Tuesday, November 05, 2013

History of Pulse Oximetry and the 5th vital sign

Back in 1991 I visited the emergency room because my asthma was acting up.  It was my first visit to the ER since I was discharged from my six months stay at the asthma hospital (NJH/NAC) in 1985.  An RT came into my room to give me a breathing treatment and he slipped this thing over my finger.  That was my first exposure to pulse oximetry -- the fifth vital sign.

I had been to emergency rooms many, many times for asthma in my life, and never had I seen such an object.  A temperature, blood pressure, and heart rate check were common, but to slip something that glows red, and causes a red number to appear on this little box the RT held, or that sat on a bedside table, was something new.

"What is that?" I asked back then.  

Pulse Oximeter Finger Probe
The RT attempted an explanation that zoomed over my head.  "It's called a pulse oximeter.  It tells me how well you're oxygenating and gives me your heart rate."  

I wasn't satisfied with that answer.  I wanted to know more.  What was it?  How did it work?  What significance was it?  What did the number 98 mean anyway?  I asked these questions and, once again, the answer wafted over me like a cool breeze.

In the summer of 1993 I was a journalist for a small weekly newspaper.  The local fire department received funding to get a new piece of equipment that was supposed to help them help sick people.  The objects were quite bulky, pretty heavy, and covered by a large, black case.  

A cord loomed from both, and the EMT I was interviewing slipped the probe over my finger.  "This is what we call a pulse oximeter," he said.  He attempted an explanation, yet again the answer flew over my head.  I ended up taking a picture of the EMT holding the new pulse oximeters.  On the caption under I wrote:  "EMT Josh Paramedic is holding two new pulse oximeters purchased with money from a Grant by..."

My next exposure to the 5th vital sign came as an RT student.  Now for the first time I finally understood what it was, and the significance of it.

Modern Pulse Oximeter
What is pulse oximetry?

Simply put, pulse oximetry is basically the transmitting of two lights through your finger (or ear lobe or toe), through your artery, and then the light is collected by a sensor on the other side of your finger.  The red light you can see, and the infared light you cannot see.

By calculating the amount of infrared light returning to the sensor, the pulse oximeter will tell you what percentage of hemoglobin is carrying oxygen.  Since it's measurement is made by the pulse of arterial blood, it can also give you a heart rate.

A normal pulse oximeter reading is 98%, however, anything greater than 90% is deemed acceptable.  In this way, supplemental oxygen can be administered and adjusted accordingly to maintain an adequate hemoglobin saturation.  The reading is measured as SpO2 (S = saturation, p = pulse oximetry, O2 = oxygen).

The SpO2 is a noninvasive measurement, and is an estimate of oxygen saturated hemoglobin.  To get an actual saturation, a more invasive blood gas measurement is required.  The saturation obtained from a blood gas is measured as SaO2 (S = saturation, a = arterial, O2 = oxygen).

By using the oxyhemoglobin dissociation curve, we can use the SpO2 to estimate the actual amount of oxygen in the lungs by estimating the PO2 (P = Partial Pressure, O2 = oxygen).  To do this we use the 4-5-6-7-8-9 rule as follows:sdf:
4-5-6-7-8-9 Rule
40 PO2
70% SpO2
50 PO2
80% SpO2
60 PO2
90% Spo2
In this way you can save the patient from an invasive blood draw and estimate how well the patient is oxygenating.  According to this rule, you would want to maintain an SpO2 of 90% (or 92% to be on the safe side) or better. Therefore, the lowest amount of oxygen should be utilized to obtain the desired oxygenation saturation.  

How did the pulse oximeter come to be?  

When I was a kid and was having an asthma attack, the respiratory therapist would place a nasal cannula over my face, and I'd promptly proceed to peel it off.  He'd get mad at me saying, "You really need to keep this on.  We need you to get oxygen."

