Pulse oximeter
Developed by Japanese engineer Takuo Aoyagi in 1972
Principle
- OxyHb absorbs less red light and more infrared light than deoxyHb.
- Of all light-absorbing substances, only arterial blood is pulsatile
- Application of Beer-Lambert law
2 different wavelengths used:
- 660nm (red light)
- 940nm (infrared light)
Assumption
- Negligible amount of metHb and carboxyHb
- Only arterial blood is pulsatile and contains HbO2
Setup
- 2 photodiodes
- 1 photodetector
- Diodes are switched on one at a time, then both off.
* on/off, off/on, off/off
* Sequence at ~500Hz
* When both diodes are off, ambient light may be detected and compensated for.
Linking to saturation
ac --> pulsatile component
dc --> fixed component
R = (ac660/dc660)/(ac940/dc940)
- Calibration curve links R to SpO2
* based on experimental data
- When R = 1, SpO2=85%
Evaluation
Functional saturation vs fractional saturation
Pulse oximeter measure only the functional SpO2
- Fractional saturation
= OxyHb / (DeoxyHb + OxyHb + MetHb + COHb)
- Functional saturation
= OxyHb / (DeoxyHb + OxyHb)
NB:
- Saturation does NOT include dissolved oxygen
Accuracy
+/-2% for 70-100% range
+/-5% for 50-70% range
No equally accurate at low SpO2
Disadvantage
- Measures functional saturation only
* Pulse oximeters which use only two diodes do not compensate fully for carboxyhaemoglobin or other abnormal haemoglobin.
- Not equally accurate at low oxygen saturation
- Excessive compression of digits
Source of error
- Venous pulsation (e.g. in tricuspid incompetence) may be falsely detected as arterial.
- Variability between sensors
* Less accuracy with absolute values
* Should not affect changes in SpO2 (i.e. trend)
- Fluorescent ambient light with flicker close to harmonic of diodes.
- Movement artefact
* movement
* shivering
- Compression of digits
--> Inadequate signals for analysis
- Presence of different species of Hb or other substances (see below)
Effects of species of Hb
Carboxyhaemoglobin (COHb)
- At 660nm, taken to be oxyHb
- At 940nm, no effect (almost no IR absorption)
- Thus,
--> saturation overestimated
Methaemoglobin (MetHb)
- At 660nm, taken to be deoxyHb
- At 940nm, taken to be oxyHb
- Thus,
--> add to both numerator and denominator
--> making R closer to 1
--> making the measured SpO2 closer to 85%
Other species
- Foetal haemoglobin (HbF)
- Sickle cell (HbS)
- SulphHb
--> All make little to no difference to measurement
Effects of other substances
- Methylene blue - decrease in measured SpO2
- Bilirubin - no effect
- Indigo carmine - little effect
- Dark skin - no effect
- Indocyamine green - decrease in measured SpO2
- Nail polish - decrease in measured SpO2
- Anaemia - exaggerate desaturation readings
* But still can be accurate at Hb as low as 23 g/L
Other notes
Isobestic points
Wavelengths at which radiation absorbance for the 2 forms of haemoglobin are identical
At 590nm and 805nm
--> oxyHb and deoxyHb have the same absorption
At isobestic point, absorbance depends only on the haemoglobin concentration, not on the species.