Relationship between frequency dependence of lung compliance and distribution of ventilation

The previously demonstrated empirical association between frequency dependence of lung compliance and distribution of ventilation, the latter determined by the N(2) washout technique, was confirmed by establishing a mathematical link between the two tests. By assuming a two-compartment system with known compliances and making corrections for Pendelluft and common dead space mixing effects, the ratio of dynamic to static compliance (C(dyn)/C(st)) for any respiratory frequency can be calculated from the compartmental analysis of the N(2) washout at a single respiratory frequency. By using these equations, a good correlation was found between calculated and measured C(dyn)/C(st) in dogs with artificially induced bronchial obstruction and in young smokers or young nonsmokers after carbachol inhalation. A two-compartment N(2) washout was demonstrated in 10 young healthy smokers at one or two respiratory frequencies whereas all 10 normal controls showed a single exponential curve. These findings indicate that the non-invasive N(2) washout test is capable of predicting C(dyn)/C(st) and at the same time gives a direct measure of gas distribution. Further, it appears to be a highly sensitive method for the detection of "small airway disease."