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Mediastinitis
The mediastinum is the thoracic space confined by the sternum anteriorly, the spine posteriorly, the pleural laterally, the diaphragm inferiorly, and the thoracic outlet superiorly. It can be divided anatomically into superior and inferior compartments by the sternal angle. The inferior compartment is further divided into the anterior (between the posterior sternum and the anterior pericardium), middle (the intrapericardial contents), and posterior (bounded anteriorly by the posterior pericardium and posteriorly by the spine) mediastinum ( Fig. 142.1 ).
With respect to etiology, mediastinitis may be either primary, arising without prior intervention, or secondary, occurring postintervention. Clinically, the anatomic anterior and middle compartments can be considered together, as mediastinitis occurs most commonly in these combined spaces secondarily as a postoperative complication of cardiac operations. Esophageal pathology accounts for the overwhelming majority of mediastinal infections of the posterior compartment.
(From Drake RL, Vogl AW, Mitchell AW. Gray’s Anatomy for Students , 2nd ed. Philadelphia, PA: Churchill Livingstone; 2009: Fig 3-5.)
Mediastinitis is defined as inflammation of the mediastinum, and the diagnosis, treatment, and prognosis are determined by location and etiology. Primary mediastinitis arises without prior intervention, whereas secondary mediastinitis occurs postintervention. Clinically, mediastinitis of the anterior and middle mediastinum occurs most frequently as a postoperative complication of a cardiothoracic procedure. Esophageal pathology accounts for the overwhelming majority of the mediastinal infections of the posterior compartment.
Other more unusual forms of mediastinal infections or inflammation include those that migrate into the mediastinum from adjacent contiguous spaces and those that are more indolent than acute and are characterized by chronic inflammation and fibrosis.
Accordingly, this presentation follows these anatomic and etiologic distinctions: acute anterior mediastinitis, acute posterior mediastinitis, and migratory and chronic mediastinal inflammation.
Acute anterior mediastinitis
The most common form of acute anterior mediastinitis occurs after sternotomy for a cardiothoracic operation. It may also occur after traumatic sternal fracture ; descending cervical infections; and/or teratomatous, thymic, and thyroid infections/inflammation.
The term mediastinitis after cardiothoracic surgery refers to an infection involving the space deep to the sternum. Other forms of postoperative infection can be identified as superficial sternal wound infection (SWI; above the fascia without sternal involvement) and sternal osteomyelitis (without deeper infection).
Clinically, it is sometimes unclear as to whether one is dealing with a superficial problem above the level of the fascia, a sterile dehiscence, or a deeper infection. As no impervious anatomic barrier exists between the posterior cortex of the sternum and the space behind it, any infection posterior to the sternum is considered an infection of the anterior mediastinum. More than a small amount of drainage, any sternal instability, or evidence of separation suggest at least a sterile dehiscence and the need for re-exploration, deep cultures, and appropriate reclosure.
Incidence, pathology, and prevention
The reported incidence of mediastinitis ranges from 0.24% to 4% postcardiotomy. Comorbidities that increase the risk of postoperative mediastinitis include diabetes, elevated body mass index, older age, renal failure, prolonged preoperative hospitalization, immunosuppression, chronic obstructive pulmonary disease, cigarette smoking, reoperation, and preoperative atrial fibrillation. ,
Intraoperative factors have also been shown to play a role in postcardiotomy infection. Bilateral internal mammary use for coronary bypass grafting has about a 5% risk of sternal dehiscence as compared with 1% in single internal mammary usage. In addition, off-midline sternotomy, prolonged operative time, and the use of an intraaortic balloon pump have also been shown to increase risks of mediastinitis.
Multiple studies have cited bone healing is significantly impaired by using bone wax for sternal hemostasis as compared with water-soluble polymer wax, suggesting the use of the latter as a useful alternative, because bone healing immediately postoperatively is the most critical time frame for the prevention of sternal nonunion and infection.
