Clinical signal map
V/Q mismatch, decoded.
Start with the relationship between ventilation and perfusion, then use oxygen response and the A-a gradient to narrow the mechanism.
Primary scan
PaO₂ ↓
Hypoxemia is the signal. The pattern identifies the cause.
01 / LOW V/Q
Too little airPerfusion exceeds local ventilation.02 / HIGH V/Q
Too little bloodVentilation exceeds local perfusion.03 / SHUNT
No useful contactBlood bypasses ventilated alveoli.Alveolar gas equation
PAO₂ = PIO₂ − PaCO₂ / R
PaCO₂Is alveolar ventilation adequate?
PaO₂Is oxygen reaching arterial blood?