Yet was my oxygen level really low?  The only way to tell would be to either do an invasive arterial blood gas, something that was rarely done on a kid.  I don't recall an ABG being drawn on me until I was at least in my upper teens.

Other than an ABG, the only way to know if a patient was oxygenating well was to use objective measurements such as skin color.  Usually if a person isn't oxygenating well blood will be shunted from fingertips and lips, and these areas will appear grayish or blue.  This is referred to as acrocyanosis, and is not life threatening.  Usually a low dose of oxygen -- say 2lpm -- will be all that's needed to resolve the problem.

If the core of the the body is blue or gray in color, this is referred to as central cyanosis.  Central cyanosis is critical, meaning oxygen isn't getting into that person's body at all, and oxygenated blood -- what little there is -- is shunted directly to vital organs such as the heart, kidneys and lungs.  In these situations, usually large amounts of oxygen are needed to remedy the problem, and perhaps even some form of positive pressure breathing such as from an Ambu-Bag, BiPAP or ventilator.

Obviously if you see cyanosis you know supplemental oxygen is needed.  Still, how much oxygen do you give?  When do you taper it off?  And what if the patient isn't oxygenating well yet doesn't show any cyanosis?  What do you do then?  Chances are, in the days prior to pulse oximetry, you winged it.  Some patients got too much oxygen, and some didn't get enough.  Outside of doing an invasive ABG, you really didn't have any means of monitoring and adjusting oxygen.

This was pretty much how it was even up to the early 2000s at some institutions.  Yet then came along the 5th vital sign.  This vital sign -- pulse oximetry -- made the job of respiratory therapists, nurses, and doctors "much easier," notes Gennie Ridlen in her rtmagazine.com article Pulse Oximetry:  A Historical Perspective.

She notes the history of the pulse oximetry can be traced back to 1862: (1)
  • In 1862 Hoppe-Seyler disovered that oxygen was transported by hemoglobin, and he referred to the oxygen-hemoglobin compound as oxyhemoglobin
  • In 1864 Stokes proved that oxygen was transported in the blood by hemoglobin
  • In 1862 von Vierodt invented the first pulse oximeter.  He measured oxygen consumption using transmitted light by wrapping a rubber band around his wrist to cut off circulation and shining a light on his hand, he saw the two bands of oxyhemoglobin disappeared and a band of deoxyhemoglobin appeared.  Using reflected light from a spectrometer, he measured the oxygen consumption of the living tissues by noting the time that elapsed as oxyhemoblogin changed into deoxyhemoglobin."
Reflection pulse ox (2)
John W. Severinghause, in his 2007 article in Anesthesia and Analgesia titled "Takuo Aoyagi: Discovery of Pulse Oximetry," (5) and the"Online Museum Catalog" (2) provide us with the following history:
  • 1931:  Ludwig Nicolai repeated the study of  von Vierodt, and created a device that "measured red light transmission through a hand."  (5)  The device used spectrophotometers (instraments that measure different wavelengths and intensities of light). (2)
  • 1934:  Reports of using pulse oximetry on animals (2)
  • 1939:  WWII sparks interest in need to monitor oxygen levels of pilots at high altitudes.  (2)Germans invent an 'ear oxygen meter' that used red and infared light."  The man credited with inventing the ear oximeter that used an ear probe is Karl Matthes.  (5) 
  • 1940-42:  A British researcher by the name of Glen Millikan (1906-1947) used "two wavelengths of light to produce a practical, lightweight aviation ear oxygen meter for which he coined the word 'oxymeter.'" (2)(5)
  • 1949: While working for the Mayo Clinic, Earl Wood "modified the Milliken ear piece." (5)
  • 1950s:  The system was refined and "manufactured by the Waters Company. This system was mainly used in physiology, aviation, and experimental studies" (2)
  • In the 1970s Hewlet Packard marketed a 35 pound pulse oximeter that costs over $10,000 and had a "bulky, clumsy earpiece. However, it did allow for continuous noninvasive monitoring of arterial oxygenation." (2)
So between 1862 and 1977 technology that would ultimately become the pulse oximeter was improved upon to the point that the first pulse oximeter was marketed in the U.S. in 1977.  The oximeter used a finger probe with fibre optic cables that were very sensitive to motion. (1)

By the late 1970s new probes were invented to solve the problem with motion, and a heart rate tracker was added and the device was precalibrated to make it more accurate. (1)

Who invented the first effective pulse oximeter?

