Cyanosis (skin lividity).

Cyanosis (Greek kyanos - dark blue) is a bluish tint of the skin and mucous membranes caused by an increase in the amount of reduced (deoxygenated) hemoglobin or its derivatives in small vessels of certain parts of the body. Cyanosis is usually most noticeable on the lips, nail beds, earlobes, and gums.

Cyanosis occurs in two situations: when the saturation of capillary blood with oxygen decreases and when venous blood accumulates in the skin due to the expansion of the venous section of the microcirculatory bed.

The presence of cyanosis directly depends on the total hemoglobin content in the blood: when it changes, the content of reduced hemoglobin also changes.

• In anemia, the content of both total and reduced hemoglobin is reduced, therefore, in patients with severe anemia, even with severe hypoxia, cyanosis is usually absent.
• In polycythemia, the content of both total and reduced hemoglobin is increased, so patients with severe polycythemia are usually cyanotic. The same mechanism causes cyanosis of a particular part of the body with local stagnation of blood in it, which is often accompanied by edema.

Cyanosis can be central and peripheral.

Central cyanosis

Central cyanosis is characterized by a bluish tint of both the skin and mucous membranes, and occurs when arterial blood is insufficiently saturated with oxygen or when altered forms of hemoglobin appear in the blood. Central cyanosis increases during physical exertion, since the muscles' need for oxygen increases, and blood oxygen saturation is impaired for various reasons.

Violation of arterial blood oxygen saturation occurs in the following situations.

• Decreased atmospheric pressure (high altitude).
• Disorders of lung function leading to a decrease in alveolar ventilation, perfusion of non-ventilated areas of the lungs, a decrease in perfusion of adequately ventilated areas, as well as a violation of the diffusion capacity of the lungs.
• Blood shunting in the presence of anatomical anomalies, i.e. "dumping" blood from the venous bed into the arterial bed, bypassing the microcirculatory bed of the alveoli, thereby oxygen-saturated blood is "diluted" with blood that has already given up oxygen at the periphery. This mechanism of cyanosis is characteristic of some congenital heart defects (for example, tetralogy of Fallot - shunting of blood from right to left through the ventricular septal defect (VSD) against the background of pulmonary artery stenosis). A similar situation is also possible in the presence of arteriovenous pulmonary fistulas or small intrapulmonary shunts.

Among the changes in hemoglobin itself, methemoglobinemia and sulfhemoglobinemia should be mentioned, which can be assumed after excluding all other probable causes of cyanosis.

Peripheral cyanosis

Peripheral cyanosis is the result of slowing blood flow in a particular area of the body. In this case, the saturation of arterial blood with oxygen is not impaired, but due to blood stagnation, more oxygen is "extracted" from it, i.e. the local content of reduced hemoglobin increases. This type of cyanosis occurs with vasoconstriction and a decrease in peripheral blood flow.

The causes of peripheral cyanosis are listed below.

• A decrease in cardiac output, such as in congestive heart failure, results in narrowing of the small arteries and arterioles of the skin, which acts as a compensatory mechanism aimed at centralizing blood circulation to supply blood to vital organs - the central nervous system, heart and lungs. It is for this situation that the term "acrocyanosis" is used - a bluish coloration of the distal parts of the body due to venous congestion, most often against the background of blood stagnation in the systemic circulation.
• Peripheral cyanosis is typical for venous hypertension. It occurs with obstruction of the veins of the extremities (against the background of varicose veins, thrombophlebitis), which leads to edema and cyanosis.
• Peripheral cyanosis when exposed to cold is the result of a physiological reaction of the body.
• Obstruction of the arteries of the extremities, for example, in embolism. In this case, pallor and coldness are more typical, but mild cyanosis is possible.

In many cases, the clinician is faced with the question of differential diagnosis of central and peripheral cyanosis.

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