Redistribution of pulmonary extravascular water with positive end-expiratory pressure in canine pulmonary edema.

We studied the effect of positive end-expiratory pressure (PEEP) on the quantity and distribution of pulmonary extravascular water (PEW) and on lung mechanics and gas exchange in dogs with high pressure pulmonary edema. Lung volumes, intrapulmonary shunt (Qs/Qt), and dynamic compliance (Cdyn) were measured before (baseline) and after (postinfusion) administration of Ringer's Lactate (25% body weight) to maintain pulmonary wedge pressure between 35 and 40 cm H2O in 16 dogs. Animals were then randomly assigned to a control group (n = 8), ventilated to maintain normocarbia for 1.5 h, and to a PEEP group (n = 8) similarly ventilated with the addition of 10 cm H2O PEEP. After final measurements (1.5 h) and the injection of 51Cr tagged red blood cells, the dogs were killed and right lower lobes were excised and processed for morphometric examination and calculation of PEW by the gravimetric technique. The cuff area/vascular area ratios were measured on all extra-alveolar vessels, and total cuff H2O as well as the cuff H2O/PEW ratio was calculated. Postinfusion, Qs/Qt increased and functional residual capacity (FRC) and Cdyn decreased in both groups. At 1.5 h the PEEP group showed significant improvement in Qs/Qt, FRC, and Cdyn (p less than 0.05). When compared with the control group, the PEEP group had similar amounts of PEW but a greater proportion of cuff H2O. We conclude that PEEP improves gas exchange and lung mechanics, and that it is associated with redistribution of lung water to the "extra-alveolar" interstitial space.