The first pulse oximeter was the OLV-5100 (left) that was invented by
Japanese bioengineer Takuo Aoyagi in 1975.  You can see here that the
ear probe used was large and bulky, and probably not very comfortable.
During the 1930s and 1940s physiologists invented the technology to construct ear oximeters with red and infared light to measure oxygen saturation in the blood.  The problem with these early devices is they required bulky fiber optic cables and they needed to be calibrated. (5)

This was probably the main reason such products were not marketable.  The task of solving this problem was given to a Japanese Physiological bioengineer by the name of Takuo Aoyagi.  He was ultimately the inventor of the marketable pulse oximeter.  According to Severinghaus, while studying the available knowledge he became impressed with the work of Wood, and he set out to improve upon it.

In another article, John W. Severinghaus and Yoshiyuki Honda, published in the April, 1987, issue of the Journal of Clinical Monitoring and titled "History of blood gas analysis. VII. Pulse Oximetry," explain that Aoyagi's work resulted in the first marketable pulse oximeter.  (3, page 135)

The article notes that Aoyagi was hired in 1971 to work for the Research Division of Nihon Kohden Corporation.  His work on a variety of equipment has impacted the medical profession in a variety of areas, including fetal heart monitoring during childbirth, pulmonary circulation, and cardiac output.  (3, page 135-136)

The article further explains that he worked with the idea of measuring "pulsatile changes in light transmission through living tissue to computer the arterial saturation.  He realized that these changes of light transmission at all wavelengths would solely be due to pulsatile variations of the intervening arterial blood volume.  Thus, the unpredictable absorption of light by tissue, bone, skin, and pigment would be eliminated from analysis  It was this key idea that permitted the development of instrumentation that required no calibration after its initial factory setting, as all human blood has essentially identical optical characteristics in the red and infared bands used in oximetry." (3, page 136)

Severinghaus and Honda said that Aoyagi filed for a patent in 1974, although so did another man by the name of Massaichiro Monishi who worked for Minolta Camera Company.  Our authors note that "How this competing application arose and whether Aoagi's idea was discovered and copied has never been established.  Konishi's patent application was rejected by the Japanese Patent Office on February 9, 1982.  However, Minolta applied for and obtained patent protection in the United States." (3, page 136-7)

They explain that the first pulse oximeter to be used clinically was a prototype made by Aoyagi.  It was a two wavelength ear oximeter that used the pulse to measure how much oxygen was absorbed by hemoglobin in the blood.  His product was referred to as the OLV-5100, and was commercially available in 1975. (3, page 137)  The patent was approved in 1979 (5, page 52) It was first used clinically in 1980 by Japanese anesthesiologist Yoshiya to monitor a patient's saturation during surgery.  (3, page 135, 137)(1)

Despite these facts, the invention of the first marketable pulse oximeter is often attributed to Konishi and Minolta.  Perhaps one reason for this, said Ridlen, is that "Nihon Kohden did not continue to develop or market this instrument and made no effort to patent it abroad," said Severinghaus and Honda.  (3, page 137)  Perhaps the reason for this product failure was that this "device used heavy, delicate, fiber-optic cables, which ultimately hampered its success." (1)

On the other hand (no pun intended), according to Severinghaus and Honda, Minolta did continue to seek patents and to market their product abroad.  Their initial product was released in 1977 as the Oximet MET-1471.  It had a "fingertip probe and fiberoptic cables... Nakajima and nine associates then tested and used this Minoruta fingertip pulse oximeter and described it in 1979." (3, page 137) Severinghaus explains that the Minolta company used the fingertip probe because it "took advantage of the greater pulse amplitude."

