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.
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.