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:

• 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.