Pulse oximetry

Oxyhaemoglobin saturation (SaO₂) can be estimated using a pulse oximeter; oxygen saturation measured by pulse oximetry is referred to as SpO₂.

Pulse oximetry is noninvasive and inexpensive. It has become widely used in the care of patients with unstable respiratory disease or when close monitoring of oxygenation is needed.

Pulse oximetry uses red and infrared light absorption to measure and compare the amount of oxyhaemoglobin and deoxyhaemoglobin in peripheral arterial blood. It involves applying a close-fitting cap or clip containing light-emitting diodes to the finger or earlobe, or sometimes to the forehead in certain settings with specialised pulse oximeters.

Pulse oximetry has several limitations:

• It measures only oxyhaemoglobin saturation, not oxygenation or ventilation.
• It does not measure carbon dioxide levels; arterial blood gas analysis is needed to quantify hypercapnia¹.
• Readings may be misleading in some clinical scenarios. Factors that can affect the accuracy of pulse oximetry include:
  • poor circulation due to reduced cardiac output or peripheral vasoconstriction
  • poor technique in the application of the monitoring device to the finger, earlobe or forehead
  • faulty equipment
  • nail polish—rotating the cap or clip on the end of the finger may improve accuracy
  • dark skin
  • high carboxyhaemoglobin concentration—pulse oximeters may give falsely high readings in the presence of high concentrations of carboxyhaemoglobin; causes of high carboxyhaemoglobin concentrations include carbon monoxide inhalation and chronic heavy smoking
  • high methaemoglobin concentration—pulse oximeters may give falsely high or low readings because of the complex way methaemoglobin absorbs light; causes of high methaemoglobin concentrations include genetic defect (congenital), dapsone, some antimalarial drugs, local anaesthetics (benzocaine, lidocaine), aniline dyes and nitrates
  • cyanide poisoning and severe anaemia—pulse oximeters may give falsely reassuring readings (the measured SpO₂ is correct, but oxygen is not available to tissues).