Science Fair Project Encyclopedia
Endocarditis can be classified as either infective or non-infective, depending on whether a foreign organism is causing the problem.
As the valves of the heart do not actually receive any blood supply of their own, which may be surprising given their location, defense mechanisms (such as white blood cells) cannot enter. So if an organism (such as bacteria) establish hold on the valves, the body cannot get rid of them.
Normally, blood flows pretty smoothly through these valves. If they have been damaged (for instance in rheumatic fever) bacteria have a chance to take hold.
Traditionally, infective endocarditis has been divided into acute and sub-acute endocarditis. This terminology is now discouraged, as the word sub-acute is considered an oxymoron. Nowadays, the terms short incubation (meaning about less than six weeks), and long incubation (greater than six weeks), are preferred.
Infective endocarditis may also be classified as culture-positive or culture-negative. Culture-negative endocarditis is due to micro-organisms that require a longer period of time to be identified in the laboratory. Such organisms are said to be fastidious because they have demanding growth requirements. Some pathogens responsible for culture-negative endocarditis include Aspergillus species, Brucella species, Coxiella burnettii, Chlamydia species, and HACEK bacteria.
Finally, the distinction between native-valve endocarditis and prosthetic-valve endocarditis is clinically important.
Aetiology and pathogenesis
As previously mentioned, altered blood flow around the valves is a risk factor in obtaining endocarditis. The valves may being damaged congenitally, from surgery, by auto-immune mechanisms, or simply as a consequence of old age. The damaged part of a heart valve becomes covered with a blood clot, a condition known as non-bacterial thrombotic endocarditis (NBTE).
In a healthy individual, a bacteraemia, (where bacteria get into the blood stream through a minor cut or wound,) would normally be cleared quickly with no adverse consequences. If a heart valve is damaged and covered with a piece of blood clot, this provides a place for the bacteria to attach themselves, and an infection is established.
Another group of causes result from a high number of bacteria getting into the bloodstream. Colorectal cancer, serious urinary tract infections and IV drug use, can all introduce large numbers of bacteria. With a large number of bacteria, even a normal heart valve may be infected. A more virulent organism (such as Staphylococcus aureus) is usually responsible for infecting a normal valve.
Intravenous drug users tend to get their right heart valves infected because the veins that are injected enter the right side of the heart. The injured valve is most commonly affected when there is a pre-existing disease. (In rheumatic heart disease this is the aortic and the mitral valves, on the left side of the heart.)
Clinical and pathological features
- Fever (often spiking)
- Continuous presence of micro-organisms in the bloodstream determined by serial collection of blood cultures
- Vegetations on valves on echocardiography
- septic emboli , causing circulatory problems (stroke, gangrene of fingers)
- chronic renal failure
- Osler's nodes and Janeway lesions (painful subcutaneous lesions in the distal fingers; and painless hemorrhagic cutaneous lesions on the palms and soles, respectively)
- Roth spots on the retina
- Conjunctival petechiae
- A new or changing heart murmur, particularly murmurs suggestive of valvular incompetence
Many types of organism can cause infective endocarditis. These are generally isolated by blood culture, where the patient's blood is removed, and any growth is noted and identified.
Alpha-haemolytic streptococci, that are present in the mouth will often be the organism isolated if a dental procedure caused the bacteraemia.
If the bacteraemia was introduced through the skin, such as contamination in surgery, during catheterisation, or in an IV drug user, Staphylococcus aureus is common.
A third important cause of endocarditis is Enterococcus species. These bacteria enter the bloodstream as a consequence of abnormalities in the gastrointestinal or urinary tracts. Enterococcus species are increasingly recognized as causes of nosocomial or hospital-acquired endocarditis. This contrasts with alpha-haemolytic streptococci and Staphylococcus aureus which are causes of community-acquired endocarditis.
Some organism when isolated give valuable clues to the cause, as they tend to be specific.
- Candida albicans, a yeast, is associated with IV drug users and the immunocompromised.
- Pseudomonas species, which are very resilient organisms that thrive in water, are may contaminate street drugs that have been contaminated with drinking water.
- Streptococcus bovis , which is part of the natural flora of the bowel, tends to present when the patient has bowel cancer.
- HACEK organisms are a group of bacteria that live on the dental gums, and are associated with IV drug users who contaminate their needles with saliva.
High dose antibiotics are administered by the intravenous route to maximize diffusion of antibiotic molecules into vegetation(s) from the blood filling the chambers of the heart. This is necessary because neither the heart valves nor the vegetations adherent to them are supplied by blood vessels. Antibiotics are continued for a long time, typically two to six weeks. Surgical removal of the valve is necessary in patients who fail to clear micro-organisms from their blood in response to antibiotic therapy, or in patients who develop cardiac failure resulting from destruction of a valve by infection. A removed valve is usually replaced with an artificial valve which may either be mechanical (metallic) or obtained from an animal such as a pig; the latter are termed bioprosthetic valves. Infective endocarditis is assocaited with a 25% mortality.
Non-infective or marantic endocarditis is rare. A form of sterile endocarditis is termed Libman-Sacks endocarditis; this form occurs more often in patients with lupus erythematosus and the antiphospholipid syndrome. Non-infective endocarditis may also occur in patients with cancers.
The contents of this article is licensed from www.wikipedia.org under the GNU Free Documentation License. Click here to see the transparent copy and copyright details