Pulse oximetry in dogs and cats: SpO2 of 90% and limitations
Dog with tachypnoea in the emergency room. The pulse oximeter, placed on the tongue with a good plethysmographic waveform, reads an SpO2 of 90% on room air.
Which statement is correct?
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A. An SpO2 of 90% is acceptable and does not require oxygen
Normal SpO2 on room air is ≥ 95–97%; below 95% there is hypoxaemia and 90% indicates severe hypoxaemia requiring oxygen.
B. An SpO2 of 90% corresponds to a PaO2 of about 60 mmHg: severe hypoxaemia
Correct. SpO2 95% ≈ PaO2 80 mmHg and SpO2 90% ≈ PaO2 60 mmHg. Below this point the dissociation curve becomes steep, and small falls in SpO2 mean large falls in PaO2.
C. The reading is equally reliable with pigmented mucosa, vasoconstriction or movement
Pigmentation, poor perfusion, hypothermia, movement and ambient light all affect the reading; check that the oximeter pulse rate matches the true rate and that the waveform is good.
D. A normal SpO2 rules out hypoventilation
Pulse oximetry does not measure ventilation: a patient on oxygen can have a normal SpO2 with severe hypercapnia. For ventilation you need PaCO2 or capnography.
lightbulb Key takeaway
Remember the pairs 95/80 and 90/60 (SpO2 %/PaO2 mmHg). Carboxyhaemoglobin gives falsely normal readings and methaemoglobinaemia tends to fix SpO2 around 85%; if in doubt, run an arterial blood gas.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.