Pulmonary hypertension is a severe, 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, in turn, causes strain that 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 pulmonary hypertension, and these can affect the symptoms, treatment, and prognosis of the disease.

To produce, maintain, and implement international health information standards, the World Health Organization (WHO) defined a classification for pulmonary hypertension diagnosis in 1973, based on the cause of the disease. This has seen multiple revisions, reflecting advances in disease understanding, with the most recent being presented in 2013 at the Fifth World Symposium on Pulmonary Hypertension in Nice, France.

Aims of the classification 

The international classification of diseases (ICD) is created by the organization and comprises a standard diagnostic tool for physicians, nurses, other providers, researchers, health information managers and coders, health information technology workers, policy-makers, insurers, and patient organizations. To determine the classification of pulmonary hypertension, WHO analyzed the general health situation of patients, as well as the disease’s incidence and prevalence.

The WHO classification is used for numerous purposes, including records, death certificates, information storage, diagnosis retrieval, national mortality and morbidity statistics, reimbursement, resource allocation, and other clinical, epidemiological and quality purposes. The organization expects the classification to help fight the disease.

The classification

In 1973, pulmonary hypertension was classified into only two groups. These were “primary pulmonary hypertension” if the cause of the disease was known, or “secondary pulmonary hypertension” if the cause could not be identified. Secondary pulmonary hypertension also was called “idiopathic pulmonary hypertension.”

Now the classification has been expanded to five groups, to emphasize the importance of the underlying cause of the disease. This reflects our increased knowledge of what can cause pulmonary hypertension, along with what impact this can have on disease progression and required treatment.

The five groups of pulmonary hypertension today are:

Group 1: pulmonary arterial hypertension

Group 1 includes pulmonary hypertension is associated with the narrowing of the small blood vessels in the lungs. It also is called pulmonary arterial hypertension (PAH) and includes cases where the underlying cause of the narrowing is not known (idiopathic pulmonary hypertension).

There are multiple other subgroups in group 1, including:

Group 1 also may be caused by rare blood conditions, like pulmonary veno-occlusive disease (PVOD) or pulmonary capillary hemangiomatosis (PCH), and a type of pulmonary hypertension present in babies called persistent pulmonary hypertension of the newborn (PPHN).

Group 2: pulmonary hypertension due to left heart disease

Group 2 refers to pulmonary hypertension caused by left heart disease. Long-term problems with the left side of the heart can lead to changes in the pulmonary arteries and cause pulmonary hypertension. This may include:

  • Left ventricular systolic dysfunction, when the heart cannot pump blood effectively
  • Left ventricular diastolic dysfunction, when the heart cannot properly relax to allow enough blood to flow in
  • Valvular disease, when the valves of the left side of the heart are allowing blood to leak
  • Congenital heart defects (heart defects from birth), which can lead to problems with blood flowing in or out of the heart

Group 3: pulmonary hypertension due to lung disease and/or chronic hypoxia

Group 3 includes pulmonary hypertension resulting from lung diseases or shortage of oxygen in the body (hypoxia). The common diseases associated with group 3 pulmonary hypertension are:

Group 4: pulmonary hypertension due to blood clots in the lungs

Group 4 refers to pulmonary hypertension caused by blood clots obstructing the pulmonary arteries. This also can be referred to as chronic thromboembolic pulmonary hypertension (CTEPH).

Clots are the body’s response to bleeding and injuries, but can harm the heart and lungs when they occur without an apparent cause. Pulmonary emboli are blood clots that travel to the lungs, and pulmonary thrombosis are clots that are formed in the lungs, which can block the pulmonary arteries.

Group 5: pulmonary hypertension due to blood and other disorders

Group 5 is the last category and includes other less-common causes 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
  • Systemic disorders, such as sarcoidosis (a condition that results in inflammation of different organs like the lungs and lymph nodes) and histiocytosis (a rare disorder that causes scarring)
  • Metabolic disorders, such as glycogen storage diseases and thyroid disorders
  • Other disorders, such as chronic kidney failure or tumors obstructing pulmonary arteries


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