Deep Sternal Wound Infection

Definition

Sternal wound complications have been classified by El Oakley and Wright as follows:

• 1

  Mediastinal dehiscence: median sternotomy wound breakdown in the absence of clinical or microbiologic evidence of infection.

• 2

  Mediastinal wound infection: clinical or microbiologic evidence of infected presternal tissue and sternal osteomyelitis, with or without mediastinal sepsis and with or without unstable sternum. Subtypes include superficial wound infection, wound infection confined to the subcutaneous tissue; and deep wound infection (mediastinitis), wound infection associated with sternal osteomyelitis with or without infected retrosternal space.

It is the latter subtype that will be the focus of this chapter. According to the guidelines from the Centers for Disease Control and Prevention in the United States, deep sternal wound infection (DSWI) can be defined by one of the following: (1) the presence of an organism isolated from culture of mediastinal tissue or fluid; (2) evidence of mediastinitis seen during operation; (3) one of the following conditions: chest pain, sternal instability or fever (>38° C) in combination with either purulent discharge from the mediastinum or an organism isolated from blood culture or culture of mediastinal drainage.

Incidence and Causes

The incidence of DSWI in most recent series ranges from 0.75% to 2.4%. In our institution, prospectively collected results for 23,499 sternotomies performed between 1992 and 2007 were retrospectively reviewed. A total of 267 patients presented with DSWI, accounting for 1.1% of the surgical population. Staphylococcus aureus and coagulase-negative staphylococci are the most commonly found microorganisms. In their review of 30,102 consecutive patients undergoing sternotomy for cardiac surgery between 1990 and 2003, Tang and collaborators found them to be respectively responsible for 42% and 24% of DSWI. Gram-negative bacteria and fungi are less commonly encountered. These offending organisms have been associated with different predisposing factors and modes of presentation. Coagulase-negative staphylococci colonize the wound from the normal skin flora and proliferate within a self-contained pocket protected by an extracellular polysaccharide biofilm. The infection is mostly seen in obese patients and those affected by chronic obstructive pulmonary disease (COPD), associated with a slow and late onset with resulting sternal instability but fewer systemic signs.

S. aureus infections are more aggressive in nature and more often associated with classic systemic signs and bacteremia. Perioperative contamination and nasopharyngeal colonization are important sources for this type of infection. Gram-negative bacteria are more commonly associated with a more complicated postoperative course, prolonged intensive care unit (ICU) stay, and concomitant nosocomial infections such as pneumonia, urinary tract infections, and abdominal sepsis, which were found in almost 25% of patients in our series.

Risk Factors

Host predispositions and numerous perioperative environmental and technical aspects can play a significant role in the development of DSWI. There is a vast body of literature concerning the most important of these factors, commonly divided as preoperative, perioperative, and postoperative, which have been associated with a higher incidence of DSWI.

Obesity, diabetes mellitus, COPD, heart failure, renal failure, smoking, poor dental hygiene, older age, and male gender have consistently been identified in patients more prone to DSWI. Preoperative considerations also include prolonged hospital stay or the use of an intra-aortic balloon pump.

Untimely administration of antibioprophylaxis, poor management of hyperglycemia, use of both internal thoracic arteries (ITAs), redo surgeries, excessive use of bone wax, and prolonged procedural times have all been described as perioperative contributing factors, whereas postoperative reexploration for bleeding, transfusions, prolonged ICU stay, and prolonged intubation time are commonly associated with the postoperative period. Specific mechanisms have been proposed to account for the increased risk seen in patients affected by these conditions.

Poor distribution of antibiotics in adipose tissue and inadequate skin preparation in obese patients, impaired wound healing by elevated blood glucose in diabetes mellitus, improper sternal vascularization associated with the use of both ITAs, and heart failure–induced low cardiac output are common examples. In our series, prolonged intubation was the strongest predictor of DSWI with an odds ratio of 5.9, probably reflecting composite indices of risk and patient fragility. Bilateral internal mammary artery grafting was the second strongest predictor, with an odds ratio of 2.7 in the latest period.

Diagnosis

DSWI diagnosis should be suspected whenever sternal tenderness or instability, erythema, fluid collection, wound dehiscence, or purulent discharge is found, especially in the presence of fever or leukocytosis. The diagnosis is essentially clinical and is based on the criteria listed in the previous “Definition” section. Blood cultures should be done when patients present with pyrexia, and identification of a staphylococcal bacteremia is almost pathognomonic. Any fluid discharge from the wound should also be cultured. Chest radiography has limited interest for diagnostic purposes but may reveal ruptured or malpositioned wires as well as sternal fractures or dehiscence as indirect signs of DSWI. Computed tomography (CT) can help determine the extent, depth, and localization of the infectious process. It has been associated with 95.3% sensitivity and 81.7% specificity. CT is also useful for guided needle aspiration and culture.

Nuclear imaging has not been used extensively, but leukocytes labeling with ^99m Tc-HMPAO has shown promise as a reliable method for the early diagnosis of sternal infections in a small cohort of 41 patients.

We suggest that a transthoracic and/or transesophageal echocardiogram be performed in patients presenting with DSWI following a valvular replacement, to rule out the presence of a concomitant prosthetic endocarditis.