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flowing through the system starts to be reduced rather than passing completely through. Corner frequency is related to the compliance and resistance (R) of the system, expressed as: ƒC=1/(2πRC). In the overdamped neonatal lung, ƒC is usually below the resonance frequency (ƒ0 ). Effect of time constant on the corner frequency of the lung. The time constant (τ) is the product of resistance and compliance (τ = R x C). As the time constant shortens, the corner frequency of the respiratory system increases. A 600g preterm infant with a compliance of 0.1 mL/ cmH2 O (severe HMD) and a resistance of 100 cmH2 O/L/s has a time constant of 10 ms and a corner frequency of 15.9 Hz. D-8059-2016 Corner Frequency (Hz) Time Constant (ms) 40 20 10 50 30 0 0 10 20 30 40 50 6.4.1 Corner frequency in different clinical settings Understanding the concept of corner frequency helps in the selection of frequency for different disease conditions. As time constant decreases, the corner frequency increases. Consequently, infants with RDS and non-compliant (short time-constant) atelectatic lungs have a higher corner frequency than infants with healthy, normally compliant lungs with relatively longer time constants. In contrast, infants with high airway resistance (long time-constant) will have an even lower corner frequency than the infant with healthy lungs. The most important pressure for determining the potential for barotrauma during HFOV is the peak carinal pressure. illustrates the normal fall in peak carinal pressure with increasing frequency at a constant distending pressure of 10 cmH2 O for a normal newborn infant. The figure clearly shows the shift of the corner frequency to the right for the infant with low compliance, highlighting the benefits of higher frequency selection in the infant with acute atelectatic disease. Similarly, as the atelectatic lung is recruited and compliance normalises, then the optimum ventilation frequency shifts to the left and lower frequency selection is appropriate. 6 Understanding transmission of pressure amplitude I Pressure amplitude and respiratory mechanics i i i 28 I I 29 In contrast, there are penalties for incorrect frequency selection in the presence of increased airway resistance, with a more narrow frequency range across which peak carinal pressures are minimised. High airway resistance in the newborn most commonly occurs in the setting of pulmonary interstitial emphysema or acute meconium aspiration syndrome. also shows that similar penalties are evident for the patient who has high resistance as well as low compliance, such as might occur with heterogeneous chronic lung disease, or evolving meconium aspiration when chemical pneumonitis reduces alveolar volume and stiffens the lung parenchyma. High resistance combined with low compliance shifts the corner frequency to the right (ie to higher frequencies). What is evident, however, is that for both high resistance settings, the peak carinal pressure is relatively high compared to normal and low compliance lung. High-frequency oscillatory ventilation is widely understood to achieve effective ventilation using tidal volumes of less-than or equal to anatomical dead space (ie 2-3 mL/kg). However, as many oscillators used in the clinical setting did not measure or report delivered tidal volume, many HFOV users have an incomplete understanding of how tidal volume changes with changes in ventilatory settings and/or respiratory mechanics. The principles of volume delivery during HFOV are the same as in pressure-controlled/limited ventilation at more conventional physiological rates. Understanding the determinants of tidal volume helps us to understand how tidal volume in high-frequency ventilation changes with different ventilator settings. Taking the inspired tidal volume as an example, the inspiratory tidal volume waveform resulting from V=(C∆P)(1 — e—TI /RC) shown in the plateau of this waveform is defined by the product of the compliance and the pressure amplitude; how quickly the maximum tidal volume is delivered is a function of the respiratory time constant (RC); the absolute volume delivered is determined by the elapsed inspiratory time (tI ), relative to the time constant. 7 Understanding tidal