An introduction to Pulmonary Hypertension (PH)

Pulmonary hypertension (PH) is a rare lung disorder in which the blood pressure in the pulmonary artery rises far above normal levels. At the same time as the pressure rises, the walls of the blood vessels (pulmonary arteries) become thicker. PH can occur with or without an identifiable or known cause.

The pulmonary arteries are the blood vessels carrying “oxygen-poor” blood from the right ventricle, one of the pumping chambers of the heart, to the lungs. Once oxygenated (that is, blood which is now richer in oxygen) in the little air sacks of the lungs, the blood is carried back to the left side of the heart by the pulmonary veins. The oxygenated blood is then pumped to the rest of the body by the left ventricle (through the aorta, the largest blood vessel in the body). This blood is carried to the body by the systemic arteries where the muscles and organs use the oxygen. Once the oxygen has been extracted the blood is carried back to the right side of the heart by the systemic veins (these are called the inferior and superior vena cava). This cycle then repeats itself.

During periods of exercise, such as walking, the muscles require larger amounts of oxygen to provide them with extra energy. Therefore the body adapts to this increased oxygen demand by increasing the heart rate which increases the amount of blood flowing through the lungs. For this to happen the pulmonary arteries have to be able to open wider. In other words the vessels have to be flexible.

In patients with PH the vessel walls are thicker and therefore the vessels become less flexible. This makes it harder for the right ventricle to pump more blood through the lungs. The right ventricle is a muscle and like any other muscle in the body, if it is worked harder it becomes bigger. However if the heart has to work harder than usual over a long period of time (months and often years) without a rest, it begins to work much less effectively.

When the right ventricle contracts it pressurises the blood within its chamber. The pressure forces the blood through the pulmonary arteries, in turn increasing the pressure within them. After the right ventricle has contracted, it relaxes causing the pressure in the pulmonary arteries to decrease. In other words the pressure in the pulmonary arteries increases when it contracts and decreases when it relaxes. In a healthy person the pressure increases to about 20 mmHg (millimetres of mercury) and decreases to as low as about 5 mmHg.

Often when doctors talk about the pressure in the pulmonary arteries they talk about a single figure. This figure is what is known as the mean (average) pulmonary artery pressure (this is often abbreviated to mPAP) and is a number between the highest and lowest pressures. In the healthy person the mean is about 14 mmHg. The diagnosis of PH may be considered when the mean pulmonary artery pressure rises above 25 mmHg whilst the patient is resting or 30mmHg on exercise.

Although in the majority of cases we do not know the exact cause of PH, we do know that it occurs often in people with other specific diseases. For instance we know that people with a condition called “systemic sclerosis” may have a life time risk of developing PH of 10-20% which is higher than the normal risk.  PH can also be said to occur in association with portal hypertension (liver problems), HIV infection and most commonly in patients with congenital heart disease.  PH can also be genetically inherited, although this is very rare.