Journal of Applied Physiology, Vol 61, Issue 1 75-80, Copyright © 1986 by American Physiological Society
Modification of pulmonary gas mixing by postural changes
H. A. Jones, E. E. Davies and J. M. Hughes
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.
