An oximeter. The little device a health practitioner clips onto a patient’s finger that affords a clear and accurate determination of the patient’s oxygenation status.

Most of us go along all day long with our blood oxygenation levels at 100% of saturation. But, occasionally, if we become terribly ill, our oxygen levels drop. And, we need supplemental oxygen. Until the recent past, that situation was when the patient was suffering from pneumonia.

Lately, however, we’ve needed to intervene (using mechanical ventilation) for those folks suffering from COVID-19. Using a pulse oximeter made it clear when such mechanical assist was necessary.

But, the pulse oximeter is only about 50 years old. No- actually, clinicians used ear oximetry back in the 1930’s and 1940’s. Back then, these devices (mostly the result of the work of Greg Millikan, based upon red and infrared measurements of light) were used to detect problems among the pilots of military aircraft- because if they suffered oxygen deprivation due to flying at higher altitudes, they could lose consciousness and crash their vessels. (Hemoglobin [the key protein in blood that carries oxygen] absorbs light differently as it binds with oxygen.  Hence  the ability to use light to discern oxygen saturation of the blood.)

Prior to the development of the oximeter, health practitioners relied on four patient vital signs- temperature, blood pressure, pulse, and the respiratory rate. Once the oximeter was developed, oxygen saturation levels became the fifth vital sign.

The best analogy I can consider (of course, because it’s mine) is that prior to the development of Colyte/Golytely, bowel surgery was a messy business (pun intended).  Infection rates and severe complications as a result of such surgery were almost 90%; death resulted in at least 20% of the cases.  Once the colon lavage was available, deaths dropped to (way less) less than 1% and infection rates were just a little higher than that.  Once the oximeter afforded accurate patient oxygen saturation levels, acute-care procedures were revolutionized and patient outcomes were dramatically improved.  Consider it an early warning system for all those COVID-19 patients who are rapidly losing respiratory capabilities.

It was left to Takuo Aoyagi to develop the modern oximeter. He had the dream to develop a modern oximeter- without drawing patient blood, to help determine when patients would need ventilation support. (Back in the 1970’s and early 1980’s, we helped develop on-line blood gas systems- specifically because oxygen saturation was such a critical piece of patient data. And, because on-line oximetry was not yet available.)

Aoyagi used dye dilution to monitor cardiac output (the volume of blood pumped by the heart). This required him to draw blood to discern the dye concentration. Which he then compared to the results of those early oximeters that were used for military pilots.

There was a problem, though. Our heart pumps blood in pulsative fashion- blood flow is not continuous akin to tap water coming out of the sink. This makes reading the dye levels a problem- Aoyagi called this ‘noise’ in his calculations. So, he set about developing mathematical processes to eliminate the ‘noise’.

(For those of you who recall your physics or spectrophotometry, this involved the Lambert-Beer (or Beer-Lambert) Law. That law states there is a linear relationship between the concentration (c) and the absorbance (A) of the solution. The trick is to determine the variable, ε (the molar absorption coefficient), in the equation A=εcl (l is the optical path length).

Aoyagi realized that the level of oxygen in blood was the same whether it was from pulsing blood or not- so he could eliminate the noise posed by the pulse with a simple mathematical manipulation. That simple concept made his oximeter development facile. (He also had to determine the optimal light wave length to employ. 630 nm is where the most sensitive measurements could be obtained. At 805 nm, hemoglobin and oxyhemoglobin have equal absorption. And, 900 nm manifested decreases with desaturation.)

Aoyagi presented his results to the Japanese Society of Medical Biologics and Engineering on 26 April 1974, which presentation was preceded by a patent application on 29 March. (The patent was granted in 1979.)

Unfortunately, Aoyagi’s employer, Nihon Khoden, neither made satisfactory versions of the device nor truly cared for the market.  (The management decided that this was lacking medical necessity.)  Which is why Minolta (also working on an oximeter, but their theory was not as advanced as Aoyagi’s) managed to capture the market.

Until 1985, when Nihon Khoden put Aoyagi back in charge of the product concept. (They had moved the development to another employee almost immediately after Aoyagi’s presentation in 1974.) Aoyagi perfected the concept by incorporating the nonlinearity of Beer’s law with absorption.

On a separate note, Aoyagi didn’t earn his PhD until 1993 (Tokyo University), which by then he was 57 years old. And, 9 years later, he was granted two prizes for his invention and improvement of the oximeter. (His device cut the fatalities for patients under anesthesia by some 90%. See why I used my analogy with Colyte/Golytely?)

 

 

 

 

 

 

You can see the award ceremony here

The IEEE (Institute of Electrical and Electronics Engineers) recognized Dr. Aoyagi for his innovation in health care technology in 2015. As the first Japanese person to be awarded this medal, the proclamation stated that “all of today’s pulse oximeters are based on Dr. Aoyagi’s original principles of pulse oximetry.”

Aoyagi perished on 18 April 2020, at the age of 84.

 

Today is a special day.  Probably not for most of us, because we don’t recall the significant event.  It was 75 years ago today that Hitler and his minions (except for the vile ones the US government decided to forgive to populate our space and military programs and the 22 others that were protected by Paraguay, Uruguay, and a few other South American dictatorships) were squashed.  Victory in Europe day.  VE Day.