Question 29

a) Explain the principles of operation of pulse oximetry. (40% marks)
b) Explain the technological principle used in finger probes verses forehead probes in pulse oximetry and outline how it would affect assessment in the critically ill patient. (20% marks) 

c) List eight causes of a false reading of SpO2 and outline for each cause how you would gain a more accurate value of the SPO2. (40% marks)

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College answer

Aim: To explore understanding of a ubiquitous piece of equipment.
Key sources include: Paper 2000.1 Q9, 2010.1 Q17.2, CanMEDS Medical Expert.
Discussion: The many causes of false readings were reasonably done by some candidates. Successful answers discussed the dyshaemoglobinaemias, low perfusion states and extremes of oxygen dissociation curves and various artifactual causes amongst others. Candidates who were unable to provide specific details of the underlying principles around the operation of a common and mandated piece of equipment in the ventilated sedated patient scored poorly. It is important to understand the technological basis and limitations of standard equipment used in the care of the critically ill and this sections marking was weighted accordingly.
Forehead probes are a newer technology and so were weighted less in marks as they are used less often than the finger probes. Successful answers detailed the use of reflectance technology. The reflectance sensor has emitter and detector components adjacent to one another, so oxygen saturation is estimated from backscattered light rather than transmitted light. In critically ill patients with low perfusion SpO2 is more reflective of the SaO2 for the forehead reflectance probe than for the finger probe.



For 40% of the marks, the explanation here should have been a maximum of one page of concise information, ideally in point form - something less detailed than this


  • Oxygen saturation is the ratio of reduced haemoglobin to oxyhaemoglobin
  • Reduced haemoglobin and oxyhaemoglobin absorb different wavelengths;
    • Deoxyhaemoglobin absorbs more light at 660nm
    • Oxyhaemoglobin absorbs more light at 940 nm.
  • Pulse oximeter exposes the blood in the fingerip to these two wavelengths
  • Concentration of oxyhaemoglobin and deoxyhaemoglobin can be determined from their absorption of the two wavelengths using Beer law (the concentration of a given solute in a solvent is determined by the amount of light that is absorbed by the solute at a specific wavelength)
  • Tissue and venous absorption is eliminated by processing the signal and rejecting non-pulsatile components
  • Signals generated by the probe are interpreted into saturation readings using a lookup table of values collected from healthy individuals


For 20% of the marks, "explain the technological principle" and "outline how it would affect assessment" seems like a lot.  Forehead probes, unlike finger probes, do not rely on the transmission of light (and light absorbance) - instead, they measure the reflected light, which means the patient's head does not need to be transilluminated like a scrotum. So:

  • Technological principle: reflected light is measured (reflectance oximetry), rather than absorbed light
  • How it affects assessment: this technology is more reliable in patients with poor digital perfusion. 


EIGHT problems? AND their solutions?

  • Technical problems
    • Poor calibration - replace the device (usually there is no option to recalibrate)
    • Damage to sensor or leads - replace the damaged components
  • Interference
    • Ambient lighting - turn down the lights
    • Patient movement - change the position of the probe / negotiation / sedation
  • Poor signal quality due to decreased access to blood
    • Poor perfusion - change to a reflectance oximeter, also perhaps fix the shock state, that should help
    • Nail polish - obviously, remove nailpolish, or attach probe to earlobe/nostril
  • Abnormal blood contents:
    • Carboxyhaemoglobin
    • Methaemoglobin
    • Methylene blue dye
    • Indocyanine blue dye
    • The solution to all of the above would be to use a co-oximeter instead


Lee, Hooseok, Hoon Ko, and Jinseok Lee. "Reflectance pulse oximetry: Practical issues and limitations." Ict Express 2.4 (2016): 195-198.

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.