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monitoring is a useful adjunct monitoring variable to facilitate stability of PaCO2 if ventilation frequency is changed to accommodate different stages of lung disease and/or mechanics. Measurement of lung mechanics is more complex in HFOV than in conventional ventilation due to the damping of the oscillatory pressure waveform from the airway opening to the lung, and the dependence of this damping on both ventilatory settings and mechanics of the intubated respiratory system. When using intermittent conventional cycles as sigh breath, the tidal volume and the difference between PIP and PEEP measured during the conventional inspiration can be used to calculate dynamic compliance (Cdyn) provided an inspiratory pressure plateau is achieved5 . Although it is tempting to apply a similar approach to assessment of compliance during HFOV in the absence of such intermittent conventional breaths, frequency dependent damping of the oscillatory pressure waveform makes this impossible to apply or interpret in a meaningful manner across the frequency spectrum. Indeed, at frequencies of 15 Hz, tidal volume is relatively independent of lung compliance. 10.2 Monitoring of lung volume With the importance of optimising lung volume being increasingly recognised as paramount to safe and lung-protective application of HFOV, tools to assess lung volume remain a focus of HFOV research. Periodic chest radiographs provide a crude indication of lung expansion: lung volume is optimal when expanded to the 8th or 9th posterior rib assessed in the mid-clavicular line (depending on gestation 5 Cdyn= VT,CV ⁄(PIPCV—PEEP) 48 I 10 Monitoring during HFOV I I 49 Lung volume and mean airway pressure and the presence/absence of pulmonary hypoplasia). Respiratory inductance plethysmography and electrical impedance tomography are proven in the research setting as useful tools to monitor relative and temporal changes in lung volume during initial volume recruitment procedures. Electrical impedance tomography is also useful for the detection of pneumothorax. Lung ultrasound is used increasingly in the setting of conventional ventilation to assess lung parenchymal pathology; application to HFOV is likely in the near future. 10.3 Monitoring of circulatory system and systemic perfusion The continuous application of moderately high distending pressures during HFOV places additional burdens on the interaction of the circulatory and respiratory systems. Additional beneficial monitoring in seriously ill infants on HFOV thus includes regular or continuous central venous pressure and system blood pressure. Monitoring during HFOV Ventilatory parameters Frequency ΔP and ΔPmax (Volume Guarantee mode) Mean airway pressure Set VT and delivered VT (Volume Guarantee mode) I:E ratio (or % inspiratory time) DCO2 (frequency x VT 2 ) FiO2 Blood gases Transcutaneous pO2 and pCO2 SpO2 Chest X-ray Respiratory inductance plethysmography Electrical impedance tomography Oscillatory pressure ratio (ΔPtrachea/ΔPventilator) Cdyn (intermittent mandatory sigh breath) Heart rate Systemic arterial blood pressure Central venous pressure Urine output Gas exchange Lung volume & respiratory mechanics Circulatory system & systemic perfusion Table 2: Monitoring during HFOV. Increased central venous pressure may provide warning of cardiorespiratory decompensation in the presence of very high mean airway pressures. Compromised cardiac function may also be evident from prolonged capillary filling time and reduced urine output. Intermittent functional echocardiography also provides information about cardiac contractility, cardiac output and right-ventricular pressure: this functional information may help clinicians identify early any adverse circulatory effects of changes in mean airway pressure strategy. Near infra-red spectroscopy (NIRS) recording of cerebral and peripheral tissue oxygen saturations may also inform on systemic impact of changes in HFOV strategy. 11 Indications for HFOV Oscillatory ventilation has been applied in the clinical setting since the early 1980s. HFOV was used initially for rescue of infants failing conventional ventilation, and in many countries this remains the