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.

Extravascular lung water wihout PEEP

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.

Extravascular lung water with PEEP

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).

References

Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (was well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.

The chapter from Tobins was actually surprisingly unenlightening. In that book, information on this topic is scattered across about 2000 pages. If you need something to-the-point, I recommend this section (5) from an online textbook of anaesthesia. It is a brief and robust introduction to the subject matter.

R. Rodriguez-Roisin, A. Ferrer "Effects of mechanical ventilation on gas exchange" - Chapter 37 (p.759) in Tobin - Principles and Practice of Mechanical Ventilation (2md ed., 2006)

Soni, N., and P. Williams. "Positive pressure ventilation: what is the real cost?." British journal of anaesthesia 101.4 (2008): 446-457.

Oakes, Dennis L. Physiological Effects of Positive Pressure Ventilation. AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH, 1992. -this is somebody's Masters of Science thesis! They received their degree in 1992, but one expects that the fundamentals of physiology have remained the same since then.

Kumar, Anil, et al. "Continuous positive-pressure ventilation in acute respiratory failure: effects on hemodynamics and lung function." New England Journal of Medicine 283.26 (1970): 1430-1436.

Malo, J. A. C. Q. U. E. S., J. A. M. E. E. L. Ali, and L. D. Wood. "How does positive end-expiratory pressure reduce intrapulmonary shunt in canine pulmonary edema?." Journal of applied physiology 57.4 (1984): 1002-1010.