This was one of the older SAQs,  Question 9 from the first paper of 2000. Co-oximetry was also asked about in Question 17.2 from the first paper of 2010. Generally, the trainees have been expected to know how the pulse oximeter and co-oximeters differ in their function, and what may give rise to false pulse oximeter readings.


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.
  • As always, LITFL do it better


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


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.