A–a O₂ Gradient
PAO₂ ≈ (P_atm − 47)×FiO₂ − PaCO₂/0.8; A–a = PAO₂ − PaO₂ (RQ = 0.8).
Calculator
How it works
The alveolar gas equation estimates alveolar PO₂; subtracting measured arterial PaO₂ gives the A–a gradient, a teaching index of oxygen transfer efficiency.
PAO₂ ≈ (P_atm − 47) × FiO₂ − PaCO₂ / 0.8 A–a = PAO₂ − PaO₂
Limits
Does not replace shunt calculation, mixed venous content modeling, or exact altitude tables; FiO₂ must match the blood gas sampling conditions.
Educational tool only—not for diagnosis or treatment decisions.