This tool applies the quick clinical rule of thumb for checking whether a patient's oxygen level is appropriate for the amount of oxygen they're receiving — especially useful on high FiO₂ where the standard A-a gradient reference breaks down.
On 100% oxygen, expect a PaO₂ above 500 mmHg. A PaO₂ below 300 mmHg on 100% oxygen suggests a significant shunt.
Measured PaO₂ is appropriate for the FiO₂.
Slightly below expected.
Below expected — a gas-exchange problem is likely.
Well below expected — suggests significant shunt.
Given: FiO₂ 100%, measured PaO₂ 450 mmHg.
500 mmHg.Multiply the FiO₂ percentage by five to get the roughly expected PaO₂.
The normal gradient itself widens substantially at high FiO₂, so the usual age-based reference no longer applies. The 5× rule of thumb is a quicker sanity check in that setting.
Greater than about 500 mmHg in healthy lungs. A PaO₂ below 300 mmHg on 100% oxygen points to a significant shunt.
Multiply the FiO₂ percentage by 5 to get the approximate PaO₂ a healthy patient should reach — so 40% ≈ 200 mmHg, 100% ≈ 500 mmHg. Falling well short suggests a gas-exchange problem.
No — it's a bedside approximation for quickly judging whether oxygenation matches the oxygen delivered, not a validated diagnostic equation.