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that I:E ratio has on the absolute tidal volume delivered also depends on frequency and lung mechanics. As the steepest part of the change in tidal volume occurs early in the inspiratory phase, there is a larger percentage increase in tidal volume when I:E ratio is change from 1:2 to 1:1 at high frequencies than at low frequencies. Similarly, because the absolute time to reach maximum tidal volume increases as time constant of the lung increases, there is a larger percentage increase in tidal volume with an I:E ratio change from 1:2 to 1:1, as the time constant of the lung increases. 7.1.4 Lung mechanics As the inspiratory time during ventilation at conventional physiological rates usually exceeds 5 time constants, there is sufficient time for tidal volumes in the order of 4-6 mL/kg to be delivered to the lung. However, during HFOV, the inspiratory times are much shorter than those used during conventional ventilation, and usually the inspiratory times during HFOV are well below the time taken for 3 to 5 time constants of the lung to elapse. Furthermore, the absolute inspiratory time usually changes with change in oscillatory frequency because the clinician sets the percent inspiratory time or defines an inspiratory/ expiratory time ratio, rather than setting an absolute inspiratory time. Cycle length decreases as frequency increases: consequently, providing I:E is held constant, the absolute inspiratory time becomes shorter as frequency increases. Figure 7-5 illustrates how limiting inspiratory time in HFOV results in the delivery of much smaller tidal volumes during HFOV than during conventional ventilation. Figure 7-5 also shows how the mechanical properties of the respiratory system markedly influence the tidal volume delivered during HFOV. The lung with very short time constant (ie low compliance), achieves the maximum tidal volume rapidly whereas the healthy and high resistance lungs may not reach their maximum potential tidal volumes before the inspiratory phase of the HFOV waveform is terminated. 7 Understanding tidal volume delivery during HFOV I Lung volume and mean airway pressure D-8056-2016 Figure 7-5: Effect of lung mechanics on inspiratory tidal volume during HFOV. Three lines are shown, representative of infants with healthy compliant (1.0 mL/cmH2 O) lungs (blue line), very low compliance (0.1 mL/cmH2 O) lungs (green line), or high resistance (250 cmH2 O/L/s), reduced compliance (0.6 mL/cmH2 O) lungs (eg BPD: orange line). Tidal Volume (mL/kg) Time (ms) healthy C R C 4 2 0 0 20 40 60 80 100 34 I I 35 8.1 Determinants of distending lung volume during HFOV Distending lung volume is changed by altering the mean intrapulmonary pressure – ie the distending pressure within the lungs (PL ). However, as the intrapulmonary pressure is not easily measured, it is the mean airway opening pressure (Pao) (the pressure measured at the airway opening or “patient wye”) that is used by the clinician as a guide to the pressure in the lung. Understanding the determinants of both of these pressures is essential to understanding how to optimise lung volume under different clinical settings. 8.1.1 Control of mean airway pressure Mean airway pressure during HFOV is usually adjusted directly by the clinician using a ventilator control. The ventilator achieves these increases or decreases in mean airway pressure settings by adjusting either the bias flow and/or the resistance to flow in the expiratory valve. 8.1.2 Determinants of mean intrapulmonary pressure During conventional ventilation, the mean airway pressure at the airway opening is usually very similar to the mean intrapulmonary pressure (the pressure actually distending the lung). However, during HFOV, mean pressures in the lung may be either higher or lower than the mean pressure set or shown on the ventilator from recorded at the airway opening. A mean intrapulmonary pressure higher than the mean airway opening pressure is usually a result of applying insufficient mean airway pressure and development of gas trapping. Increasing the mean airway pressure to splint the airways open will prevent the air from becoming trapped in the lung periphery. More commonly, mean intrapulmonary pressure is lower than the pressure measured at the