Describe the principles of how the pulse oximeter determines "arterial oxygen saturation". List causes of the false reading of SpO2.

[Click here to toggle visibility of the answers]

College Answer

(a) The candidate should have been aware of the basic principles of pulse oximetry. 
Pulse oximetry is based on the Beer-Lambert Law which states that. the concentration of an absorbing substance in solution can be determined from the intensity of light transmitted through the solution, given the intensity and wavelength of incident light, the transmission path length and the characteristic absorbency at a specific wavelength. 
To   arrive  at   oxygen  saturation,  the   relative   concentrations  of   reduced  Hb   and oxyhaemoglobin must  be  calculated. At  wavelengths of  660nm and  940nm  there is 
.maximum separation·of absorption. These wavelengths also penetrate tissue and LEDs emitting these wavelengths are readily available.                                                . 
The pulse oximeter thus has two LEDs emitting light  of  these wavelengths through a vascular bed. A photodiode detector detects the intensity of transmitted light. It rejects the absorption from tissue and venous blood by sensing the pulsatile or AC components and 
rejecting the fixed or DC component. 
Factory calibration is based on nomograms·from young normals.

{b) False readings may be caused by: 
• Optical interference eg. abnormal haemoglobin, dye 
•  Signal artefact eg. fluorescent light 
•  False assumptions/calibration eg. inaccurate saturation's below 90%

Pulse Oximetry: principles and limitations. American J ofEmerg Med 17,1;59-67.

Discussion

Physical principles of pulse oximetry:

  • Oxygen saturation is the ratio of reduced haemoglobin to oxyhaemoglobin
  • Reduced haemoglobin and oxyhaemoglobin absorb different wavelengths;
    • Reduced Hb absorbs red light (660nm)
    • Oxygenated Hb absorbs infra-red light (940nm)
  • When fingertip blood is exposed to these two wavelengths, one can measure the absorption of red and infra-red light, and from this infer the concentration of the two types of haemoglobin.
  • Tissue and venous absorption is eliminated by processing the signal and rejecting non-pulsatile components

Causes for false readings of the pulse oximeter:

  • Technical problems
    • Poor calibration
    • Damage to sensor or leads
  • Interference
    • Ambient lighting
    • Patient movement
  • Poor signal quality due to decreased access to blood
    • Poor perfusion
    • Nail polish
  • Abnormal blood contents:
    • Carboxyhaemoglobin
    • Methaemoglobin
    • Methylene blue dye
    • Indocyanine blue dye

References

Tremper, Kevin K. "Pulse oximetry." CHEST Journal 95.4 (1989): 713-715.

 

Sinex, James E. "Pulse oximetry: principles and limitations." The American journal of emergency medicine 17.1 (1999): 59-66.

 

Ralston, A. C., R. K. Webb, and W. B. Runciman. "Potential errors in pulse oximetry III: Effects of interference, dyes, dyshaemoglobins and other pigments*." Anaesthesia 46.4 (1991): 291-295.