Density dependence of maximal flow in dogs with central and peripheral obstruction

In 12 anesthetized, tracheotomized, vagotomized, open-chested, mongrel dogs we measured end and side hole airway pressures during forced expiration using a Pitot static probe. Volume was obtained as the integral of flow from a dog plethysmograph with frequency response adequate to 20 Hz. Equal pressure points (EPPs) and choke points (CPs) were located with dogs breathing air or a mixture of 80% helium-20% oxygen (HeO2) before and after partial obstruction of the trachea and intravenous histamine and propranolol. At 50% of vital capacity (VC) the CP was in the trachea in 11 of 12 dogs. Partial obstruction of the trachea decreased flow during the plateau of the maximum expiratory flow-volume curve (MEFVC) with the CP remaining in the trachea. The MEFVC plateau was extended to a lower lung volume. At 50% of VC the EPP moved downstream and density dependence remained high. Histamine and propranolol caused EPPs and CPs to move towards the periphery and density dependence to decrease. The shape of the MEFVC changed as the plateau was shortened and, in some instances, abolished. A plateau on the MEFVC could be regenerated by partial obstruction of the trachea. This was accompanied by return of the CP to the trachea and an increase in density dependence. Changes in density dependence were found to be a result of both the relocation of sites of flow limitation and differences in local CP areas with HeO2 and air.