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