Regardless, while Konishi and Minoruta often get the credit, "All pulse oximeters today are based on Dr. Aoyagi's original principle of pulse oximetry," according to the Nihonkohden.com. (4)

What did initial studies show?

Minolta introduced the Oximet MET-1471, and it was used in clinical studies.  Severinghaus reports that the pulse oximeter "was reported to be linear and accurate to within 5% by Suzukawa et al. in 1978 and Yoshiya in 1980.

Yet other studies questioned the accuracy of pulse oximetry when the saturation was below 70%, as noted by Severinhaus:
When studied at Stanford in 1980 by Sarnquist et al. a Minolta model 101 [identical to Oximet MET-1471] seriously underestimated the severity of hypoxia: At 50% actual Sao2 it read about 70%. Yamanishi wrote me (personal communication, 2006): “The Sarnquist data was the very important trigger for us to improve the accuracy of our pulse oximeter.” In 1984, Y. Shimada (then anesthetist of Osaka University, nowprofessor of Nagoya University) with Minolta’s K. Hamaguri, I. Yoshiya, and N. Oka published data using a Minolta Oximet MET-1471 that agreed with Sarnquist’s evidence of under-reporting the degree of desaturation. (5, page 53)
This knowledge was used to refine the technology, yet it was also later acknowledged that pulse oximetry should not be considered as accurate when the reading is less than 70%.  Even to this day when I'm using a pulse oximeter, if the reading is less than 70% I'm skeptical of the results.

Yet it really doesn't matter, because if it's that low you know you either have a bad reading, or the oxygenation is low.  The best remedy when you question the results of the pulse oximeter is to assess your patient.  If the clinical picture shows the patient is not oxygenating, regardless of the reading on the pulse oximeter, you should treat the patient appropriately.

In 2010 Ben J. Wilson et al. studied the results of pulse oximetry on patients diagnosed with sepsis and septic shock.  The study concluded that pulse oximetry "overestimated measured SaOby a mean of 2.75%."  These researchers recommend that "when SaOneeds to be determined with a high degree of accuracy in such patients arterial blood gases are recommended. (6)

Most studies that I've seen suggest that pulse oximetry is usually accurate withing +/- 2%.  However, when the reading is suspected to be inaccurate, or when the the spO2 does not match the clinical picture, an arterial blood gas should be drawn.  

When did pulse oximetry catch on?

As with any new discovery in medicine, it took a while for this new tool to catch on.  Severinghaus and Honda explain that "few foresaw its value in anesthesiology, intensive care, and other emergent situations, probably because conventional oximetry had never been convenient enough for these uses.  The Hewlett-Packard ear oximeter had been used almost entirely in pulmonary function laboratories and other physiologic environments.  Indeed, the initial work on pulse oximetry in the United States by several firms (Minolta, Corning, biox) considered these types of laboratories to be the probable market."

It took ten years for the pulse oximeter to make any real impact in the medical industry.  Interest finally started growing in the early 1980s as the device was determined useful when a patient was sedated to monitor oxygen saturation, and by 1986 pulse oximetry was recommended by the American Society of Anesthesiologists anytime a patient was sedated.  Pulse oximetry, therefore, became standard practice on sedated patient, operating rooms, critical care units and emergency rooms in 1986.  (1)

It was perhaps at this point that it became the 5th vital sign, alongside heart rate, respiratory rate, temperature, and blood pressure.  In 1994 the American Association for Respiratory Care released the first guidelines for the use of this 5th vital sign. (1)

The devices continued to be improved upon, and by 1995 small finger probes were introduced to the market.  (1) They were small enough to fit into your pocket, an d effective enough to produce an accurate oxygen saturation.  This made it easy to monitor saturation at any point in the clinical picture, including at home by patients when the prices dropped.  These products can now be purchased at Walmart for under a $100, and often times patients can be found with their own pulse oximeters.

