Infective endocarditis, Duke criteria, microbiology, antimicrobials and surgical indications

Definition / Overview

• Infective endocarditis (IE) is a microbial infection of the endocardial surface of the heart, most commonly involving cardiac valves but also potentially affecting mural endocardium, septal defects, or intracardiac devices.
• Classified broadly as native valve endocarditis (NVE) or prosthetic valve endocarditis (PVE), and further as acute (aggressive, typically S. aureus) or subacute (indolent, typically viridans streptococci).
• Early PVE is arbitrarily defined as onset within 12 months of valve surgery; predominantly caused by coagulase-negative staphylococci, S. aureus, gram-negative bacilli, and Candida spp. reflecting intraoperative or perioperative inoculation.
• Late PVE shares the microbial spectrum of community-acquired NVE.
• Incidence is rising globally, driven by ageing populations, more intravascular devices, and increased IV drug use.

Pathophysiology

• Endothelial disruption, from turbulent flow, instrumentation, or congenital/degenerative structural disease, allows deposition of platelets and fibrin, creating a sterile thrombotic nidus.
• Transient bacteraemia seeds this nidus; organisms adhere via surface adhesins (notably S. aureus fibronectin-binding proteins).
• The infected vegetation is poorly penetrated by host defences and many antimicrobials, necessitating prolonged, bactericidal therapy at high serum concentrations.
• Consequences arise through three mechanisms:
  1. Local destruction: valve regurgitation, perivalvular extension (ring abscess, fistula), conduction system involvement
  2. Embolic events: vegetations fragment and seed distant organs (brain, kidney, spleen, coronary arteries, lungs in right-sided disease)
  3. Immune-complex deposition: glomerulonephritis, vasculitis, Osler nodes, Roth spots

Clinical Features & Diagnosis

Symptoms and Examination

• Fever is near-universal; constitutional symptoms (malaise, night sweats, weight loss) dominate subacute presentations.
• Peripheral stigmata: their presence strongly raises the pre-test probability for IE and should be sought systematically:

• New or changing cardiac murmur is a key sign; aortic regurgitation developing acutely is haemodynamically catastrophic.
• PR interval prolongation on ECG suggests aortic root/perivalvular abscess extending toward the AV node, a finding demanding urgent imaging and surgical review.
• Splenomegaly, microscopic haematuria, and new neurological deficits are common accompaniments.

Modified Duke Criteria

• The modified Duke criteria integrate microbiological, echocardiographic, and clinical data to stratify diagnostic probability.
• Clinical judgment must always inform interpretation, the criteria are a guide, not a substitute for reasoning.

Major Criteria

Microbiological:

• Two separate blood cultures growing a typical IE organism (viridans streptococci, Streptococcus gallolyticus, S. aureus, HACEK group organisms, or community-acquired enterococci) in the absence of a primary focus elsewhere.
• Persistently positive blood cultures: ≥2 cultures drawn >12 hours apart, OR all of 3 or a majority of ≥4 cultures with first and last drawn ≥1 hour apart.
• Single positive blood culture or phase I IgG titre ≥1:800 for Coxiella burnetii (Q fever endocarditis).

Echocardiographic:

• Oscillating intracardiac mass on a valve, supporting structure, or in the path of a regurgitant jet, without an alternative anatomic explanation.
• Perivalvular abscess.
• New partial dehiscence of a prosthetic valve.
• New valvular regurgitation (an increase or change in pre-existing murmur alone does not suffice).

Minor Criteria

• Predisposing cardiac condition or IV drug use.
• Fever ≥38°C.
• Vascular phenomena: arterial emboli, septic pulmonary infarcts, mycotic aneurysm, intracranial haemorrhage, conjunctival petechiae, Janeway lesions.
• Immunological phenomena: glomerulonephritis, Osler nodes, Roth spots, positive rheumatoid factor.
• Microbiological evidence not meeting major criteria (positive culture but atypical organism or pattern), or serological evidence of an IE-consistent organism.

