This vital diagnostic tool has come a long way in the past 100 years.
The oximeter, used to measure hemoglobin’s oxygen saturation in the blood or tissue, is an astonishing photoelectric device based on the Lambert-Beer Law (known more commonly as Beer’s Law). The law illustrates the correlation between light transmission and optical density. This law relates the concentration of a solute to the intensity of light transmitted through it. A pulse oximeter passes light through body tissues at two different frequencies, 660 nm (red) and 900 nm (infrared). The instrument then reads the amount of each type of light absorbed by the tissues. Hemoglobin that is carrying oxygen (oxyhemoglobin) absorbs one wavelength of the pulse oximeter’s light output, and hemoglobin that is not carrying oxygen (deoxyhemoglobin) absorbs the other wavelength. Backgrounds such as fluid, tissue, and bone are factored out of the measurement. Using the resulting measurement, the oximeter can accurately determine the percentage of oxygen saturation of the arterial blood.
As recently as 10 years ago, RCPs treated patients without the advantages of immediacy, portability, and noninvasiveness that pulse oximetry confers. As a result, pulse oximetry has made the life of the RCP much easier and has drastically improved patient outcomes.
Oximetry can be traced back to 1862, when the oxygen-transport function of hemoglobin was demonstrated by Hoppe-Seyler,1 who showed that the color changes produced by the aeration of hemoglobin solutions were due to the absence or presence of links between oxygen and hemoglobin. The red pigment that became dominant upon aeration, originally named globin, was renamed hemoglobin by Hoppe-Seyler after he was able to crystallize it and describe its color spectrum. He demonstrated that the crystalline forms of hemoglobin differed among animal species. Using his own newly constructed gas pump, he found that oxygen formed a loose, dissociable compound with hemoglobin, which he called oxyhemoglobin. In 1864, Stokes showed that hemoglobin was the component in blood that carried oxygen.
According to Severinghaus et al,2 by 1874, von Vierordt had spectroscopically measured oxygen consumption using transmitted light. Wrapping a rubber band around his wrist to cut off circulation and shining a light on his hand, he saw that two bands of oxyhemoglobin disappeared and a band of deoxyhemoglobin appeared. Using reflected light from a spectrometer, he measured the oxygen consumption of the living tissues by noting the time that elapsed as oxyhemoblogin changed into deoxyhemoglobin.
It would be 55 years before this method was tried again, this time by Nicolai, with better equipment. In 1932, Nicolai resurrected von Vierordt’s work, adding photoelectric light detection. His student, Kramer, introduced the new German barrier-layer photocells to record saturation in vivo by transilluminating the arteries of animals.
By 1935, the first in vivo oximeter of Matthes used two light wavelengths (one to factor tissue absorbancy and the other to factor oxygen). On the basis of Nicolai’s prior studies, he constructed the first device to measure the saturation of human blood continuously in vivo by transilluminating the ear or other tissue. Matthes and his collaborator, Gross, were the first to use two spectral regions, one of which was not affected by oxygen, to compensate for changes in tissue thickness, blood content, light intensity, and other variables. At first, they used blue-green light, as Nicolai had done, but later switched to infrared. In 1939, they described the first red and infrared ear oxygen saturation meter. Unfortunately, it was large, inconvenient to use, and difficult to calibrate because it used an infrared detector that was composed of a gas-filled phototube covered with a filter (for blue-green light) that transmitted only infrared light.
By the late 1930s, Squire had introduced a self-calibrating oximeter that eliminated this problem. While studying at University College Hospital, London, Squire reported on an oximeter that could be used on the web of the hand. Red and infrared light transmission were compared by changing filters. Squire also used a method later adopted by Wood: setting the two optical channels to zero by compressing the tissue to squeeze out the blood. This was the first step toward the absolute-reading oximeter.
