Causes

The typical heart has two upper chambers (atria) and two lower chambers (ventricles). Each time blood passes through the heart, the lower right chamber (right ventricle) pumps blood to the lungs through a large blood vessel (pulmonary artery).

In the lungs, the blood releases carbon dioxide and picks up oxygen. The blood typically flows easily through blood vessels in the lungs (pulmonary arteries, capillaries and veins) to the left side of the heart.

However, changes in the cells that line the pulmonary arteries can cause the walls of the arteries to become stiff, swollen and thick. These changes may slow down or block blood flow through the lungs, causing pulmonary hypertension.

Pulmonary hypertension is classified into five groups, depending on the cause.

Group 1: Pulmonary arterial hypertension (PAH)

Causes include:

• Unknown cause (idiopathic pulmonary arterial hypertension)
• Changes in a gene passed down through families (heritable pulmonary arterial hypertension)
• Use of some prescription diet drugs or illegal drugs, such as meth
• Heart problems present at birth (congenital heart disease)
• Other conditions such as HIV infection, chronic liver disease (cirrhosis) and connective tissue disorders (scleroderma, lupus, others)

Group 2: Pulmonary hypertension caused by left-sided heart disease

Causes include:

• Left-sided heart valve disease such as mitral valve or aortic valve disease
• Failure of the lower left heart chamber (left ventricle)

Group 3: Pulmonary hypertension caused by lung disease

Causes include:

• Chronic obstructive pulmonary disease (COPD)
• Scarring of the tissue between the lung's air sacs (pulmonary fibrosis)
• Obstructive sleep apnea
• Long-term exposure to high altitudes in people who may be at higher risk of pulmonary hypertension

Group 4: Pulmonary hypertension caused by chronic blood clots

Causes include:

• Chronic blood clots in the lungs (pulmonary emboli)
• Other clotting disorders

Group 5: Pulmonary hypertension triggered by other health conditions

Causes include:

• Blood disorders, including polycythemia vera and essential thrombocythemia
• Inflammatory disorders such as sarcoidosis and vasculitis
• Metabolic disorders, including glycogen storage disease
• Kidney disease
• Tumors pressing against pulmonary arteries

Eisenmenger syndrome and pulmonary hypertension

Eisenmenger syndrome is a type of congenital heart disease that causes pulmonary hypertension. It's most commonly caused by a large hole in the heart between the two lower heart chambers (ventricles), called a ventricular septal defect.

This hole in the heart causes blood to flow incorrectly in the heart. Oxygen-carrying blood (red blood) mixes with oxygen-poor blood (blue blood). The blood then returns to the lungs — instead of going to the rest of the body — increasing the pressure in the pulmonary arteries and causing pulmonary hypertension.