In this BoardsCast episode, we continue Tobias Chapter 103 — Lungs by confronting the ICU trap everyone falls into:

You crank FiO₂ to 100%, you increase ventilation, and the patient is still hypoxemic.

Because you’re trying to treat an oxygen-transfer problem with an airflow solution. 

This episode rebuilds hypoxemia into a board-clean classification framework:

All hypoxemia comes from 5 mechanisms — and each one needs a different fix.

You’ll learn:

•  The 5 causes of hypoxemia (not 50): low FiO₂, hypoventilation, diffusion impairment, V/Q mismatch, shunt 
•  Why panting can still be hypoventilation (effort ≠ alveolar ventilation; dead space trap) 
•  Fick’s Law in plain language: surface area, gradient, thickness, gas properties
•  Why O₂ fails before CO₂: CO₂ diffuses ~20× faster (sprinter vs slow walker) 
•  The bedside decision tools: CO₂ first (hypoventilation = CO₂ high), then A–a gradient to prove gas-exchange failure 
•  The defining clue: hypoxemia resistant to 100% O₂ = true shunt until proven otherwise 

Key takeaway: Ventilation treats CO₂. Diffusion determines O₂.

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