There are many causes of PH. Sorting out (also known as classifying) and labeling different types of PH is very important for many reasons:

• It helps doctors and scientists communicate better
• It helps doctors understand how different types of PH are related
• It helps doctors make a clearer diagnosis of PH
• It makes the treatment of PH clearer
• It helps doctors better understand how a patient might respond to a treatment

In the past, PH was simply classified into primary or secondary:

• Primary PH (PPH). An older term. In a small number of PH patients, there is no known cause for PH. These patients were said to have PPH. In the current 2003 Venice classification, these patients would be diagnosed with idiopathic PAH (IPAH)
• Secondary PH (SPH). An older term. SPH indicated that there was a known cause for PH, such as heart disease (for example, heart failure), lung disease (for example, emphysema), or a general medical condition (such as cirrhosis of the liver)

The most commonly used classification system today is the 2003 World Health Organization (WHO) Venice PH Classification System. Groups in the 2003 Venice classification system include:

• WHO Group 1: PAH = pulmonary arterial hypertension. A newer term, which specifically indicates that PH is due to narrowing of the pulmonary arteries of the lungs. PAH is a broad category which includes many patients with different types of PAH:
  • IPAH = idiopathic PAH. A newer term for the old term, PPH. In a small number of PH patients, there is no known cause for PH. They are said to have IPAH
  • FPAH = familial PAH. In some PAH patients, PAH was inherited from their parents. There is often a history of PAH in other close members of the patient’s family (for example, parents, sibilings, or children)
  • CTD-PAH. PAH associated with connective tissue disease (CTD), such as scleroderma
  • HIV-PAH. PAH associated with HIV infection
  • CHD-PAH. PAH due to heart defects present at birth (also known as congenital heart disease = CHD)
    
• WHO Group II: PVH = pulmonary venous hypertension. PH due to disease of the left-side of the heart, including the left ventricle (LV; lower left chamber) of the heart, left atrium (LA; upper left chamber), or disease of the mitral valve or aortic valve on the left-side of the heart
  
• WHO Group III: PH due to pulmonary arteries being blocked by blood clots (also known as pulmonary embolism). This type of PH is often referred to as chronic thromboembolic PH (CTEPH). In rare patients, PH can be due to other types of material blocking pulmonary arteries, including cancer cells or eggs of the parasite infection, schistosomiasis.
  
• WHO Group IV: PH due to lung disease and low blood oxygen levels. These lung diseases include emphysema of the lung (usually due to smoking), scarring (also known as fibrosis) of the lung, or breathing problems during sleep (for example, sleep apnea)
  
• WHO Group V: PH due to various other rare lung conditions and other rare general medical conditions