It is easy to take for granted some of the technologies we use every day. The pulse oximeter was invented 40 years ago and has become such a routine part of medical practice that oximetry measurements have often been referred to as the “fifth vital sign.” This article takes a look at the brief history of pulse oximetry, describes how the technology works, and details the nuances of using pulse oximetry in everyday pediatric practice.
From humble beginnings
Modern day pulse oximetry had its beginnings in avionics research. In the early days of high altitude flight, it was important to study the effects of cabin pressure on the oxygenation of blood in the circulatory system of pilots. In 1935, Karl Matthes, a German scientist, introduced the first ear oxygen saturation meter using 2 photo sensors and lights initially in the red and green wavelengths, subsequently changed to light in the red and infrared wavelengths. Several years later, a US scientist, Glenn Millikan, produced a lightweight portable ear oxygen meter for use in pilots. It was Millikan who coined the term oximeter to describe his device.1 The oximeter was first commercialized in the 1960s by Hewlett Packard (Palo Alto, California) who sold a $10,000 device for use in pulmonary and sleep laboratories.
It was not until the early 1970s that 2 Japanese bioengineers, Takuo Aoyagi and Michio Kishi, serendipitously discovered that the transmission of the red and infrared frequencies of light through an earlobe showed variations corresponding with the pulsatile flow of arterial blood perfusing the tissue. They established that by measuring the pulsatile component of light transmission through living tissues they could eliminate the variable absorption of light by bone, skin, and venous circulation. They also discovered that the transmission of light in the red and infrared wavelengths could be used to calculate oxygen saturation in the arterial circulation of tissue being analyzed.2
The first pulse oximeters were manufactured in Japan in the late 1970s by Nihon Kohden and Minolta, but their clinical utility was unknown at the time. Following studies indicating that these devices could have widespread medical applications, pulse oximeters were commercialized in the United States in 1981 by Biox/Ohmeda (Ohmeda Medical; Boulder, Colorado) and by Nellcor (Covidien; Mansfield, Massachusetts) in 1983.3
The devices began to be used clinically, mostly by anesthesiologists to monitor patients undergoing sedation and anesthesia. Over the next several years, accumulated data indicated that pulse oximeters could prevent 2000 to 10,000 anesthesia deaths each year from undetected hypoxemia. In 1986, the American Society of Anesthesiologists recommended that these devices be used to monitor patients undergoing anesthesia.4 Over the next decade, pulse oximetry spread from the operating room to the emergency department, and then came to be used routinely in medical offices.
An oximetry sensor consists of red and infrared light emitting diodes and a photodetector placed on opposite sides of a measurement site, usually the finger in adults and children but the palm or foot in neonates and toddlers. The ratio of red to infrared light that passes through the tissue depends on the percentage of oxygenated versus deoxygenated hemoglobin in the arterial circulation of the tissue. In turn, the percentage of oxygen saturation displayed by a pulse oximeter is determined by an algorithm in the microprocessor of the device based on saturation measurements obtained by sampling a large population of patients breathing mixtures of decreased oxygen concentrations.
These algorithms are unique for each manufacturer. Pulse oximeters take hundreds of readings over a 3- to 6-second time period and update their measurements every 0.5 to 1 second.2 In the best of circumstances, pulse oximeter readings come within 2% to 3% of those produced by co-oximetry, the measurement of arterial blood directly by a blood gas analyzer.
When using oxygen saturation clinically, it is important to recall the oxygen dissociation curve we learned in medical school (Figure). The upper “bend” in the oxygen dissociation curve occurs at a pO2 of 60 mm Hg of oxygen, which corresponds to an oxygen saturation of 90%. Therefore, one needs to be aware that saturation levels of 90% and below are associated with hypoxemia.
In the office, we use pulse oximetry to determine whether a patient has respiratory compromise, that is, when evaluating a child presenting with signs and symptoms of pneumonia, bronchiolitis, croup, and asthma. Pulse oximetry helps determine the severity of respiratory distress and is used to monitor how well patients respond to treatment. It should be our goal to achieve a pulse oximetry reading of 92% or higher in our patients, and to be aware of the many conditions that can interfere with oximeter readings.