By 1995 you had a choice between bulky bedside monitors and finger probes that could fit into your pocket. Some modern pulse oximeters are so advanced they provide accurate readings even on patients with poor circulation, and even on patients who are wriggly and squirmy, such as kids. In 2013 a new Oxitone oximetry device the utilizes no ear or finger probes, and can be wrapped around the wrist like a watch, was introduced to the market.  This new product was inspired by Steve Jobs.  This new oximeter even allows for the pulse rate and heart rate to be observed from an iPhone.

So depending on how much money you want to dole out, there are quite a few options available.  Even many patients are purchasing small pocket sized oximeters to use at home, so they know when to turn up or down their home oxygen flowmeters, and when to call their physician or 911.

Is pulse oximetry worth the investment?

It's very difficult to determine if a person is hypoxic simply by using the naked eye. Most studies indicate even the most well educated person has a difficult time recognizing hypoxemia until the saturation is below 80%., according to Amal Jibran in his 1999 article in Critical Care titled "Pulse Oximetry."(7)  For this reason, the pulse oximeter is a viable tool to not only determine that supplemental oxygen is necessary, but also how much oxygen to give. (7, page 11)

Of interest, another study showed that of those patients who desaturated to less than 90% for five minutes within their first 24 hours admitted to the hospital, had a threefold increased risk of mortality as compared to patients who did not desaturate.  So it is clearly evident that pulse oximetry is worth the investment. (7, page 11)

Studies have also confirmed that use of oximetry at the various points of patient care -- emergency room, operating room, critical care room -- greatly diminish mortality and morbidity caused by poor oxygenation. In fact, according to Jubran, with the proliferation of pulse oximeters in different locations of the hospital throughout the 1980s, several investigators demonstrated that episodic hypoxemia is much
more common than previously suspected with an incidence ranging from 20–82%. So, again, pulse oximetry is clearly worth the investment. (7, page 11)

Now it's obvious that even while pulse oximetry shows the risk of pulse oximetry, it does not show the cause.  However, pulse oximetry does provide medical caregivers, and sometimes the patients themselves, with the comfort of knowing the oxygenation of a patient, thus providing them with the ability to do something about it.  Or, at the very least, to investigate the possible cause.

Overall, pulse oximetry, and the use of the 5th vital sign, has been very useful in monitoring oxygenation in the patient population both in the hospital and the home setting.  Each time we're using one of these devices we ought to thank all the wonderful people who have invested time and money to the various projects that have brought us the 5th vital sign.

Further Reading and references:
  1. Ridlen, Gennieer, "Pulse Oximetry:  A Historical Perspective," rtmagazine.com article, August/ September, 1998, issue of RT: For Decision Makers in Respiratory Therapy.
  2. "Reflection Oximeter," Virtual Museum Catalog,  ttp://medicine.nus.edu.sg/anaesthesia/virtual_museum/reflection_oximeter.htm, accessed 5/11/13
  3. Severinghaus, John W. Yoshiuki Honda, "History of blood gas analysis," Journal of Clilnical Monitoring, vol 3, no. 2, April, 1987, page 135-138
  4. "History: 1970," nihonkohden.com, http://www.nihonkohden.com/company/history/1970s.html, accessed 5/11/13
  5. Severinghaus, John W., "TakuoAoagi: Discovery of Pulse Oximetry," Anesthesia and Analgesia, vol. 105, issue 106, December 6, 2007, pages 51-54
  6. Wilson, Ben J, et al., "The Accuracy of Pulse Oximetry in emergency department patients with severe sepsis and septic shock: a retroactive cohort study," BMC Emergency Medicine, 2010, biomedcentral.com, http://www.biomedcentral.com/content/pdf/1471-227X-10-9.pdf
  7. Jubran, Amal, "Pulse Oximetry," Critical Care, 1999, vol. 3, no. 2, pages 11-17


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