In this BoardsCast episode, we continue Tobias Chapter 103 — Lungs with the trap that kills patients while the monitor looks “fine”:

Perfect lungs. perfect PaO₂. perfect SpO₂…and the tissues are still suffocating. 

Because oxygen entering the blood doesn’t matter if it isn’t being carried and moved.

This episode installs the single governing framework for oxygen delivery:

DO₂ = Cardiac Output × CaO₂

And then we break down the part most people misunderstand: CaO₂ is almost entirely hemoglobin, not dissolved oxygen. PaO₂ is a “warehouse” number — but hemoglobin is the truck fleet, and cardiac output is the highway. 

You’ll learn:

•  Why no hemoglobin = no delivery (period) 
•  CaO₂ math in plain terms: ~98% carried on hemoglobin, ~1–2% dissolved (PaO₂ barely moves the needle) 
•  Why does doubling PaO₂ barely increase total oxygen content (you doubled the “rounding error”) 
•  4 classic delivery failures the boards love: 
  • Severe anemia (missing trucks) 
  • Carbon monoxide poisoning (trucks hijacked + won’t unload; pulse ox can lie) 
  • Shock/low CO (highway jam) 
  • Left shift (doors won’t open: alkalosis/hypothermia/low CO₂) 
•  The bedside pivot: if PaO₂ is normal but the patient is hypoxic, stop chasing lungs and start chasing Hb and flow

Key takeaway: oxygen doesn’t move itself — hemoglobin moves it, and the heart delivers it.

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