心内膜炎 I: はじめに

Infective endocarditis is a microbial infection of the endocardium, the inner lining of the heart and its valves. Pathogens adhere to heart valves or chamber lining, forming vegetation that can lead to severe complications. The condition primarily affects the endocardium, distinguishing it from myocarditis, which involves the heart muscle, and pericarditis, which affects the outer lining.

Staphylococcus aureus and viridans group streptococci are the predominant causative agents of infective endocarditis. Staphylococcus aureus typically causes acute forms with rapid progression, while viridans streptococci usually cause subacute endocarditis that develops more slowly. Fungal infections, though rare, can also cause the disease, with Candida species being the most notable fungal pathogen.

Microorganisms can enter the bloodstream through several routes, including central venous catheter use, injection drug use with unsterile needles, and dental procedures such as tooth extractions. Endoscopic examinations can also serve as a pathway for pathogens to infect the heart. These modes of entry allow bacteria to travel through the circulatory system and attach to heart valves.

Acute infective endocarditis progresses rapidly, often within days to a few weeks, and is commonly caused by highly virulent organisms like Staphylococcus aureus. Subacute endocarditis progresses slowly over weeks to months with less severe symptoms, typically caused by less virulent bacteria such as viridans streptococci. The clinical features and disease progression differ significantly between these two forms.

Prosthetic valve endocarditis occurs after heart valve replacement surgery when bacteria are introduced during or after the procedure, infecting the artificial valves. This type presents unique diagnostic and therapeutic challenges compared to native valve infections. Patients with prosthetic valves require careful monitoring and preventive measures to reduce infection risk.

Intravenous drug use-associated endocarditis is prevalent because unsterile needles introduce bacteria directly into the bloodstream, bypassing normal defense mechanisms. Staphylococcus aureus is commonly involved in this form of endocarditis. Right-sided heart valves are frequently affected in injection drug users, and this population faces significant risk for developing severe infection.

Numerous factors increase the likelihood of acquiring infective endocarditis, including advanced age, congenital heart defects, and damaged or abnormal heart valves. A history of previous endocarditis and the presence of a new heart valve post-surgery also elevate risk. Long-term intravenous lines and invasive procedures serve as conduits for infection, further contributing to clinical manifestations and diagnostic challenges in high-risk populations.