Since the early 1980s, when pulse oximetry was introduced, this non-invasive method of monitoring the arterial oxygen saturation level in a patient’s blood (SpO2) has become a standard method in the clinical environment because of its simple application and the high value of the information it provides.
Before the advent of pulse oximetry, the common practice was to draw blood from patients and analyse the samples at regular intervals—several times a day, or even several times an hour—using large hospital laboratory equipment. These in-vitro analysis instruments were either blood gas analysers or haemoximeters. Blood gas analysers determine the partial pressure of oxygen in the blood (pO2) by means of chemical sensors. Haemoximeters work on spectrometric principles and directly measure the ratio of the oxygenated haemoglobin to the total haemoglobin in a sample of blood (SaO2).
Hewlett-Packard/Agilent HSG (Now Philips) pioneered non-invasive pulse oximetry
Hewlett-Packard pioneered the first in-vivo technology to measure a patient’s oxygen saturation level without the need of drawing blood samples in 1976 with the HP 47201A eight-wavelength ear oximeter (see photograph).
An ear probe was coupled through a fibreoptic cable to the oximeter mainframe, which contained the light source (a tungsten-iodine lamp and interference filters for wavelength selection) and receivers. This instrument served as a "gold standard" for oximetry for a long time and was even used to verify the accuracy of the first pulse oximeters in clinical studies.
The real breakthrough came in the 1980s with a new generation of instruments and sensors that were smaller in size, easier to use, and lower in cost. These new instruments used a slightly different principle from the older, purely empirical multiwavelength technology. Instead of using constant absorbance values at eight different spectral lines measured through the earlobe, the new pulse oximeters made use of the pulsatile component of arterial blood at only two spectral lines. The necessary light was easily generated by two light-emitting diodes (LEDs) with controlled wavelengths. Small LEDs and photodiodes made it possible to mount the optical components directly on the sensor applied to the patient, avoiding the necessity of clumsy fibreoptic bundles.