What are technical considerations for conducting pulse testing?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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Answer

Answer

Carboxyhemoglobin (CoHb) and methemoglobin (metHb) absorb light at the same wavelength as deoxyhemoglobin, causing a very significant overestimation of SaO2 when these are elevated. Pulse oximetry has other shortcomings. It does not provide information about the oxygen content of the arterial blood. Tissue hypoxia can exist when SpO2 is normal when anemia is present. Elevated levels of dysfunctional hemoglobins (CoHb, metHb) can cause significant overestimation of the actual SaO2.

Additionally, pulse oximetry does not address the adequacy of ventilation, which can be assessed only by evaluation of the partial pressure of carbon dioxide in arterial gas (PaCO2). Motion of the finger within the probe can cause a motion artifact secondary to equal rhythmic absorption of red and infrared light that most oximeters interpret as an SpO2 reading of 85%. Disposable finger probes fixed to the probe site with adhesive and fixed positioning of the probe site during walking can minimize this.

Pulse oximetry tends to overestimate SaO2. One reason for this is the fact that pulse oximetry expresses the percentage of oxyhemoglobin, typically without consideration for CoHb or metHb (see below). One pulse oximetry manufacturer now offers options that allow reporting of total hemoglobin, oxygen content, CoHb and metHb, but these are not yet in widespread use. See below.

SpO2 = oxyhemoglobin/(oxyhemoglobin + reduced hemoglobin [rHb])

In contrast, spectrophotometrically determined oxygen saturation from an ABG sample expresses oxygen saturation as the percentage of the sum of reduced hemoglobin, oxyhemoglobin, CoHb, and MetHb. See below.

SaO2 = oxyhemoglobin/(oxyhemoglobin + rHb + CoHb + metHb)

This significant difference generally results in pulse oximeters reporting an oxyhemoglobin value that is 2-3% higher than the spectrophotometrically determined oxygen saturation, even when the pulse oximeter is functioning perfectly.


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