[Study of maximal expiratory flows with light gases. Theoretical aspects]

The modification of maximum expiratory flows with low gas density breathing was proposed a decade ago as a test of small airway obstruction. This short review discusses the theoretical bases of the method. Three kinds of approaches have been proposed to interpret the findings: the "equal pressure point" concept of Mead et al. has stressed the role of the resistance upstream and of elastic recoil; the "flow limiting segment" concept of Pride et al. has underlined the role of central airways compliance (the compressed segment acting as a resistor); recently, the "choke point theory" proposed by Dawson and Eliott discusses the role of wave speed propagation in the walls of elastic tubes. All these three interpretations take into account the influence of the density of the gas breathed--a factor governing the drop in transmural pressure between the alveoli and the central airways, and a determinant of the speed of pressure wave propagation. In principle, every reduction in gas density should lead to an increase in maximal flow; this modification would be less marked if the flow in the upstream segment is essentially laminar due to obstruction in the peripheral airways. Several points complicating the interpretation are discussed: a) the flow in the small airways is never strictly laminar; b) the change in gas density (usually-breathing a He-O2 mixture) also influences the length of the upstream segment and the pressure wave speed; c) the influence of the cross sectional area and compliance of central airways cannot be neglected.(ABSTRACT TRUNCATED AT 250 WORDS)