Mediastinitis

Background

• Infection of the structures in the mediastinum
  • The mediastinum refers to the space between the lungs which contains the heart, thymus, esophagus, and trachea
• Confusing nomenclature, and includes mediastinal infections caused by extension of a cervical infection (necrotizing mediastinitis or descending mediastinitis) and mediastinal infection after sternotomy (sternal infection or deep sternal wound infection)

CDC Definition

• One of the following:
  • Organisms cultured from mediastinal tissue or fluid
  • Gross anatomical or histopathologic evidence of mediastinitis
  • Fever, chest pain, or sternal instability
• Plus one of the following:
  • Purulence from the mediastinal area
  • Mediastinal widening on imaging

Microbiology

• Post-cardiac surgery: often mixed
  • Staphylococcus aureus is by far most common, followed by coagulase-negative staphylococci
  • Can also involve other Gram-positive cocci and Gram-negative bacilli (including SPICE organisms and Pseudomonas aeruginosa)
• Esophageal rupture: polymicrobial, with oropharyngeal and gastrointestinal flora including anaerobes
• Can also occur from contiguous extension of pneumonia, pancreatitis, subphrenic abscess, or epidural abscess, with the microbiology determined by the source of infection

Epidemiology

• Post-surgical deep sternal wound infections occur after about 1% of cardiac surgeries
  • Incidence increases with the complexity of the operation, and is highest with CABG = valve replacement or aortic surgery
  • Risk increases with BMI, diabetes, and number of platelet transfusions

Clinical Manfestation

• Usually presents within 30 days of surgery
• Fever, chest pain, drainage from surgical site
• See CDC definition, above

Prognosis

• For deep sternal wound infections after cardiac surgery
  • Mortality is about 15% in the short term, but is also higher than those without infection in the years following surgery
  • Mortality is higher when Gram-negatives are isolated

Investigations

• CT chest can show evidence, including sternal wire displacement, sternal disruption, free gas, or fluid collections
• Surgical exploration, with fluid sent for culture

Management

• In general, surgical exploration with debridement should always be performed as quickly as possible
• Empiric antibiotics should be broad, including coverage for MRSA and resistant Gram-negatives
  • Antimicrobials directed by culture results
• Duration of therapy unclear but depends on adequacy of source control, presence of sternal osteitis, and presence of metal or other foreign bodies
  • Probably somewhere between 3 and 12 weeks
  • Following sternal resection and pectoral flap: maybe 2 to 3 weeks
  • After debridement without resection, or with residual bone infection: probably 4 to 6 weeks