What are the principles of oximetry ?

Oximetry is a non-invasive, painless test that measures the level of oxygen saturation in your blood. It can quickly analyze how efficiently oxygen is delivered to the parts of the body furthest from the heart, such as the fingers and toes. Oxygen saturation should generally be above 95%. However, saturation varies from person to person. If you have a respiratory disease or heart disease you will have an oxygen saturation of less than 95%.

Colorimetric oximetry is done with a pulse oximeter. It is a small clip that is placed on a finger, a toe or an earlobe. Pulse oximeters are used in hospitals for patients undergoing anesthesia, in monitoring rooms after medical procedures, in emergency rooms, in intensive care units, by certain physicians such as pulmonologists, and by individuals.

Oximetry can be used to monitor people with all types of conditions that affect oxygen in the blood, such as:

Chronic obstructive pulmonary disease (COPD),
Asthma,
Pneumonia,
Lung cancer,
Anemia,
Heart attack or heart failure,
Congenital heart defects.
This method can be used in several situations, including:

To determine if a lung medication is having the desired effect,
To determine if a person needs help breathing,
To monitor oxygen levels after or during surgery,
To identify the effectiveness of oxygen therapy,
To determine if breathing capacity is sufficient for physical activity,
To analyze apneas during your sleep. This is called nocturnal oximetry.

How does an oximeter work?
The oximeter consists of a sensor, a monitor and a cable. The sensor, in the form of a clip, is most often placed on the fingertips. But it can also be used on the toes or earlobes. There are sensors adapted to each area.

In our blood vessels, hemoglobin circulates in a form charged with oxygen and in a form discharged with oxygen. The color of the blood varies according to the amount of oxygen it contains. It is the two beams of red and infrared light of the saturometer that will allow the detection of the level of oxygen present in the blood.

In fact, these two beams of light allow the pulse oximeter to detect the color of the arterial blood and to determine the oxygen saturation. However, there are many areas of the finger that will absorb light (such as venous blood, bones, skin, muscles, etc.), so to determine the color of arterial blood, a pulse oximeter looks for the color change caused by a heartbeat pushing arterial blood into the finger.

This color change is very small, so pulse oximeters work best when there is a strong pulse in the finger. If the signal is too weak, the measured oxygen saturation may be unreliable and lower than the true saturation.

How to read an oximeter?
It is very easy to decipher the results obtained during a pulse oximetry test.

The pulse oximeter has a screen that displays two statistics. The first is "PR", which stands for pulse rate or heartbeat per minute. The second statistic is represented by the "SpO2" indicator, the abbreviation used for oxygen saturation. The oximeter has a large OLED display, backlit to give you good visibility.

If you use an oximeter at home, it is important to record the values you get in a logbook, specifying the date and time of the pulse oximetry test. This way, when you go to see your doctor, you can show him or her your results to keep him or her informed about your health. Most healthy people have an SpO2 reading of 95% with a PR of 70 to 80 beats.

Here are the oxygen saturation ranges that determine if your oxygen level is correct:

Between 94 and 98 percent: oxygen saturation is good,
Between 90 and 93%: it is said to be "good enough",
Below 90%: it is insufficient. This is called desaturation.