Fig. 4
A Pressure/Time and Gas Flow/Time curves for Volume Assist-Control mode set and adjusted using the ARDSNet method. Key features include an inspiratory/expiratory ratio of 1:3. Plateau pressure is not extended, so peak inspiratory pressure is brief. Positive end-expiratory pressure (set-PEEP) and FiO2 are adjusted using oxygenation as the trigger for change [4]. B Pressure/Time and Gas Flow/Time curves for the airway pressure release ventilation (APRV) mode that is set and adjusted using the time-controlled adaptive ventilation (TCAV) method. Key features include an inspiratory/expiratory ratio as high as ~ 12:1, generating a prolonged inspiratory and short expiratory time. The continuous positive airway pressure (CPAP) phase is often ~ 90% of each breath. A tidal volume (VT) is not set, rather it is influenced by changes in (i) respiratory system compliance (CRS), (ii) the CPAP Phase pressure, and (iii) the duration of the Release Phase. The Release Phase is set as a percentage (75%) of the peak expiratory gas flow, which creates a very brief expiratory duration (Flow/Time curve, red arrowhead). Although this percentage is the same for most patients, the duration of the Release Phase can vary substantially in response to changes in CRS. The slope of the expiratory flow curve in the Gas Flow/Time curve provides a breath-to-breath measure of CRS. The lower the CRS, the steeper the slope of the expiratory flow curve, and the shorter the Release Phase. The slope of the expiratory flow curve becomes less steep as the patient’s CRS improves, which causes the Release Phase to lengthen (Fig. 7). The short Release Phase does not allow the lung time to depressurize fully, maintaining a time-controlled positive end-expiratory pressure (TC-PEEP, red dotted line). TC-PEEP is ~ 50% of the CPAP Phase pressure [91]. (Permission to republish requested)