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How Pulse Oximetry Influenced Medicine and How Its Evolution Will Influence Medicine

June 10, 2021

Joe Kiani, BSEE, MSEE
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.

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In 1974, Japanese bioengineer Dr. Takuo Aoyagi made one of the most impactful inventions in patient monitoring and with it improved patient safety1. Dr. Aoyagi was in pursuit of noninvasive cardiac output monitoring, but instead invented pulse oximetry. Prior to Dr. Aoyagi’s invention, oxygen saturation monitoring was relegated to the laboratory due to the bulkiness of the devices that measured it, such as the 8-wavelength ear oximeter developed by Hewlett Packard. Dr. Aoyagi filtered the arterial pulse signal and normalized it against all of the variables that the 8-wavelength ear oximeter had to account for (such as tissue thickness, skin pigmentation and other factors) by dividing the filtered arterial pulse information by all the information the photo detector measured2. This was Dr. Aoyagi’s so-called “AC/DC” invention. Then dividing the ratio at one wavelength (sensitive to oxygenated hemoglobin) by the ratio at another wavelength (sensitive to both oxygenated and deoxygenated hemoglobin) gave us the “ratio of ratios,” which is correlated to arterial oxygen saturation (empirically derived from laboratory studies). Through his pioneering work and meticulous research, Dr. Aoyagi invented conventional pulse oximetry. Today many consider pulse oximetry to be the “fifth vital sign.”

Dr. Aoyagi’s invention of pulse oximetry was a significant advancement in patient monitoring and safety. Before the advent of pulse oximetry, the rates of anesthesia-related deaths and brain injury were very high because the only methods available to the anesthesiologist to check arterial oxygen saturation were intermittent blood lab tests or observing the color of the lips, purple indicating low oxygen saturation. Although a 20,000-patient study showed no difference in mortality of patients with or without pulse oximetry3, anesthesia-related fatalities dropped from 1-in-10,000 to 1-in-1 million4 after the advent of pulse oximetry. Clinicians immediately recognized the immense value of pulse oximetry and it quickly became a standard of care in operating rooms and intensive care units.

Dr. Aoyagi’s pulse oximeter paved the way for Masimo’s invention of Measure-through-Motion and Low Perfusion technology5. Continued innovation is paramount to improving patient outcomes. While Dr. Aoyagi’s invention assumed all blood that pulsates is arterial blood, Masimo’s invention accounted for venous blood during motion by separating venous blood pulsation from arterial blood, hence providing increased accuracy for specific patients such as the poorly perfused in the OR, ICU patients such as neonates, and awake patients in post-surgical wards and at home.

Multiple clinical studies using the Measure-through-Motion technology have demonstrated a host of successful patient outcomes, including a dramatic reduction in Retinopathy of Prematurity (ROP)6 in NICUs, the ability to detect Critical Congenital Heart Disease (CCHD)7, and to detect patient deterioration from Opioid Induced Respiratory Depression (OIRD)8,9. Pulse oximetry is no longer limited to the hospital. Clinicians can now monitor their post-surgical patients that have been prescribed opioids or COVID-19 patients who may not need ICU care in the comfort of their homes.

In the future, pulse oximetry may indeed become the most important of the five vital signs. Not only because oxygen is necessary for life, but its measurement and principles of measurement have resulted in a greater understanding of the heart, lungs, and systemic issues. Today the modern pulse oximeter, which we call the Pulse CO-Oximeter, has the capability to measure not just SpO2, but Pulse Rate, Respiratory Rate, Fluid Levels, Respiration Effort, Total Hemoglobin, Carboxyhemoglobin, Methemoglobin, Perfusion Index, and Oxygen Reserve Index.

Takuo Aoyagi’s curious fascination and ardent research gave the world the gift of pulse oximetry. He had an idea and he made it his life’s work to realize it. In the years before he passed away, he even brought out his noninvasive cardiac output monitor. Clinicians, patients and engineers owe Dr. Aoyagi a debt of gratitude. We will remember Dr. Aoyagi for his boundless work, significant contributions to clinical care, and his kindness and humility.


Joe Kiani, BSEE, MSEE,
Founder, Masimo & Patient Safety Movement Foundation


  1. Ritman EL. Earl Wood–a research career noted for development of novel instruments driven by the power of the indicator dilution concept. J Appl Physiol (1985). 2014;117(9):945-956.
  2. Severinghaus JW. HIGH LIFE: High altitude fatalities led to pulse oximetry. J Appl Physiol (1985). 2016 Jan 15;120(2):236-43.
  3. Moller JT, Johannessen NW, Espersen K, Ravlo O, Pedersen BD, Jensen PF, Rasmussen NH, Rasmussen LS, Pedersen T, Cooper JB, et al. Randomized evaluation of pulse oximetry in 20,802 patients: II. Perioperative events and postoperative complications. Anesthesiology. 1993 Mar;78(3):445-53.
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  5. Barker SJ. “Motion-resistant” pulse oximetry: a comparison of new and old models. Anesth Analg. 2002 Oct;95(4):967-72
  6. Castillo A, Deulofeut R, Critz A, Sola A. Prevention of retinopathy of prematurity in preterm infants through changes in clinical practice and SpO₂technology. Acta Paediatr. 2011 Feb;100(2):188-92.
  7. de-Wahl Granelli A, Wennergren M, Sandberg K, et al. Impact of pulse oximetry screening on the detection of duct dependent congenital heart disease: a Swedish prospective screening study in 39,821 newborns. BMJ. 2009;338:a3037.
  8. Taenzer AH, Pyke JB, McGrath SP, Blike GT. Impact of pulse oximetry surveillance on rescue events and intensive care unit transfers: a before-and-after concurrence study. Anesthesiology. 2010 Feb;112(2):282-7.
  9. McGrath SP, McGovern KM, Perreard IM, Huang V, Moss LB, Blike GT. Inpatient Respiratory Arrest Associated With Sedative and Analgesic Medications: Impact of Continuous Monitoring on Patient Mortality and Severe Morbidity. J Patient Saf. 2020 Mar 14.
Read more articles from this special collection hosted by the APSF on Pulse Oximetry and the Legacy of Dr. Takuo Aoyagi.