WHO Group 1 pulmonary hypertension is also known as pulmonary arterial hypertension (PAH) and is distinguished by high blood pressure that specifically occurs in the vessels that supply the lungs. This is distinct from more generalized hypertension which is characterized by high blood pressure in vessels throughout the body. In PAH, the blood vessels in the lung stiffen and become narrower, which increases the pressure in the arteries and causes the heart to pump harder in order to pass blood through them. Over time, the added stress from heightened blood pressure in the lungs can cause the heart to weaken. Patients with PAH commonly experience symptoms such as shortness of breath, fatigue, chest pain, dizziness and/or fainting. These patients can also experience difficulties in completing normal activities such as climbing stairs.