Outline the indications for high frequency oscillation in Intensive Care, and the mechanism of gas exchange when using high frequency oscillation.

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College Answer

Indications: usually as part of experimental therapy or as part of a controlled clinical trial.  Potential rescue therapy for ARDS in Intensive Care Units who have suitable equipment available and are experienced in its use, where “open lung” ventilation strategy (adequate recruitment and avoidance of overdistension) is desirable.  In Paediatric Intensive Care as rescue therapy for severe respiratory failure.

Mechanism of gas exchange: normal bulk flow is much less important, as the tidal volumes used are much  smaller  than  anatomical  deadspace;  gas delivery  into  the system  (as bias flow)  will  stillprovide some gas exchange.   Other potential mechanisms described (many of which may work simultaneously) include Taylor dispersion (dispersion of molecules beyond the bulk flow front), augmented diffusion (gas mixing within alveolar units), coaxial flow patterns (net flow one way through centre of airway, other direction via periphery) and Pendelluft mixing (between lung units, mixing of gas due to impedance differences).

Discussion

This question is not entirely identical, but in its spirit very similar to Question 23 from the second paper of 2010.