Pulmonary Hypertension WHO Classification

Pulmonary hypertension (PH) is a rare lung disease characterized by high blood pressure in the pulmonary arteries, which deliver blood from the heart to the lungs. The high pressure causes the heart to work harder to pump blood. This strain can lead to the heart becoming larger and weaker, and eventually result in right heart failure.

There is a wide range of potential causes for PH, and they can affect disease symptoms, and its treatment and prognosis.

The World Health Organization classification divides PH patients into five groups, according to the underlying cause of the disease. These groups are as follows:

Group 1: Pulmonary arterial hypertension (PAH)

Group 1 includes PH associated with the narrowing of the small blood vessels in the lungs. This is called PAH, and includes cases in which the underlying cause of the narrowing is not known (idiopathic PAH).

There are multiple other subgroups in group 1, including:

  • Familial, or heritable PAH
  • PAH caused by certain drugs or toxins
  • PAH associated with other conditions, such as connective tissue diseases like scleroderma or lupus, congenital heart problems, chronic liver disease, and certain infections including HIV and schistosomiasis, a parasitic disease.

Group 1 also may be caused by rare blood conditions, like pulmonary veno-occlusive disease or pulmonary capillary hemangiomatosis. A type of PH present in infants is called persistent PH of the newborn.

Group 2: PH due to left-sided heart disease

Group 2 refers to PH due to left heart disease, also known as pulmonary venous hypertension. Long-term problems with the left side of the heart can lead to changes in the pulmonary arteries and cause PH. This may include:

  • Left ventricular systolic dysfunction, in which the heart cannot pump blood effectively
  • Left ventricular diastolic dysfunction, when the heart cannot properly relax to allow enough blood to flow into it
  • Valvular disease, when the valves of the left side of the heart do not work properly, reducing blood flow or allowing blood to flow backwards

Group 3: PH due to lung disease or chronic hypoxia

Group 3 includes PH resulting from lung diseases or a shortage of oxygen in the body (hypoxia).

Chronic hypoxia can stimulate more widespread constriction in the blood vessels of the lungs. This makes it more difficult for the blood to move through the lungs, increasing blood pressure and putting the heart under increased strain to pump the blood through the lungs.

Chronic hypoxia also can trigger inflammation that damages blood vessels. This can lead to scarring, which narrows and stiffens blood vessels and causes further increases in blood pressure.

Causes of pulmonary hypoxic hypertension include various lung diseases, such as chronic obstructive pulmonary disease and diffuse parenchymal lung disease.

Sleep-disordered breathing, a group of diseases that affect breathing during sleep, like obstructive sleep apnea (OSA), are also included in this group. Other causes include repeated exposure to high altitudes, lung developmental abnormalities, and alveolar hypoventilation disorders (marked by a buildup of carbon dioxide and a reduction of oxygen in the lungs’ tiny air sacs).

Group 4: PH due to chronic blood clots

Group 4 refers to PH caused by blood clots obstructing the pulmonary arteries. Clots are the body’s response to bleeding and injuries, but can occur without an apparent cause.

Pulmonary embolism occurs when blood clots travel to the lungs from the legs or other parts of the body; these clots can block the pulmonary arteries. This group can also be referred to as chronic thromboembolic PH (CTEPH).

Group 5: PH induced by other health conditions

Group 5 includes causes of PH that do not fit into any of the other four groups. These are widely split into four categories:

  • Blood disorders, such as some types of anemia and chronic myeloproliferative diseases (bone marrow disorders, wherein excess amounts of blood cells are produced)
  • Systemic disorders, such as sarcoidosis — a condition marked by inflammation in various organs, like the lungs and lymph nodes; vasculitis, or blood vessel inflammation; neurofibromatosis, which cause tumors to grow on nerve tissue; and pulmonary Langerhans cell histiocytosis, a rare disorder that progresses to lung scarring
  • Metabolic disorders, such as glycogen storage diseases that include Gaucher disease (in which fatty molecules accumulate in cells and certain organs), and thyroid disorder
  • Other conditions such as end-stage renal (kidney) disease, or tumors obstructing pulmonary arteries


Last updated: July 1, 2021


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