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mean intrapulmonary pressure will be lower than the set mean airway pressure. 8.2 Achieving optimal lung volume 8.2.1 Approaches to recruiting and optimising lung volume during HFOV There are several different methods for recruiting lung volume during HFOV. Most commonly, a stepwise incremental and then decremental change in mean airway pressure is used as this approach is the most effective in fully recruiting the lung and finding the optimal distending pressure on initiation of HFOV in the atelectatic lung. An example of how the stepwise approach is used to recruit the lung of an infant with atelectatic lung due to respiratory distress syndrome is outlined in Figure 8-2. 8.2.2 Avoiding volutrauma during lung volume recruitment As the lung is recruited, the compliance of the lung will change: initially, the compliance increases as the lung volume increases on the steep part of the inflation limb (step 2 in Figure 8-2). As the opening pressure is achieved and the lung starts to become overdistended (step 3 in Figure 8-2), the compliance falls (step 3 in Figure 8-2). Then, as the pressure is reduced, the compliance will again increase, before falling again as the closing pressure is reached (step 4 in Figure 8-2). S i I 39 These changes in compliance have important consequences for tidal volume. Figure 8-3 shows that the tidal volume will change markedly during recruitment If Volume Guarantee is not being used, and the oscillatory amplitude remains the same throughout the recruitment procedure. Fluctuating tidal volumes may result in similar marked fluctuations in arterial partial pressure of carbon dioxide, and consequent unwanted fluctuations in cerebral blood flow. It is therefore very important to monitor carbon dioxide, tidal volume and the DCO2 continuously during a recruitment procedure and to continuously adjust the oscillatory amplitude to avoid potentially adverse consequences of abrupt changes in cerebral blood flow or turn on Volume Guarantee. 8.2.3 Maintaining optimal lung volume in HFOV Once the lung is optimally recruited, it remains important to maintain the lung at this optimal volume to minimise the risk of ongoing iatrogenic lung injury. The optimal distending pressure will decrease as lung disease improves: consequently, mean airway pressure is reduced gradually until the patient is able to maintain gas exchange with mean airway pressure at approximately the same level as pressure used for continuous positive airway pressure (CPAP). D-8065-2016 Figure 8-3: Change in tidal volume and PaCO2 during lung recruitment. This figure illustrates changes observed if amplitude is held constant as mean pressure is first incremented (solid line/ filled circles), then decremented (broken lines, open circles) during a volume recruitment manoeuvre. Adapted from (17). 38 I Figure 8-2: Algorithm for recruitment of the atelectatic lung during HFOV. Adapted from (16). Pre-surfactant optimal pressure (CDPOPT) Obtain chest X-ray & give surfactant Post-surfactant closing pressure (CDPC) Pre-surfactant opening continious distanding pressure (CDPO) Post-surfactant opening pressure (CDPO) Pre-surfactant closing pressure (CDPC) Post-surfactant optimal continious distending pressure (CDPOPT) Set mean airway pressure (MAP) at 6-8 cmH2 O and adjust FiO2 to give an SpO2 90-95%. 1 Recruit the lung again by returning to the known CDPO (step 3) for 2-3 min then decrease MAP to 2 cmH2 O above the closing pressue (CDPC) from step 4. 5 Increase MAP by 1-2 cmH2 O every 2-3 min. Reduce FiO2 stepwise by 0.05-0.10 to maintain SpO2 90-95%. 2 5-10 min after surfactant, reduce MAP by 1-2 cmH2 O every 5 min until SpO2 falls. 6 Stop increasing MAP when FiO2 is smaller than 0,25 or when FiO2 is not able to be decreased with an increase in MAP. 3 Increase M AP by 1-2 cmH2 O every 5 min until SpO2 returns to previous value at end of step 5. 7 Decrease MAP by 1-2 cmH2 O every 2-3 min until SpO2 falls. 4 Set MAP at 2 cmH2 O above post-surfactant closing pressure (CDPC) identified in step 6. 8 Mean Airway Pressure (cmH2 O) Mean Airway Pressure (cmH2 O) 5 9 8 7 6 4 8 10 12 14 16 18 20 Tidal Volume (mL) PaCO2(kPa) 8 10 12 14 16 18 20 5 7 6 4 8 Distending lung volume and mean airway pressure I Lung