Laboratory Studies
A complete blood count (CBC) shows leukocytosis, often with a left shift on the white blood cell (WBC) count differential. The hematocrit value decreases if bleeding has occurred. The platelet count increases in the early stages of sepsis or decreases as sepsis worsens or disseminated intravascular coagulation (DIC) occurs.
Bacteremia can be observed, and blood cultures should be obtained as clinically indicated. Results from properly collected blood cultures should be reflexive in the workup when mediastinitis is considered, especially in the postoperative patient several days after cardiothoracic surgery in the presence of sepsis.
Samples of any sternal drainage should be sent for Gram stain and culture. This is helpful for establishing a diagnosis and tailoring antimicrobial therapy. At operative exploration, additional cultures should be taken to direct antibiotic therapy.
Mediastinal pacing wires should be sent for culture if they are still present and no longer needed. One study found that results from pacing wire culture have a sensitivity of 75%, a specificity of 83%, a positive predictive value of 12%, and a negative predictive value of 99%. [24]
Imaging Studies
Delays in the diagnosis of mediastinitis greatly increase morbidity and mortality. The condition is typically recognized because of high clinical awareness in susceptible populations. Occasionally, diagnostic imaging studies, including computed tomography (CT) of the chest, can be helpful if the diagnosis is in question. [12] Local wound exploration is the predominant method of distinguishing between superficial wound infection and deep sternal wound infection.
Chest radiography
Findings include pneumomediastinum and air-fluid levels within the mediastinum. Air-fluid levels are often best seen on lateral films. Mediastinal widening is not a reliable sign of mediastinitis, especially postoperatively.
Computed tomography
CT is more accurate for helping to identify air-fluid levels and pneumomediastinum. A CT scan may demonstrate sternal separation and substernal fluid collections. These examinations can be helpful when the diagnosis is in question or in the late postoperative period. They should not take the place of prudent wound exploration to identify a deep sternal wound infection.
The later the scans are performed after surgery, the more accurate the results. [25] If performed after postoperative week 2, CT scans have a sensitivity and specificity of almost 100%, though most wound infections occur before this time. [25] The specificity of CT findings is clearly time-dependent.
CT findings consistent with mediastinitis can also be found in patients without sternal wound infections after cardiac surgery for as many as 21 days following the procedure. This makes integrating CT findings with clinical data and awareness critical.
Hosokawa et al, in a study aimed at assessing CT findings in pediatric patients diagnosed with mediastinitis after cardiovascular surgery, found that sternal destruction and capsular ring enhancement were observed more in patients with mediastinitis than in those without mediastinitis. [26]
Magnetic resonance imaging
Magnetic resonance imaging (MRI) is poorly suited as a diagnostic modality in persons with mediastinitis. Postoperatively, patients may have sternal wires, vascular clips, metallic valves, and pacing wires that contraindicate MRI. In addition, it is difficult to perform an MRI study on an intubated, critically ill patient.
Nuclear medicine
Findings from nuclear medicine scans/radioactive WBC scans involving labeled WBCs are reported to have very high specificity; however, few studies have been performed on postoperative patients.
Histologic Findings
As mediastinitis develops, an increasingly thick layer of fibrin is formed, causing the mediastinal structures to become progressively less mobile. As the infection spreads throughout the mediastinum through sinus tracts, a growing area of dead space develops beneath the sternum. The belief that this retrosternal dead space must be obliterated to achieve a cure after mediastinitis develops has become more popular.
A more indolent form of mediastinitis, termed chronic fibrosing mediastinitis, occurs as a complication of granulomatous infections, most commonly H capsulatum. [17] Rupture of mediastinal lymph nodes and the release of caseous material generate an intense inflammatory reaction. The patient becomes symptomatic from obstruction of major mediastinal structures, especially the superior vena cava. [27]
A significant minority of patients may be asymptomatic and present with an isolated mediastinal mass on chest radiograph.
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The frequency of various microbiological pathogens isolated in cases of postoperative mediastinitis.
