Determinants of tidal recruitment/derecruitment assessed by electrical impedance tomography in spontaneously breathing acute respiratory distress syndrome patients.

RATIONALE: Tidal recruitment/derecruitment (R/D) is a well-known mechanism of lung injury in acute respiratory distress syndrome (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

This is a secondary analysis of a previous study including lightly sedated ARDS patients ventilated on PSV at 2 positive end-expiratory pressure (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, 1 was excluded due to unstable end-expiratory lung impedance, leaving 29 patients for analysis. Median PEEP was 10.0 (IQR 8.0-12.0) cmH2O with PEEPEIT and 8.0 (IQR 5.0-10.0) cmH2O with PEEPTABLE. Overall, tidal R/D was 14.3% (IQR 9.0-33.0), but it was lower at PEEPEIT vs PEEPTABLE (11.3% [IQR 4.8-20.5] vs 21.9% [IQR 6.8-31.7]; P = .008). Recruitment/derecruitment 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 PEEPs 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 nonfocal infiltrates presented higher tidal R/D (P = .005). At multivariable analysis, PLEnd-Exp, lung collapse, and nonfocal 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 nonfocal infiltrates.