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Pulse Oximetry: The Redness of Blood in Patient Monitoring, Health Screening, and Sports Physiology
Pulse oximetry uses non-invasive photon technology to measure the redness of human blood. Oxygen saturation is essential when monitoring hospitalized patients. New applications of this technology have replaced invasive testing when screening patients for pulmonary and cardiac conditions. Recently developed portable pulse oximeters are now being used by extreme athletes to maximize physical performance.
The oxygen atom powers the main energy generation system in our cells and is necessary for our survival. The heart, lungs, blood vessels, and red blood cells are part of the elaborate oxygen delivery system that keeps us alive. Interruption of this vital oxygen supply can lead to cerebral infarction (stroke), myocardial infarction (heart attack), or death. To understand why the reactivity of oxygen is crucial in human physiology, you have to start with basic chemistry and the Periodic table. A single atom of oxygen (element “O”, atomic number 8) has 8 protons and 8 electrons. Two of these electrons are unpaired, making O highly reactive because it is more stable when these electrons are shared with other atoms. An atom of O will readily combine with other nearby atoms to create other molecules such as H2O (water), CO2 (carbon dioxide), Fe2O3 (iron oxide, rust), and even with itself to form O2 (what we commonly call oxygen). An oxygen atom wants to combine with everything and anything, driving many chemical reactions inside and outside the body.
Mitochondria are masters of oxygen. In the presence of oxygen, these intracellular organelles generate the adenosine triphosphate energy (ATP) that powers your muscles and vital organs. Mitochondria are the oxygen-consuming high-efficiency furnaces (inside each of your 37 trillion cells) that convert sugar and body fat into cell energy. Without oxygen, cells revert to anaerobic glycolysis for net gain of only 2 ATP per glucose molecule. With oxygen, mitochondria can produce 34 ATP per glucose…17 times more energy!
Since we are so reliant on oxygen, it is easy to assume that oxygen was a prerequisite for the origin of life. However, the leading theory is that life originated on Earth without oxygen. Oxygen would have been too reactive and resulted in far too simple chemicals, rather than the more complicated biochemicals required to construct the first organisms. The classic Miller-Urey experiment (first performed in 1953) demonstrated that if the gases presumed to be present in Earth’s primitive atmosphere (water, methane, hydrogen, carbon dioxide, and ammonia, but not oxygen) are exposed to electrical spark, many organic molecules are produced including: 22 amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA), and sugars. Life first developed when these organic chemicals combined in shallow cesspools at the edges of the oceans, under anaerobic (zero oxygen) conditions.
The oxygen rich atmosphere we currently enjoy would come much later. Two of the greatest leaps in evolution were photosynthesis (the capture of solar energy and production of oxygen) and oxidative phosphorylation (the use of oxygen to create more energy). Without oxygen and without mitochondria, our early ancestors would literally not have had the energy to crawl out of the primordial ooze. When you exercise it is easy to take for granted the air you breathe, but inside every one of your cells, your mitochondria turn the reactivity of oxygen into athletic power. Packed inside each mitochondrion are complex enzyme systems that convert food and body fat (using oxygen) in to ATP energy: the Krebs cycle, the electron transport chain, and ATP synthase.
Oxygen is transported by the hemoglobin in your red blood cells. Each hemoglobin molecule can hold four oxygen molecules. Hemoglobin is a dynamic molecule that changes shape as it binds and releases oxygen. This allows hemoglobin to bind oxygen faster in lung alveoli and release oxygen faster in capillaries. Medical students are taught the hemoglobin dissociation curve with the postage stamp analogy. The four bound molecules of oxygen are like a square of four postage stamps. Once the first stamp detaches (two tears) it facilitates release of the other three stamps (one tear each). Likewise, when oxygen binds hemoglobin it alters its chemical configuration, turning the hemoglobin bright red. The fingertip clamp of a pulse oximeter shines and measures light to detect this redness to calculate oxygen saturation. The inventors of this technology, Masimo corporation, further refined the accuracy of this non-invasive photon-based technology, making it the industry leader in pulse oximetry and patient monitoring.
In the hospital situation, pulse oximetry is part of the standard of care when monitoring critically ill patients and during surgery or general anesthesia. More immediate than blood pressure and more telling than heart rate alone, falling oxygen content in the blood tells healthcare workers that the patient is in serious trouble. When the oxygen desaturation alarm goes off, doctors and nurses pay attention. It could be a problem with the heart or lungs and must be remedied quickly.
Cardiac stress testing is becoming a more important part of health screening in the aging baby boomer population. Because heart disease is still the leading cause of death, clinicians should stress test more people, since there are now less invasive treatments that can save and/or improve lives. Just listening to your heart and checking your cholesterol and blood pressure during a physical exam is not nearly enough. Pulse oximetry is a key part of patient monitoring during cardiac stress testing.
