Cardiology
Infective Endocarditis
~11 min read4 sections
Infective endocarditis (IE) is a life-threatening infection of the heart valves and endocardium caused by bacterial, fungal, or rarely viral pathogens that gain access to the bloodstream and seed the cardiac valves. The condition affects approximately 3-10 cases per 100,000 person-years in developed countries, with incidence increasing due to aging populations, increased prevalence of prosthetic valves, and healthcare-associated bacteremia from central venous catheters and hemodialysis access. IE remains a significant cause of morbidity and mortality despite modern antibiotics, with in-hospital mortality rates of 15-20% and substantial morbidity from embolic and hemodynamic complications. Understanding the epidemiologic shifts (increasing proportion of healthcare-associated IE and fungal IE) and evolving diagnostic criteria is essential for residents, as early recognition and appropriate empiric therapy dramatically improve outcomes. The disease presents a diagnostic challenge due to its protean manifestations and requires integration of clinical, laboratory, and imaging findings using the Modified Duke Criteria for diagnosis.
The development of IE involves a multistep process requiring both host factors and pathogen virulence characteristics, culminating in progressive valve destruction and systemic manifestations:
- Endothelial damage and thrombus formation: The initiating event typically involves turbulent blood flow from pre-existing valvular disease (rheumatic, degenerative, or congenital abnormalities), prosthetic valves, or intracardiac devices that causes mechanical trauma to the endocardium. This injury disrupts the normal endothelial lining and exposes the underlying extracellular matrix (collagen, fibronectin, von Willebrand factor). Tissue factor (TF) release triggers the coagulation cascade, leading to deposition of platelets and fibrin that form an uninfected vegetation (non-bacterial thrombotic endocarditis [NBTE]). In predisposed individuals, bacteremia from transient mucosal breakdown (dental procedures, poor dentition, intravenous drug use) or healthcare-associated sources introduces pathogens into the circulation. These thrombi essentially serve as a "nidus" that is avascular and thus poorly penetrated by immune cells and antibiotics.
- Bacterial adhesion and colonization mechanisms: Once bacteremia occurs, specific bacterial adhesins (surface proteins) interact with exposed host cell adhesion molecules (integrins, selectins) and matrix proteins on the vegetation surface. Streptococcus viridans and other streptococci express lipoteichoic acids and serine-rich repeat proteins that bind fibronectin and collagen. Staphylococcus aureus, particularly methicillin-resistant strains, expresses coagulase and other virulence factors that promote rapid colonization. Enterococci express adhesins that facilitate binding to heart valve tissues. Pathogenic gram-negative organisms (HACEK group: Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella) employ their own specific mechanisms. Once adherent, bacteria undergo phenotypic changes—they reduce metabolic rates and form biofilm-like structures within the vegetation, making them metabolically dormant and less susceptible to beta-lactam antibiotics that target actively dividing cells. This explains why bacteriostatic agents (e.g., vancomycin) are often added to bactericidal agents for synergy in IE therapy.
- Progressive valve destruction and immune response: As bacteria multiply within the vegetation, they produce hydrolytic enzymes and toxic metabolites that directly damage valve tissue. Bacterial lipopolysaccharide (LPS, in gram-negatives) and peptidoglycans act as damage-associated molecular patterns (DAMPs) that activate complement via both classical and alternative pathways, generating C3a and C5a and recruiting neutrophils and monocytes. This inflammatory cascade is responsible for many systemic manifestations of IE. Immune complexes (circulating antigen-antibody complexes) deposit in small vessels, causing vasculitis and the characteristic Osler nodes (tender nodules on fingertips), Janeway lesions (painless macules on palms/soles), and splinter hemorrhages (linear hemorrhages under nails). Rheumatoid factor appears in 50% of subacute IE due to chronic immune stimulation. The vegetation progressively extends, causing valve regurgitation (most common) through mechanical damage and perforation of leaflets, or rarely stenosis. The infection may erode into the conduction system, causing heart block, or into myocardium/pericardium, causing abscess formation or purulent pericarditis.
- Septic embolism and metastatic infection: Fragments of infected vegetation continuously embolize into systemic circulation, lodging in distal organs and causing septic infarcts. Unlike bland emboli from atrial fibrillation, IE emboli contain viable organisms that can initiate infection in distal tissues. Septic emboli to the brain cause septic cerebritis, microabscesses, and mycotic aneurysms (misnomer; these are infected aneurysms, not fungal, at branch points of medium vessels). Pulmonary emboli from right-sided IE cause septic pneumonia with abscess formation. Renal emboli cause infarction and hematuria. Splenic emboli cause abscess and infarction. The phenomenon of progressive emboli despite appropriate antibiotics is due to the protected nidus of bacteria within the vegetation and explains why some patients require urgent surgical intervention.
