Between Issues

Aoyagi’s Legacy in the US: What Influenced Its Acceptance?

June 10, 2021

Jeffrey B. Cooper, Ph.D.
Adapted from Special Feature on Pulse Oximeters: The invention that changed the paradigm of patient safety around the world. (LiSA (1340-8836) vol28 No3 Page237-308, 2021.03 (in Japanese)

View more articles from this special collection hosted by the APSF on Pulse Oximetry and the Legacy of Dr. Takuo Aoyagi.

Disclaimer: The information provided is for safety-related educational purposes only, and does not constitute medical or legal advice. Individual or group responses are only commentary, provided for purposes of education or discussion, and are neither statements of advice nor the opinions of APSF. It is not the intention of APSF to provide specific medical or legal advice or to endorse any specific views or recommendations in response to the inquiries posted. In no event shall APSF be responsible or liable, directly or indirectly, for any damage or loss caused or alleged to be caused by or in connection with the reliance on any such information.

While Takuo Aoyagi’s name was not widely known in the US, his invention of a practical way to monitor oxygen saturation has had a huge impact on healthcare, on anesthesiology in particular. He is highly deserving of the honors that have been bestowed on him for this contribution. Now, on his passing, it is fitting that we pay tribute and recognize him for the countless lives his invention has saved and for his creativity and persistence that brought this gift to the world.

I am not an historian. I don’t have all the facts or even empirical evidence to claim exactly what influenced the relatively rapid acceptance of this new technology by a profession that typically is skeptical of the latest technology fad and demands evidence before action. And, in my experience, anesthesiologists pride themselves on individual responsibility, competence and vigilance to maintain the safety of the care of their patients. Attention to vital signs that could be easily observed, including heart rate, blood pressure, ECG and skin color were thought by many to be sufficient for the attentive anesthesiologist. This is especially so for those who had grown up in a time where there was little technology and before the term “patient safety” was coined.

How did so many come to accept pulse oximetry in the absence of definitive trials of its effectiveness in preventing harm? Yes, there were some studies to point to its usefulness, but little to justify the expense on a cost/benefit basis. What influences led to almost ubiquitous use of pulse oximetry in just a few years after the introduction of the first practical commercial monitors in the US? I will explore what I think were a few key factors, but without the data I cannot draw definitive conclusions.

I think the most critical factor for accepting pulse oximetry is that it tapped into a visceral desire for all anesthesiologists to eliminate the guesswork to ascertain the state of a patient’s oxygenation. I don’t know who first realized the huge potential, but Bill New, an anesthesiologist and engineer certainly played a key role as did his colleague and partner in the business, Mark Yelderman, also an anesthesiologist (I don’t have evidence about their individual contributions). From their own clinical experience, they recognized that having such information, updating continuously within a few heart beats, would make any anesthesiologist feel a bit less anxious, especially during critical moments.

Much credit has to be given to the design of the N-100, especially its simplicity and brilliant human factors design features (Figure 1). Getting such critical information from a single number, clearly displayed via LED’s (a relatively new technology at the time), was a game-changer. The ease of application of the sensor also contributed to easy use and acceptance. The look and function of the large adjusting knob was elegant and practical. Perhaps the most appealing feature was the changing sound of the pulse tone with decreasing saturation. My guess is that the combination of these attributes were the most important reasons for quick adoption in anesthesia. This is a good example of remarkable human factors being heavily responsible for a lifesaving technology, not just the measurement itself.

Figure 1: N-100 Pulse Oximeter

Figure 1: N-100 Pulse Oximeter

I suspect many believe that the standards for minimal monitoring during anesthesia established by the American Society of Anesthesiologists in October 1986, played a role in dissemination of this innovation.1 While they may have had some influence on raising awareness of the need for some monitoring technologies, the standards did not at first require pulse oximetry or even strongly encourage its use. The ASA standard was a direct evolution from the standard promulgated by the Anesthesia Departments of Harvard Medical School in 1985; that standard did not mandate pulse oximetry. The rationale in developing a standard (I was there and promoted that rationale) was to first establish the precedent for a basic standard. To achieve acceptance, that meant including only items that had stood the test of time and that no reasonable anesthesiologist could argue against. We thus could not immediately require pulse oximetry because there was still even little widespread personal experience among anesthesiologists. While including a requirement for it would have been prescient, in the absence of data or wide experience, the idea of a standard may have been successfully opposed by vocal skeptics who might have a strong voice in opposition to any standards.

The rationale for the original ASA “Standards for Basic Intra-Operative Monitoring” was similar, as described in an article in the Spring, 1987 APSF Newsletter:

“The ASA committee debated whether to include capnography and pulse oximetry as the “standard of care’ ‘At that time, it was felt impractical to mandate very specific (and very expensive) high tech equipment when the greatest focus of the effort was the general extension of the vigilance of the anesthesiologist. The committee also considered the questions of the consistency of performance of these two instruments and the availability at that time relative to the potential demand. However, E. C. (“Jeep’) Pierce, M.D., committee member and past president of the ASA, now states, “Capnography and oximetry are becoming so widespread that they will be functional standards. Projecting current trends, it is likely that by the end of 1988, enough oximeters will have been sold for there to be one available for every operating room in the country”.

It was not until 1989 that the ASA standard was amended to require pulse oximetry for all anesthetics. It was likely that almost all anesthesiologists were by then using pulse oximetry for every anesthetic.

Over the years, I’ve heard it said that The Anesthesia Patient Safety Foundation strongly influenced adoption of pulse oximetry. Being one of the founders of APSF, I know that we were careful not to promote specific technologies over others because of concerns of any appearance of conflict of interest since some of those companies donated to the organization. I do suspect that APSF’s strong advocacy for patient safety and focus of attention on errors in practice raised awareness of all anesthesia providers about the need to take stronger measures than just relying on their own vigilance.

I think there was another strong force that was responsible for relatively rapid spread of pulse oximetry in anesthesia. Fineberg et al, in 1978, reported on a survey of how new ideas were adopted into practice in anesthesiology using three different examples.2 While publications are generally the most influential means to persuade anesthesiologists to adopt new ideas into practice, learning from colleagues is a close second influence. Those findings likely applied to the spread of pulse oximetry, which was introduced only a few years after that study. I suspect that it often happened that anesthesiologists heard excited anecdotes from their colleagues about how valuable was this addition to their practice. Perhaps they heard of a great save resulting from early discovery of an error or of the confidence during intubation on hearing that so recognizable tone indicating the state of saturation. Such word of mouth may have contributed greatly to pulse oximetry “spreading like wildfire” even in the absence of much empirical evidence of its effectiveness.

Regardless of exactly how the idea spread so quickly, all anesthesiologists and, more so, all patients having an anesthetic, are indebted to Takuo Aoyagi for his marvelous ingenuity that has so greatly contributed to the dramatic improvements in anesthesia patient safety.


Jeffrey B. Cooper, Ph.D.
Professor of Anaesthesia
Harvard Medical School
Department of Anesthesia, Critical Care & Pain Medicine
Massachusetts General Hospital, Boston, MA
Executive Director Emeritus & Senior Fellow
Center for Medical Simulation


  1. Eichhorn JH. ASA Adopts basic monitoring standards. APSF Newsletter, 2, 1987. Last accessed 1/7/2021
  2. Fineberg HV, Gabel RA, Sosman MB. Acquisition and application of new medical knowledge by anesthesiologists: three recent examples. Anesthesiology 1978: 48(6): 430-436.
Read more articles from this special collection hosted by the APSF on Pulse Oximetry and the Legacy of Dr. Takuo Aoyagi.