Causes of pulmonary hypertension  

Pulmonary hypertension is caused by changes to the pulmonary arteries, which are blood vessels that carry blood from your heart to your lungs.

The walls of your pulmonary arteries can become stiff and thickened, or the blood vessels may get blocked by blood clots. This makes it difficult for your heart to pump blood through these arteries, increasing pressure inside them and leading to pulmonary hypertension.

There can be many different reasons for the changes to your arteries. The World Health Organization (WHO) has classified pulmonary hypertension into five different types, depending on the underlying cause. They are:

  • pulmonary arterial hypertension (PAH)
  • pulmonary hypertension associated with left heart disease
  • pulmonary hypertension associated with lung disease and hypoxia 
  • pulmonary hypertension due to blood clots
  • pulmonary hypertension due to other causes 

These are described below.

Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is caused by problems with the smaller branches of the pulmonary arteries. In PAH, these changes are usually the main problem, rather than another condition.

As the pulmonary arteries are directly affected, they can be treated immediately, whereas in other types of pulmonary hypertension, the underlying cause must be treated first.

PAH can be further divided into specific subtypes, which are described below.

Idiopathic PAH

Idiopathic means that the cause is unknown. If tests cannot find an underlying reason for your condition, you may be diagnosed with idiopathic PAH. This is an extremely rare condition, only affecting around two people in every million each year.

Inherited PAH

Some cases of PAH run in families. A particular genetic mutation (a change in one of the genes you inherit from your parents) is sometimes thought to cause PAH. There may be a family history of the condition in up to 10% of idiopathic PAH cases.

PAH associated with other conditions

Other conditions or treatments associated with PAH include:

  • connective tissue diseases that affect the structure or composition of your body tissue such as scleroderma (a disorder that causes hardening of the skin)
  • congenital heart problems such as a hole in the heart
  • portal hypertension  blood pressure inside the liver is abnormally high, which causes veins to become swollen
  • HIV  about 1 in 200 people with HIV are thought to develop PAH
  • certain medications or drugs  such as fenfluramine (a medication no longer available) and amphetamines
  • thyroid gland disorder
  • sickle cell disease and related conditions
  • glycogen storage disorders (glycogen is a carbohydrate that produces short-term energy)

PAH can also be associated with rare conditions that affect your blood vessels, such as:

  • pulmonary veno-occlusive disease  a condition that causes high blood pressure in the lungs
  • pulmonary capillary hemangiomatosis  where tiny blood vessels (capillaries) grow within the lungs, causing blockages

Persistent pulmonary hypertension of the newborn

It is estimated that persistent pulmonary hypertension of the newborn affects around 2 in every 1,000 newborn babies.

Conditions that may also be associated with persistent pulmonary hypertension of the newborn include:

  • severe pulmonary hypoplasia  where your baby’s lungs are seriously underdeveloped  
  • hypoglycaemia  an abnormally low level of glucose in the baby’s blood  
  • sepsis  a life-threatening illness caused by very severe infection
  • meconium aspiration syndrome  where a newborn baby breathes in a mixture of amniotic fluid (the fluid that surrounds the baby in the womb) and meconium (their first faeces), which can block their airways

Left heart disease

Pulmonary hypertension is sometimes associated with diseases that affect the left side of the heart (the side that pumps blood around the whole body, apart from the lungs). The left side of the heart consists of:

  • the left atria (upper chamber)
  • the left ventricle (lower chamber) 
  • the aortic and mitral valves  one-way valves that allow blood to flow through the heart in the correct direction

As blood flows through the pulmonary arteries to the lungs and then to the left side of the heart, any problem could have a backwards effect on this circuit and affect the pulmonary arteries. This in turn could cause pulmonary hypertension. 

Up to 60% of people with a condition that causes severe left ventricle dysfunction have pulmonary hypertension. Almost all people with a severe mitral valve condition, and nearly two-thirds of those with a severe aortic valve condition, have pulmonary hypertension.

Lung disease and hypoxia

Pulmonary hypertension is also sometimes associated with lung diseases or hypoxia (a shortage of oxygen in the body). This includes:

  • chronic obstructive pulmonary disease (COPD)  a term that describes a number of lung diseases that affect breathing; up to half of people with advanced COPD may also have pulmonary hypertension, although it's usually mild  
  • interstitial lung disease  a group of lung disorders that cause scarring of the lung tissue, which makes it difficult to get enough oxygen into your body; about a third of people with interstitial lung disease may have pulmonary hypertension 
  • sleep-disordered breathing  conditions that affect breathing while you are in deep sleep, such as obstructive sleep apnoea (OSA)

These types of conditions can restrict the amount of oxygen able to get into your lungs and enter your blood. A low level of oxygen in the blood causes the pulmonary arteries to constrict (get narrower). As these vessels are narrower, the blood is squeezed into a smaller space, which increases the blood pressure and results in pulmonary hypertension.

Blood clots

Pulmonary hypertension is sometimes the result of a blood clot that causes narrowing or a blockage in the pulmonary arteries or veins.

A blood clot that blocks one of the blood vessels that supply your lungs is known as a pulmonary embolism.

Other causes

Other, less common, causes of pulmonary hypertension include: 

  • sarcoidosis  a condition that causes inflammation of different organs, including the lungs and lymph nodes
  • histiocytosis X  a rare condition that causes scarring (granulomas) and air-filled cysts, predominantly in the lungs
  • compression of the blood vessels in the lungs  for example, due to a tumour

Page last reviewed: 23/02/2015

Next review due: 23/02/2017