[Closing volume and inhomogeneity of the ventilatory mechanical system (author's transl)].

The use of the closing volume (VF) to detect small airway lesions is based on physiological data : it would reflect a special and physiological distribution of the pulmonary inhomogeneity. The aim of this work is to discuss the closing volume as used to determine a pathological process or, in other words, the relationship between the observed profile of closing volume and other functional parameters, whose abnormalities are likely to reflect the inhomogeneity of the ventilatory mechanical system. In 126 patients, who represent a wide range of pathological processes, the authors calculated the closing volume and the following functional parameters : the VC/theoretical VC ratio, the RV measured by dilution method and by plethysmography, the FEV1.0/VC ratio, the expiratory total pulmonary resistance (RPTE), the efficient resistance (R), the airway resistance (Raw), the effective compliance (Ce) measured at the spontaneous respiratory frequency, and its variation in relation with respiratory frequency (f), and a distribution index of inspired gas. After discussion of the procedure and of the reproducibility of the closing volume measurements, the authors recall the significant of the lack of phase IV during the closing volume estimation and expose the reasons which allow to think that closing volume extent and inhomogeneity of the ventilatory mechanics have a parallel evolution. The increase in distribution inhomogeneity of the pulmonary time constants (shown by the slope of the Ce variation in relation with f and gas distribution index) is concomitant with an increase in closing volume. The results show that although the lack of phase IV does not have a univocal signification (and this is a limit to the utilization of the closing volume alone as a detection test) the quantification of the closing volume brings, as the Ce, f relation does, an original element, but the evaluation of Ce, f is more difficult to realize in practice.