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volume, provided the maximum pressure amplitude remains at or above the oscillatory amplitude required to deliver that volume. As indicated above, using frequency to change carbon dioxide elimination may have unwanted effects. In the absence of volume targeting, decreasing frequency will increase the tidal volume for any given oscillatory amplitude – but also increases the iatrogenic cost of ventilation with increased pressure transmission to the distal airways a source of potential barotrauma. It is important to remember that the ventilator displayed oscillatory pressure amplitude is a poor and somewhat misleading indicator of pressure changes happening distally. Lower ventilator amplitudes at lower frequencies do not mean that the distal oscillatory pressure amplitudes within the airways and alveolar compartments have decreased! (see section 6). In contrast, in the presence of volume targeting, changing frequency has minimal, if any, effect on tidal volume delivery within the limits of the power of any given device. Consequently, a decrease in frequency in volume targeted HFOV will actually decrease DCO2 and PaCO2 will rise! 46 I 9 Management of HFOV I I 47 Lung volume and mean airway pressure 9.3 Humidification Adequate heating and humidification (90% relative humidity) of the inspiratory gas is essential to avoid irreversible damage to the epithelial surfaces of the respiratory tract. In the absence of sufficient humidification, viscous secretions develop that obstruct the bronchi, promoting consolidation and collapse of the alveolar compartment and impaired gas exchange. The resultant reduced compliance also increases the risk of barotrauma as low compliance promotes increased transmission of the oscillatory pressures to the lung periphery. In contrast, excessive humidification promotes condensation in the patient circuit, the tracheal tube and the airways, and compromises delivery of adequate tidal volumes. 9.4 Weaning from HFOV As with any ventilatory modality, the goal of weaning during HFOV is to gradually withdraw support and encourage spontaneous breathing. Although many clinicians still wean from high-frequency back to conventional ventilation (possibly due to relative availability of different ventilators), weaning from HFOV directly to a noninvasive mode of respiratory support is possible, and often the preferred mode. Weaning from HFOV to spontaneous ventilation is an intuitive process. Mean airway pressure is gradually reduced until levels consistent with the airway support provided with non-invasive respiratory support is attained. Oscillatory amplitude is gradually reduced until the patient assumes primary responsibility for CO2 elimination through gentle spontaneous breathing. Although a theoretical argument can be put forward to decrease frequency as lung mechanics normalise and the corner frequency decreases, alterations to frequency are generally not necessary during the weaning phase in HFOV. The time required for weaning will depend on the underlying pulmonary disease. In acute illnesses such as respiratory distress syndrome and persistent pulmonary hypertension of the newborn, weaning may be extremely rapid, lasting only hours. More chronic diseases such as bronchopulmonary dysplasia may require weaning of HFOV support over a period of days to weeks, and will vary from baby to baby according to concomitant morbidities. 10 Monitoring during HFOV Careful continuous monitoring is essential to clinical management of any patient on mechanical respiratory support. Monitoring is especially important during HFOV as the reliance of gas exchange on the square of the tidal volume exaggerates changes in PaCO2 with any change in the mechanics of the respiratory system and ventilatory circuit. 10.1 Monitoring of gas exchange and respiratory mechanics Essential monitoring tools that should be in place prior to commencement of HFOV include peripheral oxyhaemoglobin saturation (SpO2 ), transcutaneous carbon dioxide, and flow/tidal volume monitoring. Ideally, the tidal volume display should include a field for tidal volume/kg. As CO2 removal during HFOV is dependent on the DCO2 (frequency x VT 2 ), DCO2