AKT · Infections/Key pathogens & antimicrobials

Infective endocarditis

Infection of the endocardium/valves — organism depends on the CONTEXT

Overview

Infection of a heart valve/endocardium, diagnosed on the modified Duke criteria (blood cultures + echo vegetation). The exam pivots on organism-by-context: S. aureus (commonest overall, acute, IV drug users → tricuspid/right-sided); S. viridans (subacute, abnormal native valve after dental work); S. bovis/gallolyticus (→ colorectal cancer); coagulase-negative staph (S. epidermidis) (early prosthetic-valve, <1 yr); Enterococcus (after GI/GU procedures); HACEK & Coxiella/Bartonella (culture-negative). Stigmata: splinter haemorrhages, Osler's nodes, Janeway lesions, Roth spots, new murmur, emboli.

Recognise

Fever + new/changing murmur; peripheral stigmata (splinters, Osler's nodes = painful, Janeway lesions = painless, Roth spots), splenomegaly, microscopic haematuria
IV drug user → right-sided (tricuspid) IE with septic pulmonary emboli; prosthetic valve → device/staph
Complications: valve destruction/heart failure, embolic stroke, mycotic aneurysm, glomerulonephritis

Red flags

Acute S. aureus IE with valve destruction → heart failure/cardiogenic shock → emergency surgery
Embolic phenomena (stroke, limb ischaemia) or large mobile vegetation → early surgical referral

Differentials & how to tell them apart

Rheumatic/other valve diseasemurmur without fever/positive cultures/vegetation

Atrial myxomaconstitutional symptoms + emboli, but a mass not a vegetation on echo

Culture-negative endocarditisprior antibiotics or Coxiella/Bartonella/HACEK — serology needed

Investigations

THREE sets of blood cultures (before antibiotics, from different sites); transthoracic then transoesophageal echo (vegetation); ECG (conduction — root abscess); FBC/CRP, urinalysis; modified Duke criteria. If S. bovis → colonoscopy.

Management

Three blood-culture sets + echo, then prolonged organism-targeted IV antibiotics; surgery for HF/uncontrolled infection/emboli

1Take THREE sets of blood cultures before antibiotics and image the valve (TTE→TOE). Apply the modified Duke criteria; start empirical then targeted prolonged IV antibiotics.Gate: Blood cultures BEFORE antibiotics (3 sets) — do not blindly start antibiotics in stable subacute IE, or you convert it to culture-negative and lose the target.
2Refer early for surgery if heart failure, uncontrolled/prosthetic infection or large/embolising vegetations; investigate associations (colonoscopy for S. bovis; source of S. aureus bacteraemia).

Empirical amoxicillin ± gentamicin (native) / vancomycin+gentamicin+rifampicin (prosthetic) — start after cultures; then target to the organism

Flucloxacillin (MSSA) / vancomycin (MRSA or prosthetic) — prolonged IV course (4–6 weeks)

Benzylpenicillin/amoxicillin ± gentamicin (streptococcal/enterococcal) — sensitivity-guided; enterococcus often needs synergy

Valve surgery — heart failure, uncontrolled infection, large vegetation/recurrent emboli, prosthetic dehiscence

Key points

Organism = context: IVDU → S. aureus (tricuspid); dental + abnormal valve → S. viridans (subacute); prosthetic <1 yr → S. epidermidis; S. BOVIS → colorectal cancer; post-GI/GU → Enterococcus; culture-negative → Coxiella/Bartonella/HACEK. Duke criteria = cultures + echo. Osler's = painful (Ouch), Janeway = painless.

Monitor & prognosis

Repeat cultures for clearance, echo for vegetation/valve, inflammatory markers, complications (HF, emboli, conduction).

Depends on organism, valve and complications; S. aureus and prosthetic-valve IE carry the highest mortality.

Source: BSAC/ESC endocarditis guidelines; modified Duke criteria (cross-ref cardiovascular)