Pulmonary resistance and dynamic compliance as functions of respiratory frequency

The behavior of pulmonary resistance (RL) and that of dynamic compliance (Cdyn) as functions of respiratory frequency (f) were compared in normal subjects and in patients with chronic obstructive pulmonary disease (COPD). Although both RL and Cdyn varied with f in most COPD patients and in some normal subjects, Cdyn appeared to be more sensitive than RL to variations of frequency; no significant changes in RL could be demonstrated in subjects in whom Cdyn was frequency dependent. The degree of frequency dependence of Cdyn was correlated with the severity of respiratory impairment (as quantified by conventional pulmonary function tests); in contrast, RL was not detectably frequency dependent in some subjects with advanced COPD. Compared with Cdyn, RL was more commonly nonlinearly related to respiratory frequency, often increasing at high breathing rates. Our results indicate that changes in RL with frequency are less predictable and more difficult to detect than the corresponding changes in Cdyn.