Journal of Applied Physiology, Vol 51, Issue 2 243-247, Copyright © 1981 by American Physiological Society
Hindrance to diffusive gas mixing in the lung in hyperbaric environments
H. D. Van Liew, E. D. Thalmann and D. K. Sponholtz
Diffusivity of a gas is inversely proportional to atmospheric pressure. We
studied pulmonary gas mixing in hyperbaric environments (5.5 and 9.5 ATA)
as a means of understanding the role played by diffusion in normal
situations and also as a means of determining whether persons in hyperbaric
environments will be handicapped by poor diffusive mixing. Our subjects
took single breaths of a mixture of indicator gases (5% each of SF6, Ar,
Ne, and He; 20% O2, balance N2). Recordings of expired volumes and
concentrations showed that heavier indicators were less well mixed than
lighter ones, as evidenced by a slower fall during the transition between
dead space and "alveolar" gas and a steeper slope of the alveolar plateau.
Differences between light and heavy gases increased as pressure increased.
Amounts of the indicators retained in the functional residual capacity
(FRC) or residual volume after a single breath had a weak positive relation
to diffusivity; the amounts (A) in the FRC (as fraction of inspired
amounts) were well fitted by a simple equation. ARFC/AI = 0.55 -
(0.0010/D), where D is molecular diffusivity. We conclude that the changes
of distribution of inspired gas that occur with large changes of
diffusivity have only a minor effect on the amount of gas exchanged between
the inspirate and residual gas in the FRC.
