The SpO2 Monitor – An Alternative Name for the Pulse Oximeter

If you’re interested in oxygen monitoring and plan to acquire your own pulse oximeter, you have probably heard the term SpO₂. What is SpO₂ and how exactly does it relate to the blood oxygen monitor?

SpO₂ means Saturation of Peripheral Oxygen. In the medical world, this term is used to describe the blood oxygen saturation readings obtained non-invasively via pulse oximetry. If the oxygen levels are measured by using arterial blood tests, the term SaO₂ is used.

Each type of pulse oximeter (fingertip, handheld or tabletop model) has on its display the SpO₂ label. Next to it, after each measurement, the device will display the real-time oxygen levels. The normal SpO₂ reading of a healthy, non-smoking person at sea level ranges between 95% and 99%. If the oxygen levels drop below 90%, it can be a sign of respiratory failure or hypoxia.

Because it is a device which measures the SpO_2, the blood oxygen monitor is also called SpO₂ monitor – so when you see this term on the internet or in your physician’s office, you will know that it is another name of the pulse oximeter.

The oxygen saturation in the blood represents the ratio of oxyhemoglobin (the hemoglobin saturated with oxygen) to the total hemoglobin concentration present in the bloodstream. Hemoglobin is an iron-containing protein found in the red blood cells; its main function is to transport oxygen from the lungs to all the other parts of the body, supporting the normal functioning of all the cells of the organism.

How is the SpO₂ calculated? The SpO₂ reading is calculated by using a formula based on the ratio of absorption of two different light frequencies: red and infrared. Oxyhemoglobin (HbO₂) absorbs infrared light, while red light is absorbed by unsaturated hemoglobin.

For obtaining the results, the SpO₂ monitors are equipped with two light emitting diodes (LED) which generate the red and infrared light frequencies through a translucent body part (finger, earlobe, toe or the bridge of the nose). A clipped fingertip oxygen monitor functions as follows: on one side of the device the light wavelengths will be emitted: after passing through the finger tissues, the light will reach the photosensor on the other side of the monitor. After interpreting the absorption rate of both light frequencies, the oximeter will display the results as a SpO₂ percentage.

Simply put, SpO₂ readings depend first of all on the quality of the breathing process of the person and the strength of his/her blood circulation. However, there are many other factors that can influence one’s blood oxygen levels: the percentage of oxygen in the ambient air, altitude, physical effort, age and certain medical conditions such as respiratory (asthma, pneumonia, emphysema) or cardio-vascular diseases.

The SpO₂ monitor and pulse oximetry are not the only way of measuring the oxygen saturation levels and the pulse rate (or perfusion index). For this purpose, specialists use other methods like the CO-oximeter (or the CO₂ monitor), the Capnometer or the arterial blood gas analysis (ABG). I’ll write more about each of these methods in my future posts.

However, even if all these other methods offer extensive and reliable results, they are intended for clinical use, being time-consuming and expensive. The main advantages of an oxygen monitor (or the SpO₂ monitor) are speed, convenience and affordability.