The measurement of the rate of oxygen saturation is an examination which makes it possible to evaluate the function of hematosis: the oxygenation of the blood. This analysis of oxygen saturation is particularly used in people with respiratory pathology.

The blood supplies oxygen to all tissues and carries carbon dioxide to the lungs for elimination from the body. A small amount of oxygen is carried by the plasma. Most of it is carried by hemoglobin in red blood cells.

Blood oxygen is expressed in three ways:

• the percentage of saturation of its main transporter hemoglobin (SaO2),
• the pressure exerted in the dissolved blood (PaO2)
• its amount in the blood (CaO2).

In respiratory dysfunction, the blood contains less oxygen and more carbon dioxide. The degree of oxygenation can be measured with two means: oxygen saturation (SaO2, measured in arterial blood, SpO2 measured by a pulse oximeter or saturometer) and oxygen partial pressure (PaO2).

Oxygen saturation (SaO2) represents the percentage share of hemoglobin saturated with oxygen (oxyhemoglobin) in relation to the total amount of hemoglobin present in the blood. Oxygen saturation is measured to assess the function of hematosis: oxygenation of the blood.

The rate of oxygen saturation can be measured in two ways:

By taking arterial blood (blood gas measurements).

This involves taking a blood test from an artery. It is the only technique which allows a reliable and definitive measurement of blood gases. The realization of an arterial gas measurement allows the analysis of the acid-base balance (pH) and the measurement of the arterial pressure in oxygen (PaO2) and that of carbon dioxide (PaCO2) which makes it possible to know the state respiratory. The saturation of hemoglobin with oxygen measured by arterial blood sampling is expressed in Sao2. Oxygen saturation is measured directly in red blood cells.

With a pulse oximeter or saturometer (easiest way to use)

A pulse oximeter or oximeter is a device that non-invasively measures the oxygen saturation of the blood. This device is very frequently used in hospitals to monitor patients who have respiratory distress or who are on invasive or non-invasive ventilatory supports (oxygen therapy). It is equipped with an emitter and a light receiver which makes it possible to determine the blood oxygen saturation.

It transmits a ray of light through tissue, most commonly a finger or toe in adults, but also the nose or earlobe, or the hand or foot in young children. The oxygen saturation of hemoglobin measured by pulse oximetry is expressed as SpO2 (the p signifying pulsed saturation). We speak of pulsed saturation of hemoglobin with oxygen.

There are several indications for measuring the rate of oxygen saturation by saturometer in adults:

• during anesthesia or in the monitoring room after an operation
• In emergency medicine departments
• In intensive care, especially for people placed on ventilation or likely to be.

In children, the measurement of the oxygen saturation rate also has several indications:

• assessment of the severity of a respiratory pathology (bronchiolitis, pneumonia, asthma, etc.)
• assessment of the severity of infant bronchiolitis; a saturation of less than 94% is one of the severity indicators
• evaluation of the effectiveness of an aerosol
• detection of a possible heart disease in a cyanotic newborn

Arterial gas measurement is performed in the presence of a severe respiratory state and in the presence of a suspicion of a major metabolic disorder.

Normal oxygen saturation for a healthy person is between 95% and 100% depending on age. SpO2 (Pulsed saturation measured with a pulse oximeter). It is insufficient below 95%. We are talking about hypoxemia. The concept of hypoxemia applies to any insufficiency of oxygenation of the blood and therefore as soon as the SpO2 is less than 95%. The 90% limit marks hypoxemia corresponding to the equivalent of respiratory failure.

Normal arterial oxygen saturation (SaO2) is between 96% and 98% in a young adult is 95% in a person over 70 years old. When it is less than 90%, the person is said to be in desaturation. Desaturation also corresponds to a drop of 4 saturation points compared to the base value (for example during an effort).

A “normal” SpO2 for a child corresponds to a value greater than 95%. A SpO2 level of less than 94% in a child is a criterion of seriousness and leads to hospitalization. Measuring SpO2 is very important in children, because a child only appears cyanotic (bluish complexion) when the SaO2 is less than 75% and because arterial gas measurements are rarely performed in children. The pulse oximeter is essential for detecting early hypoxia.

We speak of hypoxemia when the oxygen saturation value is less than 93%. The main risk is that of cellular suffering (ischemia) resulting from an insufficient supply of oxygen to the various tissues of the body. Acute hypoxemia can occur following an acute exacerbation of asthma, acute heart failure, pneumonia or an acute exacerbation of Chronic Obstructive Pulmonary Disease (COPD), following pulmonary embolism, pleural effusion, pneumothorax .

Hypoxemia (oxygen saturation rate less than 93%) is manifested by shortness of breath, rapid shallow breathing, bluish skin (cyanosis) but all of these signs are less specific and sensitive than pulse oximetry.

COVID-19 can cause a low oxygen saturation rate. The most severe cases of COVID can cause pneumonia causing acute respiratory distress syndrome. The symptoms are quite subtle at first. This is why doctors can monitor the level of oxygen saturation with an oximeter. Difficulty breathing and shortness of breath are signs that you should call emergency services.

Warning: Using a pulse oximeter also presents risks of errors and it is best to learn how to use it with a healthcare professional.

Too much oxygen supply during oxygen therapy can lead to hyperoxia. Hyperoxia is dangerous for people with respiratory failure.

In the event of hypoxemia (oxygen saturation less than 93), treatment with oxygen therapy may be implemented. Oxygen can be administered by the nasal route (glasses) or by the nasal and oral routes (masks) but also by artificial ventilation (ventilator, intubation) or by extracorporeal circulation (ECMO). The amount of oxygen delivered is guided by arterial blood gases or pulse oximetry to maintain Pao2 between 60-80 mmHg (92-100% saturation) without causing oxygen toxicity.