Pulmonary Hypertension: Overview Make an Appointment Find a Doctor Ask a Question Reviewed by Marjorie Patricia George, MD (July 01, 2019) Pulmonary hypertension refers to a condition in which high blood pressure exists within the vessels of the lungs. Normally, venous (low oxygen) blood returns from the body to the right side of the heart. The blood is pumped to the lungs via the pulmonary arteries. Oxygen is transferred into the blood at the alveolus-capillary interface, and then returns through the pulmonary veins to the left side of the heart, where it is pumped to the rest of the body to deliver oxygen to organs and tissues. Classifications of Pulmonary Hypertension There are many different types of pulmonary hypertension, and these types are categorized into different groupings, known as the World Health Organization (WHO) classification system. These different groupings reflect the different causes of the disease (and therefore different recommended treatments). These categories include: WHO Group I: Pulmonary hypertension due to a disease of the arteries themselves. Examples include idiopathic (of unknown cause), heritable or related to autoimmune disease WHO Group II: Pulmonary hypertension due to left-sided heart disease (e.g., heart failure or heart valve problems) WHO Group III: Pulmonary hypertension due to lung disease and/or low oxygen levels, including sleep disorders WHO Group IV: Pulmonary hypertension due to blockages in the pulmonary arteries (e.g., chronic blood clots in the lung) WHO Group V: Pulmonary hypertension associated with diseases that don’t fall into the other four categories such as sickle cell disease. See how your heart can affect your breathing in our infographic explaining the heart/lung connection. Learn about the Pulmonary Hypertension Program at National Jewish Health. Associated Conditions Pulmonary hypertension can occur in isolation or, more commonly, with diseases of the lungs and heart. Pulmonary hypertension in the absence of other diseases is very rare and is often idiopathic, associated with autoimmune disease or familial in nature. This kind of pulmonary hypertension is referred as pulmonary arterial hypertension (PAH). Pulmonary arterial hypertension can also be associated with drug use, such as methamphetamines or diet drugs; human immunodeficiency virus (HIV); liver disease; and congenital heart disease. Pulmonary hypertension is commonly associated with a variety of lung conditions with low oxygen levels. These include COPD, emphysema, interstitial lung disease, chronic pulmonary blood clots or sleep apnea. When pulmonary hypertension arises from cardiac conditions such as heart failure or heart valve disease, it is sometimes referred to as pulmonary venous hypertension. And while there are specific types of pulmonary hypertension, it is important to note that pulmonary hypertension can be associated with multiple causes. Programs & Services Cardiology Programs Pulmonary Hypertension Program Clinical Trials For more than 100 years, National Jewish Health has been committed to finding new treatments and cures for diseases. Search our clinical trials.