Diagnostic Classification

Exam point: Pathological criteria (culture or histology of a vegetation or intracardiac abscess demonstrating a microorganism, or histology showing active endocarditis) independently define definite IE regardless of clinical criteria.

Investigation & Monitoring

Blood Cultures

• Three sets from separate venepuncture sites, ideally at the peak of fever and before antibiotics, 85-90% of cases are identified from the first two sets.
• Allow each set to incubate for at least 5 days; notify the laboratory if fastidious organisms (HACEK, Brucella, Coxiella, Bartonella) are suspected.
• Culture-negative IE (approximately 10%) requires serological and PCR investigation for atypical pathogens.

Echocardiography

• Transthoracic echocardiography (TTE) is the initial study; sensitivity for NVE vegetations is approximately 60-75%.
• Transoesophageal echocardiography (TOE) is mandatory when:
  • TTE is non-diagnostic but clinical suspicion remains high.
  • Prosthetic valve IE or intracardiac device IE is suspected (TOE sensitivity >90%).
  • Perivalvular complications (abscess, fistula, dehiscence) need evaluation.
  • S. aureus bacteraemia: TOE should be performed routinely given the high rate of cryptic IE.

Other Investigations

Microbiology

Common Organisms

Management

Principles

• Multidisciplinary "endocarditis team": infectious diseases, cardiology, cardiothoracic surgery, and microbiology, should be engaged early; evidence supports improved outcomes with this model.
• Targeted IV bactericidal antimicrobials for 4-6 weeks from the date blood cultures clear and source control is achieved.
• Empiric therapy is required when haemodynamic instability, aggressive organism, or rapidly destructive course precludes awaiting cultures.

Antimicrobial Therapy

Empiric Regimens (Pre-culture or Culture-Negative)

Targeted Regimens

Streptococcal NVE (fully penicillin-sensitive, MIC ≤0.125 mg/L):

• Benzylpenicillin 1.2-2.4 g IV q4h for 4-6 weeks.
• Addition of gentamicin for the first 2 weeks permits a shortened 2-week course in low-risk patients.
• Ceftriaxone 2 g IV once daily is an alternative allowing outpatient completion.

Streptococcal NVE (reduced penicillin sensitivity):

• Benzylpenicillin + gentamicin for 4-6 weeks; or vancomycin + gentamicin if penicillin-resistant or allergic.

Staphylococcal NVE (methicillin-sensitive, MSSA):

• Flucloxacillin 2 g IV q4h for ≥4 weeks. Flucloxacillin is superior to vancomycin for MSSA and should be used whenever allergy permits.
• Routine addition of gentamicin is not recommended for NVE (increased nephrotoxicity without mortality benefit).

Staphylococcal NVE (MRSA or penicillin allergy):

• Vancomycin, target AUC/MIC 400-600; or daptomycin 8-10 mg/kg IV once daily.

Staphylococcal PVE (MSSA):

• Flucloxacillin + rifampicin 300-450 mg q12h for ≥6 weeks, with gentamicin for the first 2 weeks.
• Rifampicin's role is biofilm penetration; it should never be started as monotherapy given rapid resistance emergence.

Enterococcal IE:

• Amoxicillin 2 g IV q4h + gentamicin 3 mg/kg/day for 4-6 weeks (6 weeks for PVE).
• High-level aminoglycoside resistance: substitute gentamicin with ampicillin + ceftriaxone (dual beta-lactam synergy, particularly for Enterococcus faecalis).
• Vancomycin-resistant enterococcus (VRE): daptomycin ± ampicillin; seek specialist ID input.

HACEK organisms:

• Ceftriaxone 2 g IV once daily for 4 weeks (NVE) or 6 weeks (PVE).

Fungal IE:

• Liposomal amphotericin B or an echinocandin (depending on susceptibility); step-down to long-term azole suppression. Surgical valve replacement is almost universally required.

