Causes of pulmonary hypertension

Pulmonary hypertension is caused by changes tothe 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 Air embolism . This makes it difficult for your heart to pump blood through these arteries, increasing pressure inside them andleading 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 ofpulmonary hypertension,the underlying cause must betreated first.

PAH can befurther divided into specific subtypes, which are describedbelow.

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 affectingaround two people in every million each year.

InheritedPAH

Some cases of PAH run in families.A particular genetic mutation (a change in one of the genes you inherit from your parents) issometimes thought tocause PAH. There may be a family history of the conditionin up to10% ofidiopathic PAHcases.

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 about1 in 200 people with HIVare thought todevelop 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 isestimated that persistentpulmonary hypertension of the newborn affects around2 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 babys lungs are seriously underdeveloped
• hypoglycaemia an abnormally low level of glucose in the babys 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 thatsurrounds the babyin 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 thatpumps 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

Asblood flows throughthe 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 to60% ofpeople 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 thelung 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 yourlungs 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 issometimes the result of a blood clot that causes narrowing or a blockagein 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