Peripheral Cyanosis // Practical viva // Clinical
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Introduction
Cyanosis is a pathologic condition that is characterized by a bluish discoloration of the skin or mucous membrane. The word cyanosis is a derivative of the word cyan, a blue-green color. The presence of cyanosis can pose a serious diagnostic challenge. A careful and thorough evaluation with the proper diagnostic tools can help discern the cause.
Etiology
Cyanosis, broadly speaking, is caused by disorders of deoxygenated hemoglobin and disorders of abnormal hemoglobin. Oxygen might not reach hemoglobin in an adequate or sufficient amount as a result of conditions affecting the respiratory system, cardiovascular system, and the central nervous system (CNS).
Disorders of deoxygenated hemoglobin are further divided into two broad groups: central cyanosis and peripheral cyanosis. Central cyanosis occurs when the level of deoxygenated hemoglobin in the arteries is below 5 g/dL with oxygen saturation below 85%. The bluish hue is generally seen over the entire body surface and visible mucosa. In contrast, peripheral cyanosis is usually only seen in the upper and lower extremities where the blood flow is less rapid. In peripheral cyanosis, there is a significant difference in the saturation between the arterial and venous blood. This occurs as a result of increased oxygen extraction by the peripheral tissue in the capillary bed. Low cardiac output, venous stasis, and exposure to extreme cold causing vasoconstrictions are some of the conditions that can cause peripheral cyanosis.
Common Causes of Central Cyanosis
Pulmonary
Impaired gas exchange secondary to pneumonia
Embolism and ventilation perfusion mismatch
Impaired gas diffusion via the alveoli
High altitude
Anatomic shunts
Right to left shunt in congenital heart disease
Arteriovenous malformation
Intrapulmonary shunt
Furthermore, cyanosis can be caused by the presence of abnormal hemoglobin. Hemoglobin is the major carrier of oxygen in the blood. It is made up of four subunits. Each of the four subunits is made of polypeptide chains, two alpha and two beta. At the center is a heme group which contains iron. The presence of abnormal hemoglobin causes significant impairment in the oxygen-carrying capacity of the blood. This can cause tissue hypoxia which can manifest clinically as cyanosis.
Methemoglobinemia is a condition that can produce congenital or acquired cyanosis. The condition arises when the iron in hemoglobin is converted from the ferrous (Fe2+) to the ferric (Fe3+) state. Approximately 2% of hemoglobin is present in this form. The presence of methemoglobin can impart a bluish tinge to the skin color. Methemoglobinemia can be triggered by exposure to the topical anesthetic agent dapsone, nitroglycerin, or other strong oxidizing agents. Congenital methemoglobinemia type I and II is an autosomal recessive condition that is caused by a mutation in the gene for cytochrome b5 reductase enzyme. The condition is extremely rare. The lack of enzymatic activity by cytochrome b5 reductase causes decreased reduction of methemoglobin.
Sulfhemoglobin is another rare cause of cyanosis that arises from sulfur binding to hemoglobin. This causes the uncoupling of oxygen from hemoglobin to be very difficult. The iron in the hemoglobin remains unchanged in its ferrous state in sulfhemoglobinemia.
Pseuodcyanosis is another uncommon condition that occurs as a result of contact with drugs such as amiodarone. Exposure to gold or silver salts also can cause pseuodcyanosis. Diagnosis can be easily established by careful review of medications.