2004B Question 11
Explain how a pulse oximeter works. Do NOT discuss the potential sources of error.
Examiner Report
29.9% of candidates achieved a pass in this question.
The major domains assessed in this question were:
- System components
- Beer-Lambert law
- Differential absorption
- Isolation of the pulsatile component
- Converting ratio R to SpO2
Common problems with the answers included:
- Incorrect use of the terms “SaO2”, “SpO2”, and “PaO2”
- The misconception that only infrared light is used in pulse oximetry
- Limited or no application of the Beer-Lambert law to pulse oximetry
- Incorrect or contradictory statements about the absorption spectra for oxyhaemoglobin and deoxyhaemoglobin
- Absent or insufficient explanation of the pulsatile or “alternating current” (AC) compartment.
Model Answer
Structure:
- Physics
- Device
Physics
| Property | Mechanism |
|---|---|
| Beer-Lambert principle | - of the absorbing substance in the medium (Beer’s law) - (Lambert’s law) - Describes the basis for, but not the functioning of, the pulse oximeter |
| Known absorption spectra | - 940nm: O2Hb > HHb - 660nm: O2Hb < HHb |
Device
| Property | Mechanism |
|---|---|
| Components | - Probe on the finger, nose, ear - Two monochromatic LEDs on one side, rapid on and off - Photodetector on the other side - Electrical cable - Processor - Display: SPO2 and plethysmograph |
| Function | 1. Pulsatile signal isolated - Excludes venous and tissue absorption 2. Ratio of absorbances calculated
3. Ratio correlated with SaO2 derived from experiments - R 0.4 → 100% - R 1 → 85% - R 2 → 50% |