Fig. 3

The ability of the TCAV method to stabilize and then open the lung is based on opening and collapse time constants, which are greatly altered if pulmonary surfactant is deactivated, and the viscoelastic system by which the lung changes volume. A Viscoelastic lung volume change. EXPIRATION (Lung Derecruitment): Viscoelastic volume change can be modeled using the spring connected in parallel with a dashpot. When airway pressure begins to fall during the Release Phase (Fig. 2B), there is a very short time delay before alveolar collapse begins, followed by rapid collapse (spring), and then a gradual, continual collapse over time (dash moving slowing through the pot). INSPIRATION (Lung Recruitment): When airway pressure is reapplied during the CPAP Phase (Fig. 2B, CPAP Phase), the reverse sequence of events occurs during lung opening: slight delay → rapid recruitment → gradual progressive recruitment. B Diagram of viscoelastic lung opening and collapse over time. If the expiratory time is very brief (≤ 0.5 s), lung tissue will not have time to collapse (green box). Lung tissue will continue to recruit without a change in airway pressure for as long as the CPAP Phase is applied. (yellow boxes). C A ventilator monitor showing typical TCAV method Pressure/ Flow/ Volume/ Time curves. Using the TCAV method the extended CPAP Phase continually 'nudges' the lung open over time (yellow boxes) (Additional file 3, Additional file 4) and establishes durable lung recruitment by not giving the lung sufficient time to collapse during the brief Release Phase (green boxes). D A blow-up of the expiratory and inspiratory flow curves is seen on the ventilator monitor (black box). The animal being ventilated had ARDS so the slope of the expiratory flow curve (Slope_EF) was very steep (ARDS, yellow line) as compared to the Slope_EF in a healthy lung (NORMAL, blue dashed line). At the termination of the brief Release Phase (green star) the lung is rapidly reinflated to the set CPAP Pressure. Panel A Adapted from Reference [29], under terms of the Creative Commons Attribution 4.0 International License