Monitoring During Therapy

• Vancomycin: AUC-guided dosing preferred; monitor renal function twice weekly; watch for nephrotoxicity, especially with concurrent gentamicin.
• Gentamicin: Serum levels and renal function every 48-72 hours; minimise duration to reduce nephrotoxicity and ototoxicity; generally avoid beyond 2 weeks.
• Rifampicin: Anticipate drug-drug interactions (CYP3A4 inducer), LFTs at baseline and during therapy.
• Repeat blood cultures 48-72 hours after starting therapy to confirm clearance; persistent bacteraemia demands reassessment (undrained focus, metastatic seeding, wrong antibiotic).

Surgical Indications

• Approximately 50% of IE cases require cardiac surgery during the index admission.
• Early surgical consultation is critical; delay increases mortality when surgery is indicated.

Indications and Timing

• Neurological injury complicates the timing decision: haemorrhagic stroke is generally a contraindication to surgery for at least 4 weeks; ischaemic stroke without haemorrhagic transformation may permit earlier surgery if the indication is compelling and the infarct volume is small.

Complications & Special Considerations

Right-Sided IE (Tricuspid Valve)

• Predominantly in people who inject drugs; S. aureus is overwhelmingly the commonest pathogen.
• Presents with septic pulmonary emboli (pleuritic chest pain, haemoptysis, cavitating lung infiltrates).
• Surgery is less frequently needed; 4-6 weeks of IV therapy is standard but some data support 2-week regimens in uncomplicated MSSA tricuspid IE in people who inject drugs.

Device-Related IE (Pacemaker/ICD)

• Requires complete device removal in addition to antimicrobial therapy; extraction should be performed at a centre with surgical backup.
• Pocket infection without lead involvement may be managed more conservatively in selected cases.

IE in Pregnancy

• Rare but high maternal and foetal mortality; S. aureus and streptococci predominate.
• Penicillins and cephalosporins are safe; avoid aminoglycosides (foetal ototoxicity) when possible; avoid daptomycin (limited safety data).
• Surgery carries significant foetal loss risk but should not be withheld when the maternal indication is compelling.

Colorectal Malignancy Screening

• S. gallolyticus bacteraemia/IE has a strong association with occult colorectal neoplasia; colonoscopy is mandatory after the acute episode has been treated.

Antibiotic Prophylaxis, Current Position

• Routine antibiotic prophylaxis prior to dental or other invasive procedures is no longer recommended for patients at risk of IE in current Australian and New Zealand guidelines.
• Patients at elevated risk (prosthetic valves, prior IE, complex congenital heart disease, hypertrophic cardiomyopathy) should receive clear education about:
  • Maintaining meticulous oral hygiene.
  • Symptoms that may indicate IE and when to seek urgent review.
  • Risks associated with invasive and non-medical procedures (body piercing, tattooing).

Long-Case Integration: Consultant Reasoning Approach

• Pre-test probability: Elevated by structural heart disease, prosthetic valves, intravascular devices, IV drug use, recent dental or invasive procedures, or prior IE. Frame your history to uncover these.
• Blood culture stewardship: Never withhold cultures to expedite empiric therapy in a stable patient, the microbiological diagnosis drives everything that follows.
• Echocardiography decision: TOE rather than TTE when clinical probability is high and TTE is negative, or whenever PVE or device IE is in question. A negative TOE in a patient with true high probability does not exclude IE, consider repeat imaging at 5-7 days.
• Surgical timing viva: Be prepared to articulate why and when surgery is needed, the competing risks of operating during active infection versus waiting, and how neurological events modify the calculus.
• Prognosis: In-hospital mortality is approximately 20% overall; S. aureus carries approximately 30% mortality, bowel organisms approximately 14%, and streptococci approximately 6%. Recurrence occurs in approximately 15% by 2 years, disproportionately in people who continue to inject drugs.
• Equity lens: People who inject drugs are a high-risk and often marginalised group. Non-judgmental care, harm reduction counselling, and addiction medicine involvement improve outcomes and represent best practice, address this explicitly if the long case presents a patient in this context.