Septic Pulmonary Embolism

Septic pulmonary embolism is the embolisation of infected thrombus or vegetation material to the pulmonary arteries from a distant source, producing multiple peripheral nodules or infarcts that often cavitate. It is uncommon, and the diagnosis is frequently delayed.

Sources are tricuspid valve endocarditis (especially in people who inject drugs), infected central venous catheters, pacemaker leads and implanted devices, septic thrombophlebitis of peripheral, jugular (Lemierre syndrome) or pelvic veins, infected dialysis fistulas or grafts, liver abscess, and periodontal and ear infections. The commonest organisms are Staphylococcus aureus (including meticillin resistant), streptococci, and, in Lemierre syndrome, Fusobacterium necrophorum. Gram negative organisms and fungi are less common.

Pathophysiology. Infected material detaches and lodges in the small pulmonary arteries, causing infarction, abscess formation and cavitation, and pleural involvement (empyema, pneumothorax if a subpleural cavity ruptures). Continued seeding sustains bacteraemia.

Fever (often high and persistent), rigors, pleuritic chest pain, cough, purulent sputum, haemoptysis and dyspnoea, over days to weeks. Look for the source: a new murmur (tricuspid regurgitation), injection sites, central lines, neck pain and swelling (Lemierre syndrome), a swollen vein or limb, pelvic infection, dental or throat infection. Peripheral stigmata of endocarditis may be present.

Septic shock, respiratory failure or neurological signs (septic emboli to the brain) need urgent assessment.

• Blood cultures: at least 3 sets before antibiotics, from different sites.
• Chest radiograph: multiple bilateral peripheral nodular opacities of varying size, some cavitating, and wedge shaped infarcts.
• CT chest (contrast): multiple peripheral, ill defined or well defined nodules (0.5 to 3.5 cm) with cavitation (in 50 to 100%), the "feeding vessel sign" (nodule linked to a vessel), wedge shaped subpleural consolidation, pleural effusion or empyema.
• Echocardiography: transthoracic, then transoesophageal for vegetations on the tricuspid or pulmonary valve (or leads).
• Source search: CT or ultrasound of neck veins, abdomen and pelvis, Doppler of limbs, and tracking of catheters and devices. Doppler or CT of the internal jugular vein in suspected Lemierre syndrome.
• Inflammatory markers, renal and liver function, HIV, hepatitis B and C serology.
• Sputum and pleural fluid culture, and fungal and mycobacterial tests when a cavity is present.

Differential diagnosis.

• Pulmonary embolism with infarction (bland)
• Metastases or lymphoma
• Granulomatosis with polyangiitis
• Fungal pneumonia
• Tuberculosis or nocardiosis
• Multiple lung abscesses from aspiration

• Start empirical intravenous antibiotics immediately after blood cultures: flucloxacillin 2 g every 6 hours (for meticillin sensitive Staphylococcus aureus), or vancomycin 15 to 20 mg/kg every 8 to 12 hours (target trough 15 to 20 mg/L, or area under the curve 400 to 600) for suspected meticillin resistant organisms or until sensitivities are known. Alternatives for resistant strains: linezolid 600 mg every 12 hours, or ceftaroline 600 mg every 12 hours.
• Lemierre syndrome and anaerobic sources: piperacillin-tazobactam 4.5 g every 6 to 8 hours, or ceftriaxone 2 g daily with metronidazole 500 mg every 8 hours, or ampicillin sulbactam 3 g every 6 hours.
• Duration: at least 4 to 6 weeks of intravenous therapy from the first negative culture, directed by response, vegetations and cavities. Right sided endocarditis in people who inject drugs may be shortened to 2 weeks in selected uncomplicated meticillin sensitive cases, as the specialist advises.
• Source control, which is essential: remove infected catheters, leads or grafts, drain abscesses, treat dental and ear sources, and surgical debridement or valve repair or vegetation removal (percutaneous aspiration or surgery) for persistent bacteraemia, large vegetations (over 2 cm) with recurrent emboli, or heart failure.
• Drain empyema or large effusion, and manage pneumothorax with a chest drain.
• Anticoagulation: used for Lemierre syndrome with extensive thrombosis, for example for 3 months (apixaban 5 mg twice daily after initial therapy), though evidence is limited. In endocarditis, avoid starting anticoagulation without another indication, since haemorrhagic transformation of emboli is a risk.
• Support: oxygen, fluid, ventilation if needed, and nutrition.
• Addiction treatment for people who inject drugs (opioid agonist therapy: methadone 20 to 30 mg daily titrated, or buprenorphine 4 to 8 mg sublingual), needle exchange and harm reduction, and HIV and hepatitis testing.

Complications and prognosis. Lung abscess, empyema, pneumothorax, respiratory failure, haemoptysis, cerebral emboli, and recurrence if the source is not controlled. Mortality is about 10 to 20% (up to 30% with delay or resistant organisms).

Monitoring. Repeat blood cultures every 48 to 72 hours until negative, weekly CT or radiograph in the first weeks, vancomycin trough levels and renal function, and repeat echocardiography at the end of therapy.

Referral and escalation. Refer to infectious diseases and cardiology or cardiothoracic surgery, with intensive care for septic shock, and addiction services.