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volume and mean airway pressure 40 I 8.2.4 Avoiding atelectasis Atelectatrauma is a consequence of ventilating the collapsed (atelectatic) lung. Atelectasis during HFOV may follow aspiration of secretions, overly enthusiastic weaning of mean airway pressure or consequent to prolonged HFOV with low tidal volumes in the absence of spontaneous breathing effort. Careful attention to maintaining appropriate humidification may reduce the risk of developing thick tenacious secretions that block small airways and promote collapse. Attention to prompt re-recruitment of the lung after suction is important to avoid unintended adverse lung collapse. Infrequent sigh breaths (approximately 1 per minute) with a moderate peak inspiratory pressure may facilitate maintenance of lung volume during prolonged HFOV with low tidal volumes. 8.3 Avoiding overdistension Volutrauma during HFOV may result from both static overdistension as well as cyclic overdistension of the lung. The peak alveolar distension is the volume reached at the peak of inspiration and should not exceed 90% of the total lung capacity as shown in Figure 8-4 (15). As the lung is kept open at a level above functional residual capacity at high mean airway pressures, the MAP needs to be reduced if lung disease improves in order not to overdistend the more compliant lung. D-8066-2016 Figure 8-4: Difference in distending lung volume during HFO and conventional ventilation. The lung is maintained closer to total lung capacity (TLC) than functional residual capacity (FRC) during HFO compared to conventional ventilation. I 41 9 Management of HFOV 9.1 Transition from conventional ventilation Prior to commencing HFOV, it is important to consider the nature of the underlying disease and to have a clear plan for ventilation strategy as different diseases benefit from slightly different approaches to delivery of HFOV. The guidelines given below are appropriate for recruitment of the homogeneously derecruited lung such as occurs with RDS due to surfactant deficiency. Suggested approaches for other lung diseases are provided in section 12. 9.1.1 Prepare the patient and the monitoring environment Ensure that appropriate monitoring is available and ready to facilitate informed adjustment and optimisation of HFOV ventilator settings after initiation of HFOV (see section 10). It is useful to note the current ΔP (PIP-PEEP) and mean airway pressure applied during conventional ventilation, as these variables guide initial selection of HFOV ventilator settings. Airway secretions present a major impediment to achieving adequate tidal volume during HFOV (much more so than during conventional ventilation) and pose a risk for increased oscillatory pressure transmission to proximal alveoli (see section 6.2.2). Consequently, it is advisable to also perform tracheal suction prior to commencement of HFOV to avoid clinical deterioration on commencement of HFOV. 9.1.2 Setting the mean airway pressure Mean airway pressure is the primary determinant of distending lung volume, and also influences pulmonary vascular resistance, and hence the flow of blood through the pulmonary capillaries. Consequently, mean airway pressure is the primary determinant of oxygen transfer from the alveolus to the pulmonary capillary. A strategy for identification of optimal mean distending pressure for first intention HFOV treatment in the setting of neonatal respiratory distress syndrome is shown in Figure 8-2. A similar approach can also be applied in the initial recruitment of the atelectatic lung when HFOV is used in the rescue setting. In the rescue setting, as useful guide for initial mean airway pressure approximately the same (I:E of 1:1) or 2-3 cmH2 O higher (I:E of 1:2) than the mean airway pressure used during conventional ventilation. 9.1.3 Setting oscillatory pressure amplitude (pressure controlled HFOV) If commencing HFOV at 10 Hz, a ΔP on the HFOV ventilator approximately 1.5 times the ΔP (PIP-PEEP) used during conventional ventilation provides a reasonable conservative starting oscillatory pressure amplitude during HFOV. Higher initial FRC TLCLung Volume Volutrauma Atelectatrauma 8 Distending lung volume and mean airway pressure I