OXYGEN SATURATION normal, high and low values

Oxygen saturation

What is blood saturation and what is it for

In the health sector, when we speak generically ­of oxygen saturation in the blood (SaO2 or SpO2) means the relationship, expressed in percentage, between the'hemoglobin tied oxygen and the total hemoglobin circulating within arterial blood vessels. It is therefore a numerical value that can theoretically vary from 0 to 100 and that can undergo slight fluctuations even in normal conditions based on age and breathing quality.

Measuring saturation can be of great importance as a method of monitoring bloodless (through the use of a pulse oximeter, also called oximeter by finger or oximeter) of any changes in blood oxygenation in subjects with respiratory problems. A reduced quality of breath, as present in people with pathologies respiratory, leads to a minor ventilation of the pulmonary circulation and therefore to a reduction of gas exchange in the alveoli; this reduced exchange leads to the impossibility of hemoglobin to bind the molecules of oxygen and therefore of to carry these same molecules towards the tissues of the organism, thus compromising the fundamental function energy oxygen.

In particular, monitoring of saturation levels is especially useful in acute cases of pulmonary edema, in asthmatic attacks and exacerbations of COPD, or in particularly severe pneumonia. The saturation value is relevant to understand the severity of the subject's respiratory picture (the degree of hypoxemia), on the basis of which a more or less aggressive therapy against the disease can be established.

Principles of operation and method of measurement

In order to fully understand the meaning of saturation and the working principle of the saturation meter (pulse oximetry), some basic concepts of physiology, biochemistry and of physics, which will be outlined below in a general way:

  • Concepts of physiology and biochemistry. Oxygen (O2), essential for the production of energy within all cells of the body, is transported by hemoglobin and spread throughout the body through the blood. Hemoglobin it's a protein contained in large quantities within red blood cells (erythrocytes), and is able to bind four oxygen molecules through its four subunits (two alpha chains and two beta chains); the moment it binds the oxygen molecules, the protein undergoes a modification conformational (it slightly changes its structure compared to what it has when it does not bind oxygen molecules).
  • Concepts of physics. A determined protein, when subjected to a beam of light of a specific wavelength, it interacts with this beam absorbing it. Through this phenomenon it is possible to measure the quantity of a protein present in a liquid by projecting a beam of light and verifying how much light can pass.

To measure saturation oxyhemoglobin arterial can then be used a pulse oximeter: it is a small electronic tool that is usually applied to the finger and which, through the issue and the detection (through a sensor) of details beams of light, manages to estimate the ratio between oxygen saturated hemoglobin and unbound hemoglobin. The wavelengths that are used are 660 nanometers (red light, visible) and 940 nanometers (infrared light, not visible): these two values were decided on the basis of experiments, in which it was established that at these two wavelengths maximum the sensitivity instrument in detecting the amount of bound and unbound hemoglobin in the blood.

Hand-held finger pulse oximeter, used for a quick and fairly accurate measurement of blood oxygen saturation.

Hand-held finger pulse oximeter, used for a quick and fairly accurate measurement of blood oxygen saturation.

Limits of the finger oximeter

Unfortunately, under some conditions, the saturation value can not be reliable o not be useful for clinical investigation purposes:

  • Anemia: anemia refers to a reduced concentration of hemoglobin in the blood; when hemoglobin is low, it is more easily saturable by oxygen as fewer molecules are needed to bind all the proteins. Therefore, in the presence of anemia, a high saturation value can still indicate a picture of low gas exchange capacity in the lungs.
  • Carbon monoxide poisoning (CO): carbon monoxide is a molecule with a very high affinity for hemoglobin (an affinity 200 times higher than that of oxygen), but it cannot play the same role that oxygen plays in tissues; furthermore, the carboxyhemoglobin that is formed is not even able to carry oxygen. It can therefore lead to a condition of even severe hypoxia, up to death. The finger pulse oximeter not is able to distinguish oxygen-binding hemoglobin from carbon monoxide-binding hemoglobin; for this it is possible that the saturation is maximum although in reality the oxygen bound to the hemoglobin is little.
  • Absence of flow: in order to calculate saturation, the instrument needs to be there variations into the bloodstream. These normally occur in the healthy subject thanks to the pulsations determined by the heartbeat and the patency and adequacy of the flow at the level of the fingers. In the presence of an inadequate heart rate (heartbeats or bpm) or people with very pale or cyanotic fingers it is possible that the flow variations are insufficient to calculate a correct saturation.
  • Presence of enamel: Since the measurement is made at the level of a finger of the hand and is based on the passage of light, the presence of colored enamel can interfere with a correct measurement. In these cases, it is possible to position the oximeter sideways (with the two surfaces on the sides of the nail and not above and below) or position it at the level of a lobe ear.

