History of Pulse Oximetry
Oximetry is the measurement of transmitted light through a translucent measuring site to determine a patient's oxygen status
noninvasively. Oximetry measurements can be traced to the early 1930's when German investigators used spectrophotometers (instruments that measure different wavelengths and intensities of light) to research light transmission through human skin. In 1934, one investigator reported measuring oxygen saturation in blood flowing through closed vessels in animals.
In 1939, German researchers reported use of an "ear oxygen meter" that used red and infrared light to compensate for changes in tissue thickness, blood content, light intensities and other variables. However, it was not until World War II that interest in oximetry took hold.
At that time there was a need to evaluate oxygenation of high altitude pilots.
Between 1940 and 1942, a British researcher, Millikan, used two wavelengths of light to produce a practical, lightweight aviation ear oxygen meter for which he coined the word
"oximeter". He noted that light transmitted through a red filter was oxygen-saturation-sensitive and light passing through a green filter was independent of oxygen saturation. It was later determined that oxygen insensitive-signals were not due to the green light filter but instead to infrared light.
The system went through many modifications during the 1940's and 1950's and was eventually manufactured by the Waters Company. This system was mainly used in physiology, aviation, and experimental studies.
In 1964, a San Francisco surgeon developed a self-calibrating, 8- wavelength oximeter that was marketed by Hewlett Packard in the 1970's. This system was used in clinical environments but was very large, weighing approximately 35 lbs., and had a bulky, clumsy earpiece. The unit was also very expensive (approximately $10,000.00). However, it did allow for continuous noninvasive monitoring of arterial oxygenation.
In the early 1970's, Takuo Aoyagi, a Japanese bioengineer, was trying to develop a noninvasive method to determine cardiac output using cardiogreen dye and measuring light passing through an earpiece. He found that light transmitted through the ear exhibited pulsatile variations that made it impossible to compute cardiac output. Fortunately, he was also interested in oximetry and was familiar with previous oximetry work. He recognized that he might be able to use the pulsating changes in the light transmission through the ear to measure arterial oxygen saturation. He then went on to develop a pulse oximeter and applied for a Japanese patent. At the same time, another Japanese researcher with Minolta was working on the same concept and applied for a patent a month later. This patent was denied in Japan but approved in the U.S.
In the late 1970's, the Biox Corporation in Colorado made significant advances in pulse
oximetry, 2-wavelength measurements. They first introduced the use of Light Emitting Diodes (LED's) for the red and infrared light sources. They marketed their device directly to respiratory therapists and anesthesiologists who could see the benefit of continuous, real time, noninvasive oxygen saturation readings. Ohmeda Corporation purchased
Biox, and in the 1980's, along with Nellcor (started by an anesthesiologist, Bill New), and
Novametrix, continued to make significant advances in size reduction, cost, and development of multiple site probes.
Today there are many manufacturers of pulse oximeters. All offer a variety of different oximeter boxes with SpO2
(oxyhemoglobin) and pulse rate readings, waveform displays, alarms, etc. While the boxes and the displays may differ, they use a similar method of measuring oxyhemoglobin saturation by two wavelengths of light in the red and infrared range. But while the two-wavelength method is used to start the SpO2 measurement process, the way the signals are processed after that point, play a major role on how accurate the readings will be, especially through motion and low perfusion. During the late 1990's and into the next decade, 'new generation' pulse oximeters have been introduced that have elevated the accuracy of pulse oximeter readings significantly.
Note: When arterial oxyhemoglobin saturation is measured by an arterial blood gas it is referred to as
SaO2. When arterial oxyhemoglobin saturation is measured non-invasively by pulse
oximetry, it is referred to as SpO2.