Modification of pulmonary gas mixing by postural changes.

Mixing for two gases of markedly different gaseous diffusivity, helium (He) (mol wt = 4) and sulfur hexafluoride (SF6) (mol wt = 146) has been studied by a rebreathing method in different postures. In nine normal subjects duplicate measurements were made in the erect (seated), supine, and lateral decubitus posture, at a constant tidal volume (700 ml) and frequency (1 Hz) starting from functional residual capacity (FRC). Additional measurements were made on four of the subjects, rebreathing seated erect at a volume similar to the relaxed FRC supine and supine at a volume similar to the relaxed FRC seated. In the supine posture the mean breath number to reach 99% equilibrium (n99), was not significantly different for the two gases, 8.9 for He and 9.8 for SF6. There was a difference (P less than 0.01) when erect; n99 (He) = 8.2 and n99 (SF6) = 10.9. The greatest He-SF6 difference (P less than 0.001) was in the lateral decubitus position n99 (He) = 10.1 and n99 (SF6) = 15.9. The mean relaxed FRC as percent of seated was 71% supine and 75% in lateral decubitus posture. Rebreathing seated at a lower volume did not abolish the He-SF6 mixing difference nor did rebreathing at a higher volume when supine induce a He-SF6 mixing difference. Thus the effect of posture on gas mixing cannot be due solely to lung volume and must represent a convective and diffusive dependent change in the distribution of ventilation per unit lung volume.