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WHO Group 1: Pulmonary Arterial Hypertension (PAH)

PAH is a type of PH that is specifically due to narrowing of the pulmonary arteries (blood vessels) that carry blood to the lungs. 

The exact cause of PAH is not well understood. Narrowing of pulmonary arteries in PAH is due to multiplication and growth of cells in the walls of the pulmonary arteries, as well as scar tissue formation in the pulmonary arteries. Tremendous ongoing research has identified many biologic abnormalities in PAH, including genes, proteins, and small chemicals in the body.

PAH is a broad category which includes many patients with different types of PAH. These include:

  • Idiopathic
  • Heritable (genetic)
  • Associated with connective tissue disease
  • Associated with other conditions

WHO Group 2: PH Associated with Left-Sided Heart Disease

Diseases of the left side of the heart result in failure to properly pump blood to the rest of the body. This causes a back-up of blood in the lungs, resulting in PH.

What causes PH Due to Left-Sided Heart Disease?

  • Left-sided congestive heart failure
  • Mitral valve disease
  • Other heart diseases

WHO Group 3: PH Associated with Lung Disease

Lung diseases can lead to hypoxia (lower blood oxygen levels). Hypoxia causes vasoconstriction (narrowing of pulmonary arteries due to spasm), resulting in PH.
What causes PH Due to Lung Disease?

  • COPD
  • Pulmonary fibrosis
  • Sleep apnea
  • Other lung diseases

WHO Group 4: Chronic Thromboembolic Pulmonary Hypertension (CTEPH)

In most patients who have blood clots in the lungs (pulmonary embolism, PE), the body is able to resolve (break down) the blood clots. In a minority of patients, PE does not resolve and can cause scarring in the pulmonary arteries, which leads to the development of CTEPH.

Click here to find out more about CTEPH and how it's treated.

WHO Group 5: PH Caused by Various Other Diseases

PH can sometimes be caused by various other diseases. The exact mechanisms of PH in most of these conditions are unclear.

What are some examples?
  • Certain lung diseases (e.g. sarcoidosis, others)
  • Certain blood diseases (e.g. sickle cell anemia, others)
  • Chronic renal failure
  • Vasculitis