The History of Pulse Oximetry
From the First Steps toward Pulse Oximetry to Modern Miracles
Pulse oximetry is a vital and versatile part of modern medicine. A wide range of medical conditions including asthma, emphysema, cancer, congestive heart failure, altitude sickness, COVID-19, sleep apnea, and severe blood loss from bodily trauma can cause oxygenated blood levels to fall to dangerously low levels.
In rare cases, overexposure to oxygen gas can cause a deadly oversaturation in the blood, but low blood oxygen levels can be equally dangerous if not more. That’s why pulse oximetry is crucial, but where did it all begin? Let’s look back on its history and the people who’ve made it possible.
Who Invented the Pulse Oximeter?
Like many scientific advances, modern pulse oximetry wasn’t the brainchild of a lone genius. Scientists and medical engineers made a series of slow advances in measuring blood oxygenation levels starting in the mid-1800s that continued into the 1980s. Over this period spanning more than a century, these breakthroughs built upon one another. The major pioneers such as Karl Matthes, Glenn Millikan, and Takuo Aoyagi could all be fairly called the inventor of the pulse oximeter.
Blood oxygenation measurement started with crude, painful, slow, and impractical methods and eventually culminated in the rapid, portable, and non-invasive pulse oximetry methods we have today.
Early Methods of Pulse Oximetry and the Discovery of Hemoglobin
With modern, sophisticated pulse oximetry technology, a doctor can determine a patient’s blood oxygenation levels almost instantly by putting a small instrument on a patient’s finger. In the mid-to-late 1800s, things weren’t so easy, and scientists had barely just learned the body’s method of absorbing oxygen and distributing it throughout the body.
The seeds of modern pulse oximetry were planted in 1840 when Friedrich Ludwig Hunefeld, a member of the German Biochemistry Association, discovered crystalline structures in the blood that transports oxygen. The crystals were hemoglobin, a term that Felix Hoppe-Seyler coined in 1864. Hoppe-Seyler’s investigations on hemoglobin would spur George Gabriel Stokes, an Irish-English mathematician and physicist, to look into “the reduction and oxidation of the colouring matter of the blood.” Before their experiments, scientists knew that blood carries oxygen, but they weren’t sure if it was dissolved in the blood or bound to a substance that carried it along.
In separate experiments and investigations, Stokes and Hoppe-Seyler figured out how to answer this question with an experiment: if oxygen were simply dissolved in the blood, its presence or absence in blood wouldn’t noticeably affect the way that light passes through it. But if oxygen were chemically bound to a molecule in the bloodstream, light would behave differently when hitting oxygen-rich and oxygen-poor blood.
The scientists hypothesized that hemoglobin bound to oxygen would reflect a different wavelength of light—and therefore a different color—than hemoglobin that’s not. This means that blood should change color when oxygen is added or taken away from it.
When Stokes and Hoppe-Seyler exposed blood with varying oxygenation levels to light, they found different wavelengths of light were emitted from oxygen-rich and oxygen-poor blood, and that the same blood sample could change its color when exposed to different levels of oxygen. This proved definitively that hemoglobin binds to oxygen.
This realization wasn’t enough to give us the modern pulse oximeter. At the time Stokes and Hoppe-Seyler conducted their experiments, the only way to measure the oxygenation level of a patient’s blood was still to extract a blood sample and analyze it. This method was painful, invasive, and too slow to give doctors enough time to act on the information it provided.
The fundamental principle of modern pulse oximeters has been discovered. What followed then was a series of significant breakthroughs in creating a device capable of measuring how much oxygen is in the blood. Or the pulse oximeter as we know it.
1935: Karl Matthes Invents the First “Blood Oximeter”
German physician Karl Matthes implemented the basic principle of modern pulse oximetry, measuring blood oxygenation through light, in 1935. He created a device that detected the presence of oxygenated and deoxygenated hemoglobin, which connected to a patient’s earlobe to easily shine through a patient’s blood without taking a blood sample.
Matthes’s device used two wavelengths of light, originally green and red: one that detects the presence of oxygenated hemoglobin and another that isn’t affected by its presence. Matthes later found red infrared light better suited for this purpose.
While ingenious, Matthes’s machine was hard to calibrate and limited in providing saturation trends instead of absolute numbers. Nonetheless, the device is regarded as the first oxygen saturation measuring device.
