Mechanisms of Gas Transport and Exchange

The mechanisms governing gas flow, gas mixing, and pressure transmission during high-frequency oscillatory ventilation (HFOV) are fundamentally different to ventilation at conventional respiratory breathing frequencies. During CV, the main mechanisms of gas exchange are bulk convection and diffusion. During HFOV, there are 5 additional mechanisms of gas exchange, namely, asymmetric velocity profiles, Taylor dispersion and turbulence, cardiogenic mixing, pendelluft effect, and collateral ventilation [4] (Fig. 4.2). A basic understanding of these gas transport mechanisms represents essential background for clinical HFOV use. With the exception of molecular diffusion, all other mechanisms depend on convective fluid motion.

Bulk Convection

Bulk convection is the concerted, collective movement of groups of molecules within fluids. Unlike CV, bulk convection plays a relatively small role in gas transport during HFOV, although it is likely to contribute significantly to ventilatory

Fig. 4.2 Gas transport mechanisms during high-frequency oscillation. The major gas transport mechanisms that are operative under physiologic conditions within each region are shown (convection, convection and diffusion, diffusion alone). There are seven mechanisms of gas transport during high-frequency oscillation: turbulence in the large airways, causing enhanced mixing; direct ventilation of close alveoli; turbulent flow with lateral convective mixing; pendelluft (asynchronous flow among alveoli due to asymmetries in airflow impedance); gas mixing due to velocity profiles that are axially asymmetric (leading to streaming of gas toward the alveoli along the inner wall of the airway and the streaming of alveolar gas away from the alveoli along the outer wall); laminar flow with lateral transport by diffusion (Taylor dispersion); and collateral ventilation through nonairway connections between neighboring alveoli (Reproduced with permission from Pillow [4])

exchange in the most proximal lung units [4]. Research in animal models showed that decreasing delivered V_t to a level below the HFOV-circuit-related rebreathing volume (as opposed to the larger anatomic dead space) causes a sudden rise in PaCO₂. This suggests CO₂ elimination efficiency is dependent on net oscillatory volume and that bulk convection has an essential role during HFOV [9].