The Critical Transmural Pressure of the Airway

Publisher Website
Google Scholar
Abstract
The critical transmural pressure (Ptm.) is defined as the transmural pressure of the airway at the site where and when flow is limited during a forced expiration. According to the presented theory, the maximal expiratory flow (V̇max) 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 V̇max was calculated at different values of Ptm and compared with actual flow, i.e. measured V̇max for the entire airway. In the downstream part of the airway, the actual flow equalled calculated V̇max at most Ptm values. The site of flow limitation, being the most upstream point where actual flow equals calculated local V̇max 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. V̇ was regarded as a function of Ptm and the elastic recoil pressure of the lung (Pel). Measured and calculated iso‐Pel, Ptm‐V̇ curves agreed well with one major exception: when Ptm < Ptm measured curves were distorted due to a concomitant downstream compression of the collapsible airway.