Describe the principles of measurement of arterial haemoglobin oxygen saturation using a pulse oximeter. Outline the limitations of this technique.

[Click here to toggle visibility of the answers]

College Answer

The main points expected for a pass included a brief description of the following:
· The system components
· The principles of light absorbance and the Beer-Lambert Law
· The differential absorbance of Hb species in red/infrared spectrum, and their use to
calculate the amount of reduced and oxygenated Hb present
· LED emitting 660/940/off cycles at 450-900Hz, averages data over several cycles to
eliminate ambient light, and detect pulsatile and non pulsatile elements
· Pulse added absorption of each cycle compared as ratio “R” at different wavelengths
· Calibration curve to compare “R” to SaO2 data from healthy volunteers
· The limitations of the technique including:
- quality of product, bias, precision and accuracy
- insensitivity to PaO2
- false readings and their causes
Diagrams gained marks only with sufficient labelling and explanation