Discrepancy between SaO2 and SpO2

Article

Author: Munir Karjaghli, Elmar Paetzold

Date of first publication: 02.08.2019

Oxygen saturation can be assessed by measuring SaO2 or SpO2. SaO2 is the oxygen saturation of arterial blood, while SpO2 is the oxygen saturation as detected by the pulse oximeter.

Causes for discrepancies

Typically the discrepancy between SaO2 and SpO2 is about 2% to 3% (Jubran A. Pulse oximetry. Crit Care. 2015;19(1):272. Published 2015 Jul 16. doi:10.1186/s13054-015-0984-81).

There are different causes that may lead to this discrepancy in oxygen saturation.

Insufficient signals or the quality of the SpO2 probe used (see the article linked below: Reliability of SpO2 sensor signal readings)
Blood sample used was in fact venous
Blood sample was taken from a site affected by localised hypoxemia, e.g., ischaemic limb
Excessive oxygen consumption following blood sample collection (e.g. massive leucocytosis or thrombocytosis)
Carbon monoxide poisoning
Methemoglobinemia → the SpO2 value might be false high, but PaO2 will not be affected

Corrective action

Check the quality index of the sensor (example: Set the PEEP and Oxygen controllers to manual and try the pulse oximeter on yourself)
In your ABG sampling, assess PaO2, PaCO2, pH and Hb status, then adjust the SpO2 target shift to achieve your clinical goal
Switch the PEEP and Oxygen controllers to manual

Relevant devices: HAMILTON-G5/S1; HAMILTON-C6; HAMILTON-C3; HAMILTON-C1/T1
Relevant software: HAMILTON-G5/S1 SW v2.81; HAMILTON-C6 SW v1.1.4; HAMILTON-C3 SW v2.0.5; HAMILTON-C1/T1 SW v3.0.x

Footnotes

References

1. Jubran A. Pulse oximetry. Crit Care. 2015;19(1):272. Published 2015 Jul 16. doi:10.1186/s13054-015-0984-8

Pulse oximetry.

Jubran A. Pulse oximetry. Crit Care. 2015;19(1):272. Published 2015 Jul 16. doi:10.1186/s13054-015-0984-8

Link to article

Pulse oximetry is universally used for monitoring patients in the critical care setting. This article updates the review on pulse oximetry that was published in 1999 in Critical Care. A summary of the recently developed multiwavelength pulse oximeters and their ability in detecting dyshemoglobins is provided. The impact of the latest signal processing techniques and reflectance technology on improving the performance of pulse oximeters during motion artifact and low perfusion conditions is critically examined. Finally, data regarding the effect of pulse oximetry on patient outcome are discussed.