1940s: Glenn Millikan Invents the Portable Oximeter
American inventor and physiologist Glenn Millikan developed an earpiece that came to be known as the first portable oximeter. He also coined the term oximetry.
The device was born out of the need for a practical device for World War II pilots who sometimes flew to oxygen-starved high altitudes. Millikan’s ear oximeter was primarily used for military aviation.
Without compact computer circuits, it was difficult to make an analog device that could reliably keep track of oxygenated blood levels in pilots. Milikan’s oximeter was unreliable as it only measured static levels of oxygenated blood, offering only a snapshot of the pilot’s condition.
1948–1949: Earl Wood Improves Millikan’s Oximeter
Another factor that Millikan overlooked in his device was the need to establish a high volume of blood in the ear.
Mayo Clinic doctor Earl Wood developed an oximetry device that squeezed more blood into the ear with pneumatic pressure to obtain a more accurate and reliable reading in real-time. This earpiece is part of Wood’s ear oximeter system advertised in the 60s.
1964: Robert Shaw Adds Wavelengths to His Oximeter Sensor
Robert Shaw, a surgeon in San Francisco, experimented with adding more wavelengths of light to the oximeter, improving on Matthes’s original method of detection that used two wavelengths of light.
Shaw’s device included eight wavelengths of light, which added more data for the oximeter to calculate oxygenated blood levels. This device is considered the first absolute reading ear oximeter.
1970: Hewlett Packard Unveils the First Commercial Oximeter
Shaw’s oximeter was deemed costly, heavy, and had to be wheeled from room to room in hospitals. However, it showed that the principles of pulse oximetry were well-understood enough to be sold in a commercial package.
HP commercialized the eight-wavelength ear oximeter in the 70s and went on to offer pulse oximetry devices.
1972–1974: Takuo Aoyagi Develops a New Principle for Pulse Oximeters
While researching methods to improve a device that measures arterial blood flow, Japanese engineer Takuo Aoyagi stumbled upon a discovery that had significant implications for another problem—pulse oximetry. He realized that the level of oxygenation in arterial blood could also be measured by the heart’s pulse rate.
Aoyagi presented this principle to his employer Nihon Kohden, which later developed the Oximeter OLV-5100. Launched in 1975, the device was regarded as the world’s first ear oximeter that used pulse oximetry, based on Aoyagi’s principle. The device was not commercially successful, and his insight was overlooked for a time.
In 1977 Minolta launched the first fingertip pulse oximeter, OXIMET Met 1471. In the 1980s Nellcor and Biox Technology marketed pulse oximeters such as this.
By 1987, Aoyagi became renowned as the inventor of modern pulse oximetry devices. Aoyagi believed in “the development of non-invasive continuous monitoring technology” for patient monitoring. Modern pulse oximetry embraced this principle, and devices today are quick and painless for patients.
The Limits of Modern Pulse Oximetry
Modern pulse oximetry may seem miraculous compared to the clumsy methods of the past, but it still has room for improvement. Since pulse oximetry doesn’t directly measure the presence of oxygen in the blood, instead measuring the presence of the hemoglobin molecules that carry it, it can give an incomplete or even false picture of a patient’s true blood oxygen levels.
For example, it’s difficult for a pulse oximeter to determine the difference between a patient with carbon monoxide or cyanide poisoning and one with a high blood oxygen level. This is because both cyanide and carbon monoxide bind with hemoglobin, taking up precious oxygenation capacity in the blood and sometimes killing a patient through hypoxia. Hemoglobin that’s chemically bound to these dangerous chemicals reflects light similarly to oxygenated blood, and a pulse oximeter’s sensor detects it in the same way. A poisoned patient could receive a high reading on a pulse oximeter when in fact, their organs are being starved of oxygenated blood.
Modern Pulse Oximetry
Pulse oximeters have become well-adapted to the many complications that can arise when attempting to measure a patient’s oxygenated blood levels. They have benefited greatly from the ever-shrinking size of computer chips, allowing them to analyze the light reflection and heart pulse data they receive in an ever-smaller package. Digital breakthroughs have also given medical engineers the opportunity to come up with tweaks and advances that improve a pulse oximeter reading’s accuracy.