The Missing Dimension in Lung-Protective Ventilation: Physiologically Directed Inspiratory and Expiratory Time

Acute respiratory distress syndrome (ARDS) remains a major clinical problem, and treatment is largely supportive with mechanical ventilation. When ventilator settings are poorly matched to lung physiology, ventilator-induced lung injury can occur and worsen outcomes. Two key features of ARDS limit the effectiveness of conventional ventilation strategies: (1) elevated pleural pressure (Ppl), particularly in dependent lung regions, and (2) time- and pressure-dependent (T-PD) mechanics, in which injured lung units open slowly but collapse quickly at any given airway pressure. These features may limit the effectiveness of standardized approaches such as ARDS Network low tidal volume ventilation, which was not designed to adapt to high Ppl or T-PD dynamics. Airway pressure release ventilation (APRV) has shown promising results in animal studies, meta-analyses, computational models, and small clinical trials, although no large multicenter randomized controlled trial has yet to demonstrate a mortality benefit. APRV can be individualized to evolving lung pathology using the time-controlled adaptive ventilation (TCAV) method. TCAV is designed to address elevated Ppl through a prolonged inspiratory phase (THigh) that enables viscoelastic recruitment by creep, and a brief expiratory release (TLow) that prevents the newly opened alveoli from re-collapsing. Recruitment may be further facilitated by transient airway pressure perturbations generated at the collision of inspiratory and expiratory gas flow (the “gas hammer” hypothesis). Recruitment first occurs in areas with low Ppl, such that transpulmonary pressure (PTP) is positive. Lung tissue recruitment reduces the mass load from edema and atelectasis, progressively lowering Ppl and resulting in a positive PTP. Gradually decreasing Ppl from nondependent to dependent regions as the lung recruits modifies the airway pressure gradient, allowing the entire lung to be gradually reopened. The TLow is personalized using the slope of the expiratory flow curve, providing a real-time indicator of respiratory system compliance and mechanical behavior analogous to passive spirometry. The TCAV method supports progressive recruitment within T-PD lung regions, gradually reducing Ppl, increasing PTP, and promoting more homogeneous ventilation distribution across the injured lung.