Positive end-expiratory pressure ventilation increases extravascular lung water due to a decrease in lung lymph flow

Positive end-expiratory pressure (PEEP) is used to improve gas exchange, increase functional residual capacity, recruit air spaces, and decrease pulmonary shunt in patients suffering from respiratory failure. The effect of PEEP on extravascular lung water (EVLW), however, is still not fully understood. This study was designed as a prospective laboratory experiment to evaluate the effects of PEEP on EVLW and pulmonary lymph flow (Q_L) under physiologic conditions. Twelve adult sheep were operat ively prepared to measure haemodynamics of the systemic and pulmonary circulation, and to assess EVLW. In addition, the lung lymphatic duct was cannulated and a tracheostomy performed. The animals were then mechanically ventilated in the awake-state without end-expiratory pressure (PEEP 0). After a two-hour baseline period, PEEP was increased to 10 cmH₂O for the duration of two hours, and then reduced back to 0 cmH₂O. Cardiopulmonary variables, Q_L, and arterial blood gases were recorded intermittently; EVLW was determined two hours after each change in PEEP. The increase in PEEP re sulted in a decrease in Q_L (7±1 vs 5±1 ml/h) and an increase in EVLW (498±40 vs 630±58 ml; P<0.05 each) without affecting cardiac output. As PEEP was decreased back to baseline, Q_L increased significantly (5±1 vs 10±2 ml/h), whereas EVLW returned back to baseline. This study suggests that institution of PEEP produces a re versible increase in EVLW that is linked to a decrease in Q_L.