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HFOV pressure waveform parameters: Amplitude is usually determined as the difference between peak and trough of the pressure waveform. Amplitude MAP Pressure Time 3 Interestingly, the frequency at which this transition occurs depends on the ratio of the metabolic rate to dead space: consequently the transition frequency is lower in adults than for infants. 16 I 5 HFV ventilator parameters and control variables I I 17 Pressure amplitude and respiratory mechanics 5.3 Oscillatory frequency Oscillatory frequency (f) is a ventilator parameter measured in units of Hertz (Hz = 1/s). The frequency influences the oscillatory volume: as frequency increases, the available time for inflation and deflation decreases, limiting the delivered tidal volume for any given oscillatory amplitude and prevailing lung and circuit mechanics. Frequency selection has important implications for the proportional transmission of pressure amplitude to the alveolar compartment (see Chapter 6), with smaller pressure amplitudes delivered as frequency increases. This fact is often not appreciated by clinicians, as the required amplitude (displayed by the ventilator) usually decreases as frequency is lowered. Although amplification of alveolar pressure amplitudes may occur at frequencies close to the resonance frequency (11), amplification is rarely a problem in the overdamped neonatal lung. Optimal oscillatory frequency needs to be adjusted according to the prevailing mechanical condition of the lung and associated disease characteristics. Frequency selection is covered in more detail in section 6.3 and section 7.1.2. D-8070-2016 A I:E = 1:1 A = B B Pressure Time A I:E = 1:2 A ≠ B B Pressure Time Figure 5-2: Influence of I:E ratio on symmetry of the oscillatory pressure waveform. At an I:E of 1:1, the peak and trough of the oscillatory pressure waveform are equidistant from the mean airway pressure. However, at I:E of 1:2, the trough of the pressure waveform is approximately half as far below the mean airway pressure as the peak of the pressure waveform is above the mean airway pressure. A larger inspiratory ΔP is required at I:E of 1:2 to achieve equivalent tidal volume delivery to an I:E of 1:1. 5.4 Inspiratory to expiratory ratio The inspiratory to expiratory ratio (I:E) determines the proportion of time spent when oscillatory pressure is above (inspiratory) or below (expiratory) the mean airway pressure, resulting in similar fluctuations of positive and negative flow. The peak and trough of the oscillatory pressure waveform are similarly distant (in opposing directions) from the mean airway pressure when inspiratory and expiratory cycle durations are of equal value (I:E = 1:1). However, equidistance of the peak/trough from the mean does not apply when using I:E other than 1:1, to ensure that the area under the flow curve is the same during inflation and deflation and complete removal of any delivered volume. At I:E of 1:2 for example, the trough of the oscillatory pressure waveform will be approximately half the distance of the peak of the oscillatory pressure waveform away from the mean airway pressure, but the relative negative pressure will be maintained for twice as long (see Figure 5-2). I:E of 1:2 were initially used in HFOV due to concerns about potential gas trapping at high-frequencies. However, gas trapping is not a relevant concern when active expiration is used. As the peripheral pressures are damped, the difference between inspiratory peak pressure and expiratory trough pressures is much smaller than seen at the airway opening, resulting in negligible differences between inspiratory and expiratory resistance. If mean airway pressures are high enough, the airways will be stented sufficiently to prevent collapse during expiration. Importantly, intra-thoracic expiratory pressures remain positive, despite potential for subatmospheric pressures at the airway opening (12). An exception occurs when extremely low mean airway pressures are set, creating opportunities for choke point formation with resultant airway collapse and gas trapping. I:E ratio may have implications for gas mixing efficiency and potential shear effects,