Once the clip of a pulse oximeter goes on your finger (or toe or earlobe), a doctor or nurse will wait a few seconds for two numbers to pop up. After a few seconds more, they will make a note of these numbers and begin making treatment decisions based on them.
What are these mysterious numbers that hold so much power?
Using red and infrared light sources and a light sensor, pulse oximeters determine the percentage of hemoglobin (red blood cells) in your bloodstream that is carrying oxygen to various parts of your body.
Surprise! Your blood is not stagnant in your veins. Therefore, it is nearly impossible to time a pulsed light with your heart rate to determine your blood oxygen saturation. Instead, a pulse oximeter uses a steady stream of light to measure absorption and a sophisticated calculation to measure the “pulsed absorbance” while filtering out interference from muscle, tissue, and fingernail. Since it only measures oxygen absorption during blood flow, it reads a pulse rate in addition to oxygen saturation.
What does it mean?
Breathing normal room air at your normal respiration rate, you should have an oxygen saturation somewhere between 95 and 100 percent and a pulse rate between 60 and 100. This means your heart is beating normally all the while carrying enough oxygen to the rest of your body to keep you healthy. If one or both of these numbers are outside of normal limits, it is an indicator that something is wrong that is preventing your body from effectively oxygenating your bloodstream.
There are a variety of conditions that can affect PO2 levels. Asthma, pneumonia, Chronic Obstructive Pulmonary Disease (COPD), lung cancer, chemotherapy, and even mild cold or flu can adversely affect oxygen saturation and should be treated. However, marathon runners often have a heart rate lower than 60 but an oxygen saturation over 97 percent on room air. Babies and toddlers often have a heart rate over 100 but normal oxygen levels. In both cases, treatment is not necessary for heart rate or oxygen levels since this is considered “normal” for this population.
Also, pulse oximetry provides a single snapshot in time. It does not tell you what happens when the person sleeps, stands, walks, or runs. It also does not tell you how accurate of a reading it is for that person. Perhaps their circulation is poorer on the one hand than another. Perhaps their nail polish is interfering with the reading. Perhaps they are slightly anemic and therefore do not have enough hemoglobin in their bloodstream to give an accurate measurement.
Carbon dioxide can also interfere with an accurate reading. Since CO2 bonds with a person’s hemoglobin, replacing oxygen, the pulse oximeter cannot distinguish between the two. It might show an oxygen saturation level of 95 percent, but 15 percent of the hemoglobin is carrying carbon dioxide instead of oxygen. This is why a pulse oximeter is not the best tool for measuring oxygen saturation in heavy smokers.
Despite its limitations, a pulse oximeter still provides a look at your overall health that can be crucial in making treatment decisions.