High positive end-expiratory pressure: only a dam against oedema formation?

INTRODUCTION: Healthy piglets ventilated with no positive end-expiratory pressure (PEEP) and with tidal volume (V(T)) close to inspiratory capacity (IC) develop fatal pulmonary oedema within 36 h. In contrast, those ventilated with high PEEP and low V(T), resulting in the same volume of gas inflated (close to IC), do not. If the real threat to the blood-gas barrier is lung overinflation, then a similar damage will occur with the two settings. If PEEP only hydrostatically counteracts fluid filtration, then its removal will lead to oedema formation, thus revealing the deleterious effects of overinflation. METHODS: Following baseline lung computed tomography (CT), five healthy piglets were ventilated with high PEEP (volume of gas around 75% of IC) and low V(T )(25% of IC) for 36 h. PEEP was then suddenly zeroed and low V(T )was maintained for 18 h. Oedema was diagnosed if final lung weight (measured on a balance following autopsy) exceeded the initial one (CT). RESULTS: Animals were ventilated with PEEP 18 ± 1 cmH(2)O (volume of gas 875 ± 178 ml, 89 ± 7% of IC) and V(T )213 ± 10 ml (22 ± 5% of IC) for the first 36 h, and with no PEEP and V(T )213 ± 10 ml for the last 18 h. On average, final lung weight was not higher, and actually it was even lower, than the initial one (284 ± 62 vs. 347 ± 36 g; P = 0.01). CONCLUSIONS: High PEEP (and low V(T)) do not merely impede fluid extravasation but rather preserve the integrity of the blood-gas barrier in healthy lungs.