- Hemodynamic consequences: Progressive valve destruction causes acute or subacute valve regurgitation, particularly aortic insufficiency in left-sided IE. The large volume load from aortic regurgitation causes eccentric left ventricular hypertrophy and eventually systolic dysfunction if severe and prolonged. Right-sided IE causes tricuspid regurgitation leading to right ventricular dilation and hepatic congestion. Vegetations may obstruct valve orifices causing stenosis, though this is less common. Myocardial involvement through abscess extension impairs contractility and predisposes to cardiogenic shock.
- Microbiological determinants of disease expression: The bacterial species involved determines the tempo and severity of disease. S. aureus (particularly healthcare-associated MRSA) causes acute, rapidly progressive IE with large friable vegetations, extensive myocardial involvement, and septic emboli—accounting for up to 40% of healthcare-associated IE cases. S. viridans and other viridans streptococci cause indolent subacute disease with smaller vegetations and fewer emboli. Enterococci cause slowly progressive disease. HACEK organisms cause predominantly prosthetic valve IE with subacute course. Fungi (Candida, Aspergillus, Cryptococcus) cause indolent disease typically in immunocompromised hosts or following cardiac surgery, with large vegetations and high embolic rates. This pathogen-dependent variability in presentation is crucial for understanding why empiric therapy and diagnostic urgency differ depending on clinical context.
Understanding the etiology requires differentiating native valve endocarditis (NVE) from prosthetic valve endocarditis (PVE) and hospital-acquired endocarditis, as microbiology and risk factors differ:
- Streptococcus viridans and other alpha-hemolytic streptococci: These are the most common cause of subacute NVE (50-60% of cases), constituting part of normal oral flora. Infection typically follows dental procedures (extraction, cleaning, implantation) or poor dental hygiene leading to transient bacteremia. Risk factors include pre-existing valvular disease (rheumatic heart disease, most common in developing nations; mitral valve prolapse with regurgitation; degenerative valve disease in elderly), congenital heart disease, and bicuspid aortic valve. These organisms have low virulence and preferentially infect abnormal valves. Prophylactic antibiotics are recommended for high-risk patients undergoing dental procedures.
- Staphylococcus aureus: Now the most common cause of acute IE overall (30-40% of cases), reflecting epidemiologic shifts toward healthcare-associated disease. Methicillin-susceptible S. aureus (MSSA) is more common in community-acquired IE, while methicillin-resistant S. aureus (MRSA) dominates healthcare-associated and injection drug use (IDU)-associated IE. S. aureus uniquely infects normal valves, causing rapidly progressive, destructive disease with high rates of septic emboli, myocardial involvement, and mortality (20-40%). Right-sided IE in IDU patients typically causes S. aureus tricuspid endocarditis with septic pulmonary emboli. Risk factors include: intravenous drug use (most common modifiable risk factor; accounts for ~10% of IE with increasing trend), central venous catheters, hemodialysis access (infected vascular catheters are a major source), cardiac devices (pacemakers, ICDs, implantable cardioverter-defibrillators), recent cardiac surgery, and healthcare exposures.
- Enterococcus species: Account for 5-15% of IE, typically in elderly or immunocompromised patients. E. faecalis (>80% of enterococcal IE) is associated with urinary tract infections, biliary procedures, and colorectal disease. E. faecium is increasingly prevalent in healthcare settings and often carries vancomycin resistance. Enterococcal IE is typically left-sided, subacute, and associated with high risk of embolic complications and treatment failure due to intrinsic resistance to cephalosporins and many other agents. Ampicillin-susceptible E. faecalis may be treated with ampicillin or penicillin; ampicillin-resistant strains and E. faecium require vancomycin, though resistance is emerging.
- Staphylococcus epidermidis and other coagulase-negative staphylococci: These are the most common cause of early prosthetic valve endocarditis (within 60 days of surgery; ~50% of early PVE cases) and are typically nosocomially acquired via contamination during cardiac surgery or through intracardiac device infection. They preferentially infect prosthetic material and are difficult to eradicate due to biofilm formation. Late prosthetic valve endocarditis (>60 days) is typically caused by the same organisms as NVE (viridans streptococci, S. aureus). Coagulase-negative staph is also the leading cause of infected pacemakers and ICDs.
- HACEK group organisms (Haemophilus influenzae, Aggregatibacter actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, Kingella kingae): These fastidious, slow-growing gram-negative organisms account for only 1-3% of IE but are highly associated with prosthetic valve endocarditis and intracardiac device infection. They are part of normal oral flora and can follow poor dentition or oral procedures. HACEK IE characteristically presents subacutely with embolic phenomena. Culture requires extended incubation periods (10-14 days), and these organisms may grow in special media.
- Streptococcus bovis/gallolyticus: This organism is strongly associated with colorectal malignancy and polyps and should trigger colonoscopy even in the absence of gastrointestinal symptoms. S. bovis IE presents subacutely and typically involves normal or mildly abnormal valves. The association with colorectal disease may relate to disruption of the colonic mucosa allowing bacteremia.
