Effects of Positive Pressure Ventilation on Pulmonary Physiology

The act of forcefully pushing air into somebody's chest cavity has a range of effects, of which some will hopefully be beneficial. Usually, by subjecting somebody to pressurised oxygen torture we expect to achieve some sort of improvement in the pulmonary mechanics or gas exchange. That, after all, is why we subject people to this pressurised gas torture. Specific indications for positive pressure ventilation (and the indications for mechanical ventilation in general, which are subtly different) are discussed elsewhere.

This chapter is concerned purely with the effects of positive pressure on the respiratory system. Specifically, generic "positive pressure" effects will be discussed. The effects of PEEP, for example, are slightly different from the effects of positive pressure in general (and are discussed in greater detail elsewhere.

Positive End-Expiratory Pressure (PEEP)

PEEP is airway pressure artificially kept above atmospheric pressure at the end of inspiration.

PEEP

Why do we use PEEP?

Because it improves oxygenation (by recruiting collapsed alveoli and decreasing shunt), because it decreases the work of breathing due to intrinsic PEEP and increased airway resistance, and because it has several interesting (and sometimes useful) haemodynamic effects.

For the pusposes of physiological discussions, the distinction between PEEP and positive pressure in general is a fairly arbitrary one (it's all positive pressure, innit); however there are certain situations where end-expiratory pressure plays a specific role. For example, it is the end-expiratory pressure that protects alveoli from collapse; and it is end-expiratory pressure that decreases the effort of initiating a breath due to intrinsic PEEP.

Effect of positive pressure ventilation on lung volume

Effect of positive pressure ventilation on lung compliance

Effect of positive pressure ventilation on work of breathing

References

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.