# Infective Endocarditis: Duke Criteria and Management Dilemmas

## Introduction

Infective endocarditis (IE) is a microbial infection of the heart valves or endocardial surface that carries high morbidity and mortality despite advances in diagnosis and treatment. The internist plays a critical role in clinical suspicion, appropriate blood culture collection, and coordinating multidisciplinary care. Delayed diagnosis remains a major contributor to poor outcomes.

## Epidemiology and Risk Factors

### Predisposing Conditions

- **Degenerative valve disease** has replaced rheumatic heart disease as the leading risk factor in developed countries
- **Prosthetic heart valves**: highest risk in the first year after implantation
- **Injection drug use (IDU)**: predominantly right-sided (tricuspid) involvement; *S. aureus* is the most common pathogen
- Intracardiac devices (pacemakers, defibrillators), poor dentition, hemodialysis catheters
- **Bicuspid aortic valve** and mitral valve prolapse with regurgitation

## Microbiology

- **Staphylococcus aureus**: most common cause overall; acute presentation; high mortality
- **Viridans group streptococci**: subacute presentation; associated with dental procedures
- **Enterococcus faecalis**: older adults, genitourinary tract source
- **HACEK organisms**: fastidious gram-negative rods; subacute course
- **Culture-negative endocarditis** (5-10%): prior antibiotics, Coxiella burnetii, Bartonella, fungi

## Diagnosis

### Modified Duke Criteria

- **Definite IE**: 2 major, or 1 major + 3 minor, or 5 minor criteria
- **Possible IE**: 1 major + 1 minor, or 3 minor criteria

| Category | Criterion | Details |
|----------|-----------|---------|
| **Major** | Blood cultures | Typical organism from 2 separate cultures, or persistently positive cultures |
| **Major** | Endocardial involvement | Positive echo (vegetation, abscess, dehiscence) or new valvular regurgitation |
| **Minor** | Predisposition | Predisposing heart condition or injection drug use |
| **Minor** | Fever | Temperature >= 38.0°C |
| **Minor** | Vascular phenomena | Septic emboli, Janeway lesions, mycotic aneurysm, conjunctival hemorrhage |
| **Minor** | Immunologic phenomena | Osler nodes, Roth spots, glomerulonephritis, positive RF |
| **Minor** | Microbiologic evidence | Positive blood cultures not meeting major criteria |

### Major Criteria

- **Blood cultures**: typical organism from 2 separate cultures, or persistently positive cultures
- **Endocardial involvement**: positive echocardiogram showing vegetation, abscess, or new valvular regurgitation

### Minor Criteria

- Predisposing heart condition or injection drug use
- Fever >= 38.0 C
- Vascular phenomena: septic emboli, Janeway lesions, mycotic aneurysm, conjunctival hemorrhage
- Immunologic phenomena: Osler nodes, Roth spots, glomerulonephritis, positive rheumatoid factor
- Positive blood cultures not meeting major criteria

![Transesophageal echocardiogram showing a large mitral valve vegetation](/images/residency/endocarditis-tee-vegetation.jpg)

### Blood Culture Technique

- Obtain **at least 3 sets (6 bottles)** from separate venipuncture sites before antibiotics
- Space collections over 1-2 hours if the patient is stable
- Do **not** draw cultures from indwelling lines if possible; peripheral cultures are preferred
- Hold antibiotics until cultures are drawn unless the patient is critically ill

### Echocardiography

- **Transthoracic echo (TTE)**: initial study; sensitivity 50-75% for native valve IE
- **Transesophageal echo (TEE)**: sensitivity > 90%; indicated when TTE is negative but suspicion remains high, prosthetic valves, or suspected complications
- Repeat TEE in 5-7 days if initial study is negative and clinical suspicion persists

## Management

### Antibiotic Therapy

| Organism | Preferred Regimen | Duration |
|----------|------------------|----------|
| Empiric (native valve) | Vancomycin + ceftriaxone | Until culture results |
| MSSA (native valve) | Nafcillin or cefazolin | 6 weeks |
| MRSA | Vancomycin or daptomycin | 6 weeks |
| Viridans streptococci (MIC ≤ 0.12) | Ceftriaxone (or penicillin G) | 4 weeks (or 2 weeks with gentamicin) |
| Enterococcus | Ampicillin + ceftriaxone (preferred) | 6 weeks |
| Prosthetic valve | Pathogen-directed + rifampin | ≥ 6 weeks |

- **Empiric therapy** (native valve): vancomycin + ceftriaxone pending culture results
- **MSSA native valve**: nafcillin or cefazolin for 6 weeks
- **MRSA**: vancomycin or daptomycin for 6 weeks
- **Viridans streptococci**: ceftriaxone for 4 weeks; 2-week regimen with gentamicin if fully susceptible
- **Enterococcal IE**: ampicillin + ceftriaxone (preferred) or ampicillin + gentamicin for 6 weeks
- Prosthetic valve IE requires longer courses (>=6 weeks) and often surgical intervention

![Algorithm for empiric and directed antibiotic therapy in infective endocarditis](/images/residency/endocarditis-antibiotic-algorithm.jpg)

### Surgical Indications

- **Heart failure** due to valvular dysfunction (most common indication)
- **Uncontrolled infection** despite appropriate antibiotics (persistent bacteremia > 7 days)
- **Large vegetation** (> 10 mm) with embolic events or high embolic risk
- Perivalvular abscess, fistula, or prosthetic valve dehiscence
- Fungal endocarditis or highly resistant organisms
- Early surgical consultation improves outcomes; do not delay referral

## Complications

- **Embolic stroke**: occurs in 20-40% of left-sided IE; risk decreases after ART initiation
- **Mycotic aneurysms**: screen with CT angiography if neurologic symptoms
- **Perivalvular abscess**: suspect with persistent fever and new conduction abnormalities (PR prolongation)
- **Splenic abscess**: consider in persistent bacteremia; CT abdomen for evaluation
- **Acute kidney injury**: embolic infarction, immune complex glomerulonephritis, antibiotic nephrotoxicity

![CT head showing septic embolic infarcts in a patient with left-sided endocarditis](/images/residency/endocarditis-septic-emboli-ct.jpg)

## Key Clinical Pearls

- Always obtain blood cultures before antibiotics; at least 3 sets from separate sites
- TEE is required when TTE is negative but clinical suspicion is high, and in all prosthetic valve IE
- S. aureus bacteremia mandates echocardiography and a minimum 2-week IV course even without IE
- New conduction delay in a patient with IE suggests perivalvular abscess; obtain urgent TEE
- Multidisciplinary endocarditis team involvement improves mortality

## References

1. Baddour LM, Wilson WR, Bayer AS, et al. Infective endocarditis in adults: diagnosis, antimicrobial therapy, and management of complications. *Circulation*. 2015;132(15):1435-1486.
2. Habib G, Lancellotti P, Antunes MJ, et al. 2015 ESC Guidelines for the management of infective endocarditis. *Eur Heart J*. 2015;36(44):3075-3128.
3. Holland TL, Baddour LM, Bayer AS, et al. Infective endocarditis. *Nat Rev Dis Primers*. 2016;2:16059.
4. Cahill TJ, Baddour LM, Habib G, et al. Challenges in infective endocarditis. *J Am Coll Cardiol*. 2017;69(3):325-344.