The pulse oximeter

The name "oximeter" was first applied to a device measuring haemoglobin oxygenation by Glen Millikan in 1942 when he developed a lightweight ear oxygen meter. Until the pulse oximeter was developed by Dr Takuo Aoyagi in 1973, oximetry could only measure static haemoglobin oxygen concentration in blood.

The origins of the oximeter lie in several important discoveries. Johann Lambert described in Latin, the relationship between the absorption of light and the amount of absorbent, in his book published in Augsburg, Germany, in 1760. Robert Bunsen and Gustaf Kirchhoff, professors of chemistry and physics at Heidelberg, invented the spectroscope in 1860 in order to measure the wavelengths in gases emitted by different elements when passed through a flame. Measurement of the intensity of spectral emissions was originally done by visual assessment but precise colorimetric analysis was not possible until refinement of the photoelectric cell that had been invented by Alexander-Edmond Becquerel in 1839.

In 1864, Felix Hoppe-Seyler, the founder of physiological chemistry in Germany, coined the term "haemoglobin" to describe the oxygen carrying pigment in blood. He showed that it absorbed both green and blue light, and that the absorption changed when a solution of crystallised haemoglobin was shaken with air.

Thus, in 1876, Karl von Vierordt was able to apply the principle of oximetry to measure the absorption of light by oxyhaemoglobin and haemoglobin in the finger. After occluding the circulation with a rubber band, the two absorption bands of oxyhaemoglobin disappeared and the band of deoxyhaemoglobin appeared. By timing the change, he was able to determine oxygen consumption in the tissues.

This pioneering research went unnoticed, and while many experiments were conducted on in vitro measurement of oxyhaemoglobin, it wasn't until Ludwig Nicolai developed the forerunner of the oximeter in 1931. He used alternating wavelengths of blue-green light and a photoelectric cell to obtain exponential curves for the decay of oxyhaemoglobin and rise of...