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Overinflation is the most frequent complication and cause for failure of oscillatory ventilation. It occurs most often in obstructive lung diseases such as meconium aspiration syndrome and pulmonary interstitial emphysema. Careful attention to appropriate frequency selection for the nature and stage of the underlying disease is essential to avoid this complication. 13.2 Relative contraindications Acute pulmonary obstruction is the only relative contraindication to the use of HFOV. Acute obstruction may be present in the early stages of meconium aspiration, as well as during pulmonary haemorrhage, RSV bronchiolitis and bronchopulmonary dysplasia. Use of HFOV in the presence of obstruction risks barotrauma to the proximal airways, acutely inadequate gas exchange and exacerbation of gas trapping if a ball valve effect is present due to secretions. 13.3 Limitations of HFOV Assuming a disease and patient appropriate strategy is selected, the success of HFOV is dependent on the capabilities of the high frequency ventilator to meet the proposed strategy. Even adults can be oscillated with an appropriately powered ventilator. Key to this goal is the achievement of adequate oscillatory volumes at disease specific optimal frequencies. Compliance of the patient circuit has a crucial impact on delivery of oscillatory tidal volumes: cyclic tidal volumes delivered to the patient are increased considerably with a low-compliant circuit including the use of a low volume humidifier chamber. Patient lung mechanics additionally influence the ability of the ventilator to deliver adequate tidal volumes for efficient gas exchange. The same ventilator that easily ventilates a large healthy patient may struggle to deliver adequate tidal volumes to a smaller patient. In this setting, effective ventilation may be achieved using increased I:E (eg I:E = 1:1) and lower frequency. Recent and emerging modifications including volume targeting and inclusion of capability to use sigh breaths may also allow refinement of the strategy to achieve oxygenation and ventilatory goals whilst minimising iatrogenic complications. Practical Tips for the optimal set-up: The set-up of patient circuit has a higher impact on ventilator performance during high frequency ventilation compared to during conventional ventilation. The patient circuit can be optimized as follows: ■ Use breathing circuits with very stiff tubing walls and minimal volume in order to reduce the compressible gas volume and minimize compliance of the breathing circuit: • Use a circuit as short as possible. • Use a circuit with stiff tubing walls (smooth bore, no standard corrugated circuit). ■ Prepare humidifier chamber as follows in order to reduce compressible volume to a minimum: • Use a small humidifier chamber to minimize chamber volume The chamber shall be filled with water to the maximum level as fluids are not compressible. • Avoid the use of incubator extensions as these increase tubing compliance and compressible volume. If the incubator temperature is high and extensions are needed, select the shortest extension with stiff tubing walls. ■ Use heated circuits, if possible: • Use no or low-volume water trap in order to reduce the compressible volume. ■ Avoid condensation in the breathing circuit to minimize resistance to air flow: • Avoid temperature drops in patient circuit by air conditioning or air draft. • Avoid restrictions of tube diameters by hose kinks or adapters. T 13 Complications, relative contraindications and limits of HFOV I Lung volume and mean airway pressure 58 I 14 Failure of HFOV I I 59 14 Failure of HFOV 15 The future directions of HFOV In most instances, HFOV improves the critical respiratory condition, at least temporarily. Failure is often a consequence of inappropriate strategy – that does not fully consider the patient’s disease, the ventilator’s capabilities, or the interactions between the two. Inappropriate HFOV strategy can lead to a rapid worsening of the patient’s condition. Patients with homogeneous lung diseases are more likely (70-87%) to respond to HFOV than inhomogeneous diseases (50-79%), airleaks (63-80%), PPHN (39-69%), or CDH (22-27%) (21). Early identification of HFOV failure (persistently high FiO2 , oxygenation index, PaCO2 , or mean airway pressure and/or low A/a ratio) allows for prompt commencement of alternative therapies (eg high-frequency jet ventilation in inhomogeneous lung disease), and if necessary, transfer to a medical centre offering extracorporeal membrane oxygenation.