Pulmonary hypertension is a rise in pressure within the pulmonary artery that is caused either by a reduction in volume of the pulmonary circulation or by an elevation of pressure in the left atrium or left ventricle. Pulmonary heart disease is said to be present if the right ventricle enlarges in response to the pulmonary hypertension and if the hypertension is due to a disorder of the pulmonary circulation rather than of the left side of the heart.

Pulmonary hypertension may result from several processes that reduce the volume of the pulmonary circulation. One such process is surgical removal of large amounts of lung tissue. Pulmonary blood vessels also can be occluded with thromboemboli. Alternatively, they may be replaced by scar tissue in patients with idiopathic pulmonary fibrosis. Finally, the pulmonary circulation may be eaten away by emphysema.

The most common cause of pulmonary hypertension is narrowing of the pulmonary capillaries in response to a low oxygen pressure in adjacent alveoli. In patients with localized disease processes such as pneumonia this response favours gas exchange by forcing blood to better ventilated regions of the lung. However, when all the alveoli have a low oxygen pressure, as occurs a high altitude and in severe respiratory disorders such as chronic bronchitis and emphysema, pulmonary vessel narrowing becomes generalized. The right ventricle therefore must generate a higher pressure to perfuse the pulmonary circulation. This prompts it to increase in size to pump more forcefully.

Despite this protective mechanism, heart muscle performance worsens. At the same time, pressure rises even further within the right ventricle as its output falls. Pressure then increases in the veins that return blood to the right ventricle from the periphery of the body, forcing water to leak from them and collect in tissues. This process first is noticeable as edema in the legs but eventually may involve the entire body. Left ventricular output also diminishes because less blood is traversing the lungs.