When blood clots in the legs travel up to the lungs, they can cause life threatening pulmonary embolism by preventing adequate blood from reaching the lungs. Not long ago the standard test was drawing blood directly from an artery with a needle and sending the sample to the lab (arterial blood gases). This was an invasive test that was painful, could damage the artery, and took valuable time while waiting for the results. Now, emergency room physicians can use non-invasive pulse oximetry to stratify high risk from low risk patients suspected of having pulmonary embolism. Pulse oximetry allows measurement of oxygen saturation painlessly, immediately, and accurately.
There are some newborn babies that may have congenital heart defects that are not detectable on routine postnatal physical exam and may be discharged home, undiagnosed. Several hospitals and some states have instituted screening of newborns with pulse oximetry to help detect congenital heart disease in the first 24 to 48 hours of life.
During high altitude flight, loss of cabin pressure can lead to hypoxia, irrational behavior, and loss of consciousness in pilots. Pulse oximetry detects low blood oxygen in pilots before severe symptoms occur.
As mentioned in my book with Greg LeMond, the recent miniaturization of this technology allows oxygen measurement in athletes outside of the laboratory setting, so oxygen saturation can now be recorded when athletes train outdoors in practical situations. When researching our book, we evaluated Masimo’s then new iSpO2 portable pulse oximeter. Although healthy athletes do not desaturate (turn blue) when exercising, there are several possible niche uses of oxygen monitoring in athletes:
- High altitude training: The US Olympic training camp is in Colorado Springs well over a mile above sea level. Living at high altitude causes some adaptive changes to low pO2 such as increasing hemoglobin. Pulse oximetry could help monitor for O2 desaturation and detect those at risk for altitude sickness when exercising intensely at the lower pO2 found at high altitude.
- Fitness trainers who train their athletes at altitude or push them very, very hard: Trainers like to have data and might be interested in even small changes in O2 The iSpO2 makes this data very precise and repeatable.
- Extreme mountain climbing: where high altitude, high exertion, hypothermia, and sometimes supplemental O2 combine to make O2 a key element of performance and safety.
- Patients with emphysema and chronic obstructive pulmonary disease (COPD): It is well known that some COPD patients are CO2 retainers and have their respiratory rate controlled more by O2 levels in the blood than CO2. We also know that they can desaturate faster and lower than normal people. Too much supplementary O2 can suppress respiration. Just because someone has COPD does not mean that they cannot exercise. In fact, exercise can improve many respiratory conditions. Pulse oximetry might help people with COPD/emphysema exercise within safe parameters.
- Patients with heart disease: This is another growing subset of people who would benefit from light to moderate exercise but are limited in what they can do. Pulse oximetry would warn them if they are over doing it, just as we use pulse oximetry to monitor heart patients during stressful situations or during procedures.
Masimo pulse oximetry is used by athletes such as Dotsie Bausch (2012 Olympic silver medalist in women’s team pursuit cycling) and Stig Severinsen, PhD (Guinness Book of World Records holder for longest underwater breath hold at 22 Minutes).
In the patent infringement case (which began in 2009) Masimo Corporation sued Philips Electronics North America Corporation for using its pulse oximetry technology without license or permission. Philips’ weak defense was that Masimo’s patents were void because they were “obvious” and described inadequately. In 2014 a jury decided that Philips did not prove those claims and awarded Masimo $467 million, a figure that was upheld on appeal in 2015. This lawsuit proves the value of this ground-breaking technology. The global patient-monitoring device industry is expected to be worth $22.2 billion by 2018. More important than these huge monetary figures, the practical use of oxygen technology has saved countless human lives.
Full disclosure: When I was writing my book with Greg LeMond, The Science of Fitness, Masimo had simultaneously developed a new portable pulse oximeter. Because our book is based on oxygen consumption by mitochondria, I contacted Masimo and each author received a gratis iSpO2 device to evaluate. No money was exchanged. We reviewed the device in our book and Masimo mentioned our book in a corporate product press release.
This article first appeared on Memeburn.com. Click here for the original.
Dr. Mark Hom is a Johns Hopkins University trained biologist, an award-winning medical illustrator, an interventional radiologist, an educator of young doctors, an Elsevier author, and an avid fitness cyclist. Dr. Hom’s work with Greg LeMond in their recent book The Science of Fitness: Power, Performance, and Endurance explains how the human body, various organ systems, and individual cells function in the biologic process of exercise. He is currently a member of the Department of Radiology at Virginia Commonwealth University in Richmond, VA, USA.
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