Relationship between frequency dependence of lung compliance and distribution of ventilation

Abstract

The previously demonstrated empirical association between frequency dependence of lung compliance and distribution of ventilation, the latter determined by the N2 washout technique, was confirmed by establishing a mathematical link between the 2 tests. By assuming a 2 compartment system with known compliances and making corrections for Pendelluft and common dead space mixing effects, the ratio of dynamic to static compliance (Cdyn/Cst) for any respiratory frequency can be calculated from the compartmental analysis of the N2 washout at a single respiratory frequency. By using these equations, a good correlation was found between calculated and measured Cdyn/Cst in dogs with artificially induced bronchial obstruction and in young smokers or young nonsmokers after carbachol inhalation. A 2 compartment N2 washout was demonstrated in 10 young healthy smokers at one or 2 respiratory frequencies whereas all 10 normal controls showed a single exponential curve. These findings indicate that the noninvasive N2 washout test is capable of predicting Cdyn/Cst 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'. (25 references.)