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What is PEEP : normal physiological PEEP created by the anatomy of the airway is about 3-5 cm. Consider the fact that an artificial airway/endotube creates a direct unobstructed (think: comparatively low turbulence) air flow. A normal un-intubated resistance to exhalation (think of all those twist/turns in the airway/nasal turbinates, etc) creates "normal PEEP". You can feel PEEP yourself by exhaling through a straw into a liquid beverage. That resistance to exhalation is PEEP. You (could) put a pressure manometer inline to the straw and measure the pressure. Applying a 5 cm of PEEP is just replacing what was bypassed by the endotube being in there. Its just a bit of positive airway pressure, without volume.