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

Introduction Healthy piglets ventilated with Tidal Volume (VT) close to inspiratory capacity (IC; maximal physiological lung inflation) always largely increase their lung weight within 24-36 hours [1], as for fluid filtration through a disrupted blood-gas barrier. We have recently noted that when a similar degree of lung inflation is mainly due to high Positive End-Expiratory Pressure (PEEP), lung weight never changes. Aim To clarify whether PEEP acts only as a dam, preventing fluid filtration through a blood-gas barrier that is invariably damaged when lungs are inflated up to their physiological limit. If that is the case, removal of PEEP will rapidly lead to lung edema formation. Methods Three healthy piglets (21±2 Kg) had their IC measured with lung computed tomography (CT). They were then ventilated with high PEEP (around 75% of IC) and low VT (25% of IC). After 36 hours, PEEP was removed and ventilation continued with low VT for 18 hours. Lung weight was measured at the beginning (CT) and end (balance, following autopsy) of the study. Results IC was 1023±136 ml. Accordingly, animals were initially ventilated with PEEP 19±1 cmH2O (volume of gas 898±211 ml) and VT 213±12 ml. Following PEEP removal, VT was 217±21 ml. As shown in figure, lung weight never increased. InitialFinalLung Weight ( g)200300400 Figure. Individual changes in lung weight. p = 0.13 ( Paired t-test ) Conclusion Our preliminary findings suggest that high PEEP does not merely act as a dam, as its removal is not followed by an increase in lung weight. We therefore hypothesize that ventilation with high PEEP and low VT somehow protects the blood-gas barrier. [1]: Protti et al. AJRCCM, 2011; 183(10):1354-1362.