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reduction in oscillatory pressure amplitude 6 Understanding transmission of pressure amplitude I Pressure amplitude and respiratory mechanics Factors influencing damping of the oscillatory pressure waveform amplitude. The extent to which the oscillatory pressure waveform is damped is influenced by the mechanical characteristics of the respiratory system. Atelectatic alveoli experience higher oscillatory pressure amplitudes than normally aerated alveoli, for the same unit change in cyclic volume. Increased peripheral resistance increases the oscillatory pressure transmitted to proximal airways and neighbouring alveolar units. Adapted from (3). D-8073-2016 CONVECTION & DIFFUSION CONVECTION DIFFUSION Oscillatory pressure applied at airway opening is damped by flowdependent resistance and inertance of tracheal tube and central airways High peripheral resistance increases pressure transmission to more proximal airways and nearby alveoli Alveoli distal to a zone of increased peripheral resistance see low pressures due to decreased flow Proximal alveolar units are exposed to central airway oscillatory pressures Atelectatic compartments exposed to increased oscillatory pressures Expanded & aerated alveoli protected from high oscillatory pressures 6.2.4 Inhomogeneous lung disease High frequency oscillatory ventilation has cemented its place in the recruitment and ventilation of the homogeneously collapsed or atelectatic lung. However, the value of HFOV in the inhomogeneous lung is less certain. As suggested by , co-existence of high and low compliance alveolar compartments and areas of increased peripheral resistance may co-exist within the same lung. Areas with longer time constants will have reduced oscillatory pressure transmission whilst higher oscillatory pressures are transmitted to the short-time constant (eg low compliance) segments. Although increased oscillatory pressure transmission to low compliant lung zones increases risk of barotrauma in those areas, the increased oscillatory pressures may also promote recruitment of alveolar volume if the peak oscillatory pressures exceed the opening pressure of that lung region. D-8081-2016 Effect of oscillatory frequency on transmission of oscillatory pressure. The proportion of the oscillatory pressure amplitude at the airway opening transmitted to the alveolus is higher at low frequency (eg top panel) than at higher frequencies (eg lower panel). Importantly, although the absolute oscillatory pressure displayed by the ventilator may be lower at the airway opening than that required for high frequencies, the absolute alveolar oscillatory pressure amplitude may actually be higher at lower frequencies, as a higher tidal volume is required to achieve equivalent CO2 exchange (ie to maintain DCO2 ). Airway Opening Pressure (cmH O) 2 5 35 Large Airways 5 35 5 35 Alveoli 5 35 0.2 0.4 0.6 0.8 1.0 35 5 0.2 0.4 0.6 0.8 1.0 Time (s) 5 35 0.2 0.4 0.6 0.8 1.0 6 Hz 12 Hz 26 I I 27 is achieved at the expense of higher oscillatory pressure amplitude to the more proximal components of the respiratory system, such as the large airways. A graphical representation of the relation between corner frequency and the time constant of the lung is shown below. As time constant increases, the corner frequency decreases. Damping Damping refers to the reduction, restriction or prevention of oscillations within or upon that system. Damping in physical systems is a result of a process that dissipates the energy stored within the oscillation. In an overdamped system the system returns (exponentially decays) to equilibrium without oscillating. Resonance frequency The resonance frequency of a system is the frequency at which the response amplitude to a defined input is a relative maximum compared to the response at other frequencies. At resonance frequencies, small periodic driving forces may produce large amplitude oscillations due to storage of vibrational energy. Resonance frequency is calculated as ƒ0=1(2π√——IC) where ƒ0 is the resonance frequency, C is the compliance and I is the inertance. Corner frequency Corner frequency represents the point (boundary) within the frequency response of any system, at which the energy