Pulmonary hypertension: Pulmonary hypertension refers to high blood pressure in the blood vessels that carry blood to the lungs. Blood pressure in other parts of the body is normal or sometimes even low. The condition may be caused by such things as lung conditions (e.g. emphysema, chronic bronchitis), heart conditions (e.g. congestive heart failure, birth defects involving heart), AIDS or medications such as fenfluramine (a diet drug). Sometimes it occurs for no apparent reason and is called primary pulmonary hypertension.