Infective Endocarditis

The term "infective endocarditis" has replaced the older term "subacute bacterial endocarditis." It is the most serious complication of valvular heart disease, an infection of the cardiac valves caused most often by bacteria and occasionally by fungi but not by viruses. Cardiac valves are not the only sites of infection; congenital heart lesions are also susceptible. It is unusual for endocarditis to develop on healthy valves since microorganisms tend to settle on damaged endocardium.

Bacteria in the bloodstream (bacteremia) is common even in health. A variety of organisms normally present on the skin and in the mouth, nose, and rectum may enter the bloodstream as a result of minor cuts or punctures. The body's defense mechanisms take care of eliminating the bacteria unless they can find a vulnerable spot in the inner lining of the heart and blood vessels; even then only occasionally can they take sufficient hold to cause infection. Certain conditions or procedures are likely to introduce large numbers of microorganisms into the bloodstream and thus carry a higher-than-average risk of starting an infection:

dental surgery

surgical procedures or manipulations involving the gastrointestinal and genitourinary tracts

intravenous injection of drugs among drug abusers

superficial infections such as abscesses

Invasion of microorganisms into a damaged endocardium produces a local reaction. Blood cells accumulate, including platelets and fibrin, which along with small thrombi form a structure, called a vegetation, that attaches itself to the valve. Single vegetations have the appearance of small pearls, but they tend to aggregate, producing larger structures sometimes resembling a bunch of small grapes. These vegetations can damage valvular tissue by producing holes in the valves or actually chewing up portions of them. Moreover, they may detach themselves and float away in the bloodstream, producing emboli. The consequences of infection of valves and the extent of damage are related to the type of microorganism, its aggressiveness (virulence), the extent of blood contamination, and the time antibiotic therapy is started.

The onset of endocarditis may be sudden, characterized by chills and high fever (acute endocarditis). More often, however, endocarditis develops inconspicuously and may not be recognized for weeks or even months (subacute endocarditis). In such cases patients may be aware of a certain lassitude or lack of pep. Fever is almost always present, though it may be slight. Anemia often accompanies subacute endocarditis. The correct diagnosis is frequently apparent, despite the vague symptoms, if the patient is known to be at risk for endocarditis because of valvular or congenital heart disease. However, if the presence of heart disease is undetected—as in the case of trivial valvular lesions—considerable difficulties may arise in interpreting the symptoms.

Endocarditis affects patients in several ways. The infection may spread and cause death. (The mortality rate from endocarditis is about 20 percent.) The destructive process on valves may produce acute, severe valvular regurgitation, leading to heart failure or even shock. Loose vegetations may produce embolic damage to distant organs such as the brain, kidneys, or spleen. Prompt antibiotic therapy can cure the infection without any aftereffects.

Infective endocarditis is suspected in cases of unexplained fever in patients with valvular or congenital heart lesions. A more direct clue to the diagnosis is a heart murmur not present previously. Also of aid in diagnosing the disease is echocardiography, which can detect vegetations provided they have reached significant size. The confirmatory test is a blood culture to identify the organism responsible for the infection. A sample of the patient's blood is placed in a medium, such as broth, on which bacteria thrive and is incubated at body temperature. Once the organism has been identified, the proper antibiotic therapy can be chosen and the prognosis determined.

The commonest cause of subacute infective endocarditis is the green streptococcus (Streptococcus viridans ). But almost every known microorganism capable of invading the body can, under appropriate circumstances, produce endocarditis. Some of them can be cultured and identified within 24 to 48 hours. Others may need special culture mediums and techniques. In 10–20 percent of cases the infecting organism cannot be identified.

Antibiotic therapy is required for all cases of infective endocarditis and is started as soon as the microorganism is identified. If prompt identification from blood culture is not forthcoming, interim therapy with some broadly effective antibiotics should be administered. The effect of therapy is often apparent within a few days, as the fever and malaise disappear; however, antibiotic therapy should be continued for a few weeks after the symptoms have passed, to ensure that all viable infecting organisms have been eliminated from the body. The initial therapy is usually administered intravenously and is later replaced by oral antibiotics. Follow-up blood cultures help in monitoring the success of treatment.

Whether therapy other than antibiotics is indicated depends on the extent of damage to the valves or other infected structures. Endocarditis caused by the green streptococcus is relatively benign and usually requires only antibiotic therapy. Fungi, yeasts, and more-aggressive bacteria may cause extensive valvular damage, often requiring surgical removal of the infected valve and its replacement with a prosthesis. In an emergency such as severe heart failure the surgery may be performed despite the infection, but it is preferable to wait until the organisms are destroyed by antibiotics.

Prosthetic valves themselves can be targets of bacterial infection; infected prostheses usually require replacement.

Preventive therapy should be undertaken in all patients at risk of infective endocarditis. It consists of administering an antibiotic before procedures that may introduce microorganisms into the bloodstream and continuing treatment briefly thereafter. Among conditions requiring antibiotic prophylaxis are dental extractions and surgery, diagnostic and therapeutic procedures in the gastrointestinal and genitourinary tracts, and childbirth. Antibiotic prophylaxis does not offer total protection but does reduce the probability of infecting the heart.