The condition may also develop secondary to lung disease or hypoxemia, such as chronic obstructive pulmonary disease, interstitial lung disease, and obstructive sleep apnea. Pulmonary hypertension may develop as the result of chronic pulmonary thromboembolism. Pulmonary hypertension of this type may be of unknown cause (so-called primary pulmonary hypertension) or secondary to conditions, such as connective tissue disease, sarcoidosis, and certain inherited conditions. The term applies particularly to diseases that affect the small pulmonary arteries and markedly increase their resistance to blood flow. Pulmonary hypertension is present when the pulmonary artery pressures are elevated above normal. If these pressure elevations are sufficiently severe or sustained, right-sided heart failure may develop. A number of disease processes affect the pulmonary circulation and increase the pressure levels in the pulmonary arteries and right ventricle. The normal pulmonary artery systolic pressure is 20 mm Hg or less, and the normal mean (average) pulmonary artery pressure is 12 mm Hg. This critical circulation of blood through the vessels of the lungs-the pulmonary arteries, capillaries, and veins-normally occurs at much lower pressures than blood flow through the systemic circulation to the rest of the body. The right side of the heart collects venous blood from the body and pumps it into the lungs, where oxygen is taken up from inspired air and carbon dioxide is released in the expired air.
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