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

Abstract

The expiratory time constant (RC_EXP) 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, RC_EXP 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 RC_EXP, 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 RC_EXP 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 RC_EXP 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 RC_EXP and proposes future directions for its integration into ventilator management.

Keywords: Expiratory time constant; Flow-volume waveform; Mechanical ventilation; Positive end-expiratory pressure; Review.

Fig. 1

Physiological values of RC_EXP in passive, healthy postoperative patients under pressure-controlled ventilation (PCV) and volume-controlled ventilation (VCV) using different methods to obtain RC_EXP. The upper and lower limits of the reference range are displayed as dashed red lines [14]

Fig. 2

Effects of PEEP on RC_EXP in terms of compartments in A patients with COPD—increasing PEEP levels reduces the heterogeneity of regional lung emptying and thus promotes exhalation. B patients with ARDS—effect of ZEEP (dotted lines) on τ of the total respiratory system including artificial airways (τ tot) and τ of the respiratory system alone (τ RS) and PEEP 15 cmH₂O (dashed lines). COMP-1–5—compartments 1–5, ZEEP—zero end-expiratory pressure, PEEP—positive end-expiratory pressure, COPD—chronic obstructive pulmonary disease. Figures were reconstructed from the original study by Kondili et al. [23] (A), and Guttmann et al. [6] (B)

Fig. 3

PEEP titration and recruitability patterns in COVID-19 patients with moderate to severe ARDS in the prone and supine positions. Mean values of RC_EXP are displayed at each PEEP in both positions. Figures were reconstructed from Depta et al. [15]

Fig. 4

Common clinical scenarios and ventilator adjustments based on RC_EXP. A. Short RC_EXP typical of acute pulmonary edema and "stiff" lungs, such as nonrecruitable ARDS and pulmonary fibrosis, B. Normal RC_EXP typical for recruitable lung (i.e. ARDS, pneumonia) including mixed pathologies creating "pseudonormal" RC_EXP, C. Long RC_EXP associated with expiratory flow limitation due to high airway resistance