[Variation of effective compliance with respiratory frequency. Test of inhomogeneity of the ventilatory mechanical system (author's transl)].

A decrease in effective compliance (Ce) with increase in respiratory frequency (f) is considered as representing an inhomogeneity in the distribution of the pulmonary time constants. The Ce variation in relation with f reflects the inadequacy of the first order linear mono-alveolar model usually used to describe the ventilatory mechanical properties. Such an assertion is widely accepted, but few attempts have been done to validate this point in patients and to determine the various components likely to affect the Ce, f relationship. In 171 patients, selected in order to sample different types of lesions of some pulmonary structures, the authors calculated the next functional parameters : VC/theoretical VC ratio, RV by dilution method and by plethysmography, airway resistance and distribution index of inspired gas. Moreover, by means of a computerised system, the effective compliance and its variation with respiratory frequency have been calculated. After discussion of the procedure and of the method used to express the compliance-frequency relationship (linear regression), the authors develop the arguments which allow to assert that despite the elementary type of expression the relation is a good discriminative parameter. Its signification is discussed by analysing results obtained in patients and by simulating a mathematical model of the ventilatory mechanics where the mechanical parameters are distributed. It seems likely that a negative slope of the compliance-frequency relationship expresses an important mechanical inhomogeneity. Taking into account the fact that the whole frequency-compliance relationship gives little information, at this moment, in the face of an unpleasant test for the patients, the authors propose a simplified procedure, adapted to the usual practice.