Q: Can I use the P/F ratio from a venous blood gas?

No — venous PO₂ is significantly lower than arterial and does not reflect oxygenation. Only an arterial blood gas (or an arterialised capillary sample in certain settings) provides a valid PaO₂.

Q: How do I calculate FiO₂ for HFNO (high-flow nasal oxygen)?

The FiO₂ set on the HFNO device is accurate (range 0.21–1.0). For standard nasal cannula, the approximate FiO₂ is 0.24 + (0.04 × flow rate in L/min) up to about 0.44 at 5 L/min. For a non-rebreather mask, estimate approximately 0.60–0.80 depending on seal and flow.

Question 1

What FiO₂ should I use for a patient on room air?

Accepted Answer

Room air is FiO₂ = 0.21 (21%). A patient on room air with PaO₂ <60 mmHg has a P/F ratio <286, which is abnormal.

Question 2

Can I use the P/F ratio from a venous blood gas?

Accepted Answer

No — venous PO₂ is significantly lower than arterial and does not reflect oxygenation. Only an arterial blood gas (or an arterialised capillary sample in certain settings) provides a valid PaO₂.

Question 3

How do I calculate FiO₂ for HFNO (high-flow nasal oxygen)?

Accepted Answer

The FiO₂ set on the HFNO device is accurate (range 0.21–1.0). For standard nasal cannula, the approximate FiO₂ is 0.24 + (0.04 × flow rate in L/min) up to about 0.44 at 5 L/min. For a non-rebreather mask, estimate approximately 0.60–0.80 depending on seal and flow.