Describe the principles of how the pulse oximeter determines "arterial oxygen saturation". List causes of the false reading of SpO2.
(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.
Physical principles of pulse oximetry:
Causes for false readings of the pulse oximeter:
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.