Mechanisms of gas transport during ventilation by high-frequency oscillation.

Ventilation by high-frequency oscillation (HFO) presents some difficulties in understanding exactly how gas is transported in the lung. However, at a qualitative level, five modes of transport may be identified: 1) direct alveolar ventilation in the lung units situated near the airway opening; 2) bulk convective mixing in the conducting airways as a result of recirculation of air among units of inhomogeneous time constants; 3) convective transport of gases as a result of the asymmetry between inspiratory and expiratory velocity profiles; 4) longitudinal dispersion caused by the interaction between axial velocities and radial transports due to turbulent eddies and/or secondary swirling motions; and 5) molecular diffusion near the alveolocapillary membrane. These modes of transport are not mutually exclusive and certainly interact. It is therefore difficult to make quantitative predictions about the overall rate of transport. Qualitatively, it may now be stated with confidence that convective transport in the tracheobronchial tree is very important during HFO as in normal breathing and that increasing tidal volume is more effective than increasing frequency in improving gas exchange during HFO. To optimize the gas transport efficiency of HFO, future research should focus on identifying the rate-limiting mode of transport for a given set of geometric and dynamic conditions.