High-frequency oscillatory ventilation: Mechanisms of gas exchange and lung mechanics

OBJECTIVE:Overview of the mechanisms governing gas transport, mechanical factors influencing the transmission of pressure and flow to the lung, and the measurement of lung mechanics during high-frequency oscillatory ventilation (HFOV) in acute respiratory distress syndrome. DATA SOURCES AND STUDY SELECTION:Studies indexed in PubMed illustrating key concepts relevant to the manuscript objectives. Pressure transmission during HFOV in the adult lung was simulated using a published theoretical model. DATA SYNTHESIS:Gas transport during HFOV is complex and involves a range of different mechanisms, including bulk convection, turbulence, asymmetric velocity profiles, pendelluft, cardiogenic mixing, laminar flow with Taylor dispersion, collateral ventilation, and molecular diffusion. Except for molecular diffusion, each mechanism involves generation of convective fluid motion, and is influenced by the mechanical characteristics of the intubated respiratory system and the ventilatory settings. These factors have important consequences for the damping of the oscillatory pressure waveform and the drop in mean pressure from the airway opening to the lung. New techniques enabling partitioning of airway and tissue properties are being developed for measurement of lung mechanics during HFOV. CONCLUSIONS:Awareness of the different mechanisms governing gas transport and the prevailing lung mechanics during HFOV represents essential background for the physician planning to use this mode of ventilation in the adult patient. Monitoring of lung volume, respiratory mechanics, and ventilation homogeneity may facilitate individual optimization of HFOV ventilatory settings in the future.