Tasty Morsels of EM 074 – #FRCEM Oxygen Extraction Ratio & Aa Gradient

20 Jul

I’m entering a few months prep for the UK and Ireland exit exam in Emergency Medicine: the FRCEM. I’ll be adding lots of little notes on pearls I’ve learned along the way. A lot of my revision is based around the Handbook of EM as a curriculum guide and review of contemporary, mainly UK guidelines. I also focus on the areas that I’m a bit sketchy on. With that in mind I hope they’re useful.

You can find more things on the FRCEM on this site here.

What is meant by DO2, VO2 and O2ER?

Mainly from LITFL

  • DO2/VO2 relationships
    • DO2 = oxygen delivery
    • VO2 = oxygen consumption
    • O2ER is oxygen extraction ratio and is ratio of VO2 to DO2 [O2ER = VO2 / DO2]
    • Normally VO2 = 250ml/min and DO = 1000ml/min therefore O2ER = 25%. In other words oxygen delivery is usually well in excess of oxygen consumption
    • the critical O2ER is about 70% and beyond this things don’t go well

What is the Alveolar-arterial gradient?

  • The Alveolar-arterial gradient
    • Aa Gradient = PAO2-PaO2
    • PAO2 comes from the alveolar gas equation
      • PAO2 = PiO2 – PACO2/0.8
      • remember PiO2 = 0.21 x (760 – 47) [at sea level]
    • PaO2 comes from your ABG
    • Normal Aa gradient increases with age.
    • In a hypoxic patient a normal Aa gradient suggests hypoxia is due to 1) hypoventialtion (eg opiates) or 2) reduced FiO2
    • Increased Aa gradient found in V/Q or shunt or increased O2ER or diffusion defect


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