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of the respiratory system. In the clinical setting, elastance (E) is more commonly understood as compliance (C) of the lung where C = 1/E. 5 HFV ventilator parameters and control variables I Pressure amplitude and respiratory mechanics THE COEFFICIENT OF GAS TRANSPORT ■ DCO2 is the critical determinant of the effciency of gas mixing. ■ The removal of carbon dioxide in HFOV is approximately f * VT 2 . ■ DCO2 is an absolute value and depends on the tidal volume. ■ CO2 removal is most efficiently achieved by increasing the tidal volume. S i 20 I I 21 6.2.1 Lung compliance Clinical understanding regarding marked damping of the oscillatory pressure waveform (illustrated in Figure 6-1A) arose out of studies in healthy compliant lung models. However, HFOV was developed and applied initially for the extremely non-compliant and immature lung of the preterm infant with respiratory distress syndrome (RDS). Low compliance increases impedance to flow. Consequently, in the presence of low compliance (Figure 6-1B), higher pressure amplitudes are required to generate the same tidal volume and achieve effective gas exchange. Clinically, the importance of compliance to damping of the oscillatory pressure waveform was demonstrated clearly in animal models. Studies using alveolar capsules to measure alveolar oscillatory pressure swings in preterm lambs during lung volume recruitment showed the proportionally higher transmission of oscillatory pressure amplitude from the airway opening to the alveolus when compliance was low (14). Similarly, the oscillatory pressure ratio4 is lowest at the point of maximal compliance during the inflation and deflation limbs of a pressure volume recruitment manoeuvre (29). 6 Understanding transmission of pressure amplitude I Pressure amplitude and respiratory mechanics 4 ratio of oscillatory pressure amplitude at the distal end of the tracheal tube to amplitude at the airway opening Low lung compliance and HFO If HFO is applied to immature, low-compliant lungs: ■ higher ventilator pressure amplitudes (compared to more compliant lungs) are required to generate the same tidal volume. ■ the inspiratory and expiratory lung time constants are shorter with lower compliance. ■ the percentage transmission of pressure amplitude from the airway opening to the alveoli is higher compared to normally compliant lungs. D-8064-2016 : Pressure damping during HFOV. At physiological breathing rates (black line), there is sufficient time for the pressure waveform to be completely transmitted from the airway opening (patient wye) to the alveoli. In contrast, during HFOV (blue line), the very short inspiratory times are insufficient to fully transmit the complete pressure waveform. Instead, there is progressive damping of the pressure waveform as it travels from the airway opening to the alveolar compartment . In the presence of low-compliance, the inspiratory and expiratory time constants are shorter and higher peak inspiratory pressures (or ΔP) needs to be delivered to the airway opening to achieve the same tidal volume as is achieved in the compliant healthy lung using physiological breathing rates. For HFOV, not only are higher delta pressures required at the airway opening, but the extent of pressure damping is reduced. Consequently, in the poorly compliant lung, there is a higher percentage transmission of pressure from the airway opening to the alveolus. Airway Opening Pao (cmH O) 2 5 25 Large Airways 5 25 5 25 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 Compliant Non-compliant Pao(cmH O) 2 A B High Frequency Ventilation Conventional Ventilation 6.2.2 Resistance Peripheral Resistance Increased peripheral resistance, associated with interstitial emphysema or collapsed peripheral airways also increases impedance to flow at the point of obstruction and beyond. Increasing peripheral resistance reduces the cyclic volume change in the alveolus accompanied by reduced alveolar oscillatory pressure amplitude. Consequently, higher ventilator oscillatory pressures are required to restore tidal volume to the same level as obtained prior to development of the increased peripheral resistance (14).