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The pulse oximeter, Covid-19 and the legacy of Dr, Takuo Aoyagi

By Rob Cooper.

In the midst of a global pandemic that has seen hundreds of thousands lose their lives to respiratory failure, the Japanese medical device giant Nihon Kohden quietly announced the death at 84 of their long-time employee, Dr. Takuo Aoyagi: the inventor of the pulse oximeter.

The pulse oximeter, which will be familiar to many as the small sensor that clips over a patient’s finger and emits a gentle red glow, is by any measure one of the most successful medical instruments in history. It provides a continuous measurement of pulse rate and blood oxygen saturation- a measure of the level of oxygen in the blood that is a key marker of respiratory and heart failure. A pulse oximeter can be found at the side of every hospital bed and in every operating theatre, while similar technology is increasingly common to consumer devices from fitness trackers to baby monitors.  The pulse oximeter has saved countless lives. Its introduction in the late 1970s coincided with a 90% decrease in deaths during anaesthesia, which is just one area of application.

Blood, Light and Oxygen

The pulse oximeter is, like all great inventions, remarkably simple. It uses two colours of light (usually red and near-infrared) shone through the finger to measure changes in the concentration of the oxygen-rich and oxygen-poor forms of haemoglobin: the molecule that carries oxygen around the bloodstream. This is possible because haemoglobin that is rich with oxygen is a much brighter red colour than haemoglobin that has had its oxygen stripped away by the cells of the body.

The idea that light could be used to yield measurements related to oxygen saturation dates back to the 1870s, and was built upon extensively during WWII in an effort to develop a system to warn allied pilots if their oxygen levels became perilously low. But it was not until 1974, and the work Dr. Aoyagi, that a method was developed to take these optical signals and produce a measure of oxygen saturation that was accurate enough to impact patients.

Noise to Signal

Dr. Takuo Aoyagi was born on 14th February, 1936 in Niigata Prefecture in Japan.  He studied electrical engineering at Niigata University, graduating in 1958. Soon after, Dr. Aoyagi came across a quote from the founder of the Nihon Kohden Corporation, Dr. Yoshio Ogino, which read “a skilled physician can treat only a limited number of patients, but an excellent medical instrument can treat countless patients across the world”. By 1971, Dr. Aoyagi was leading a small team at Nihon Kohden, and had been issued a simple challenge by his manager: “develop something unique”.

Dr. Aoyagi set about developing a sensor that could inform doctors when a patient’s respiratory system was failing to the point where they needed artificial ventilation. Previous efforts to make use of light to measure oxygen levels had failed to yield a useful device because of their dependence on convoluted calibration procedures. One such approach relied on the repeated compression and release of the upper arm.

Dr. Aoyagi’s initial investigations were severely hampered by noise in his optical measurement, noise that was due to the regular surge of oxygenated blood that flows through our bodies with every heartbeat. His real breakthrough came with the realisation that this pulsatile signal could itself provide a method of calibration. By comparing the size of the peaks and troughs that came with each heartbeat, Dr. Aoyagi was able to demonstrate a simple and accurate measurement of the oxygen saturation.

Happy Hypoxia

In the last few weeks, the importance of pulse oximeters in the treatment of coronavirus patients has led to a surge in interest in the now-ubiquitous device. In a New York Times op-ed publish two days after the death of Dr. Aoyagi, emergency physician Dr. Richard Levitan argued that “all persons with cough, fatigue and fevers should have pulse oximeter monitoring even if they have not had virus testing”.  This was in response to patients presenting with what has been misleadingly dubbed “happy hypoxia” – a state of perilously low oxygen saturation that seems to occur in some Covid-19 patients without being accompanied by a shortness of breath.

Dr. Aoyagi’s pulse oximeter would normally read between 90 and 99% in a healthy person, but there have been reports of Covid-19 patients exhibiting oxygen saturations at low as 50% without overt breathing difficulties.

While many clinicians have expressed concern that a surge in the at-home use of pulse oximeters could do more harm than good, Dr. Levitan’s suggestion has been echoed by others, including by some doctors in the UK NHS, who argue that distributing pulse oximeters to the most vulnerable could help identify those developing severe cases of Covid-19.

No matter the outcome of this debate, it is clear that the importance of this small, little-celebrated device has never been greater, and that Dr. Aoyagi’s death has coincided with a reaffirmation of the impact he has had on countless patients across the world.