Airway and regional alveolar pressures in constant flow ventilation (CFV)

During CFV in anesthetized dogs, bidirectional flow occurs in central airways and alevolar pressure (P(A)) exceeds airway opening pressure (Pao) by a larger amount (about 6 cm H₂O at 1 l/s) than can be attributed to expiratory flow resistance and Bernoulli effect. We investigated the nature of this large resistance to bidirectional flow (V̇) in a plastic airway model. Expiratory pressure-flow relationships were curvilinear and fit a logorithmic relationship P α V̇(a) where 'a' was about 1.9; during CFV, the airway pressure drop (ΔP) was 4 times greater at the same VE, and 'a' was about 1.4. Helium-oxygen reduced ΔP during expiratory flow in proportion to the reduced density, but had less effect on ΔP during CFV. These results suggest that bidirectional flow increases shear rate and reduces hydraulic diameter in central airways, necessitating a large P(A) and lung volume to effect expiratory flow equal to the constant inflow during CFV. In the dogs, P(A) as measured via intrapulmonary needles varied among lobes by 9 cm H₂O during CFV. Conceivably this non-uniform inflation is due to non-homogeneous streaming of CFV, and it may account for part of the increased V(A)/Q variance and hypoxemia.