Pathophysiology

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The cardiac valve leaflets are the portion of the heart most susceptible to infection because of their limited blood supply. Endocarditis can occur with normal valves but is more common with congenital and acquired valve disease and prosthetic valves. Bacteria and fungi gain entry to the circulation through various routes and settle on valvular tissue. A platelet-fibrin matrix forms, and further growth of the organisms forms a vegetation on the valve that makes the organisms inaccessible to normal cellular host defenses. Risk factors for infective endocarditis include congenital or acquired valvular heart disease, intravenous drug abuse, prosthetic valves, hemodialysis or peritoneal dialysis, indwelling venous catheters, postcardiac surgery, and calcific valve degeneration that occurs with increasing age. Rheumatic heart disease, although still important, is declining in frequency.

Infective endocarditis can be divided into acute and subacute forms, depending on the virulence of the infecting organism. Subacute disease more commonly infects abnormal valves, while acute disease more commonly infects previously normal valves. In the acute form, devastating complications are more common, including rapid disruption of the valve, leading to incompetence and heart failure. Embolism of the vegetations is responsible for many of the clinical features of the disease in both forms. Younger patients are more likely to have acute endocarditis, while older individuals are more likely to have subacute disease. In the subacute form, anemia is common and is probably a reflection of the chronicity of the disease. Antibodies form in reaction to foreign antigen, and immune-complex injury to basement membranes of the kidney may result in glomerulonephritis, which can occur in both acute and subacute disease.

Endocarditis can be further divided into left and right heart disease. Left-sided disease (aortic and mitral involvement) is the most common, except in injecting drug users. The most common organisms include S. viridans (declining in frequency), S. aureus (increasing in frequency), Enterococcus, and fungal organisms. Pseudomonas and Serratia are important etiologic agents in intravenous drug users in certain areas of the United States, especially Detroit and San Francisco, respectively. Cardiac failure is the most common cause of death in left-sided disease, but deaths due to neurologic complications are increasing. Patients with aortic involvement are more prone to ring abscess and atrioventricular block. Vegetations may embolize from the left heart, causing neurologic complications, systemic infarction, or metastatic infection.

Right-sided disease is usually seen in intravenous drug abusers (60 percent) and is caused by S. aureus (75 percent) and Streptococcus pneumonia (20 percent), gram-negative organisms (4 percent), and fungal organisms.16 Vegetations may embolize from the right heart, causing pulmonary infection or infarction. The fatality rate for right-sided endocarditis is lower than that for left-sided disease because the incidence of cardiac failure is less than that for left-sided disease.

Children with endocarditis most commonly have complex congenital heart disease (35 percent) or unrepaired ventricular septal defect (14 percent). Echocardiography may fail to uncover vegetations despite the presence of acute infection. Staphylococcus aureus is the most common organism isolated.

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