Effects of Positive Pressure Ventilation on Extravascular Lung Water
These effects are common to all forms of positive pressure ventilation. However, practically speaking this discussion has the greatest relevance for PEEP and CPAP because the continuousness of positive pressure is important to achieving the maximal therapeutic effect.
Effect of positive pressure ventilation on alveolar fluid content
Behold, the two compartments of extravascular lung water.
One is the compartment directly abutting the alveolus, which is a reasonably small compartment. This is the alveolar interstitium.
Let us say that it is oedematous for whatever reason.
The extra water in this compartment occupies space, and delays the diffusion of gases (Ficks law of diffusion).
Now let us apply some pressure to this compartment.
The pressure squeezes the water out of the alveolar interstitium and pushes it into the more compliant peribronchial (and perihilar) interstitium. The result is an improvement of diffusion!
Gas exchange can now take place because there is no longer a lake of oedema fluid in the way. This is backed up experimentally. Malo, Ali and Wood reported on this effect in a series of thoracotomized dogs. Not only that, but the gas exchange membrane surface increases (there is more surface area per alveolus when they are distended like this).