Opening pressures and atelectrauma in acute respiratory distress syndrome

Open lung strategy during ARDS aims to decrease the ventilator-induced lung injury by minimizing the atelectrauma and stress/strain maldistribution. We aim to assess how much of the lung is opened and kept open within the limits of mechanical ventilation considered safe (i.e., plateau pressure 30 cmH₂O, PEEP 15 cmH₂O).

Prospective study from two university hospitals. Thirty-three ARDS patients (5 mild, 10 moderate, 9 severe without extracorporeal support, ECMO, and 9 severe with it) underwent two low-dose end-expiratory CT scans at PEEP 5 and 15 cmH₂O and four end-inspiratory CT scans (from 19 to 40 cmH₂O). Recruitment was defined as the fraction of lung tissue which regained inflation. The atelectrauma was estimated as the difference between the intratidal tissue collapse at 5 and 15 cmH₂O PEEP. Lung ventilation inhomogeneities were estimated as the ratio of inflation between neighboring lung units.

The lung tissue which is opened between 30 and 45 cmH₂O (i.e., always closed at plateau 30 cmH₂O) was 10 ± 29, 54 ± 86, 162 ± 92, and 185 ± 134 g in mild, moderate, and severe ARDS without and with ECMO, respectively (p < 0.05 mild versus severe without or with ECMO). The intratidal collapses were similar at PEEP 5 and 15 cmH₂O (63 ± 26 vs 39 ± 32 g in mild ARDS, p = 0.23; 92 ± 53 vs 78 ± 142 g in moderate ARDS, p = 0.76; 110 ± 91 vs 89 ± 93, p = 0.57 in severe ARDS without ECMO; 135 ± 100 vs 104 ± 80, p = 0.32 in severe ARDS with ECMO). Increasing the applied airway pressure up to 45 cmH₂O decreased the lung inhomogeneity slightly (but significantly) in mild and moderate ARDS, but not in severe ARDS.

Data show that the prerequisites of the open lung strategy are not satisfied using PEEP up to 15 cmH₂O and plateau pressure up to 30 cmH₂O. For an effective open lung strategy, higher pressures are required. Therefore, risks of atelectrauma must be weighted versus risks of volutrauma.

Clinicaltrials.gov identifier: NCT01670747 ( www.clinicaltrials.gov ).