In the 1940s, Millikan’s first lightweight ear oximeter was used for aviation research during pilot training in World War II. Important contributions were made by both Millikan and Kramer, whose research was greatly accelerated during the war. The air forces of both Allied and Axis powers needed to monitor blood oxygen saturation because pilots were blacking out at high altitudes during dogfights. The US military became interested in Millikan’s research and the development of the oximeter only after learning the Germans had such a device. Millikan developed an oxygen-delivery system with a demand valve that responded to altitude and activity. The oximeter was constructed using a servo system in which the oximeter reading controlled the oxygen supply to the aviator’s mask, which was built with an oximeter attached to it. Millikan’s ear oximeter used two German ideas: Kramer’s copper oxide barrier photocells and Matthes’s red and green filters. By accident, his green filter and photocell responded only to infrared light; this was the start of modern oximetry.
In 1948, the most important subsequent work in oximetry began in the laboratory of Wood at the Mayo Clinic, Rochester, Minn. Geraci, with Wood’s guidance, modified Millikan’s earpiece, improving the infrared filter and adding a pressure capsule (inflatable balloon) that could stop blood flow to the ear for an initial zero setting. By 1950, the Mayo group had reported use of the new oximeter in many clinical settings.
In 1951, Nahas, working in Wood’s laboratory, devised a special 0.1-mm light path cuvette for the Beckman spectrophotometer and published a report on a method of measuring oxygen saturation that became the accepted standard.
The same year, according to Hough,3 Stephen was the first to use ear oximetry in the operating room to report frequent and significant intraoperative desaturation. His instrument required a dedicated technician for frequent calibrations, was delicate, and sometimes burned the patient’s ear.
The concept of using multiple wavelengths to distinguish among the pigments of carboxyhemoglobin, methemoglobin, and oxyhemoglobin was introduced in 1964 by Shaw, a San Francisco surgeon and inventor. Shaw began design and construction of a self-calibrating, eight-wavelength ear oximeter. His unique idea was to solve the simultaneous equations by using one wavelength more than the number of separate forms of hemoglobin that needed to be identified. In the late 1960s, Lubbers developed the catheter tip and cuvette fiber-optic sensors for oxygen tension, carbon dioxide tension, and pH. By the early 1970s, a self-calibrating, eight-wavelength ear oximeter was introduced. Cumbersome and expensive, it heated the tissue to “arterialize” venous blood.
In 1972, Aoyagi, while working on a device to measure intravenous dye washout using light transmission/absorbance through the ear, noticed pulsatile artifacts in absorbance. He realized the implication and developed a two-wavelength ear pulse oximeter, which made use of heart pulsations to detect and measure arterial blood absorbance. His ear oximeter was made available in 1975. Although his idea was correct, his device used heavy, delicate, fiber-optic cables, which ultimately hampered its success.
In 1974, Konishia submitted a patent application based on a similar idea. His application was rejected by the Japanese Patent Office in 1982. In 1976, he was able to obtain US patent protection for a photoelectric oximeter.
In 1975, Nakajima et al invented a pulse oximeter that eliminated the need for calibration because it used only two wavelengths, and it responded only to the pulsatile changes in transmitted red and infrared light. The first commercial instrument was made available in 1975 as an ear oximeter by Aoyagi et al.4 The company marketing Aoyagi’s oximeter, however, did not continue to develop or market this instrument and made no effort to patent it abroad.
In 1977, the world’s first pulse oximetry device was marketed and introduced to the United States by a competing company. This oximeter used a fingertip probe and fiber-optic cables that were very motion sensitive.
In the late 1970s, important advances were made when Wilber, while trying to make a better heart-rate counter for joggers, developed the light-emitting-diode-based pulse oximeter, eliminating the need for fiber-optic cables. He also improved the device’s accuracy by making it precalibrated.
Pulse Oximetry Makes an Impact
Pulse oximetry sparked only limited interest during its first 8 to 10 years because of its inconvenience. Few saw its value in anesthesiology, intensive care, and other emergency situations. In the early 1980s, however, enormous interest developed in pulse oximetry, thanks to New (an anesthesiologist) and Lloyd (an engineer). New recognized the potential importance of (and market for) a convenient pulse oximeter in the operating room and in all other hospital and clinic sites where patients are sedated, anesthetized, unconscious, comatose, paralyzed, or in some way limited in their ability to regulate their own oxygen supply.
