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Lung volume and mean airway pressure 42 I I 43 D-8067-2016 Figure 9-1: Target range for frequency and DCO2 relative to frequency and patient weight. Graph indicates estimated lower (orange) and upper (blue) targets for each variable. DCO2 (mL2 /s) Weight (kg) 800 400 200 1000 600 0 0 1 2 3 4 5 Tidal Volume (mL/kg) Frequency (Hz) 4 2 1 3 0 0 5.0 7.5 10.0 12.5 9 Management of HFOV I Lung volume and mean airway pressure oscillatory pressure amplitudes need to be set on the ventilator at frequencies above 10 Hz, whereas lower amplitudes are required for frequencies below 10 Hz. Immediately after commencing HFOV, adequacy of the oscillatory ΔP should be reassessed by the delivered tidal volume: as the tidal volume required varies according to frequency, the clinician is referred to the table below for approximate “target” tidal volumes for specific frequencies. In the absence of tidal volume monitoring in HFOV, visual confirmation of adequate chest wall vibration (“wiggle”) is a useful alternative guide to adequacy of oscillatory amplitude. Regardless, close monitoring of arterial CO2 via transcutaneous measurement of pCO2 is essential to ensure amplitude is set such that changes in pCO2 are gradual: rapid fluxes in pCO2 may be accompanied by sudden and deleterious changes in cerebral blood flow. Table 1: Different combinations of frequency and tidal volume to reach same DCO2 for permissive hypercapnoea. Initial DCO2 for different patient weights Weight (kg) 5 Hz 7 Hz 10 Hz 15 Hz 3.46 mL/kg 2.93 mL/kg 2.45 mL/kg 2.00 mL/kg 0,5 15 15 15 15 1 60 60 60 60 2 240 240 240 240 3 540 540 540 540 4 960 960 960 960 Inadequate or excessive chest wall motion should result in prompt readjustment of the ΔP to prevent hypocapnoea or hypercapnoea. If tidal volume monitoring is in place, adequacy of delivered tidal volumes and DCO2 should be considered. In the event that HFOV is commenced in the setting of significant hypercapnoea, oscillatory ΔP should be adjusted to achieve a gradual fall in transcutaneous pCO2 of about 2-3 mmHg/min (0.3-0.4 kPa/min) to avoid very rapid changes in cerebral blood volume that may result in intracerebral haemorrhage. 9.1.4 Setting oscillatory tidal volume (volume targeted, pressure limited HFOV) Commencement of volume targeted pressure limited HFOV (HFOV+VG) focuses similarly on a controlled optimisation of pCO2 . Rather than selecting a starting oscillatory amplitude, the strategy is to set the volume target (Volume Guarantee) appropriate to the selected frequency for oscillation. A DCO2 of 40-60 mL2 /kg2 /s will generally achieve PaCO2 in the mild permissive hypercapnoeic range. Suggested tidal volumes to achieve mild permissive hypercapnoea during volume targeted HFOV are provided in Table 1 and illustrated in Figure 9-1. Importantly, larger tidal volumes are required at lower frequencies to achieve the same DCO2 and consequently rate of CO2 removal from the lung. Therefore, the decision to use a low frequency needs to be offset against the potential of increased risk of volutrauma to the alveolus because the higher tidal volumes required have potential to “overdistend” the immature alveolus. Oscillators delivering volume targeted HFOV usually do so in a pressure-limited setting. For volume-targeted pressure-limited HFOV, setting the maximum oscillatory pressure amplitude (ΔPmax) is the equivalent of setting the maximum PIP during volume-target conventional ventilation. During HFOV, ΔPmax should be adjusted after the initial stabilisation period, to remain approximately 5 cmH2 O above the ΔP used to deliver the targeted volume required at the selected frequency that achieves the desired target PaCO2 . The 5 cmH2 O buffer in ΔPmax provides the ventilator with capacity to maintain ventilation in the event of brief, or sustained minor deterioration in oscillatory tidal volumes. It is important to avoid setting the ΔPmax more than 5 cmH2 O above the average ΔP, to avoid a significant deterioration in lung mechanics (and hence aeration of the lung) without notification to the clinician/ 9.2 Continuation of HFOV Once HFOV is established, careful monitoring is required to maintain optimal patient condition. In particular, attention