SpO2 is one of many useful measurements used to build an understanding of patient status but like every other vital sign SpO2 must be critically evaluated each time you take it.Mechem (2015) cautions, "Interpretation of pulse oximetry readings must account for a variety of factors that may artifactually influence the results. The best defense against these potential sources of error is a high index of suspicion"

In other words, it's not an accurate SpO2 until you decide it is.

How SpO2 works: The probe emits two wavelengths of light from one side of the probe and measures the amount of each of those wavelengths that reaches the other side. As the light travels through blood in a finger the oxyhemoglobin absorbs one wavelength and deoxyglobin absorbs the other. The results are microprocessed into a percentage of bound Hb and presto: SpO2.

Remember, SpO2 is not a measure of overall patient oxygenation or wellbeing”you also need to consider if oxygen is being extracted at the cellular level. For example, in cyanide toxicity, the SpO2 is often normal, but cells are unable to extract oxygen from hemoglobin and cellular respiration is inhibited (cells are unable to utilize the oxygen that is present in the blood)

You should consider the following factors that render an SpO2 a potentially unreliable measure of patient oxygenation:

Color- other absorbent colors in the probe's environment

• Nail polish and/or fake nails, particularyly dark colors, can alter SpO2 results
• Dark pigmentation of skin has affected readings in the past but newer technology has improved reliability in this regard
• Ambient lighting like fluorescent, infrared, incandescent can be picked up by the probe's sensor

Reduce perfusion - other absorbent colors in the probe's environment

• Inflation of BP cuff on same limb
• Elevation of limb
• Peripheral vasoconstriction due to hypothermia, medications, shock, injury

Abnormal hemoglobin situation

• Anemia”if there is less Hb overall in patient™s system then a 100% SpO2 isn™t reassuring
• Sickle hemoglobin”patients at particular risk of hypoxemia despite œnormal SpO2 reading
• Hypovolemia due to blood loss
• Poisoning”Hb carries stuff other than O2, and the SpO2 only measures if the Hb is bound, not whether it's bound to actual O2. If something else has displaced oxygen on the Hb, the SpO2 can™t tell the difference. Carbon monoxide poisoning is an example (smoke inhalation or smokers). A life-threatening arterial desaturation can be masked by a falsely reassuring SpO2.
• Acidosis”when patient is acidotic there is an increased affinity between Hb and oxygen, meaning that although there may be a normal SpO2, cellular respiration may be altered as oxygen will not dissociate from hemoglobin. As a result, the hemoglobin retains oxygen, the SpO2 is normal, but cells are unable to extract oxygen and revert to anaerobic metabolism

Probe limitations

• SpO2 is averaged over several seconds, therefore you may have a newly hypoxemic patient and the SpO2 has not yet caught up to the true reading. Think: hypoxia during intubation.
• Fit/displacement of probe”incomplete capture of both wavelengths of light can result from ill-fitting or slightly displaced probe resulting in falsely inflated or deflated readings.

If your defib isn™t configured to automatically display the SpO2 waveform you can dial it up easily on each call. A curvaceous waveform at least tells you the SpO2 reflects movement of blood through the patient™s finger.

So to recap: when you are measuring SpO2 you must always consider whether the result is actually a reliable measure of patient oxygenation or if it is CRAP!

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References: Mechem, C.C. (2015). Pulse Oximetry. Up To Date. Retrieved from http://www.uptodate.com/contents/pulse-oximetry?source=machineLearning&search=oxygen+saturation &selectedTitle=1%7E150&sectionRank=1&anchor=H16#H16

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