Pulmonary hypertension is a medical term for a complex group of diseases in which the pressure in the pulmonary vessels is increased. It can occur either as a vascular lesion of the pulmonary circulation itself or in the progression of other diseases. The disease is often caused by a lack of vasodilating substances and an increase in vasoconstricting substances, which cause the pulmonary vessels to become constricted, or by structural changes in the pulmonary vessels and the formation of blood clots (called “thrombi”) in the pulmonary vessels, which increase the pressure in the pulmonary vessels and reduce blood flow from the heart to the lungs. The disease process progresses gradually, and eventually right heart failure occurs. Pulmonary hypertension and what is often referred to as “hypertension” are two distinct diseases. The term “hypertension” refers to high blood pressure in the arteries of the body circulation, which can be easily measured and diagnosed by measuring the upper extremities with a cuff. Pulmonary hypertension, on the other hand, is more difficult to diagnose. In the later stages of pulmonary hypertension, as the blood flow from the pulmonary circulation to the heart decreases, the blood pressure in the body circulation also decreases and becomes “hypotensive”.