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
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.