Oximetry is an exam that allows measuring the oxygen saturation of the blood, that is the percentage of oxygen that is being transported in the bloodstream. This test is important when diseases that impair or interfere with the functioning of the lungs, heart disease or neurological diseases, for example, are suspected.
Generally, oximetry above 90% indicates good blood oxygenation, however, it is necessary for the doctor to evaluate each case. A low blood oxygenation rate may indicate the need for treatment at the hospital with oxygen, and may indicate a life-threatening condition if not properly corrected. Understand the consequences of lack of oxygen in the blood .
There are two ways to measure oxygen saturation:
1. Pulse oximetry (non-invasive)
This is the most used way to measure oxygen saturation, as it is a non-invasive technique that measures the amount of oxygen through a small device, called a pulse oximeter, which is placed in contact with the skin, usually at the tip of the finger.
The main advantage of this measure is that it is not necessary to collect blood, avoiding bites. In addition to oximetry, this device may also be able to measure other vital data, such as the amount of heartbeat and respiratory rate, for example.
- How it works: The pulse oximeter has a light sensor that captures the amount of oxygen that passes in the blood under the place where the test is being done and, in a few seconds, indicates the value. These sensors take immediate, regular measurements and are designed to be used on the fingers, toes or ear.
Pulse oximetry is widely used by doctors and other health professionals during clinical evaluation, especially in cases of diseases that cause difficulty in breathing, such as pulmonary, cardiac and neurological diseases, or during anesthesia. The oximeter can also be purchased at medical or hospital supply stores, and is available in different brands and prices.
2. Oximetry / arterial blood gases (invasive)
Unlike pulse oximetry, arterial blood gas analysis is an invasive way to measure the rate of oxygen in the blood, as it is done by collecting blood into a syringe, and for this a needle stick is necessary. For this reason, this type of examination is less frequent than pulse oximetry.
The advantage of arterial blood gases is a more accurate measure of oxygen saturation levels in the blood, in addition to being able to provide other important measures, such as the amount of carbon dioxide, pH or amount of acids and bicarbonate in the blood, for example.
- How it works: it is necessary to perform an arterial blood collection and then this sample is taken to be measured in a specific device in the laboratory. The blood vessels most used for this type of measurement are the radial artery, in the wrist, or femoral, in the groin, but others can also be used.
This type of measurement is usually used only in cases where the patient needs to be monitored continuously or more accurately, which is more common in situations such as major surgery, severe heart disease, arrhythmias, generalized infection, sudden changes in pressure blood pressure or in cases of respiratory failure, for example. Learn what respiratory failure is and how it can decrease blood oxygenation .
Normal saturation values
A healthy person, with adequate oxygenation of the body, usually has an oxygen saturation above 95%, however, it is common that due to mild conditions, such as colds or flu, the saturation is between 90 and 95%, without reason of concern.
When the saturation reaches values below 90%, it may indicate the presence of some more serious disease that is capable of affecting the oxygen values in the body, such as asthma, pneumonia, emphysema, heart failure or neurological diseases, for example.
In arterial blood gases, in addition to the measurement of oxygen saturation, partial oxygen pressure (Po2) is also evaluated, which must be between 80 and 100 mmHg.
Care for a more accurate result
It is very important that the devices that measure oxygen saturation are calibrated regularly, to avoid altered results. In addition, when using the pulse oximeter, some precautions to avoid altering the exam include:
- Avoid using enamel or false nails, as they alter the passage of the light sensor;
- Keep the hand relaxed and below the level of the heart;
- Protect the device in a very bright or sunny environment;
- Make sure that the appliance is properly positioned.
Before taking the exam, the doctor should also investigate other diseases such as anemia or impaired blood circulation, which can interfere with the measurement of blood oxygenation.
