The inventor of the pulse oximeter, an essential tool in monitoring blood oxygen levels in critical care patients, has died, The New York Times reports.

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.

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