory quotient, minute ventilation, breathing frequency, heart rate or blood pressure, and the improved oxygen transfer could not be attributed to changes in cardiac output or mixed venous oxygen content in the 1 subject in which they were measured. There may be an altered distribution of ventilation during dense gas breathing, so that the ventilation-perfusion ratio (.ovrhdot.VA/Q) variance was reduced. Of several mechanisms, the one favored is that in which SF6 promotes cardiogenic gas mixing between peripheral parallel units having different alveolar gas concentrations. This mechanism allows for observed increases in arterial carbon dioxide tension and dead-space-to-tidal volume ratio during dense gas breathing, and suggests that intraregional .ovrhdot.VA/Q variance accounts for at least 1/2 of the resting AaDO2 in healthy supine young men.