Cyanosis refers to abnormal blue discoloration of the skin and mucous membranes. The term is derived from the Greek word kuaneos meaning dark blue.
In normal humans blood protein called hemoglobin serves to carry oxygen in the arteries to the different organs.
Nearly all of the oxygen in the blood is carried by hemoglobin. A small portion is carried dissolved in blood in free form.
Each gram of hemoglobin can combine with 1.34 mL of oxygen.
While oxygenated hemoglobin is bright red, reduced hemoglobin is dark blue or purple in color. Venous blood is deoxygenated and is thus darker in color. This leads to the dusky or blue color of the skin and mucous membranes. (1-5)
Cause of cyanosis
Cyanosis is caused by an increase in the deoxygenated haemoglobin level to above 5 g/dL. In fact patients who have anemia do not develop cyanosis until the oxygen saturation (also called SaO2) falls below normal haemoglobin levels.
In adults with normal hemoglobin (13.5-18 g/dL in men, 11.5-16 g/dL in women) central cyanosis is seen if oxygen saturation drops between 80 to 87%.
Patients with lower haemoglobin or anemia say with hemoglobin of 6 g/dL, the saturation has to drop as low as 60% before cyanosis becomes clinically apparent.
Symptoms of cyanosis
Cyanosis is seen in the skin, nail beds, or mucous membranes. If cyanosis is seen only in the limbs it is termed acrocyanosis or peripheral cyanosis. This is common in young infants and is a part of normal physiology.
However, central cyanosis affects the whole body. This is seen in the mucous membranes and tongue. This usually means a potentially serious and life-threatening disease is lying underneath and requires immediate evaluation.
Major systems that may be affected include the respiratory system, blood, heart or cardiovascular system and the central nervous system.
Conditions that lead to cyanosis
Peripheral cyanosis occurs due to decrease in local blood circulation and increased offloading of oxygen in the peripheral tissues. This may commonly occur if there is stagnation or stasis of blood in the peripheries.
Common examples of such conditions are:
- circulatory shock (standstill of blood circulation with severe lowering of blood pressure)
- congestive heart failure
- exposure to cold temperatures
- some diseases of blood circulation
Commonly bluish or purple discoloration of the affected area in the toes and fingers is seen. Hands and feet are often cold to touch.
Nail beds are the best place to look for peripheral cyanosis.
Central cyanosis is caused by diseases of the heart or lungs or may be caused by abnormal haemoglobin called methhemoglobinemia or sulfhemoglobinemia.
It is seen over the tongue, lips and inner linings of the mouth.
In central cyanosis there is concomitant presence of peripheral cyanosis.
Cyanosis of respiratory origin
If the underlying disease is of respiratory origin there may be breathlessness, rapid breathing, bluish or purple discolouration of the fingers and toes and mouth mucus membranes.
The hands and feet are usually normal temperature or warm.
Cyanotic heart disease in infants
Cyanosis is commonly seen in infants with a heart defect called a congenital heart disease.
Cyanotic heart disease in infants results in low blood oxygen levels and severe central cyanosis.
Normally, blood with oxygen is pumped to various organs from the heart. From the peripheries the blood returns to the heart via lungs bringing in oxygen.
The cycle continues as oxygenated blood flows out of the heart. Heart defects can change the way blood flows through the heart and lungs. (4)