Outline the principles underlying pulse oximetry. ( 80% of marks)

Briefly describe the effect of an elevated level of the following upon pulse oximetry values. (20% of marks)
a) Carboxyhaemaglobin
b) Methaemaglobin

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College Answer

Explanation of several crucial principles was expected for a good answer. These would include 
that Haemoglobin can be measured and quantified with a light absorbance technique; based 
on the Beer Lambert law (a description of this was required). In addition, oxygenated 
haemoglobin must be distinguished from reduced haemoglobin (the 2 dominant species of 
Hb) and that the oximeter determines pulsatile from non pulsatile blood. The oximeter 
accounts for ambient light and that “R”, a ratio of absorbances during pulsatile and non 
pulsatile flow is calculated and compared within a computer algorithm to standardised values 
of SaO2 to deliver a final value. 
Mention of limitations was not required except to answer the second part of question.
Common omissions included failure to describe accurately the Beer Lambert Law, and no 
explanation of how pulsatile component was detected, or ambient light accounted for. Many 
candidates understood the clinical inaccuracy associated with CO and Met HB, but failed to 
identify the spectrophotometric reason and application of the R value for this discrepancy.