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unknown. A second feature of clinical relevance is the practical ease of switching from conventional ventilation to HFOV, or even of combined conventional and HFO ventilation. The emergence of hybrid ventilators that can combine conventional and high frequency modalities will increase access to oscillatory ventilation and potentially encourage earlier a priori use of HFOV for lung protective respiratory support. A downside of this combination is that hybrid ventilators often have more limited power than standalone oscillators such as the SM3100 series. To a large extent, these limitations can be overcome by either switching to a lower oscillatory frequency, or changing from a 1:2 to a 1:1 inspiratory to expiratory ratio (I:E). Other features with practical implications for inter- and intra-hospital transport include minimum bias flow requirements and availability of inbuilt battery backup. The importance of incorporated tidal volume monitoring is recognised increasingly as clinical strategies are revised to include monitoring of volume and the diffusion coefficient to optimise ventilation and lung protection. Reversing oscillatory flow at high frequencies may distort cells due to shear stress A) Turbulent flows and formation of eddies creates high shear zones. B) Shear stress applied to the surface of an airway epithelial cell will promote deformation of the cell surface, whilst the base of the cell remains fixed. In HFOV, a reversing oscillating shear effect occurs due to shear effect in opposing directions during inspiration and expiration. Low shear zone High shear zone A B High frequency oscillatory ventilation offers unique and highly efficient gas mixing, contributing to its success in both a priori and rescue treatment of patients with severe respiratory disease and impaired gas exchange. The gas mixing efficiency of HFOV over conventional ventilation is due to fundamental differences in the behaviour of gases in the airways and alveoli. Gas mixing during conventional ventilation is primarily dependent on direct ventilation of the alveoli via bulk convective flow. To be effective, delivered tidal volumes need to be of sufficient size to fill the anatomical dead space of the airways as well as to distend and fill the volume of the alveolar compartment. Decreasing tidal volume to volumes at or below anatomical dead space volume, whilst delivering these volumes at conventional breathing frequencies, impedes clearance of the waste carbon dioxide from the alveolar compartment (dead space ventilation). Low tidal volume ventilation at conventional breathing frequency thus contributes to development of atelectasis and impaired oxygenation due to inadequate transmission of a distending pressure (reduction of mean airway pressure). 4 Gas mixing during high-frequency oscillations 2 Inertive flow refers to the variable flow occurring with sudden accelerations and decelerations accompanying reversal in flow direction at high frequencies. 10 I 4 Gas mixing during high-frequency oscillations I I 11 Pressure amplitude and respiratory mechanics In sharp contrast to mechanical ventilation at conventional breathing frequencies, HFOV offers highly effective oxygenation and clearance of waste gas, despite use of tidal volumes at or below dead space volume. The efficiency of HFOV in ventilating the lung at very low tidal volumes is attributed to highly effective mixing of fresh and exhaled gases in the airways and alveolar compartment. The mechanisms contributing to this augmented gas mixing are multiple (3), and influenced by the ventilator waveform (sinusoidal or complex, multiple harmonic) and settings (eg frequency, mean airway pressure and oscillatory amplitude) as well as the respiratory mechanics (airway and parenchymal), homogeneity of ventilation and cardiac function. 4.1 Augmented longitudinal gas transport and enhanced dispersion The concept of augmented longitudinal gas transport and potential penetration of the alveoli of gas delivered in volumes less than the anatomical dead space were foreseen by the early simple yet elegant studies by Henderson and colleagues in 1915 (4). Blowing tobacco smoke down a tube, they observe the formation