Background Lung rest has been recommended during extracorporeal membrane oxygenation (ECMO) for severe acute respiratory distress syndrome (ARDS). Whether positive end-expiratory pressure (PEEP) confers lung protection during ECMO for severe ARDS is unclear. We compared the effects of three different PEEP levels whilst applying near-apnoeic ventilation in a model of severe ARDS treated with ECMO. Methods Acute respiratory distress syndrome was induced in anaesthetised adult male pigs by repeated saline lavage and injurious ventilation for 1.5 h. After ECMO was commenced, the pigs received standardised near-apnoeic ventilation for 24 h to maintain similar driving pressures and were randomly assigned to PEEP of 0, 10, or 20 cm H 2 O ( n =7 per group). Respiratory and haemodynamic data were collected throughout the study. Histological injury was assessed by a pathologist masked to PEEP allocation. Lung oedema was estimated by wet-to-dry-weight ratio. Results All pigs developed severe ARDS. Oxygenation on ECMO improved with PEEP of 10 or 20 cm H 2 O, but did not in pigs allocated to PEEP of 0 cm H 2 O. Haemodynamic collapse refractory to norepinephrine ( n =4) and early death ( n =3) occurred after PEEP 20 cm H 2 O. The severity of lung injury was lowest after PEEP of 10 cm H 2 O in both dependent and non-dependent lung regions, compared with PEEP of 0 or 20 cm H 2 O. A higher wet-to-dry-weight ratio, indicating worse lung injury, was observed with PEEP of 0 cm H 2 O. Histological assessment suggested that lung injury was minimised with PEEP of 10 cm H 2 O. Conclusions During near-apnoeic ventilation and ECMO in experimental severe ARDS, 10 cm H 2 O PEEP minimised lung injury and improved gas exchange without compromising haemodynamic stability.

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