Pulse oximetry enters routine clinical monitoring

For most of the twentieth century, a patient's arterial oxygen saturation during surgery was a matter of clinical inference. Anesthesiologists watched for cyanosis, a late and unreliable sign of falling saturation. Arterial blood gases provided precise measurements but required needle puncture, laboratory processing, and several minutes of delay. In the interval between a sample draw and its result, a patient could sustain hypoxic brain injury without triggering any continuous alarm.

Takuo Aoyagi, an engineer at the Nihon Kohden company in Japan, presented the principle of pulse oximetry at a conference in Osaka on April 26, 1974. He recognized that the pulsatile component of the photoplethysmographic signal could isolate arterial blood from tissue background, and that the ratio of light absorption at red and infrared wavelengths in that pulsatile fraction correlated with oxyhemoglobin saturation. Turning the idea into a dependable clinical instrument took most of the next decade; an early fingertip device described in Osaka in 1980 was accurate but easily disturbed by finger movement.

Nellcor, of Hayward, California, began making and selling its N-100 finger oximeter for surgical use in 1982, after a prototype that used light-emitting diodes and a silicon photodiode in a small finger probe. Its co-founder William New, a Stanford anesthesiologist, had promoted the earlier Biox II ear oximeter in anesthesia and saw a larger market in the operating room. The N-100 sounded a tone whose pitch changed with the saturation, and Nellcor quickly took market share from Biox; its N-180 of 1987 updated the reading with every heartbeat.

Clinical impact arrived against a backdrop of simultaneous changes. Attributing any safety gains to the oximeter is complicated. Capnography was being adopted at the same time, anesthetic agents were improving, and professional training standards were tightening. In 1986 Harvard Medical School's Department of Anaesthesia published mandatory minimum monitoring standards for its nine teaching hospitals, saying no such standards had existed before, and aimed partly at rising malpractice costs. A 2014 Cochrane review of randomized trials in 22,992 patients found that oximetry detected hypoxemia but found no evidence that it changed the outcome of anesthesia.

Pulse oximeters spread well beyond the operating room. They became fixtures in emergency departments, intensive care units, post-anesthesia care units, and eventually general wards. Home oximeters became widely available to outpatients managing chronic lung disease or heart failure. During the COVID-19 pandemic the cheap finger oximeter became as common as a thermometer, and the pandemic also exposed problems with the technology's accuracy that researchers are still working to fix.