Determinants of tidal recruitment/derecruitment assessed by electrical impedance tomography in spontaneously breathing ARDS patients

Rationale: Tidal recruitment/derecruitment (R/D) is a well-known mechanism of lung injury in ARDS patients, but its bedside identification is challenging during assisted ventilation. Objectives: To investigate bedside determinants of tidal R/D assessed by electrical impedance tomography (EIT) in spontaneously breathing ARDS patients on pressure support ventilation (PSV). Methods: Secondary analysis of a previous study including lightly sedated ARDS patients ventilated on PSV at two PEEP levels selected by EIT (PEEPEIT) and low PEEP/FiO2 table (PEEPTABLE). Detailed physiological assessment was performed, including respiratory mechanics, inspiratory drive and effort, regional mechanics and tidal R/D measured by EIT. Measurements and main results: Of 30 original patients, one was excluded due to unstable end-expiratory lung impedance, leaving 29 patients for analysis. Median PEEP was 10.0 [8.0-12.0] cmH2O with PEEPEIT and 8.0 [5.0-10.0] cmH2O with PEEPTABLE. Overall, tidal R/D was 14.3% [9.0-33.0], but it was lower at PEEPEIT vs. PEEPTABLE (11.3% [4.8-20.5] vs. 21.9% [6.8-31.7]; P = .008). R/D reduction with PEEPEIT correlated with decreases in collapse (rho = 0.72; P < .001) and pendelluft (rho = 0.57; P = .002).Analyzing data from both randomized PEEP together, mixed-effects models showed higher tidal R/D at lower PEEP (P < .001), more negative end-expiratory transpulmonary pressure (PLEnd-Exp) (P < .001), larger collapse (P < .001) and lower lung compliance (P = .002). Higher respiratory drive (P = .001), effort (P = .009), and dynamic driving transpulmonary pressure (P = .009) correlated with higher tidal R/D. Patients with non-focal infiltrates presented higher tidal R/D (P = .005).At multivariable analysis, PLEnd-Exp, lung collapse and non-focal pattern were independently associated tidal R/D. Conclusion: In spontaneously breathing ARDS patients on PSV, the main determinants of tidal R/D are more negative PLEnd-Exp, higher lung collapse and non-focal infiltrates.