Normal values of oxygen saturation

First of all it is good to specify that the normal values of arterial saturation depend, in part, on theage of the subject, by the presence of pathologies pulmonary and from habit al smoke; it is therefore important to understand what are the saturation values to which the individual is accustomed and to be alarmed only if the saturation undergoes a decrease quick, within minutes or hours.

In the children, the saturation should be higher than 97%, usually around 99-100%, while in the adults even slightly lower saturations, above 95%, are acceptable, especially if smokers. In the Senior citizens reference values are also slightly lower than 95% especially as there is a high prevalence of COPD (chronic obstructive pulmonary disease), in which the normal values of oxygen saturation they may even be a few points above the 90%.

Low saturation values, lower than 90%, are almost always pathological and it is therefore necessary to investigate the possible cause and treat them quickly.

Moglobin oxygen saturation

Oxygen saturation curve of hemoglobin contained in red blood cells circulating in our blood.

Low oxygen saturation: causes and symptoms

There lack of oxygen it can depend on numerous pathological states that can determine a decreased breathing function, a reduced gas exchange surface inside the lungs or a reduced pulmonary perfusion flow. Diseases are mainly affecting the airways or lungs, and more rarely affecting the heart or vessels.

The most frequent pathologies in which there is a reduction in saturation are:

  1. COPD: in subjects affected by this disease (up to 15% of individuals over 55 years of age), due to the alteration of air flows, it is normal to find saturation values even lower than 93%; in the case of exacerbations of the disease (episodes in which there is a further acute decrease in respiratory function) the saturation may be even lower and it is necessary to treat the condition with bronchodilators and oxygen therapy (chronic oxygen therapy, with flows of 1-2 L / min, is related to an increase in patient survival).
  2. Asthma: subjects affected by asthma (often young people) can experience acute episodes in which there is a worsening, even serious, of the air flows inside the airways. It is always necessary, in these cases, to administer a bronchodilator and eventually oxygen.
  3. Lung infections: one of the most common causes of desaturation are lung infections. These can involve different levels of the upper or lower airways, but the ones that most easily lead to a reduction in the oxygenation capacity of the blood are pneumonia, for exudative filling of the alveoli and therefore reduction of the gas exchange surface, the bronchitis and the bronchiolitis (especially in children less than one year of age), due to a reduction in air flow.
  4. Airway obstructions: especially in children, it is possible that particularly small objects can enter the airways causing a sudden reduction in the flow of ambient air. Obviously the most important "therapy" is the removal of the foreign body.
  5. Pulmonary edema: this picture can depend on some different pathologies, and can present with different degrees of severity. In all cases there can be a significant reduction in saturation, which can reach values below 90%.
  6. Pulmonary embolism: Pulmonary embolism consists of an acute reduction in arterial flow in the lungs. In this case, the extent of the desaturation essentially depends on the volume of the vascular bed affected by theocclusion and therefore from the non-perfused lung volume.

Regarding the symptoms of low oxygen saturation, essentially the presence of low saturation may be accompanied by dyspnea (difficulty in breathing), respiratory failure (insufficient or inadequate breathing), cyanosis (bluish discoloration of the mucous membranes or extremities), confusion or drowsiness.

To cope with reduced oxygenation, subjects often tend tohyperventilation (increased breathing rate), which is often enough to compensate for thehypoxemia; however, following the increase in gas exchanges, a condition of hypocapnia (reduced amount of carbon dioxide in the blood, or PCO2).

High oxygen saturation

A saturation picture high in reality it is not contemplated, as the saturation is usually already at its maximum or slightly lower. However, it is possible to speak of excessively high saturation in subjects suffering from COPD, in which the saturation is normally lower than the maximum and can be more harmful increase it excessively so acute through the administration of oxygen (target values are estimated between 88 and 92%); this is because patients are used to hypoxemia. Also, as already discussed, patients in whom poisoning is suspected carbon monoxide they can have a maximum saturation (100%) although oxygenation is actually insufficient.

Difference between blood saturation and oxygenation

At the hospital level, in addition to measuring saturation, it is also possible to measure pressure partial of oxygen (PaO2) in the blood. The latter is a much more accurate measure of the amount of oxygen present in the arterial circulation, but is carried out through a method invasive which blood gas analysis arterial; this consists in the puncture of an arterial vessel (almost always the radial artery of the wrist) and in taking a small sample of arterial blood. Through the detection of PaO2 it is possible to determine the picture of insufficiency respiratory, which consists of a PaO2 lower than 60 mmHg; it is not possible to speak of respiratory insufficiency in the absence of this value and on the basis of it alone oximetry, although it is normally assumed that saturation values equal to or less than 90% may be related to a PaO2 of less than 60 mmHg.


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