In high-risk patients, sternal closure with rigid plate fixation may decrease the incidence of postoperative mediastinitis as compared with a similar population of patients whose sterna were closed with traditional wire alone. Some have shown that the use of cyanoacrylate glue can decrease the infection rates of superficial and deep surgical sites in patients who have sternal detachment and/or are at high risk for developing infection. For those advocating minimally invasive and alternative approaches to cardiac procedures, all together avoiding the sternotomy appears to also reduce the risk of mediastinal infection after cardiac operations.
Postoperatively, increased glucose levels (>200 mg/dL), , re-exploration, and prolonged ventilator use are associated with a higher incidence of deep sternal infection. As such, many quality metrics postcardiotomy are related to glucose control and time to extubation. In patients requiring complex cardiac repair, postoperative tracheostomy is frequently required. Despite the close proximity to the sternal incision, early tracheostomy for patients with ventilator dependence has not been shown to be associated with an increased incidence of mediastinitis. Tracheostomy per se is not a risk factor for sternal breakdown, but rather serves as a surrogate for respiratory failure. , As many ICUs have moved toward percutaneous tracheostomy, Hubner and colleagues evaluated the technique of percutaneous tracheostomy specifically and found that it has not been associated with a subsequent increase in mediastinal infection.
Staphylococcal species are the most common organisms seen in patients with poststernotomy deep wound infection, and these are increasingly methicillin resistant. Coagulase-negative resistant organisms are more common in patients who have prolonged hospitalizations. Gram-negative organisms may be cultured, particularly from patients with diabetes, in patients with gram-negative pneumonia before operation, or in those who require re-exploration. Given the most common organisms causing these infections, a second-generation cephalosporin is still the most accepted preoperative prophylaxis. Vancomycin is substituted in patients with penicillin allergy, and the addition of preoperative gram-negative coverage is appropriate in such cases, given vancomycin’s poor coverage of such organisms. Topical vancomycin has been shown to be effective in decreasing the incidence of sternal infections, and it is used routinely in some practices. Evidence-based guidelines from the Society of Thoracic Surgeons recommends gram-positive prophylaxis for no more than 48 hours in addition to preoperative nasal mupirocin. ,
Diagnosis
Patients with mediastinitis after sternotomy generally have clinical signs of wound drainage and sternal instability, but neither may be initially present. Fever and leukocytosis are common symptoms. Some patients manifest signs of sepsis, with mental status changes and hemodynamic compromise. Mediastinitis can appear as early as 1 day after the index operation or as remotely as months after an operation. The variable diagnostic accuracy of imaging techniques for the diagnosis of mediastinitis permits them to be supportive, but rarely, if ever, definitive. This is especially true during the early time frame (<30 days), when the vast majority of patients present. During this time, fluid collections and mediastinal soft tissue changes are common, if not universal, with both being nonspecific for infection.
Computed tomography (CT), magnetic resonance imaging (MRI), and technetium-99m leukocyte imaging can identify patients who have deep sternal wound infection (DSWI) and require surgical débridement. Cooper and colleagues showed that the patterns of intense uptake at 4 and 20 hours or increasing uptake between 4 and 20 hours were 100% sensitive and 89% specific for the detection of DSWI. It is useful in patients with suspected DSWI when clinical examination fails to confirm a diagnosis or when deep sternal aspirates of the superficial SWIs are equivocal. Leukocyte imaging is not useful for detecting superficial SWIs. Given the speed and efficiency, CT scan is favored in localizing the site of pathology and can give a better picture of the overall patient condition.
A profile of abnormal cytokine levels has been characterized, with the terminal SC5b-9 complement complex concentration being substantially higher in patients with mediastinitis and having no overlap with the values in non-mediastinitis, post–cardiac surgery controls. In difficult-to-diagnose cases, blind retrosternal, subxiphoid needle aspiration and culture have been variably employed, and aspiration with ultrasound guidance has been reported after cardiac transplantation. A recent small series suggested diagnostic success in patients without classic signs of infection by anteriorly inserting a 22-gauge needle percutaneously and aspirating between the recently closed sternal edges. Cultures and Gram stains were used to establish the presence of infection, with a high degree of specificity and sensitivity.
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