The World Health Organization (WHO) classifies the five main types of pulmonary hypertension (PH) by cause. Pulmonary hypertension refers to high blood pressure in the lungs due to any cause. Pulmonary arterial hypertension (PAH) is high blood pressure in the lungs specifically caused by narrowed arteries. PH is the collective term – all PAH is PH. PAH is a more specific term – not all PH is PAH.
Read more about causes and risk factors for pulmonary hypertension.
Normal heart and lung function
It helps to know a little about the anatomy of the heart and lungs and how they work.
There are four heart chambers – two atria (left and right) on top and two ventricles (left and right) below them. The right-side chambers of the heart circulate blood through the lungs, while the left-side chambers of the heart circulate blood through the body. The right atrium receives oxygen-depleted blood from the body and moves it into the right ventricle, which pumps it into the lungs via the left and right pulmonary arteries. The deoxygenated blood moves through progressively smaller arteries into the capillaries (tiny blood vessels) of the lung, where it picks up a fresh oxygen supply. Now rich in oxygen, blood returns to the left atrium through the pulmonary veins. The left atrium receives oxygenated blood from the lungs and moves it into the left ventricle – the largest and most powerful chamber of the heart – which pumps the blood around the body.
There are four heart valves – mitral, aortic, tricuspid and pulmonary – that control the flow between each chamber of the heart. The valves open to allow forward flow and snap shut to prevent backward flow.
Pulmonary hypertension types
WHO classifies PH into five main groups based on the cause of the condition.
Pulmonary arterial hypertension (PAH) is caused by narrowing of the pulmonary arteries, usually due to inflammation and thickening of the muscular layer of the arterial wall. There are several subcategories of PAH:
Idiopathic PAH – without known cause
Heritable PAH – caused by a genetic problem inherited from a parent
Drug- or toxin-induced PAH – caused by exposure to a medication, illicit drug, or toxin
Associated PAH – caused by another disease
Drugs and toxins that can cause PAH include:
Diseases that can cause associated PAH include:
Eisenmenger syndrome is a specific type of advanced PAH associated with congenital heart defects.
In people with group II pulmonary hypertension, PH is caused by left-side heart disease. When disease damages the left ventricle, it can no longer pump blood effectively around the body. Deoxygenated blood backs up, causing high blood pressure in the pulmonary arteries. Types of heart disease that may cause PH include mitral valve problems, coronary artery disease (CAD), congenital heart defects, and chronic hypertension (high blood pressure throughout the body).
Group III pulmonary hypertension is caused by chronic hypoxia – low oxygen levels – due to lung disease or sleep-related breathing disorders. Conditions that can cause group III PH include COPD, interstitial lung disease, and sleep apnea. In some people, group III PH can be caused by hypoxia from living at high altitudes. Group II and III PH is far more common than Group I PAH. In other words, most pulmonary hypertension is caused by heart and lung disease.
Also known as chronic thromboembolic pulmonary hypertension (CTEPH), group IV PH is caused by blood clots that have become lodged in the pulmonary arteries. The clots raise the blood pressure in the arteries, leading to PH symptoms. CTEPH is the only type of PH that can be cured with surgery in some individuals. However, not everyone with CTEPH is a good candidate for surgery.
Cases of PH in group V are caused by other conditions or multiple factors. Any disease that interferes with the circulation of blood through the lungs can potentially cause pulmonary hypertension.
Conditions that can cause group V pulmonary hypertension include: