Takuo Aoyagi and the Discovery of Pulse Oximetry

A pulse oximeter.
Photograph by Rama,
Wikimedia Commons, Cc-by-sa-2.0-fr

The pulse oximeter is among the most significant applications of physics and engineering to medical and biology. In Chapter 14 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I describe oximetry.

Near-infrared light in the range 600-1000 nm is used to measure the oxygenation of the blood as a function of time by determining the absorption at two different wavelengths…

Pulse oximeters that fit over a finger are widely used… The basic feature is that arterial blood flow is pulsatile, not continuous. Therefore, measuring the time-varying (AC) signal selectively monitors arterial blood and eliminates the contribution from venous blood and tissue.

Takuo Aoyagi,
From the Engineering and Technology History Wiki.

The pulse oximeter has a long history, but an important milestone was reached by Takuo Aoyagi, a Japanese engineer. He tells his story in “Pulse Oximetry: It’s Invention, Theory, and Future” (Journal of Anesthesia, Volume 17, Pages 259-266, 2003). Below I present excerpts from the article. As you read, notice how Aoyagi transforms an annoying artifact into a breakthrough.

In 1958, I graduated from Niigata University and was employed by Shimadzu Corporation at Kyoto. There, I became interested in patient monitoring. In 1969, I attended the summer school of physiology and measurement organized by H.A. Hoff and L.A. Geddes held at Baylor University, Houston, TX, USA. This was a very valuable experience for me. After that, I visited several institutions to see patient monitoring systems in the USA. Based on these experiences I came to have a belief that the final goal of patient monitoring must be the automatic control of patient treatment…

Just after I was employed by Shimadzu Corporation, I read a report on an interview with Dr. Yoshio Ogino, founder of Nihon Kohden Corporation, in a newspaper. I was deeply impressed by his words: “A skilled physician can treat only a limited number of patients. But an excellent medical instrument can treat countless patients in the world…”

The first order made by our Research and Development division manager, Mr. S. Ouchi, was “Develop something unique.” And he made me leader of a group of several members newly assigned to the division. In those days, research on automatic control of artificial ventilation was being carried out at Tokyo University in the Department of Anesthesiology by Professor H. Yamamura. I was very interested in this project and visited Professor Yamamura’s group. Assistant Professor M. Kamiyama explained the system and told me that, “To make this system a practical product, a reliable continuous measurement of arterial O₂ (Sa_O2 ) and CO₂ is indispensable…”

As a theme of our research group I decided to develop a high-accuracy noninvasive dye densitometer for cardiac output measurement. My new idea was to adopt the principle of Wood’s earpiece oximeter to improve the accuracy of previous earpiece dye densitometers... In Wood’s oximeter, the blood in the ear is expelled pneumatically before the measurement, and light transmitted through the blood is measured and the value is stored as a reference. Next, the blood is readmitted to the ear. After that, the optical density of the blood is calculated continuously against the reference value. Two light wavelengths, red and infrared, are used. The ratio of the optical densities at the two wavelengths is calculated and converted to Sa_O2 by using an empirical calibration curve…

I appointed Mr. K. Yamaguchi chief of this project. An experimental model was constructed. For animal experiments, secondhand monitors and instruments were brought into an old hut… Just after starting the experiments, we noticed a pulsatile variation in the tissue optical density caused by arterial pulsation. This phenomenon made us anxious…

At this point… I thought as follows:

(1) If the optical density of the pulsating portion is measured at two appropriate wavelengths and the ratio of the optical densities is obtained, the result must be equivalent to Wood’s ratio.

(2) In this method, the arterial blood is selectively measured, and the venous blood does not affect the measurement. Therefore, the probe site is not restricted to the ear.

(3) In this method, the reference for optical density calculation is set for each pulse. Therefore, an accidental shift of probe location introduces a short artifact and quick return to normal measurements.

This was my conception of the pulse oximeter principle... It was December 1972.

In the 2007 article “Takuo Aoyagi: Discovery of Pulse Oximetry” (Anesthesia and Analgesia, Volume 105, Pages S1-S4), John Severinghaus writes

Greatness in science often, as here, comes from the well-prepared mind turning a chance observation into a major discovery. “One man’s noise is another man’s signal” commented the respiratory physiologist Jere Mead half a century ago.

Severinghaus concludes

Introduction of pulse oximetry coincided with a 90% reduction in anesthesia-related fatalities. Takuo Aoyagi’s invention was serendipitous. Although he could use the infrared signal to cancel pulsatile “noise” in the dye decay optical signal, hypoxic desaturation spoiled the smooth dye curve. In that noise, he recognized a useful signal—oximetry—because his mind was well prepared to understand what he saw happen. The process of turning his insight into more accurate, convenient and inexpensive saturation monitors still continues in dozens of laboratories and firms, while he continues to innovate.

Intermediate Physics for Medicine and Biology can’t teach readers how to make creative leaps leading to innovations and discoveries. But perhaps it can prepare the mind, so when you encounter a chance observation you can recognize it as an opportunity.