Early Changes of Lung Mechanics After PEEP Reduction in ARDS: A 4D-CT Study

Rationale Positive end-expiratory pressure (PEEP) balances recruitment against over-inflation in ARDS. The early response to a PEEP step-down remains under-characterized. Four-dimensional computed tomography (4D-CT) can quantify within-breath mechanics with anatomy-specific resolution. Objectives Using 4D-CT to characterize the early (3-5 min) effects of PEEP step-down (15 to 5 cmH2O) on tidal recruitment and over-inflation in ARDS patients. Methods In a prospective, single-center, physiological study, 40 patients with stabilized ARDS (77.5% with PaO2/FIO2 ≥ 150, median normalized respiratory system compliance 0.78 mL/cmH2O/kg) received lung-protective volume-controlled ventilation with median PEEP of 8 cmH2O. 4D-CT was obtained at PEEP 15 cmH2O and 3-5 min after a PEEP step-down to 5 cmH2O. We quantified tidal recruitment (−200 to + 100 HU) and over-inflation (−1000 to − 901 HU). Measurements and Main Results Tidal recruitment was low at high PEEP and minimally increased at low PEEP, from 1.15% to 1.84% of expiratory tissue weight (P < .001). End-inspiratory over-inflation decreased from 11.9% to 5.2% of end-inspiratory lung volume (P < .001), and tidal over-inflation decreased from 42.5% to 18.3% of tidal volume (P < .001). End-expiratory lung volume (EELV) substantially decreased (median −787 mL, 95% CI −952 mL to − 743 mL, P < .001) and the volume loss exceeded the passive elastic prediction, indicating substantial alveolar derecruitment. In exploratory analyses, the recruitment-to-inflation ratio did not modify the tidal recruitment response. Conclusions At 3-5 minutes after a PEEP step-down from 15 to 5 cmH2O, 4D-CT showed large EELV loss, substantial alveolar derecruitment and reduced over-inflation, while tidal recruitment slightly increased and remained low. These results are compatible with a condition in which alveolar units prone to closure respond by closing and remaining closed.