Effect of gas density on dynamic pulmonary compliance.

We measured dynamic pulmonary compliance (Cdyn( in nine asymptomatic young men breathing gases of different density. When corrected for gas inertia, Cdyn was significantly lower during dense gas breathing (sulfur hexafluoride) than during air breathing. At higher breathing frequencies (60-90 breaths/min), Cdyn was greater on helium than on air. Static compliance was not different while breathing the three gas mixtures. These results may be explained by a density dependence of airways resistance in parallel lung units which contribute to frequency dependence of dynamic compliance. We conclude that most frequency-dependent behavior occurs among intraregional lung units subtended from airways between segmental bronchi and peripheral airways.