The World Health Organization classifies pulmonary hypertension into five clinical groups based on the underlying cause and how the disease affects the blood vessels in the lungs. This system matters because each group responds to different treatments, and a correct classification guides the entire management approach. Pulmonary hypertension is now formally defined as a mean pulmonary arterial pressure above 20 mmHg at rest, measured by right heart catheterization. That threshold was lowered from the previous cutoff of 25 mmHg following the 6th World Symposium on Pulmonary Hypertension in 2018.
How the Five Groups Are Organized
The classification sorts pulmonary hypertension by what’s driving the elevated pressure. Some forms originate in the small arteries of the lungs themselves. Others are a downstream consequence of heart failure, chronic lung disease, or blood clots. The five groups share a common endpoint (high pressure in the pulmonary arteries) but differ sharply in their biology, prognosis, and treatment. Understanding which group applies to a specific patient is the single most important step after diagnosis.
Group 1: Pulmonary Arterial Hypertension (PAH)
Group 1 is what most people think of when they hear “pulmonary hypertension.” In PAH, the small arteries in the lungs narrow, stiffen, and sometimes close off entirely. This forces the right side of the heart to work progressively harder to push blood through.
Group 1 includes several subtypes:
- Idiopathic PAH: no identifiable cause
- Heritable PAH: linked to genetic mutations, most commonly in a gene called BMPR2 that normally keeps the smooth muscle cells lining pulmonary arteries from overgrowing. When this gene malfunctions, those cells proliferate and the vessels narrow. Other gene mutations (ALK-1, ENG, SMAD9, CAV1, KCNK3) account for a smaller share of heritable cases.
- Drug and toxin induced: historically tied to appetite suppressants and amphetamines
- Associated with other conditions: connective tissue diseases like scleroderma, HIV, portal hypertension from liver disease, congenital heart defects, and schistosomiasis
Two related conditions are classified as subsets of Group 1: pulmonary veno-occlusive disease (which affects the veins rather than arteries in the lungs) and persistent pulmonary hypertension of the newborn. Group 1 is the group where targeted PAH therapies, such as drugs that relax or remodel the pulmonary blood vessels, are most effective. For a subset of patients with idiopathic, heritable, or drug-induced PAH, a specialized test called vasoreactivity testing can identify those who respond well to calcium channel blockers, a simpler and less expensive treatment option.
Group 2: Left Heart Disease
Group 2 is the most common cause of pulmonary hypertension overall. When the left side of the heart cannot pump efficiently or its valves malfunction, blood backs up into the lungs. That backup raises pressure in the pulmonary arteries without any primary disease in those vessels.
Causes include heart failure (both with reduced and preserved pumping function), mitral or aortic valve disease, and cardiomyopathies. The key hemodynamic marker is a pulmonary artery wedge pressure above 15 mmHg, which signals that high filling pressures on the left side of the heart are the root problem. This is called post-capillary pulmonary hypertension because the obstruction to blood flow sits downstream of the lung’s capillary bed.
In some patients, the sustained backup pressure eventually damages the pulmonary arteries themselves, creating a combined pattern where both the left heart disease and intrinsic vascular changes contribute. When the pulmonary vascular resistance climbs to 3 Wood units or above on top of elevated wedge pressures, that combined pattern is present and typically signals more advanced disease. Treatment for Group 2 focuses on managing the underlying heart condition. PAH-specific drugs used in Group 1 have generally not shown benefit here and can cause harm.
Group 3: Lung Disease and Hypoxia
Chronic lung diseases can raise pulmonary artery pressure through two main pathways: destruction or scarring of the lung tissue reduces the number of functioning blood vessels, and low oxygen levels cause the remaining vessels to constrict. COPD and interstitial lung disease (a group of conditions that scar lung tissue) are the two largest contributors in this group.
Other conditions classified under Group 3 include sleep-disordered breathing, alveolar hypoventilation disorders (where breathing is too shallow to maintain normal oxygen levels), chronic high-altitude exposure, and developmental lung abnormalities. Sleep-disordered breathing commonly overlaps with COPD and interstitial lung disease, compounding the problem. Treatment centers on improving oxygenation and managing the underlying lung condition rather than using PAH-targeted medications.
Group 4: Chronic Blood Clots in the Lungs
Group 4, called chronic thromboembolic pulmonary hypertension (CTEPH), develops when blood clots in the pulmonary arteries fail to dissolve after a pulmonary embolism. The organized clot material physically obstructs blood flow and triggers remodeling of the vessel walls.
This group stands out because it is potentially curable without lung transplantation. A surgical procedure called pulmonary thromboendarterectomy can remove the clot material from the artery walls and, in experienced centers, restore near-normal pressures. For patients whose clots are too far into the smaller branches for surgery, balloon-based procedures and medications that target the pulmonary vessels offer additional options.
Group 5: Unclear or Multifactorial Causes
Group 5 is a catch-all for conditions that raise pulmonary artery pressure through mechanisms that don’t fit neatly into the other four groups, often involving multiple pathways at once. It includes blood disorders such as myeloproliferative diseases and chronic hemolytic anemias, systemic diseases like sarcoidosis and pulmonary Langerhans cell histiocytosis, metabolic disorders, chronic kidney failure, and conditions that physically compress the pulmonary arteries (such as fibrosing mediastinitis or tumors). Because the mechanisms are varied and often poorly understood, there is no single treatment strategy. Management targets the underlying systemic condition.
Functional Class vs. Clinical Group
A common source of confusion is the difference between the five clinical groups and the WHO functional classification. They are two separate systems used together. The clinical group identifies the cause. The functional class describes how much the disease limits daily life, scored from I to IV:
- Class I: no limitation of physical activity. Ordinary exertion doesn’t cause unusual shortness of breath, fatigue, chest pain, or near-fainting.
- Class II: slight limitation. Comfortable at rest, but ordinary activity triggers symptoms.
- Class III: marked limitation. Comfortable at rest, but even less-than-ordinary activity causes symptoms.
- Class IV: unable to perform any physical activity without symptoms. Shortness of breath or fatigue may be present even at rest, and signs of right heart failure are typically present.
A patient might be classified as Group 1, Class III, meaning they have pulmonary arterial hypertension and experience significant symptom limitations with mild exertion. Functional class can shift over time with treatment or disease progression, while the clinical group generally stays the same.
Why Correct Classification Matters
The medications approved for pulmonary arterial hypertension (Group 1) work by relaxing and remodeling the small arteries in the lungs. Using these drugs in patients whose elevated pressure is actually driven by left heart failure (Group 2) or chronic lung disease (Group 3) can worsen fluid retention, drop blood pressure, or cause other complications without improving the underlying problem. Misclassifying a Group 4 patient means potentially missing the one form of pulmonary hypertension that can be surgically cured. Accurate classification requires right heart catheterization, imaging to rule out chronic clots, pulmonary function testing, and a thorough workup for associated conditions. The process can feel extensive, but it directly determines which treatments will help and which could do harm.