Pulse oximetry (Medical students)



Pulse oximetry is a simple, non-invasive method used to monitor oxygenation by estimating the percentage of oxygen-saturated haemoglobin (SpO₂) in arterial blood.

Principles and Physiology

Oximeters operate using the principles of spectrophotometry. They measure the differential absorption of red (660 nm) and infrared (940 nm) light by oxygenated and deoxygenated haemoglobin.

During systole, pulsatile blood flow causes changes in light absorption that are detected by photodetectors on the opposite side of the emitter.

The probe is typically placed on fingertips, toes, or earlobes, using either wrap-around or clip-style designs.

Interpreting Oxygen Saturation

A normal SpO₂ is ≥95% in healthy individuals.

The oxyhaemoglobin dissociation curve is relatively flat at high saturations, but drops steeply below 90%, indicating a sharp fall in PaO₂ and thus a greater risk of tissue hypoxia.

Clinical action is usually recommended when SpO₂ falls below 92% in room air, particularly in acute respiratory illnesses (e.g. asthma, bronchiolitis, croup).

Limitations of Pulse Oximetry

Pulse oximetry does not measure:
  • Total oxygen content – e.g. in anaemia, SpO₂ may be normal despite reduced oxygen-carrying capacity.
  • Tissue oxygen delivery – shock states may impair perfusion despite normal SpO₂.
  • Ventilation – a normal SpO₂ does not exclude hypercapnia (e.g. in COPD or hypoventilation).
It also cannot distinguish between:
  • Oxyhaemoglobin and carboxyhaemoglobin – SpO₂ may be falsely elevated in carbon monoxide poisoning.
  • Normal haemoglobin and methaemoglobin.
Clinical Uses

Assessment of hypoxaemia: Pulse oximetry offers a more sensitive and earlier indicator than central cyanosis, which only becomes apparent at saturations below 75–80%.

Monitoring acute illness: Acute asthma, bronchiolitis, croup, pneumonia: SpO₂ <92% in room air should raise concern for severe disease. Aids in determining the need for supplemental oxygen.

Perioperative and procedural monitoring: Continuous SpO₂ monitoring is standard during sedation or anaesthesia.