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The ventilation that is achieved however, is delivered at the cost of increased proportional transmission of oscillatory pressure amplitudes to the airways proximal to the site of increased peripheral resistance (14). Consequently, the use of HFOV in the patient with increased peripheral resistance presents a risk of barotrauma to the proximal airways. 6.2.3 Inertance Complex (eg square wave) oscillatory pressure waveforms have higher inertive content than sinusoidal waveforms with more abrupt acceleration and decelerations in flow. Whether square waves are more beneficial or more harmful to the infant than sinusoidal oscillatory pressure waveforms remains unclear. The more abrupt changes in flow of square waveforms may enhance gas mixing efficiency, but also may exert more injurious shear effects on the airway wall and compromise airway wall stiffness. However, the complex waveform becomes more sinusoidal in morphology as it transverses the airways, hence most of the effect of different waveform morphologies (square versus sinusoidal) are likely to be effected in the large airways with minimal beneficial or adverse impacts on the distal lung parenchyma. Inertance per se does not impact the transmission of the oscillatory pressure amplitude from the airway opening to the alveolar compartment (14). Tracheal Tube and Large Airway Resistance Resistance in the tracheal tube and the large airways is flow dependent, due to the turbulent nature of high flow through a tube of narrow diameter at high-frequencies. Consequently, as flow increases, the resistive contribution of the tracheal tube and large airways to overall respiratory system impedance also increases, necessitating increased pressure amplitudes to maintain flow delivery (and tidal volumes) (14). 6 Understanding transmission of pressure amplitude I Pressure amplitude and respiratory mechanics Resistance of tracheal tube Flow dependent resistance in the tracheal tube and large airways is calculated as: RTT = k1 + k2 V . where RTT is the resistance of the tracheal tube, k1 and k2 are constants and V . is the oscillatory flow. High peripheral resistance and HFO If HFOV is applied to lungs with high peripheral resistance: ■ higher pressure amplitudes (compared to lower resistive lungs) are required to generate the same tidal volume. ■ the risk of barotrauma to proximal airways might be higher. ■ the tidal volume is lower compared to normal resistive lungs. Inertance If a certain mass needs to be accelerated, a force is required to overcome the inertia. In mechanical ventilation, inertance is the pressure difference required to change flow (i.e. acceleration/ deceleration). Inertance has a negligible contribution to total impedance during tidal breathing at normal breathing frequencies, but contributes more substantially to the overall impedance during high-frequency ventilation. Inertance primarily arises from gas flow in the tracheal tube and major airways. S i i 24 I I 25 6.3 Pressure damping and frequency As frequency increases, the proportion of the pressure waveform transmitted to the lung periphery decreases. What is not often appreciated by clinicians, is that whilst lowering frequency often results in a lower oscillatory pressure amplitude on the ventilator display the distal lung is exposed to at least the same pressure and likely higher oscillatory amplitudes: this effect occurs due not only to the higher proportional transmission of that oscillatory pressure to the lung periphery at low frequency. Higher volumes are required to maintain the same efficacy of CO2 removal (ie DCO2 ). 6.4 Corner frequency and pressure cost of oscillatory flow Impedance decreases rapidly with increasing frequency, reaching a minimum at the resonance frequency (ƒ0 ). However, in the neonatal lung, minimal additional damping of the oscillatory pressure is achieved above the corner frequency (ƒC). Consequently, targeting oscillatory frequency to the corner frequency of the lung potentially achieves the minimum pressure cost per unit of oscillatory flow (15). In more simple terms, using frequencies above the corner frequency may achieve small additional falls in the oscillatory pressure amplitude delivered to the alveolar compartment. However, this additional