- Gram-negative bacilli (E. coli, Klebsiella, Pseudomonas, other Enterobacteriaceae): These are uncommon causes of NVE but increasingly recognized in healthcare-associated IE, particularly with central lines and urinary catheters. They are more common in prosthetic valve and device-related IE. Empirically, these are covered by broad-spectrum beta-lactams or fluoroquinolones in initial therapy.
- Fungal endocarditis (Candida species most common, followed by Aspergillus, Cryptococcus, Histoplasma): Accounts for only 2-4% of IE but increasingly recognized with expanded use of immunosuppressive therapy, cardiac surgery, and long-term central lines. Risk factors include prolonged antibiotics (selecting for fungal overgrowth), immunosuppression, prosthetic valves, and cardiac surgery. Candida is the most common fungal pathogen, often from indwelling catheters or contaminated infusates. Fungal IE typically presents subacutely with large vegetations, high embolic rates, and is associated with poor outcomes without surgical intervention. Empiric coverage requires consultation with infectious disease specialists.
- Blood culture-negative endocarditis: Accounts for 5-10% of IE despite appropriate culture technique. Common causes include: prior antibiotics (even a single dose can sterilize cultures for weeks), fastidious organisms requiring special media (HACEK, Legionella, Coxiella burnetii, Bartonella, Mycobacterium, fungi), and organisms with nutritional requirements not met by standard media. Serologic testing for atypical organisms and molecular methods (PCR, 16S rRNA gene sequencing) are increasingly employed.
- Right-sided endocarditis: Predominantly affects the tricuspid valve (90% of right-sided cases), predominantly in intravenous drug users (>50% of IE in IDU is right-sided). Right-sided IE typically presents with septic pulmonary emboli causing pleurisy, hemoptysis, and infiltrates on imaging. S. aureus predominates in IDU-associated right-sided IE. Left-sided valves are occasionally infected in IDU patients, usually in those with congenital heart disease or existing valve abnormalities.
- Prosthetic valve endocarditis: Early PVE (≤60 days post-op) occurs in 1-2% of valve replacements and is usually caused by organisms introduced during surgery (coagulase-negative staph, S. aureus, gram-negatives, Candida). Late PVE is caused by the same organisms as native valve disease. Prosthetic valves have foreign material and disrupted endothelium, making them vulnerable. Mechanical prostheses may be more resistant to infection than bioprostheses.
IE presents across a wide spectrum from insidious subacute disease to fulminant sepsis, and the clinical presentation is highly variable, contributing to diagnostic delays:
- Fever: Present in 90% of cases but highly variable in character and degree. In acute IE (especially S. aureus), fever is high and remittent (reaching 39-40°C daily). In subacute IE (viridans streptococci, enterococci, HACEK organisms), fever is low-grade and often accompanied by night sweats. Elderly patients and those on antipyretics may have blunted febrile responses. Fever results from direct bacterial endotoxin effects and robust inflammatory responses. The absence of fever makes IE less likely but does not exclude it.
- Fatigue, malaise, and weight loss: Nonspecific constitutional symptoms that develop insidiously over weeks in subacute IE, reflecting the chronic inflammatory state. Weight loss results from anorexia and hypermetabolism. These vague symptoms often lead to misdiagnosis as connective tissue disease or malignancy, causing diagnostic delays.
- New or changing cardiac murmurs: A new cardiac murmur is present in 30-40% of patients and suggests valve destruction or perforation. An aortic insufficiency (AI) murmur—a high-pitched, blowing diastolic murmur best heard along the left sternal border with the patient sitting forward—is most commonly associated with IE and results from aortic valve perforation or vegetation preventing leaflet closure during diastole. A new mitral regurgitation (MR) murmur—a holosystolic murmur best heard at the apex radiating to the axilla—occurs with mitral leaflet perforation or ruptured papillary muscle. Right-sided tricuspid regurgitation produces a systolic murmur along the left lower sternal border that increases with inspiration (Carvallo sign). Importantly, in acute IE with large vegetations, murmurs may be absent initially because the vegetation acts as a space-occupying lesion rather than creating turbulent flow. The clinical pearl is that absence of a murmur does not exclude IE, particularly in the acute presentation and right-sided disease. Previously unknown murmurs may be chronic (rheumatic disease, mitral valve prolapse) and unrelated.
- Septic emboli and metastatic infection: Septic emboli occur in 15-30% of IE cases and manifest as systemic evidence of infection in multiple organ systems—a key clinical feature distinguishing IE from other causes of fever. Septic emboli to the lungs (more common in right-sided IE) cause pleurisy, pleural effusion, pulmonary nodules, and septic pulmonary infarcts (wedge-shaped infiltrates), sometimes with cavitation or empyema. Patients present with cough, dyspnea, and hemoptysis. Splenic abscesses occur in 5-15% of cases and may present as left upper quadrant pain, splenic rupture, or may be asymptomatic and detected on imaging