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
- Ambient lighting
- Patient movement
- Poor signal quality due to decreased access to blood
- Poor perfusion
- Nail polish
- Abnormal blood contents:
- Methylene blue dye
- Indocyanine blue dye