Pulse Oximetry

Pulse oximetry provides a simple, accurate, non-invasive and continuous means of monitoring arterial oxygen saturation. Despite its limitations (in comparison to an arterial blood gas assay), it gives immediate information related to oxygenation (and, therefore, hypoxia) and has been adopted universally in emergency medicine, where the early recognition of hypoxia is critical. In addition to identifying hypoxia and guiding subsequent oxygen therapy, it is useful in monitoring changes in a patient’s condition. Pulse oximetry should be instigated in all patients presenting with dyspnoea, whatever the potential underlying pathology.

A number of factors can affect the accuracy of pulse oximetry and it is important to be aware of its limitations. In conditions where the pulse waves have a very low amplitude (eg. hypothermia, hypovolaemia – just when you might need a rapid assessment of oxygenation!) readings may be unobtainable. In conditions with abnormalities of the haemoglobin molecule itself (eg. carboxyhaemoglobin, methaemoglobin) pulse oximetry is inaccurate – carboxyhaemoglobin, for example, will result in falsely elevated measured oxygen saturation (SaO2). Pulse oximetry is also affected by the ambient light – it may not be so reliable outside the ED in the pre-hospital setting.

Pitfall

A number of factors can affect the accuracy of pulse oximetry and it is important to be aware of its limitations.