Takuo Aoyagi, an inventor of the pulse oximeter, an ingenious and indispensable medical device that measures oxygen in the blood and has become a staple of hospitals around the world, emerging in recent months as a key tool in the fight against the novel coronavirus, died April 18. He was 84.

His death was announced by his longtime employer, Nihon Kohden, a Japanese manufacturer of electronic medical equipment, which did not provide additional details. The New York Times reported he died at a hospital in Tokyo, citing a niece.

For years, health professionals have been taught to measure four primary vitals signs: body temperature, pulse, respiratory rate and blood pressure. Since at least the late 1980s, when the pulse oximetry technique pioneered by Dr. Aoyagi began to gain wide acceptance, oxygen saturation has been described as a “fifth vital sign,” a crucial indicator of whether oxygen is being delivered from the lungs and heart to the rest of the body.

“It’s been absolutely revolutionary in terms of improving the safety of acute-care procedures as well as improving diagnostic training, because it’s such a useful sickness index,” said Lance Terada, chief of pulmonary and critical care medicine at the University of Texas Southwestern Medical Center in Dallas. While working in the operating room or performing a pulmonary procedure, he added by phone, “you have one eye on what you’re doing and the other eye on the pulse oximeter,” a device that can be applied to the forehead or, more often, painlessly clipped to a finger or ear.

“The most valuable thing about the pulse oximeter is that it’s one of very few pieces of data that we monitor in real time,” Terada said, enabling health-care workers to quickly respond to the worsening condition of a coronavirus patient, for instance, whose symptoms may rapidly transform from a bad cold into pneumonia and respiratory failure. A pulse oximeter can provide a “warning sign,” he added, for patients whose condition is worsening but who may still have no difficulty breathing.

Many patients with chronic illnesses also use a pulse oximeter to track their oxygen levels from home — a convenience that was all but unthinkable in the decades before Dr. Aoyagi’s breakthrough in the early 1970s. Previously, oxygen saturation was measured through a blood gas test that required samples from patients’ arteries. Imprecise ear oximeters were also developed during World War II to warn military pilots of oxygen deprivation.

At the time those devices were created, Dr. Aoyagi was a young boy living on the west coast of Japan’s main island. He was 9 when Japan’s surrender brought the war to an end, absolving any fears of Allied bombing runs near his home, and went on to experiment with ear oximeters as an electrical engineer, fascinated by the underlying science.

As he learned, a pair of lights — red and infrared — are shined through the earlobe or another translucent part of the body, and differences in the way the lights are absorbed enable the device to calculate the amount of oxygen in the blood.

Dr. Aoyagi hoped to use the oximeter to develop a noninvasive way of measuring cardiac output (the amount of blood pumped by the heart) through a method known as dye dilution, in which dye is injected into a patient. Rather than aid fighter pilots in dogfights, he hoped that his invention would signal a hospital patient’s need for artificial ventilation.

According to a 2007 essay by John Severinghaus, an authority on anesthesia, Dr. Aoyagi encountered a roadblock in the form of “noise” — variations in the flow of blood as it moves through the body’s labyrinthine plumbing system — that prevented him from accurately calculating the movement of the dye. His breakthrough was a mathematical formula, a “ratio of ratios,” that enabled him to cut through the noise and measure oxygen in the blood.

“Greatness in science often, as here, comes from the well-prepared mind turning a chance observation into a major discovery,” Severinghaus wrote. Quoting Jere Mead, a respiratory physiologist, he added: “One man’s noise is another man’s signal.”

Dr. Aoyagi created a pulse oximeter prototype with a Nihon Kohden colleague, Michio Kishi, and in 1974 their company submitted a patent application listing both men as inventors. The patent was granted five years later.

By then, however, Dr. Aoyagi had been transferred to another project. The concept behind pulse oximetry had been “denied” by a skeptical supervisor at Nihon Kohden, Dr. Aoyagi recalled, and he was kept from working on the device until 1985. Competitors such as Minolta had made refinements that helped make the pulse oximeter popular. Without their work, he wrote in a personal essay, “the idea might be buried.”

In recent years, Dr. Aoyagi was increasingly recognized for his invention, which has been championed by the World Health Organization and proved especially useful for surgeons monitoring patients under anesthesia. In 2015, the Institute of Electrical and Electronics Engineers awarded Dr. Aoyagi its Medal for Innovations in Healthcare Technology and declared that his research had led to “a fortyfold reduction in death rates in anesthesia.”

“All of today’s pulse oximeters,” the institute said, “are based on Dr. Aoyagi’s original principles of pulse oximetry.”

Dr. Aoyagi was born in Niigata prefecture on Feb. 14, 1936. He graduated from Niigata University in 1958 with a degree in electrical engineering and worked at Shimadzu, a Kyoto-based scientific instruments company, before joining Nihon Kohden in 1971. In 1993 he received a doctorate in engineering from the University of Tokyo.

Information on survivors was not immediately available.

While health workers have credited pulse oximeters with helping to save the lives of coronavirus patients, some doctors have cautioned the public from making a run on the device. The American Lung Association issued a statement Thursday warning that it had become “more difficult for consumers and even hospitals to purchase” pulse oximeters.

“Unless you have a chronic lung or heart condition that affects your oxygen saturation level on a regular basis,” said Albert Rizzo, the association’s chief medical officer, “most individuals do not need to have a pulse oximeter in their home.”