Yesterday I took my asthmatic son to the doctor. His doctor told me that his pulse ox was 95%, which is concerningly low. Together we decided whether he should be transferred to the emergency room, or if we could continue treating his asthma at home.
Pulse ox is a measure of how much oxygen is being carried in the bloodstream. It can be measured by a pulse oximeter, a small device that many Americans were encouraged to keep at home during the COVID-19 pandemic. Oxygen is required by most cells in the body to generate useable energy, it is carried by red blood cells and enters the blood from the lungs. When the lungs are struggling to get enough oxygen in or there is fluid in the lungs that slows the transport of oxygen into the blood, the level of oxygen in the blood may fall. If left untreated, the cells will starve for usable energy. Therefore, pulse oximeters are handy to have at home for individuals who are concerned about lung infection or disease, such as those with COVID-19 or those with asthma. A low reading on a pulse oximeter would signal the affected person that its time to go to the hospital.
Similarly pulse oximeters are used in clinic and hospital settings to indicate when to give patients supplemental oxygen, and how much to give. Pulse oximetry is the standard of care in triage, ICU and other clinical settings.
Taken together, pulse oximeters are both a critical and an accessible means of deciding on who needs life-saving oxygen and when.
Pulse oximeters work by emitting light, and measuring how much light is absorbed and reflected (to put it in very simple terms). This gives us information on how much oxygen is in the bloodstream because the protein that binds oxygen (hemoglobin) will change shape slightly, and therefore change the way it reflects and absorbs light, when bound to oxygen.
An analogy to help visualize is thinking about what you might wear on a sunny day. If you wear a dark shirt, the dark fabric will absorb more light from the sun, making you hotter. If you were a light-colored shirt, the light fabric reflects more of the light, keeping you cooler. In this way, the small different in shape and color of hemoglobin when it is bound or not bound to oxygen changes its reflective capacity of light.
It has been known for decades that pulse oximeters are less accurate in darker skinned individuals. (It was first reported in 1976 by Saunders and colleagues and reviewed in 1987.) Just like different fabrics, the pigment in skin contributes to reflecting or absorbing light, altering the readings from a pulse oximeter. Despite this widely held knowledge, reliance on the pulse oximeter has stayed. It was not until 2022 that investigators sought to find out if the decrease in accuracy of pulse oximetry in darker skin affected clinical outcomes. It does. Gottlieb and colleagues (2022) found significantly lower rates of supplemental oxygen delivery in hospitals for Asian, Black and Hispanic patients. I think that bears repeating. We have known for almost 50 years that these devices were less accurate in some patients, and yet continued to use them to decide who received life-saving oxygen.
During the COVID-19 pandemic, pulse oximetry was not just used to determine the use of supplemental oxygen, but also sometimes used to determine which cases would be considered “severe COVID disease” and therefore eligible for COVID-19 therapies. A 2022 study (Fawzy et al.) discovered that the inaccuracy of pulse oximeters lead to delayed or unrecognized eligibility for these vital therapies.
In researching this topic for my book (and this blog post) I looked through the A&P books that I had in my office. They each discuss pulse oximeters. They go into varying levels of detail about how they work, both the technology and the physiology. Not one of them describes the inaccuracies of pulse oximetry on darker skin.
Our students are our future. Let us arm them with both critical knowledge, and the thinking skills required to ask important questions.