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volume delivery during HFOV 6 Understanding transmission of pressure amplitude I The concept of corner frequency The concept of corner frequency High Frequency Oscillatory Ventilation undertaken at a frequency around the corner frequency will minimize pressure costs per unit of oscillatory flow. As the time constant of the lung increases, the corner frequency decreases and vice versa. The used frequency should be adjusted according to the underlying mechanical properties of the airways and the lung: ■ Lower frequencies in high resistance and/or high compliant lungs. ■ Higher frequencies in low compliant lungs with normal resistance. Peak Carinal Pressure Frequency (Hz) healthy Raw 0 0 10 20 30 Raw C C Figure 6-5: Effect of mechanical properties on the peak pressures transmitted to the carina. Adapted from (15). Corner frequency is shown by the dashed lines, representing the point at which most gain in minimising carinal peak pressures are achieved. Low compliance (C) shifts the optimal frequency to the right, whereas lower frequencies are more optimal for settings in which airway resistance (Raw) is increased. D-8080-2016 S 30 I I 31 7.1 Determinants of tidal volume With this background understanding, it is possible to understand how changing HFOV settings such as amplitude, frequency and I:E ratio alter the delivery of tidal volume to the lung. We can also understand why tidal volume delivery varies as the mechanical properties of the lung change. 7 Understanding tidal volume delivery during HFOV I Lung volume and mean airway pressure Calculation of tidal volume Delivery of tidal volume follows the general formula of: V=(C∆P)(1 — e—TI /RC) Equation where VT is tidal volume, C is compliance, ΔP is ΔP (PIP-PEEP) and R is resistance. The product of R x C is the time constant (τ). Figure 7-1: Waveform for inspired tidal volume pressure controlled conventional ventilation. The plateau (ie maximum tidal volume) delivered is determined by the product of compliance and ΔP (peak inspiratory pressure – positive end expiratory pressure). The rate at which the plateau tidal volume is achieved is determined by the time constant (τ = RC). How much tidal volume is actually delivered for any given combination of ventilatory pressure settings and respiratory mechanics is determined by the elapsed time (t). Effect of oscillatory amplitude on delivered tidal volume. For any given lung compliance, as amplitude increases, the delivered tidal volume also increases. Figure shows tidal volume/kg for a compliance of 0.1 mL/ cmH2 O, and resistance of 100 cmH2 O/L/s, and an inspiratory time of 33 ms (f=10 Hz, I:E 1:2) in a 1 kg infant. D-8060-2016 D-8079-2016 Tidal Volume (mL) Inspiratory Time (ms) 10 5 0 0 100 200 300 400 500 Slope Plateau 7.1.1 Oscillatory amplitude During conventional ventilation, ΔP is the difference between the peak inspiratory pressure and the positive end expiratory pressure, with each parameter being set independently. During HFOV, oscillatory amplitude (ΔP) is set directly at the ventilator. As oscillatory amplitude increases, the maximum achievable tidal volume will also increase Decrease in absolute inspiratory time with increasing frequency. Graph shows inspiratory times with an I:E ratio of 1:2 (ie 33 % inspiratory time). Tidal Volume (mL) 15 Hz 10 Hz 7 Hz 5 Hz Inspiratory Time (ms) 4 2 0 0 20 40 60 80 100 6 Volume (mL/kg) Amplitude (cmH2 O) 2 1 0 0 10 20 30 40 3 4 i 32 I I 33 D-8057-2016 Change in absolute inspiratory time and delivered volume as I:E ratio changes. Graph illustrates changes at 5 Hz and 15 Hz frequencies. Tidal Volume (mL) 15 Hz 1:2 1:1 5 Hz 1:2 1:1 Inspiratory Time (ms) 4 2 0 0 20 40 60 80 100 6 7.1.2 Frequency As frequency increases, the absolute oscillatory time for each volume delivery cycle decreases. If the inspiratory to expiratory ratio does not change, the absolute inspiratory time will decrease with increasing frequency. Because there is less time available, the tidal volume delivered will decrease with increasing frequency if all other ventilator settings and lung mechanical properties remain the same. 7.1.3 I:E ratio As I:E ratio increases from 1:2 to 1:1, the absolute inspiratory time also increases, allowing more time for volume to be delivered to the lung The effect