Conduction abnormalities that develop in patients with active
infective endocarditis may represent extension of infection from the
valve leaflets into the surrounding myocardium. Extension of infection
from the aortic valve into the septum can lead to significant
conduction abnormalities, with bundle-branch blocks or complete heart
block. Extension from the mitral valve is less common, and the
conduction abnormalities resulting from such extension include
low-grade atrioventricular blocks or supraventricular arrhythmias. A
study of cardiac anatomy makes it clear why aortic valve endocarditis
more commonly causes significant conduction problems than mitral valve
endocarditis. The conduction system, particularly the right and left
bundles, is closely related anatomically to the aortic valve. The
atrioventricular node, though adjacent to the mitral valve, is not as
close to this valve as the aortic valve is to the bundle branches.
Therefore, mitral valve endocarditis that spreads to perivalvular
tissue usually causes first-degree or second-degree heart block.
Third-degree heart block is unusual; if present, it is accompanied by a
narrow complex QRS. Aortic valve endocarditis can result in
first-degree or second-degree heart blocks in addition to bundle-branch
blocks, hemiblocks, and complete heart block. These latter
complications are not uncommon, especially if noncoronary and right
cusps of the valve are involved. Perivalvular extension of infection is
a serious complication of bacterial endocarditis. The most common type
of extension of the infection involves the cardiac tissue immediately
adjacent to the valve ring (also referred to as perivalvular abscess).
Other types include the development of aneurysms, intracardiac
fistulas, and valve dehiscence. Aortic valve infection most commonly
leads to perivalvular extension.