Oxygen delivery determinants - content vs saturation (anaemia, CO poisoning, methaemoglobinaemia)
Core concept
- DO2 = CO x CaO2 x 10 - normal ~1000 mL/min; VO2 ~250 mL/min -> extraction ratio ~25%
- CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2)
- Hb and SaO2 carry essentially all of it; dissolved O2 is ~1.5% (0.3 mL/dL)
- A PaO2 of 100 contributes 0.3 mL/dL; 1 g/dL of Hb contributes ~1.3 mL/dL - four times as much
- -> the three ways to be hypoxic with a normal PaO2
- Anaemic hypoxia - dec Hb (commonest)
- Carbon monoxide - Hb present but occupied
- Methaemoglobinaemia - Hb present but Fe3+ cannot bind O2
- And a fourth: histotoxic (cyanide) - O2 delivered but unusable
- Stagnant (circulatory) hypoxia - dec CO; the DO2 term nobody checks
3 more sections, plus exam facts
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