Approach Considerations
Operative exploration includes reopening the previous sternotomy and debridement of necrotic and infected tissue. The sternum is carefully separated from the ventricle bypass grafts and the aorta so as not to cause bleeding. Cultures are sent to direct antibiotic therapy.
Wound closure is usually delayed until reasonable control of infection is achieved; however, some surgeons perform closure with muscle flaps at the initial debridement, with good results. Delayed closure is usually accomplished with muscle flaps (pectoralis, rectus) and may be aided by vacuum-assisted closure. [2, 28, 29]
Sterile sternal dehiscence, which is described as a sternal nonunion, is usually not treated. Occasionally, patients have abrupt separation of the sternum in close proximity to cardiac surgery, necessitating sternal reclosure. Also, some have extreme pain or cannot tolerate the clicking and discomfort of the nonunion and require sternal reclosure.
For simple sternal dehiscence (postoperative mediastinitis), great care must be taken to exclude active infection before rewiring the sternum. Surgery is seldom recommended for cases of chronic fibrosing mediastinitis unless compression of the major mediastinal structures has occurred. In cases of sternal nonunion, surgery should be deferred except when patients have extreme pain.
Medical Therapy
Mediastinitis after cardiac surgery
Appropriate, well-directed antibiotic therapy is crucial to successful treatment of mediastinitis.
Most patients have already received prophylactic antibiotics, usually a first-generation cephalosporin. Because as many as 20% of organisms cultured from infected sternotomy sites are methicillin-resistant S aureus (MRSA) and because another 20% are gram-negative organisms, it is vital to institute very broad and deep antibiotic coverage that includes Pseudomonas species. Culture results should then guide antibiotic use; multiple regimens are available for use with patients who have mediastinitis.
Therapy is usually prolonged, ranging from weeks to months. One study suggests that 4-6 weeks of therapy is adequate for most patients. [30]
Enteral nutritional support should be instituted immediately, with a duodenal feeding tube, if necessary. Data suggest that the use of diets formulated with various anti-inflammatory compounds to include omega-3 long-chain fatty acids and arginine provide clinically important benefits for critically ill patients with sepsis. If enteral feedings are contraindicated, hyperalimentation should be considered.
Chronic fibrosing mediastinitis
Chronic fibrosing mediastinitis, which is often caused by H capsulatum infection, should be treated with close observation for signs of superior vena cava compression or other mediastinal organ compromise. [31] The role of antifungal therapy is controversial, though amphotericin B has been used.
Surgical Therapy
Mediastinitis after cardiac surgery
Effective treatment for simple sternal dehiscence without infection is rewiring the sternum. [12] This usually yields reasonable long-term results. Cultures should be taken to exclude active infection in the cases of sternal dehiscence.
Failure to adequately debride and sterilize the mediastinum during the first reoperation is the most common cause of repeat postoperative mediastinitis. Options for mediastinitis after cardiac surgery are as follows:
-
Immediate closure after sternal debridement
-
Delayed closure after sternal debridement
-
Sternal irrigation after sternal debridement
Each has its advantages and disadvantages. The optimal strategy for a given case depends on the duration of the infection, the condition of the mediastinal structures, and the experience of the surgeon.
Most surgeons prefer to leave the wound open or treat with vacuum-assisted closure for subsequent debridement efforts after initial sternal reexploration. [2, 28, 29, 32] In this case, the wound is packed daily until it appears clean with adequate granulation tissue. At this point, muscle flap closure is achieved. Usually, bilateral pectoralis flaps are used. Occasionally, the rectus abdominis that is opposite the internal mammary artery used for bypass is employed for coverage.
Both the surgeon's experience and patient factors influence the type of flap procedure used. If a large anterior retrosternal dead space exists, it must be obliterated in order to achieve cure. Although this is often achieved with a muscle flap, the omentum provides lymphocytes and angiogenesis factors that may prove beneficial. [33, 34] Disadvantages of this delayed approach are the altered thoracic mechanics, which may lead to ventilator dependence, and a risk of bleeding from the exposed heart and vessels, with muscle flap closure for mediastinitis in an attempt to decrease the incidence of this bleeding.
Some surgeons uniformly perform muscle flap closure at the initial debridement, with good results. [35] Other surgeons elect to close the wound site primarily in less advanced cases of mediastinitis and use large-bore drainage and irrigation tubes to infuse various antibiotic or antiseptic solutions for many days. Although the most commonly used solution in the past has been povidone-iodine, this should be used with caution; case reports have indicated the development of serious iodine toxicity manifesting as seizures and renal failure. [36]
The lack of a bony anterior sternal wall may be unacceptable to some patients and has prompted some surgeons to attempt sternum-sparing procedures, even in more advanced cases. This is often a difficult decision, requiring excellent surgical judgment. Clearly advanced cases of sternal osteomyelitis are extremely difficult to cure, and most patients with muscle or omental flaps do very well from a functional standpoint.
Chronic fibrosing mediastinitis
Surgery is seldom recommended for cases of chronic fibrosing mediastinitis unless compression of major mediastinal structures has occurred. Whether surgical debulking early in the process minimizes the development of superior vena cava syndrome (SVCS) or cardiac compression has not been adequately studied.
Oropharyngeal descending infections
In cases of descending mediastinitis due to infections that began in the oropharynx, some surgeons attempt to limit drainage and debridement to the cervical region. In a more advanced infection, often the best plan to offer a maximal chance of cure is to proceed with formal thoracic drainage and debridement.
Complications
Systemic sepsis is a major complication of mediastinitis and manifests with tachycardia, hypotension, poor urine output, and other signs of poor systemic perfusion. The aim of early aggressive therapy, both surgical and medical, is to prevent this often lethal complication.
Pneumoperitoneum and pneumothorax can produce serious local problems and eventual hemodynamic compromise.
If pleural effusions become infected and develop into empyema, systemic sepsis may occur.
Severe and life-threatening bleeding from ruptured vessels or the heart itself can occur when the chest is packed and left open to await definitive closure.
SVCS and compression of critical mediastinal structures are sometimes observed with chronic fibrosing mediastinitis. Endovascular management of SVCS occurring in this setting appears to be safe and feasible. [37, 38] Compression of the pulmonary vessels may give rise to pulmonary hypertension. [39]
Prevention
Measures for preventing mediastinitis include the following:
-
Strict adherence to perioperative aseptic technique
-
Attention to hemostasis
-
Precise sternal closure
-
Possible prophylactic intranasal mupirocin administered to S aureus carriers (this may reduce the rate of overall surgical-site infection by S aureus after cardiac surgery) [20]
-
Possible topical application of bacitracin ointment to sternotomy to decrease risk of mediastinitis after cardiac surgery [3]
-
The frequency of various microbiological pathogens isolated in cases of postoperative mediastinitis.
