​Pulmonary arterial hypertension (PAH) is a condition of high lung pressure in the blood vessels leading from the heart to the lung. There are many different types of pulmonary hypertension, but PAH is the most important type.

Sometimes, the exact cause of PAH is not known. In some rare cases, PAH can be inherited.

PAH is more often diagnosed in people with certain diseases. These include:

• portal hypertension (due to liver problems)
  
• connective tissue disease such as systemic sclerosis
  
• congenital heart disease
  
• human immunodeficiency virus (HIV)