Pulse Oximetry

How does pulse oximetry work?

Pulse Oximetry allows us to non-invasively measure oxygen saturations.

It works on the basis that oxyhaemogobin (HbO2) and deoxyhaemoglobin (Hb) have different absorption spectra. If we plot the absorption coefficient of Hb and HbO2 over different wavelengths, we can see that Hb absorbs red light to a much greater extent than HbO2 (hence oxygenated blood looks a brighter red than deoxygenated blood).

  • Pulse oximetry uses infrared absorption spectrometry to determine the proportion of saturated haemoglobin present in pulsatile arterial blood via a transcutaneous emitter and detector within a probe, usually on the finger or ear
  • Oxygenated haemoglobin absorbs infrared radiation at a wavelength of 940 nm to a greater extent than deoxygenated haemoglobin
  • Deoxygenated haemoglobin absorbs red light at a wavelength of 660 nm more than oxygenated haemoglobin
  • Therefore by analysing and comparing the absorption at these two wavelengths, the proportions of each can be calculated
  • This is done by the microprocessor in the machine, by comparing the absorption spectra to known validated tables of absorption spectra data from experiments using healthy volunteers breathing hypoxic gas mixtures
  • For this reason the pulse oximeter is not validated below saturations of 70%, as these values have not been accurately calibrated
  • The wavelengths at which oxy- and deoxyhaemoglobin demonstrate equal absorption are called the isobestic points and occur at:
    • 590 nm
    • 805 nm
      • Some machines may use isobestic points as a helpful calibration reference point, as the absorption is only proportional to the haemoglobin concentration at this wavelength
  • Light emitting diodes emit pulses of red and infrared light every five to ten ms, each time followed by an ‘off’ cycle to allow for subtraction of ambient light interference
  • The absorption due to soft tissue and venous blood will be constant, while the arterial absorption will be pulsatile, allowing subtraction of the constant component, and amplification of the pulsatile component (which only forms about 2% of the overall signal)
  • Then, by employing the Beer Lambert Law it can determine the degree of absorption and therefore the relative proportions of oxy- and deoxygenated haemoglobin present
  • The Beer Lambert Law states that the absorption of light through a substance is directly proportional to the length of the path and concentration of the substance doing the absorbing
    • For exam purposes, use the following:
      • Beer’s law = absorption is directly proportional to the concentration
      • Lambert’s law = absorption is directly proportional to the length
        • I remember that lambda = length, and beer is either weak or strong (concentration)

What sources of errors can occur while using a pulse oximeter?

  • Movement
  • Poor perfusion
  • Interference from abnormal haemoglobin
    • Carboxyhaemoglobin – reading falsely high
    • Methaemoglobin – reading around 85%
    • Hyperbilirubinaemia – reading falsely low
    • Carboxyhaemoglobin, hyperbilirubinaemia and methaemoglobin can be detected by CO-oximeters that use many more wavelengths
  • Intravenous dye
    • Methylene Blue = sats of 65%
  • Irregular pulse or tricuspid regurgitation causing venous pulsation
  • Doesn’t work if there is non-pulsatile blood flow, such as ECMO or bypass
  • Ambient light
  • Interference from Diathermy