Pulse oximetry: SpO2 and SaO2

This article looks at the measurement of oxygen in the blood. It discusses how this essential observation is carried out in the pre-hospital environment, and its uses and limitations. The article will also take a brief look at the history and technology of the pulse oximeter.

Arterial oxygen saturation or the discovered oxygen level in arterial blood (SaO₂) is invasive, and it is difficult to monitor trends in a practice setting. In 1972, the idea of a non-invasive pulse oximeter to measure peripheral oxygen level (SpO₂) was conceived, with a prototype completed by 1974 (Aoygi, 2003). Prior to this, a number of monitoring methods for hypoxaemia have been described by Severinghaus (2011). Up until the 1930s, dentists were using pure N₂O (unlike the 50:50 N₂O:O₂ mix in Entonox^®) to anaesthetise patients for teeth extraction. The cyanosis, or shade of blue of the patient, was used to monitor how long they could stay in this state. While the drugs improved in the 1930s, such as the use of thiopental, the monitoring did not. Respirations were impaired by the new exploration of anaesthetics, and the only change to monitoring was observing chest movement. Deaths inevitably followed.