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Pulmonary hypertension (PH) is a hemodynamic abnormality common to a variety of conditions and characterized by increased right ventricular (RV) afterload and work. The clinical manifestations, natural history, and reversibility of PH depend heavily on the nature of the pulmonary vascular lesions and the etiology and severity of the hemodynamic disorder, although individual variability exists. For example, subacute or chronic hypoxia predominantly causes increased muscularization of the small muscular pulmonary arteries and arterioles with the intima relatively intact. Relief of the hypoxia improves or occasionally reverses the process with little or no pathologic residue.1,2 In contrast, the lesions of systemic sclerosis (scleroderma), mostly confined to the intima of the small pulmonary arteries and arterioles, are usually progressive and irreversible. Unlike these two examples, which spare the pulmonary capillary bed, the pulmonary capillaries are the primary site of involvement in pulmonary capillary hemangiomatosis.3
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Because of its large capacity, its great distensibility, its low resistance to blood flow, and the modest amounts of smooth muscle in the small arteries and arterioles, the pulmonary circulation is not predisposed to become hypertensive. In normal individuals lying supine, systolic pressure is approximately 15 to 25 mm Hg; the corresponding diastolic pressure is 5 to 10 mm Hg. The mean driving pressure (ie, the difference between the mean blood pressure in the pulmonary artery and in the left atrium, the transpulmonary gradient) is usually < 10 mm Hg. Because blood flow (cardiac output) is the same in both circulations in the absence of any systemic to pulmonary communications, the pulmonary vascular resistance (PVR) is approximately one-eighth of systemic vascular resistance. The large cross-sectional surface area of the pulmonary circulation, coupled with the distensibility of its thin-walled vessels and the large recruitable vascular reserve, account for these unique characteristics. During exercise, as pulmonary blood flow increases, new regions of the pulmonary vascular bed are open and existing vasculature dilates; accordingly, the pulmonary circulation is capable of accommodating a fourfold or greater increase in resting blood flow with virtually no change in pulmonary artery pressure, with a concomitant decrease in PVR.
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When total cross-sectional area is decreased by destruction or obliteration of lung tissue or occlusive lesions in the resistance vessels, pulmonary arterial pressures increase. The degree of PH that develops is a function of the amount of the pulmonary vascular tree that has been eliminated. PH is most often associated with cardiac or pulmonary diseases. Although idiopathic pulmonary arterial hypertension (IPAH; formerly known as primary pulmonary hypertension [PPH]) is uncommon, it is well recognized as a distinctive clinical entity in which intrinsic pulmonary vascular disease is free of the complicating features of PH contributed by diseases of the heart and/or lungs. Mild PH can exist for a lifetime without becoming evident clinically. For example, native residents at high altitude, in whom mild-to-moderate PH is a natural result of sustained exposure to hypoxia, can adapt and function normally. When ...