4.2. Measurements
            4.2.3. Gas analysis
                4.2.3.2. Oxygen-specific
4.2.3.2.4. Pulse oximeter

Pulse oximeter

Developed by Japanese engineer Takuo Aoyagi in 1972

Principle

  1. OxyHb absorbs less red light and more infrared light than deoxyHb.
  2. Of all light-absorbing substances, only arterial blood is pulsatile
  3. Application of Beer-Lambert law

2 different wavelengths used:

Assumption

Setup

Linking to saturation

ac --> pulsatile component

dc --> fixed component

R = (ac660/dc660)/(ac940/dc940)

Evaluation

Functional saturation vs fractional saturation

Pulse oximeter measure only the functional SpO2

NB:

Accuracy

+/-2% for 70-100% range

+/-5% for 50-70% range

No equally accurate at low SpO2

Disadvantage

Source of error

Effects of species of Hb

Carboxyhaemoglobin (COHb)
Methaemoglobin (MetHb)
Other species

--> All make little to no difference to measurement

Effects of other substances

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.

Keywords:  Carboxyhaemoglobin ; Foetal haemoglobin ; Fractional saturation ; Functional saturation ; Isobestic point ; Methaemoglobin ; Oxygen saturation ; Pulse oximeter
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Created20050602
Reviewed20050602


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