The pulse oximeter is a ubiquitous, non-invasive medical device that quickly and continuously measures the oxygen saturation level in a person’s arterial blood, known as SpO2. This small clip or probe, often placed on a fingertip or earlobe, provides immediate feedback on oxygen delivery. This simple, painless technology is now a standard measure of a person’s respiratory and circulatory function.
Early Steps Toward Oxygen Measurement
The concept of measuring blood oxygen levels using light began long before the modern device. Research in the 1930s established that oxygenated and deoxygenated hemoglobin absorb light differently. German physician Karl Matthes developed an early ear device using two wavelengths of light, demonstrating the possibility of transcutaneous measurement.
American physiologist Glenn Millikan developed the first truly portable oximeter in the 1940s to monitor military pilots for oxygen deprivation during World War II. Millikan coined the term “oximeter” for his ear device. These early instruments were often unreliable in a clinical setting because they required calibration or were significantly affected by blood flow changes. The fundamental challenge remained isolating the arterial blood signal from the constant absorption of venous blood and other tissues.
The Pivotal 1970s Invention
The breakthrough occurred in 1974 with Japanese bioengineer Takuo Aoyagi at the company Nihon Kohden. Aoyagi was working on a cardiac output device when he noticed the measurement signal fluctuated with the patient’s heartbeat. He realized this pulsatile component—the change in light absorption with each beat—was caused only by arterial blood flow. By mathematically isolating this pulsatile absorption from the constant absorption of venous blood and other tissues, he could accurately calculate arterial oxygen saturation (SpO2). His team patented this technique, known as the “ratio of ratios,” in 1974, and the principle was later adapted by competitors such as Minolta, which began selling a finger-type device around 1978.
The Core Scientific Principle
The pulse oximeter relies on the distinct light-absorption properties of oxygenated hemoglobin (oxyhemoglobin) and deoxygenated hemoglobin (deoxyhemoglobin). The device emits two specific wavelengths of light, typically red (660 nm) and infrared (940 nm), through the tissue. Oxyhemoglobin permits more red light to pass while absorbing more infrared light, while deoxyhemoglobin does the opposite. By measuring the ratio of light absorbed at these two wavelengths, the device determines the percentage of hemoglobin carrying oxygen.
A photodetector measures the light that transmits through the tissue, which fluctuates with the blood volume changes of the arterial pulse. This pulsatile signal allows the oximeter to disregard the constant light absorption from non-pulsatile venous blood, skin, and bone. This ensures the reading reflects only the arterial oxygen saturation.
Transition to Clinical Standard
Commercialization and Adoption
The 1980s marked the period of commercialization and widespread clinical adoption, primarily driven by American companies. Nellcor, founded in 1981, developed and marketed the first user-friendly, bedside pulse oximeters, such as the N-100 model released in 1983. These devices required no complex calibration, making them practical for operating rooms and critical care units.
Standard of Care
The technology revolutionized patient safety in anesthesia by providing continuous, real-time monitoring of oxygen levels, which previously required invasive blood gas analysis. In 1986, the American Society of Anesthesiologists recommended pulse oximetry for all anesthetized patients, establishing it as a standard of care. The pulse oximeter quickly became recognized as an indispensable monitoring tool, often referred to as the “fifth vital sign.”