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.