Pulmonary Hypertension
In both the rare primary form and the more common secondary form, pulmonary hypertension is indicated by a resting systolic pulmonary artery pressure (PAP) above 30 mm Hg and a mean PAP above 18 mm Hg.
Primary or idiopathic pulmonary hypertension is characterized by increased PAP and increased pulmonary vascular resistance, both without an obvious cause. This form is most common in females between ages 20 and 40 and is usually fatal within 3 to 4 years; mortality is highest in pregnant females.
Secondary pulmonary hypertension results from existing cardiac or pulmonary disease or both. The prognosis in secondary pulmonary hypertension depends on the severity of the underlying disorder.
Causes
Although the cause of primary pulmonary hypertension remains unknown, the tendency for the disease to occur within families points to a hereditary defect. It also occurs more commonly in those with collagen disease and is thought to result from altered immune mechanisms. In primary pulmonary hypertension, the intimal lining of the pulmonary arteries thickens for no apparent reason. This narrows the artery and impairs distensibility, increasing vascular resistance.
Secondary pulmonary hypertension results from hypoxemia. (See Causes of secondary pulmonary hypertension, page 768.)
Complications
Pulmonary hypertension may ultimately lead to cor pulmonale, cardiac failure, and cardiac arrest.
Causes of secondary pulmonary hypertension
Secondary pulmonary hypertension can stem from any of the following:
Alveolar hypoventilation. This can result from diseases that cause alveolar destruction, such as chronic obstructive pulmonary disease (the most common cause in the United States), sarcoidosis, diffuse interstitial pneumonia, malignant metastases, and scleroderma. Or, it can stem from obesity or kyphoscoliosis, neither of which damages lung tissue but prevents the chest wall from expanding sufficiently to let air into the alveoli.
Either way, the decreased ventilation that results increases pulmonary vascular resistance. Hypoxemia resulting from this ventilation-perfusion mismatch also causes vasoconstriction, further increasing vascular resistance. The result is pulmonary hypertension.
Vascular obstruction. Such obstruction can arise from pulmonary embolism or vasculitis. It can also result from disorders that cause obstructions of small or large pulmonary veins, such as left atrial myxoma, idiopathic venoocclusive disease, fibrosing mediastinitis, and mediastinal neoplasm.
Primary cardiac disease. Such disease may be congenital or acquired. Congenital defects that cause left-to-right shunting include patent ductus arteriosus and an atrial or a ventricular septal defect. This shunting into the pulmonary artery reroutes blood through the lungs twice, causing pulmonary hypertension.
Acquired cardiac diseases, such as rheumatic valvular disease and mitral stenosis, result in left-sided heart failure which diminishes the flow of oxygenated blood from the lungs. This increases pulmonary vascular resistance and right ventricular pressure.
Assessment
The patient with primary pulmonary hypertension may have no signs or symptoms until lung damage becomes severe. (In fact, the disorder may not be diagnosed until an autopsy.)
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