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SpO2/FiO2 ratio

Non-invasive surrogate for the PaO2/FiO2 ratio to assess hypoxaemic respiratory failure.

SpO2/FiO2 ratio
Pulse oximetry saturation (SpO2)
%
Fraction of inspired oxygen (FiO2)
Enter as a percentage, e.g. 40 for 40 % (0.40).
%
Complete the fields above to see the result.

When to use

  • Estimating severity of hypoxaemic respiratory failure/ARDS non-invasively when an arterial blood gas is not immediately available.

Formula

SF ratio = SpO2 (%) / FiO2 (as a fraction, e.g. 0.40 for 40 %). An SF of 235 approximates a PaO2/FiO2 of 200, and an SF of 315 approximates a PaO2/FiO2 of 300.

Pearls and pitfalls

  • Only validated when SpO2 <=97 %, where oxygen saturation still tracks the steep part of the oxyhaemoglobin dissociation curve; above this the ratio underestimates severity of hypoxaemia.

References

  1. Rice TW, Wheeler AP, Bernard GR, et al. Comparison of the SpO2/FiO2 ratio and the PaO2/FiO2 ratio in patients with acute lung injury or ARDS. Chest. 2007;132(2):410-417.