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to lung distension and oxygenation inform on the need to adjust mean airway pressure; in contrast, fluctuations in transcutaneous carbon dioxide levels, high frequency tidal volume or chest wall vibration and perfusion may reflect more on the effectiveness of gas mixing and volume delivery. New equilibriums in PaCO2 with alterations in oscillatory amplitude are achieved rapidly (within minutes) due to the highly efficient gas mixing that occurs during HFOV. However, recruitment of lung volume resulting from changes in mean airway pressure occur more slowly, and after the initial recruitment period, effects of changes in mean airway pressure are best assessed after an interval of 15 minutes. Careful and informed interpretation of monitored variables during HFOV will inform required adjustments to HFOV parameters to appropriately manage the patient’s condition. 9.2.1 Management of oxygenation If oxygenation is satisfactory, the FiO2 should be weaned to approximately 0.25 – 0.30 Vol.%. Subsequently, the mean airway pressure may be carefully, and gradually reduced (1 – 2 cmH2 O every 1 – 4 hours). The cause of acute onset hypoxia will determine whether mean airway pressure needs to be increased again (eg atelectasis) or decreased (eg overinflated lung). When uncertain, it is often prudent to first decrease the mean airway pressure to avoid inadvertent progression towards overdistension. Increases in mean airway pressure, if necessary, may be achieved through a brief re-recruitment procedure, or through intermittent, sustained inflations using slightly higher airway pressures (ie increased pressure with minimal additional volume delivered to the lung). Avoidance of high mean airway pressures that impair systemic blood pressure, significantly increase central venous pressure and/or impede pulmonary blood flow is paramount. If overinflation develops and persists despite appropriate decreases in mean airway pressure, a decrease in the oscillatory frequency and use of 1:2 (or shorter) I:E ratios may permit more effective egress of air during exhalation phase (25). 9.2.2 Management of carbon dioxide Hypercapnoea may occur secondary to airway obstruction with secretions, or as a result of changes in lung volume (either over- or under-distention). Airway obstruction needs to be dealt with promptly as it markedly impedes effectiveness of HFOV, and increases risk of injury due to excessive pressure transmission to proximal segments. Deflation of the lung during suction will result in subsequent respiratory deterioration, if post-suctioning recruitment measures are not used: effective post-suctioning recruitment is often achieved with brief small increases in mean airway pressure (2-4 cmH2 O ) or transient (5 min) increases in sigh frequency (max 2-5 breaths/min) or application of 2-3 manual sustained inflations of 2-3 s each. The objective of any sigh is to raise mean pressure enough to reopen any collapsed alveoli, rather than delivery of significant tidal volumes. Consequently, the sigh may be best delivered with a long inspiratory time (ie 0.5 – 2.0 s), and a peak pressure as little as 5 cmH2 O above mean pressure. Hypercapnoea/hypocapnoea is ultimately best managed through maintenance of the DCO2 variable. Careful and constant monitoring of the delivered tidal volume (where volume monitoring is available) and/or chest wall vibration is essential in the absence of volume targeting as DCO2 may change rapidly with any change in condition of the lung (improvement or deterioration) resulting in either hypocapnoea or hypercapnoea respectively. Unless the nature of the lung condition changes, carbon dioxide elimination is optimally managed through increases in DCO2 . HFOV ventilators generally do not have a dedicated DCO2 dial that can be adjusted on the ventilator; rather changes in DCO2 are normally achieved through changes in tidal volume and frequency. Altering DCO2 via changes in tidal volume is usually the preferred option, unless the nature of the lung disease changes. In the absence of volume targeting, tidal volume (and hence DCO2 ) is altered by changing ΔPhf. In volume targeted HFOV, DCO2 is controlled directly by changing tidal