Pulmonary ventilation by high‐frequency oscillations

Abstract

Bohn et al. [Fed. Proc. Fed. Am. Soc. Exp. 38 (3), 951 (1979)] reported that paralyzed beagle dogs maintained normal gas exchange for six hours or more when small oscillatory flows were maintained at the airway opening (15 cc tidal volume @ 15 Hz). These tidal volumes were 25% of pulmonary dead space and, thereby, were too small to permit convective gas exchanges with pulmonary airspaces. To test the hypothesis that effective gas exchange can be achieved by enhanced axial diffusion promoted by high‐frequency oscillatory flows, I have modelled that process and computed the time constant for concentration equilibration in the lung as a function of flow amplitude and frequency. Knowing the distribution of oscillatory flows within the bronchial tree [Fredberg and Moore, J. Acoust. Soc. Am. 63, 954–961 (1978)], the enhanced diffusion due to oscillatory flows [Chatwin, J. Fluid Mech 71, 513–527 (1975)] may be integrated along the bronchial tree. The results indicate that gas concentration gradients within the lung can be substantially abolished within 100 to 10−2 s. This suggests that diffusion enhanced by small‐amplitude oscillatory flows (tidal volume ≪ dead space) can support effective gas exchange in the lung in the absence of convective transport (breathing). [Work supported by the Physicians' Service Incorporated Foundation, Ontario.]