Question: Under what circumstances can StO₂ read higher than a pulse oximeter (SpO₂)?

Summary

It is extremely unusual for StO₂ readings to be higher than pulse oximeter measurements (SpO₂). The most likely explanation is an artifact in the SpO₂ and/or StO₂ measurement.

The three most likely sources of measurement artifact are:
a) Intravenous administration of methylene blue
b) Elevated methemoglobin blood levels (normal level is < 1% of total hemoglobin)
c) Elevated carboxyhemoglobin blood levels (normal range is 1-3% of total hemoglobin)

All three are well-known to cause spectral interference with pulse oximeters.^1-4 Because StO₂ also uses near infrared spectroscopy it, too, is subject to spectral interference if sufficient quantities of any of these confounders are present. The artifact is further compounded when methylene blue is administered to treat methemoglobinemia.

If none of the three confounders can be identified, the pulse oximeter and InSpectra^TMStO₂ Tissue Oxygenation System should be checked to ensure proper functioning.

Background

For details on the specific effects of methylene blue, methemoglobin and carboxyhemoglobin on StO₂, refer to the FAQs:

• How does methylene blue affect InSpectra^TMStO₂ and THI measurements?

• How do the presence of methemoglobin and carboxyhemoglobin affect InSpectra^TM StO₂ measurements?

References

1. Mardirossian G, Schneider RE. Limitations of pulse oximetry. Anesth Prog. 1992;39:194-96.
2. Kamat V. Pulse Oximetry. Indian J. Anaesth. 2002;46(4):261-68.
3. Gorman ES, Shnider MR. Effect of methylene blue on the absorbance of solutions of haemoglobin. Br J Anaesth. 1988;60(4):439-44.
4. Sidi A, Paulus DA, Rush W, Gravenstein N, Davis RF. Methylene blue and indocyanine green artifactually lower pulse oximetry readings of oxygen saturation. Studies in dogs. J Clin Monit. 1987;3(4):249-56.
5. InSpectra™ StO₂ Tissue Oxygenation Monitor Instructions for Use (#5024798) and InSpectra™ StO₂ Spot Check Instructions for Use (#5024733)