Pulse Oximetry : what it is, how it is performed and normal values

Oximetry is a test capable of measuring blood oxygen saturation, which is the percentage of oxygen being transported in the bloodstream. This measure is usually necessary when diseases that harm or interfere with the functioning of the lungs are suspected, such as asthma, emphysema, pneumonia, lung cancer, pulmonary congestion or neurological diseases.

Generally, when oximetry results in an oxygen saturation above 90%, it indicates that there is good oxygenation of the blood, however, it is necessary for the doctor to evaluate each case individually. A low blood oxygen saturation may indicate the need for treatments such as catheters or oxygen masks, and in case the indicated treatment is not performed it can be a risk to the life of the individual.

There are several ways to measure oxygen saturation and these are:

1. Pulse Oximeter

It is the most used way to measure the amount of oxygen in the blood, where small devices called pulse oximeters are used, which make this measurement only with the pulse contact when placed on a finger or in the earlobe.

The main advantage of this measure is the fact that it is not invasive since it is not necessary to inject or collect a blood sample from the patient. In addition to oximetry, this device is also capable of measuring other vital data such as the amount of heartbeat and respiratory rate.

How it works: The pulse oximeter has a light sensor that captures the amount of oxygen in the blood of the arteries and the frequency of the heartbeat only with contact with the skin, indicating the value in a few seconds. These sensors make immediate and regular measurements and are developed for use on the fingers, toes or ears.

Pulse oximetry is widely used by doctors and other health professionals during clinical evaluation, mainly in cases of diseases that cause difficulty in breathing such as lung, heart, and neurological diseases, or during anesthesia.

The oximeter can be purchased at medical or hospital supplies stores and is available in different brands and prices. It is recommended to obtain reliable marks to avoid measurement errors.

2. Arterial Blood gas

An arterial blood gas is an invasive way to measure the rate of oxygen in the blood because it is done through the collection of blood in a syringe.

The advantage of arterial gasometry is a more accurate measure of blood oxygen saturation levels, in addition to providing other important measures, such as the amount of carbonic gas, electrolytes such as sodium and potassium, pH or amount of acids and bicarbonate in the blood, for example.

How it works: For blood gasometry, it is necessary to collect a blood sample in a syringe and then this sample is taken to be measured in a specific device. The most used blood vessels for this type of measurement are the radial or femoral artery, but other arteries can also be used.

This type of measurement is usually used in cases where the patient needs to be monitored continuously or more accurately, as it is in performing major surgeries, serious heart disease, arrhythmias, generalized infection, sudden alterations in blood pressure or in cases of respiratory failure.

Normal Values of Oximetry

A healthy person, with adequate oxygenation of the body, usually has an oxygen saturation above 95%, however, it is common that some mild conditions such as flu or colds, oxygenation can be between 90 and 95%, not being a matter of concern.

When saturation reaches values lower than 90%, it may indicate that the oxygenation of the blood is poor, which may arise in diseases such as asthma, pneumonia, emphysema, heart failure or neurological diseases, for example.

In arterial gasometry, in addition to measuring oxygen saturation, the partial pressure of oxygen (Po2), which must be between 80 and 100 mmHg, is also evaluated.

However, the evaluation of the doctor or health professional is always necessary, since other clinical data need to be evaluated to reach the conclusion about the cause and how the treatment should be performed.

Care During Oximetry

It is very important that the devices that perform this measurement are regularly calibrated, to avoid alterations in the results. In addition to this, during the use of the pulse oximeter, some care should be taken such as:

• Avoid the use of enamel or false nails, since they alter the passage of the light sensor;
• Protect the device if it is in a very bright or sunny environment;
• Observe that the device is well-positioned.

In addition to this, the doctor should investigate all diseases such as the presence of anemia or deficiencies of blood circulation, as they are conditions that can interfere with the measurement of oxygen found in the blood.