Cyanosis refers to bluish discoloration of skin, nail beds and mucous membranes.
Normally haemoglobin carries most of the oxygen in blood. This oxygen carrying capacity of haemoglobin in the blood (present in the arteries) is called oxygen saturation.
Once in the tissues the haemoglobin releases the oxygen and comes back darker and deoxygenated in the blood carried by the veins. This darker venous blood travels to the lungs to be reloaded with fresh oxygen from breathing and comes back to the heart only to be pumped out again in the arteries. (1-5)
When is cyanosis seen?
When the deoxygenated haemoglobin in blood reaches between 4 to 6 gm/dl, cyanosis may be seen. This blood is bluish or purple giving the skin and mucous membranes their appearance.
Normally approximately 80-87% oxygen saturation would give rise to clinically apparent cyanosis. However, in cases of anemia where haemoglobin levels are low, lower oxygen saturation may show up as cyanosis.
A haemoglobin level of 6 g/dl for example shows cyanosis only when oxygen saturation has dropped below 60%. (2)
Two types of cyanosis
Cyanosis may be of two major types: central and peripheral. (3)
Central cyanosis is caused by diseases of the heart or lungs or by abnormal haemoglobin types like methemoglobin or sulfhemoglobin etc. This is seen commonly as bluish or purple discoloration of tongue and linings of the mouth.
There may be concomitant peripheral cyanosis.
In cases where there is no peripheral cyanosis, fingers and toes are warm to touch. There may be other features like breathlessness, shallow breathing, rapid breathing etc.
Peripheral cyanosis results from decreased local blood circulation in the peripheral organs, arms and legs. This is commonly seen if the arterial blood stagnates too long in the limbs and loses most of its oxygen.
Cyanosis shows up as the levels of deoxygenated blood rises in the small blood vessels of the fingers and toes. This may be seen in heart failure, shock (sluggish or slowing of blood circulation with severe loss of blood pressure), exposure to cold temperatures and diseases of blood circulation.
Limbs appear bluish and are usually cold to touch. Peripheral cyanosis is most intense in nail beds. Warming may resolve the cyanosis.
Causes of central cyanosis
Causes of central cyanosis can be broken down into causes in new-borns and causes in adults. (1-5)
Causes of central cyanosis in new-borns
In new-borns central cyanosis is seen in cases of severe problems with airway, breathing or heart and circulation. In these babies such cyanosis requires immediate assessment and correction if possible.
Right after birth there is a transient cyanosis. This usually clears within a few minutes.
Causes of central cyanosis in new-borns lying in the heart are termed cyanotic heart diseases or congenital heart diseases.
This includes conditions like
- Transposition of great arteries,
- Fallot’s trtralogy,
- Total anomalous pulmonary venous return,
- small or hypoplastic left heart,
- Truncus arteriosus,
- Persistent fetal circulation etc.
These conditions may occur if the baby has a genetic disorder like Down syndrome, Turner syndrome, Marfan’s syndrome, Noonan syndrome etc.
It may also be seen if the other suffers an infection like German measles or rubella during pregnancy.
Women with diabetes during pregnancy and those taking some prescription and over-the-counter medications and street drugs during pregnancy have a high risk of giving birth to a baby with a congenital heart defect.
Causes of central cyanosis lying in the respiratory system include:
- birth injury or asphyxia,
- Transient tachypnoea of the newborn,
- respiratory distress syndrome,
- pulmonary or lung edema,
- accidental aspiration or choking on meconium (black stool that the baby may pass within the womb),
- diaphragmatic hernia,
- pleural effusion,
- trachea-oesophageal fistula
- obstruction of the upper respiratory tract.
Central cyanosis in new-borns may also be caused due to low blood sugar, low blood magnesium and due to infections or epileptic or other seizures.
Central cyanosis in adults
Central cyanosis in adults is caused due to any severe respiratory disease including:
- lung edema,
- lumg thromboembolism (dislodgement of a blood clot and its lodging in the lung arteries),
- high altitude sickness,
- severe pneumonia,
- acute sever attack of asthma,
- chronic obstructive lung disease,
- acute adult respiratory distress syndrome.
Abnormal varieties of haemoglobin may lead to cyanosis. Methaemoglobinaemia may be caused due to genetic abnormalities or due to drugs like antimalarial drug Primaquin or antibiotics sulphonamides.
Sulfhaemoglobinaemia is caused due to antibiotics sulphonamides. Increased blood cells called polycythemia may also cause cyanosis.
Causes of peripheral cyanosis
Causes of peripheral cyanosis include (1-5):
- Central cyanosis and its causes are all associated with peripheral cyanosis
- Decreased pumping of blood by the heart or reduced cardiac output. This is seen in heart failure or circulatory shock.
- Diseases of circulation like thrombosis or embolism.
- Constriction of blood vessels of the limbs, fingers and toes. This could be due to:
- cold exposure,
- Raynaud’s phenomenon,
- spasm of the smaller skin capillaries or arteries called acrocyanosis,
- erythrocyanosis occurring in young women and as side effects of beta blocker drugs taken for high blood pressure.