High-altitude/cabin pressure physiology - equivalent FiO2
Core concept
- Alveolar/inspired O2 depends on partial pressure of O2 (PiO2), not the FiO2 fraction alone: PiO2 = FiO2 x (Patm - PH2O)
- At altitude, Patm falls while FiO2 (21%) is unchanged -> PiO2 falls -> hypoxaemia despite breathing "normal air"
- Aircraft cabins are pressurised only partially -- typically to an equivalent altitude of ~6000-8000 feet, meaning PiO2 is lower in flight than at sea level even though passengers are not told this explicitly
3 more sections, plus exam facts
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