In 1986, intraoperative pulse oximetry became a standard of care. The American Society of Anesthesiologists strongly advocated pulse oximetry in every anesthesia location. In 1991, the American Association for Respiratory Care released clinical practice guidelines for pulse oximetry. In 1995, the world’s first self-contained finger pulse oximeter was developed.
Today, pulse oximetry is used in most health care settings, and its uses continue to grow. In 1995, Madico et al5 used pulse oximetry and the algorithm of the World Health Organization (WHO) to predict the clinical diagnosis of pneumonic and nonpneumonic acute lower respiratory infection (ALRI) in Peruvian children. Together, pulse oximetry and the WHO algorithm were successful in detecting 99 percent of pneumonic ALRI cases, 87 percent of radiologic pneumonia cases, and 94 percent of all cases of pneumonic and nonpneumonic ALRI (as diagnosed clinically).
In 1997, pulse oximetry was used for the first time to detect autonomic neuropathy in diabetes. Mithall et al6 used pulse oximetry to assess altered vascular responses to various thermal stimuli; these correlate well with the changes seen in autonomic neuropathy. They found, in diabetic patients with autonomic neuropathy, that the decrease in oxygen saturation seen on exposure to cold was slower and less intense than usual. They also determined that diabetic patients with autonomic neuropathy had no rebound increases in oxygen saturation.
Reflectance pulse oximetry could well be the next step for this diagnostic tool. The measurement of oxygen saturation using reflected light was first described by Brinkman and Zijlstra in 1949.1 Transmittance pulse oximetry, which is most commonly used at present, relies on tissue transillumination and is based on the spectrophotometric analysis of the optical absorption properties of blood, combined with the principle of photoplethysmography. This method limits its application to body areas such as the fingertips, earlobes, and toes, as it can be used only on parts of the body that are thin enough to be penetrated by the red/infrared light. In reflectance pulse oximetry, the LEDs and photodetector are positioned adjacent to each other instead of on opposite sides of the vascular bed, as in transmission pulse oximetry. The positioning of the LEDs and photodetector allows absorption on the same skin surface and the reading is determined from light that scatters back to the tissue surface. Reflectance sensors make transillumination of the tissue unnecessary and allow saturation to be measured at the hand, forearm, foot, and leg.
In the mid-1990s, Mendelson7 began developing various optical sensors, along with improved signal detection and processing techniques, for measuring arterial oxygen saturation using reflectance pulse oximetry. His long-term goal was to investigate how reflectance pulse oximetry can be used in intrapartum applications to monitor the fetus during active labor and delivery. There is currently an intrapartum fetal pulse oximeter awaiting Food and Drug Administration approval. The device will allow the clinician to assess fetal well-being by allowing direct measurement of fetal oxygen saturation during labor and delivery. The sensor for this unit is noninvasive (to the fetus) and can be placed against the fetal cheek or temple, where it gathers real-time data. Normal uterine forces, coupled with the design of the sensor, hold it in place during labor and delivery. To date, other methods of monitoring the fetus have been invasive and technically difficult and have provided only intermittent data.
Gennie Ridlen, RRT, CPFT, is president of the AirWise Asthma Clinic, Oklahoma City.
1. Severinghaus JW, Astrup PB. History of blood gas analysis. VI: oximetry. J Clin Monit. 1986;2:270-288.
2. Severinghaus JW, Astrup PB, Murray JF. Blood gas analysis and critical care medicine. Am J Respir Crit Care Med. 1998;4:S114-S122.
3. Hough SW. Pulse oximetry; 1997. http://www.mghdacc.com/BaefFolder/PulseRaw.htm.
4. Severinghaus JW, Honda Y. History of blood gas analysis. VII: pulse oximetry.
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5. Madico G, Gillman RH, Jabra A, et al. The role of pulse oximetry. Its use as an indicator of severe respiratory disease in Peruvian children living at sea level. Arch Pediatr Adolesc Med. 1995;149:1259-1263.
6. Mithall A. Pulse oximetry detects autonomic neuropathy in diabetes. J Diabetes Complications. 1997;11:35-39.
7. Mendelson Y. Pulse oximetry: theory and applications for noninvasive monitoring. Clin Chem. 1992;38:1601-1607.