Density dependence of maximum expiratory flow in the dog.

Airway lateral and impaction pressures were measured during expiratory flow limitation in six anesthetized, vagotomized, tracheally intubated, open-chest dogs with the lungs filled with air or a mixture of 80% helium-20% oxygen (HeO2). Pressures were measured in the vicinity of equal pressure points (EPP) and choke points (CP). Maximum flow (Vmax) was ensured by demonstrating no increase in flow with a 50% increase of driving pressure. At 50% vital capacity, mean density dependence (VmaxHeO2/Vmaxair) was 1.58, which was less than 1.69 predicted for fully density-dependent flow. Transmural pressure and airway area at CP and EPP (located on air) were significantly less with HeO2 than with air. Frictional losses between the alveoli and CP were 40% greater with HeO2 than with air. These enhanced losses were mostly peripheral to the EPP. Frictional loss upstream from the EPP was 47% of the total pressure loss on air and increased to 70% on HeO2. The data at 50% VC suggest that these higher frictional losses with HeO2 resulted in decreased density dependence of Vmax due to different pressure distribution along the airway with a lower transmural pressure and smaller area at the CP.