By John Schwartz and Hikari Hida

Takuo Aoyagi, a Japanese engineer whose pioneering work in the 1970s led to the modern pulse oximeter, a lifesaving device that clips on a finger and shows the level of oxygen in the blood and that has become a critical tool in the fight against the novel coronavirus, died on April 18 in Tokyo. He was 84.

His death, in a hospital, was announced by his employer, Nihon Khoden, a Tokyo-based company that makes medical equipment. A niece, Kyoko Aoyagi, confirmed the death but said she did not know the cause.

The pulse oximeter has become “an indispensable addition to medicine,” said V. Courtney Broaddus, a professor emeritus of medicine at the University of California, San Francisco. Historically, patients were measured by four vital signs: temperature, blood pressure, pulse and respiratory rate. “It has become the fifth vital sign,” she said of oxygen level.

While many coronavirus patients do feel chest pain, fever and other symptoms, Dr. Broaddus said, the pulse oximeter “has become especially important because humans do not sense a low oxygen saturation alone.”

Moreover, some Covid patients seem not to have other symptoms. As a result, when moderately or mildly ill patients test positive for the coronavirus, they may be sent home with a pulse oximeter so that they can track their oxygen level and return to the hospital if it drops.

Mr. Aoyagi’s contribution to medical science was built on decades of innovation and invention. In an essay about Mr. Aoyagi, John W. Severinghaus, a professor emeritus of anesthesia at the University of California, San Francisco, wrote in 2007 that Mr. Aoyagi’s “dream” had been to detect oxygen saturation levels without having to draw blood.

His early research tried to measure cardiac output (the amount of blood the heart pumps), using a method known as dye dilution, which involves injecting a patient with dye. Instead of withdrawing the blood downstream and measuring the concentration of dye, he attempted instead to use early oximeters, some of which were developed during World War II to help military pilots breathe at high altitudes.

Those early devices, which clamped to the ear, tended to be inaccurate, unreliable and cumbersome, but Mr. Aoyagi was fascinated by the underlying technology: using two wavelengths of light — red and infrared — to measure blood oxygen levels. (Hemoglobin, the protein in blood that transports oxygen, absorbs light differently when it binds with oxygen.)

But he soon ran into a problem. Blood does not flow smoothly like an open tap, but pulses through the body irregularly, thus preventing an accurate recording of dye levels. The problem, however, turned out to be an opportunity. By devising a mathematical formula to cancel out this “pulsatile noise,” he created a device that measured oxygen levels with greater accuracy than before.

“Greatness in science, often, as here, comes from the well-prepared mind turning a chance observation into a major discovery,” Dr. Severinghaus wrote.

Nihon Kohden applied for a Japanese patent for its pulse oximeter in 1974, with Mr. Aoyagi and a colleague, Michio Kishi, listed as inventors. It was granted in 1979.

Takuo Aoyagi was born Feb. 14, 1936, in Niigata Prefecture, on the west coast of Japan, to Monshichi and Tatsu Aoyagi. His father was a math teacher, his mother a homemaker. He graduated from Niigata University in 1958 with a degree in electrical engineering and worked for Shimadzu before joining Nihon Khoden in 1971.

He earned a doctorate in engineering from the University of Tokyo in 1993. Other honors include the JS Gravenstein Award from the Society for Technology in Anesthesia in 2013. In 2002, he received a Medal With Purple Ribbon from the Emperor of Japan, given in recognition of achievements in the arts and academics.

Survivors include his wife, Yoshiko, and three children, Yasutoshi, Midori and Kaori.



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