Describe the principles of measurement of arterial haemoglobin O2 saturation using a pulse oximeter. (60% of marks) Outline the limitations of this technique. (40% of marks)

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College Answer

A lack of understanding of the physics behind pulse oximetry was a common area of weakness amongst most candidates. Candidates were expected to mention the underlying principle of the “Beer Lambert” Law, absorption spectra and that the differential absorption of light at different wavelengths by different haemoglobin species is used to determine the fractions of haemoglobin types. Limitations should include mention of errors due to calibration as well as sources of false positive and false negative readings.