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airway opening and displayed by the ventilator. As shown in Figure 8-1, this pressure loss (shown by a negative value for PL – Pao) is only present when an I:E ratio of 1:2 (or 1:3) is used and does not occur at an I:E ratio of 1:1. Furthermore, for the same tidal volume or pressure amplitude, the pressure drop from airway opening to the alveolar compartment in the lung increases with increasing frequency or decreasing tracheal tube size. This dependence of pressure drop on the I:E ratio is important to remember when changing from conventional ventilation to high-frequency oscillatory ventilation: the mean pressure set on the ventilator needs to be set higher when using a 1:2 ratio compared to 1:1 ratio to achieve the same mean intrapulmonary pressure. 8 Distending lung volume and mean airway pressure Maintaining a well-recruited lung during HFOV that is neither overdistended or underinflated is essential to minimising injury to the lung and optimising arterial oxygenation. Lung recruitment is achieved primarily through the adjustment of mean airway pressure. Understanding the determinants of distending lung volume and how to achieve optimal distending lung volume during HFOV is essential to maximise the benefits and minimise harm when using this ventilatory modality. 8 Distending lung volume and mean airway pressure I Lung volume and mean airway pressure HFO VENTILATOR PARAMETERS AND TIDAL VOLUME CO2 removal is mainly dependent on tidal volume and frequency (see chapter 5.5). The tidal volume is usually smaller or equal than anatomical dead space (i.e. 2-3 ml/kg). Tidal volume is adjusted by changing the following control parameters: ■ Amplitude • Higher amplitude leads to higher tidal volume. ■ Frequency • Tidal volume delivered will decrease with increasing frequency (without Volume Guarantee). • Tidal volume is increased by decreasing the frequency (only if Volume Guarantee is not activated), at a cost of increased transmission (reduced damping) of the oscillatory pressure amplitude. ■ Percent inspiratory time • Decreased percent inspiratory time (I:E = 1:2 or 1:3) will reduce tidal volume as less time is available to deliver flow. • Tidal volume increases with a higher percent inspiratory time (I:E = 1:1). ■ I:E ratio • Tidal volume increases when changing from I:E ratio of 1:2 to 1:1 or 1:3 to 1:2. S 36 I I 37 Mean intrapulmonary pressure (cmH O) 2 Ventilator Amplitude(cmH2 O) -2 -4 -6 0 0 90 1:1 1:2 Ventilator Amplitude(cmH2 O) -2 -4 -6 0 0 90 15 Hz 10 Hz 5 Hz Ventilator Amplitude(cmH2 O) I:E ratio Different Frequencies Different ET Tube Size -2 -4 -6 0 0 90 2.5 mm 3.0 mm 3.5 mm D-8062-2016 Figure 8-1: Effect of I:E ratio, frequency and tracheal tube size on fall in mean intrapulmonary pressure. Left panel: There is minimal difference in mean pressure between the airway and the alveolar compartment when I:E of 1:1 are used. However, at I:E of 1:2, there is a progressive fall in mean airway pressure as ventilator amplitude increases. Middle and right panel: At I:E of 1:2, the fall in pressure from airway opening to alveolar compartment becomes steeper as frequency increases, or as tracheal tube internal diameter decreases. 8 Distending lung volume and mean airway pressure I Lung volume and mean airway pressure Resistive pressure drop Resistive pressure drop during hfov follows the general form of: Pres = k1 V . + k2 (V. )2 Where Pres is the resistive pressure drop, k1 and k2 are constants and V. is the flow in the tracheal tube. At I:E of 1:1, the inspiratory and expiratory Pres are approximately equal, and no net pressure drop results across the cycle. At I:E of 1:2, the inspiratory flow is higher than the expiratory flow, resulting in a greater pressure drop during inspiration than expiration. Mathematically, the magnitude of the pressure drop is proportional to the difference between the squared mean inspiratory and squared mean expiratory velocities (12). Mean intrapulmonary pressure and I:E ratio ■ The mean intrapulmonary pressure is the effective pressure distending the lungs. It cannot be measured directly. ■ The mean intrapulmonary pressure is closest to the set mean airway pressure at an I:E ratio of 1:1. ■ At I:E ratios of 1:2 or 1:3, the