Fortnightly Review: Pulse oximetry: a practical review

doi:10.1136/bmj.311.7001.367

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C D Hanning, senior lecturera,
J M Alexander-Williams, research fellowa

^aDepartment of Anaesthesia, University of Leicester, Leicester General Hospital, Leicester LE5 4PW

Correspondence to: Dr Hanning.

Accepted 5 August 1995

Summary points

Hypoxaemia is common, poorly detected, and harmful
Pulse oximetry is a simple, reliable, and accurate means of detecting hypoxaemia
Pulse oximetry should be widely available and routinely used

Pulse oximetry is arguably the greatest advance in patient monitoring since electrocardiography. It enables oxygenation, an important physiological variable that is poorly detected by clinical means, to be monitored continuously, simply, and non-invasively. Hypoxaemia is commonly found in all aspects of medical practice and is a major cause of organ dysfunction and death. Pulse oximetry should be widely available and used routinely in clinical practice both in primary care and in hospital.

Principles of operation

An understanding of the principles of operation is essential for the proper use and interpretation of the values displayed by an oximeter. The absorption spectra of oxygenated and reduced haemoglobin differ so that arterial blood appears red while venous blood appears “blue.” When two compounds with differing absorption spectra are together in solution, the ratio of their concentrations can be determined from the ratio of the light absorbed at two different wavelengths. The aim of oximetry is to measure the ratio of oxygenated haemoglobin to total haemoglobin in arterial blood, the oxyhaemoglobin saturation. In living tissue, however, light is also absorbed by the tissues and by the haemoglobin in venous and capillary blood. To overcome this, early instruments heated the tissues to increase flow and to “arterialise” the blood in the capillaries and veins and some used up to eight wavelengths to allow for absorption of light by tissues. These instruments were useful research tools but were too cumbersome for clinical use.