Primary Pulmonary Hypertension is an increased pressure within the pulmonary arteries without any apparent cause. Secondary Pulmonary Hypertension is a secondary disorder whose cause is known. Blood clots, schleroderma, and emphysema are examples of cardio-pulmonary diseases that have been known to cause pulmonary hypertension. The diet drug Fen-Phen is also linked to an increased incidence of PH.