"To the Editor

During a decremental positive end-expiratory pressure (PEEP) ­trial, insufficient PEEP may lead to expiratory lung collapse followed by inspiratory reopening (tidal recruitment/derecruitment), an injurious mechanism that may misleadingly increase measured respiratory system compliance (Crs), ultimately resulting in setting a PEEP level that does not prevent alveolar instability.^1-3 A combined assessment of Crs and tidal hysteresis improves interpretation of the trial, because tidal recruitment/derecruitment is indicated by a large increase in hysteresis following PEEP reduction.^1,4 In a previous post hoc analysis among patients with COVID-19 acute respiratory distress syndrome (ARDS), an absolute hysteresis threshold (≥100 mL) predicted tidal recruitment/derecruitment without requiring comparison between different PEEP levels.¹ We performed an external validation of this threshold to detect alveolar instability using a single assessment of hysteresis, independently of a PEEP trial. In addition, we sought to identify a cutoff for hysteresis normalized to the patient’s predicted body weight (PBW)."