WHO Group 3 pulmonary hypertension is caused by chronic lung disease and/or low levels of oxygen (i.e., hypoxia). Diseases that restrict the expansion of lungs when a patient breathes in (e.g., idiopathic pulmonary fibrosis, interstitial lung disease), obstructive lung diseases that make exhalation difficult (e.g., COPD), sleep apnea, and prolonged exposure to high altitude commonly underly WHO Group 3 pulmonary hypertension. In response to the impaired function of the lungs or more limited oxygen access, the arteries of the lung constrict in order to direct blood to regions of the lung that receive the most oxygen. The constriction of the vessels in the lung leads to increased blood pressure.
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.