The Critical Transmural Pressure of the Airway

Abstract
The critical transmural pressure (Ptm.) is defined as the transmural pressure of the [human] airway at the site where and when flow is limited during a forced expiration. According to the presented theory, the maximal expiratory flow .ovrhdot.Vmax can be calculated from the relation between Ptm. and the corresponding cross-sectional area of the airway (A.). By means of a pitot-static tube, Ptm-A curves were constructed for several locations in the elastic airway of a mechanical model. From these curves, local .ovrhdot.Vmax was calculated at different values of Ptm and compared with actual flow, i.e., measured .ovrhdot.Vmax for the entire airway. In the downstream part of the airway, the actual flow equalled calculated .ovrhdot.Vmax at most Ptm values. The site of flow limitation, being the most upstream point where actual flow equals calculated local .ovrhdot.Vmax could therefore be located. Theory and experiments showed positive as well as negative Ptm. not influenced by change in upstream or downstream resistance. Flow limitation could therefore be initiated at distending as well as compressing pressures across the wall of the airway. .ovrhdot.V was regarded as a function of Ptm and the elastic recoil pressure of the lung (Pel). Measured and caluclated iso-Pel, Ptm-.ovrhdot.V curves agreed well with 1 major exception: when Ptm<Ptm. measured curves were distorted due to a concomitant downstream compression of the collapsible airway.