Penetration of inhaled He and SF6 into alveolar space at low tidal volumes

To study mixing of inspired gas with lung gas, penetration of simultaneously inspired helium (He) and sulfur hexafluoride (SF6) into alveolar space was determined in normal subjects at low tidal volumes (from 50 to 500 ml) and at varied lung volumes and speeds of inspiration/expiration. The volume of inspired gas reaching the alveolar space, termed alveolar-tidal volume, VTA, was calculated from preinspiratory lung volume, inspired volume, and inspired and expired alveolar test gas concentrations. The difference between the VTA values calculated for He and SF6, VTA(He) - VTA(SF6), was influenced by tidal volume, lung volume, and the speed of inspiration/expiration, but it was always positive. The results are qualitatively explainable on the basis of easier diffusive mixing of He in lung airways compared with SF6. Since Taylor dispersion would produce deeper penetration, and therefore higher VTA, for a less diffusible gas the results provide no evidence for its implication in pulmonary gas exchange.