How Do Pulse Oximeters Work?
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Pulse oximeters are made up of two components: a sensor unit and a display. The sensor unit measures one’s pulse (or heart rate) as well as the oxygen saturation of one’s blood.
Pulse (or Heart Rate): Your pulse is the rate at which your heart beats. It is usually listed as beats per minute (bpm). The oximeter's probe detects the flow of blood through whatever part of the body it is clipped to — usually the end of a finger or toe (and, in some cases, an earlobe). The display on the oximeter will present the pulse information in two ways: a pulse wave signal that will show how regular, erratic, weak said pulse is, and as a number, which specifies the wearer’s bpm.
Blood Oxygenation: Pulse oximeters use light-emitting diodes (LEDs) that project two types of red light into the tissue of the wearer's finger, toe or ear. One side of the oximeter will hold the LED that projects that light; the other side of the oximeter will include a sensor that detects the light that has travelled through the wearer's flesh. The oximeter then calculates what percentage of your blood is carrying oxygen by measuring the change in the absorption of light by oxygenated blood (that is, the blood that’s carrying oxygen) and deoxygenated blood (blood that does not contain oxygen). The result will be displayed as a measure of Blood Oxygen Saturation (referred to as SpO2), which is given as a percentage.
In addition to being used to measure one’s performance under stress or track their daily health, pulse oximeters are used to monitor vital signs during surgery, especially while you're under anesthesia. Apart from personal devices, blood oxygenation measurements can be calculated in clinical settings through an arterial blood gas (ABG) test. Although this ABG test is extremely precise, it’s much more invasive than a personal pulse oximeter. In short, at-home pulse oximeters empower you to manage your own health and, while they shouldn’t be used for diagnostic purposes, their immediacy can certainly help you stay informed and aware.
Interpreting Pulse Oximeter Readings
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So, how do you know what your readings mean? The Mayo Clinic suggests that the normal resting pulse rate for adults should fall between 60 and 100bpm. That's a pretty wide range — and one that accommodates for variation in gender, age, weight, level of physical activity and more. Generally speaking, a pulse rate at the lower end of that range means that your heart is functioning more efficiently than a heart that is required to beat more often each minute while at rest.
At a certain point, though, lower is not better. The medical condition bradycardia is when one's resting heart rate is unusually low — that is, at or below 60bpm. Bradycardia’s symptoms include memory challenges, fainting, tiredness and chest pain.
So, what about blood oxygenation? As you may know, your blood carries oxygen from your lungs to cells throughout your body. The delivery of oxygen at the right levels is essential to life and health. The air that we typically breathe contains approximately 21% oxygen. Standard blood oxygen saturation — in someone without lung disease — is typically between 95% and 100% when measured with an oximeter and expressed as SpO2. When measured via an ABG, which expresses results in mmHg, standard blood oxygen saturation is between 80 and 100 mm Hg. Typically, a SpO2 reading of below 95% is considered low.
Breathing oxygen at too high a concentration causes hyperoxia, a potentially dangerous condition that can cause damage to your lung tissue. On the other hand, not getting enough oxygen can cause hypoxemia. Symptoms of hypoxemia include headaches; shortness of breath; rapid heart rate; coughing and wheezing; mental confusion; and a bluish tinge to your skin and lips. The consequences of hypoxemia range from relatively minor — frequent headaches and shortness of breath — to extremely severe in extreme cases, such as an interference with heart function. Hypoxemia can stem from many factors, including heart and lung conditions; asthma and emphysema; high altitudes; certain pain medications; and sleep apnea. According to the United States Food & Drug Administration (FDA), severe cases of the flu and COVID-19 may also cause oxygen levels to drop to the point where supplemental oxygen is required.
Of course, these figures are generalizations and don’t take into account one’s specific health conditions, like chronic obstructive pulmonary disease (COPD) or other lung disorders, which might impact blood saturation. For that reason, oximeter readings taken at home should never be relied on for diagnostic purposes or to determine treatment. If you are concerned by your at-home readings, seek medical advice to understand what those readings indicate in the larger context of your health.