Pulmonary hypertension

Pulmonary hypertension refers to increased blood pressure in the pulmonary circulation, more specifically a mean pulmonary arterial pressure that is greater than 25 mmHg.

The pulmonary circulation starts with the right ventricle.

From there - blood is pumped into the large pulmonary trunk, which splits to form the two pulmonary arteries – one for each lung.

The pulmonary arteries divide into smaller arteries known as pulmonary arterioles and then eventually into pulmonary capillaries which surround the alveoli - which are the millions of tiny air sacs where gas exchange happens.

At that point, oxygen enters the blood and carbon dioxide enters the alveoli.

The pulmonary capillaries drain into small veins that join to form the two pulmonary veins exiting each lung, and these pulmonary veins complete the circuit by delivering oxygen-rich blood into the left atrium.

The blood pressure in the pulmonary circulation is normally much lower than the systemic blood pressure.

The normal pulmonary artery pressure is about 25/10 mmHg with a mean arterial pressure of 15 mmHg.

Pulmonary hypertension most commonly develops as a result of left heart disease.

Here the pulmonary blood vessels are normal and undamaged, but the left side of the heart is unable to pump efficiently – for example because of heart failure or valvular dysfunction.

This causes a backup of blood in the pulmonary veins and capillary beds, which can increase the pressure in the pulmonary artery.

Another cause of pulmonary hypertension is chronic lung disease, which typically causes hypoxic vasoconstriction.

That’s when some area in the lung is diseased and is unable to deliver oxygen to the blood.

To help adapt to this, the pulmonary arterioles in that area, start to constrict - and this effectively shuttles blood away from those damaged areas of the lung, and towards healthy lung tissue. But if the problem is widespread, like in individuals with emphysema, the mechanism can backfire.

That’s because there’s widespread vasoconstriction of pulmonary arterioles, and that increases pulmonary vascular resistance in general.

Increased resistance makes it hard for the right ventricle to pump out blood – a bit like pushing water through a narrow pipe as opposed to a wider one.

So to make the same amount of blood flow through the pulmonary arterioles, the right side of the heart has to generate increased pressure and that results in pulmonary hypertension.

Another cause of pulmonary hypertension is chronic thromboembolic pulmonary hypertension - which is when there are recurrent blood clots in their pulmonary vessels.

The clots can form because of an underlying clotting disorder, and can embolize or travel to the lungs.

The clots can block pulmonary vessels which increases the resistance to blood flow, and they can also endothelial cells in the vessels to release histamine and serotonin, which constricts the pulmonary arterioles.

Together the blockage and the narrowing of the blood vessels causes a rise in the pulmonary blood pressure.

One type of pulmonary hypertension is pulmonary arterial hypertension which is when there’s elevated pressure in the pulmonary arterioles, but the pressure in their capillaries and pulmonary veins is still normal.

Some congenital heart defects can cause pulmonary arterial hypertension.

A long-standing left-to-right cardiac shunt caused by a ventricular septal defect, atrial septal defect, or less commonly, a patent ductus arteriosus can result in pulmonary hypertension and eventual reversal to a right-to-left shunt, which is called Eisenmenger’s syndrome.