What is a pulse oximeter, how does it work, and how to read its results?

A pulse oximeter is a device designed to measure the level of blood oxygenation, which is a standard procedure for patient care in hospitals and ambulances. Due to the simple mechanism of action and quick results, this medical tool is also increasingly often used at home.

What is a pulse oximeter? 

A pulse oximeter is an electronic device that measures the saturation of blood with oxygen (oxygen saturation). Another parameter measured by a pulse oximeter is a person's heart rate or pulse.

Pulse oximeter - how does it work and when should it be used? 

This inconspicuous-looking device works on the principle of pulse oximetry - it which emits light of different wavelengths: 

660 nm (red light),

940 nm (infrared light).

Hemoglobin absorbs these wavelengths to a different extent depending on whether it transports oxygen (oxyhemoglobin) or is deoxygenated (reduced hemoglobin). Based on the differences in absorption of radiation by hemoglobin, the device uses the absorption constant to calculate the level of oxygen saturation, expressed as a percentage value. Pulse oximetry also uses the presence of a pulsating signal that is produced by the arterial blood in the finger. 

Pulse oximeter - application 

Pulse oximeters are used mainly in emergency departments, intensive care units, operating theaters, and in ambulances, mainly to monitor the patient's condition in the event of breathing difficulties and during and after general anesthesia. Hospitals tend to use larger devices than the "pocket" models available on the market. Larger models are able to test blood oxygenation not only from the finger but also from the earlobe.  

Pulse oximeters are used in patients of all age groups - from newborns to the elderly. In young children, pulse oximeters are used in the form of a band with a sensor, which is put on the wrist or foot. These bands often allow for the constant measuring of the child's body temperature, even at night, which is particularly useful during the course of an infection. 

Pulse oximeters are also used by athletes who practice sports at high altitudes, such as mountain climbing, skiing, and skydiving. 

Stocking up on a home pulse oximeter is especially recommended for people suffering from respiratory or cardiovascular diseases, as well as for elderly patients with chronic diseases. People undergoing oxygen therapy and at risk of sleep apnea should also consider having this kind of device at home. 

How to read and interpret measurement results? 

The result of a pulse oximeter measurement can warn you of hypoxemia (hypoxia), meaning too low oxygen level in the arterial blood.  

Insufficient oxygen in the blood can cause irreversible damage to many organs and can lead to death. 

Pulsoksymetr – normal measurement results 

The level of hemoglobin oxygen saturation ranging from 95% to 99% is considered a normal result. At higher altitudes, these results may be lower, so pulse oximeters are calibrated between 70% and 100% typically with an accuracy of 2% to 4%. This means that readings below 70% saturation may be inaccurate. 

A finger pulse oximeter is placed over the index or middle finger with the fingernail facing up. During the measurement, you should refrain from moving. Be aware that the presence of nail polish or fake fingernails may interfere with proper pulse oximeter readings. Inaccurate results may also be caused by the patient's movement during the examination, as well as the patient having cold limbs. 

Types of pulse oximeters. Which one to choose? 

Conventional pulse oximeters read the oxygenation level of hemoglobin at two wavelengths, thus distinguishing oxyhemoglobin from reduced, i.e. oxygen-free hemoglobin. More sophisticated, newer devices can also determine the presence of methemoglobin and carboxyhemoglobin from different wavelengths. Their presence also influences the final oxygen saturation results. 

Due to the fact that the accuracy of pulse oximeters is tested mainly on healthy volunteers who are exposed to only slight hypoxia, the accuracy of pulse oximeters is only guaranteed in certain oxygen saturation ranges (usually 75% and higher). In the case of severe hypoxemia, the measurement results may not be as accurate. 

Unfortunately, there is not enough available research to exhaustively compare specific models of pulse oximeters and determine which one is the best. The purchased equipment should have the status of a medical device with a CE certificate. 

Finger or wrist pulse oximeter? 

In addition to finger pulse oximeters, or "pocket" medical devices, there are also special smartphone apps or wrist pulse oximeters placed in watches, smartwatches, and sports bands. These systems use a different mode of action, one based on the hemoglobin's reflective properties. Most studies have shown that these systems have significantly reduced accuracy. The lower the actual oxygen saturation of the patient's hemoglobin, the less accurate the results obtained, even with mild hypoxemia.  

If you feel short of breath, but your device shows a normal oxygen saturation result, do not hesitate to contact a doctor.    

Pulse oximeters and COVID-19 

Patients suffering from COVID-19 may develop what is known as "silent hypoxia," which is a state in which the patient does not feel or realize that they are hypoxic. The results of the pulse oximeter measurement may allow the patient to react quickly enough to prevent complications related to the possible hypoxia. In some countries, including Poland, asymptomatic or paucisymptomatic patients with confirmed results for the presence of the SARS-CoV-2 virus receive home pulse oximeters from the state. 

Pulse oximeters are commonly used in hospitals and ambulances primarily to monitor the condition of patients in order to detect possible hypoxemia as early as possible. These devices are also used at home, mainly by people exposed to hypoxia, but also for preventive purposes, for example by athletes. Despite many factors that may affect the accuracy of this type of device, it is worth having one at home. Remember that if you ever feel weak or have shortness of breath, despite the device showing a normal saturation result, you should contact a doctor.