Expiratory time constant in mechanical ventilation: devices, settings, and methodology impact the results

Expiratory time constant (τ) calculation enables estimation of the time required for complete expiration during mechanical ventilation, assessment of bronchodilator therapy effects, and optimization of advanced ventilatory modes. However, differences in expiratory control design among ventilator manufacturers represent a potential source of variability, as they may alter expiratory flow and consequently affect τ values. Moreover, numerous mathematical methods for τ calculation have been described, yet no reference recommendation currently exists. This study evaluated the impact of non-patient factors on τ using a standardized test lung, specifically the chosen ventilator, positive end-expiratory pressure (PEEP), tidal volume (VT), and calculation method. A thermocompensated test lung with fixed compliance and variable resistors was used to compare eight mechanical ventilator models (each tested in duplicate) at PEEP levels of 0, 5, and 15 cm H 2 O and VT of 300, 500, and 700 mL. For each setting, 10 representative breaths were manually selected and temporally aligned, and τ was subsequently calculated using 11 distinct methods. A gamma-distributed generalized linear model evaluated ventilator-model effects, and exploratory linear regressions evaluated associations with PEEP and VT. The measured τ was significantly dependent on ventilator manufacturer and model and ranged from 1.03 s to 1.53 s despite unchanged physical properties of the test lung and ventilator settings. Identical changes in PEEP or VT resulted in prolongation of τ in some ventilator models, shortening in others, or no significant change. Across calculation methods, τ estimates differed by more than twofold in most settings with extremes as high as an elevenfold difference in settings simulating high flow limitation. Non-patient factors substantially influence the measurement of the expiratory time constant during mechanical ventilation, showing that it is not solely a physiological parameter. The variability caused by ventilator design, settings, and method used for calculation may exceed clinically meaningful differences between studied patients, underscoring the need for cautious interpretation. Further research is needed to elucidate the possible implications of ventilator choice in selected groups of patients with expiratory pathology.