Expiratory time constants in mechanically ventilated patients: rethinking the old concept—a narrative review

The expiratory time constant (RCEXP) plays an important role in understanding the mechanical properties of the respiratory system in patients receiving mechanical ventilation. Initially conceived as a tool to illustrate nonlinearity in lung emptying, RCEXP has transitioned from a theoretical concept to a clinically relevant parameter, particularly within the realm of intelligent ventilation strategies. This narrative review explores the historical development of RCEXP, starting with its foundational definition based on fixed values of respiratory system resistance and compliance (i.e., the single-compartmental model). This early approach to RCEXP largely overlooked the intricate viscoelastic characteristics of the lungs. The inherent limitations of this simplified model are discussed. The review then shifts its focus to clinical evidence describing the severity of deviations in RCEXP from the ‘‘ideal’’ state in both acute lung injury and obstructive lung disease. This includes an analysis of which portions of the expiratory phase are most affected and how adjustments in tidal volume and positive end-expiratory pressure can potentially improve the homogeneity of lung emptying. The review concludes with a discussion of the clinical applications of RCEXP and proposes future directions for its integration into ventilator management.