Incidence, Pathology and Prevention

Even among large well-reported series, the incidence of post–cardiac surgery mediastinitis varies, as noted earlier, due in part to the various definitions of mediastinitis, sternal osteomyelitis, or deep sternal wound infection. On occasion, the diagnosis may be made weeks or months after hospital discharge, and that occurrence may be missed by the institution where the operation was originally performed. Over the past 2 decades, there may be a trend toward lower reported rates of mediastinitis after sternotomy (more large series with <1% incidence), but the reported range remains wide, from 0.24% to 4% of cardiac operations.^2,3,4 Increasingly, as the postoperative length of stay decreases, mediastinal infections are diagnosed days or even months after hospital discharge, with median time to diagnosis variously reported around 10 days after the index cardiac operation.

In October 2008 the Centers for Medicare and Medicaid declared post–cardiac surgery mediastinitis a “preventable condition,”⁵ but no existing data manifest it as entirely preventable. Indeed, a number of mostly unmodifiable host factors that increase the risk of post–cardiac surgery mediastinitis have been identified. Among these are diabetes, increased body mass index, older age, renal failure, prolonged preoperative hospitalization, immunosuppression, chronic obstructive pulmonary disease, cigarette smoking, reoperation, preoperative atrial fibrillation, and elevated C-reactive protein.^3,6,7

In addition to host factors, intraoperative factors influence the risk. An increased incidence of deep sternal wound infection has been associated with bilateral internal mammary use, prolonged operative time, and use of the intraaortic balloon pump.⁷ Postoperative management may also influence the risk of mediastinitis; increased glucose levels (>200 mg/dL),⁸ reexploration, and prolonged ventilator use are associated with a higher incidence of deep sternal infection.⁷ Glucose values as low as ≥130 mg/dL have been linked to such infection in children.⁹ Undoubtedly factors such as skin preparation, electrocautery use, glove changes, and attention to a host of other details account for some of the variation in infection rates from surgeon to surgeon and institution to institution. Avoiding sternotomy entirely, as can be done with less chest wall–invasive approaches, appears to drastically reduce or eliminate the risk of mediastinal infection after cardiac operations.⁷

Postoperative tracheostomy is required in some post-sternotomy patients, and many of these have some of the risk factors that are also predictive of deep sternal wound infection. Open tracheostomy for patients with prolonged ventilator dependence, once deferred 2 or more weeks after sternotomy for fear of contaminating the anterior mediastinal space, has not been shown to be associated with an increased incidence of mediastinitis (mean of 5.6 days post cardiac operation).¹⁰ Specifically, the technique of percutaneous tracheostomy has not been associated with subsequent mediastinal infection. This technique may allow an earlier, safer switch from an oral to cervical airway¹¹ in patients requiring prolonged mechanical ventilation.

Staphylococcal species are the most common organisms seen in patients with post-sternotomy deep wound infection, and these are increasingly methicillin resistant.¹² Coagulase-negative resistant organisms are more common in patients who have prolonged hospitalizations.¹³ Gram-negative organisms may be cultured, particularly from diabetics, in patients with gram-negative pneumonia prior to operation, or in those who require reexploration.¹⁴

The serious consequences of mediastinitis after cardiac operations dictate the use of prophylactic antibiotics as an established practice. Given the most common organisms causing these infections, a second-generation cephalosporin is still the most accepted prophylaxis. A commonly used regimen would include cephazolin, 2 g, within 1 hour of skin incision, with a second 1-g dose at 3 to 4 hours if the incision remains open. Vancomycin is substituted in patients with penicillin allergy, in deference to the possibility of cross-reactivity, and it may be used routinely for an interval in institutions experiencing an outbreak of resistant staphylococci. The addition of preoperative gram-negative coverage (e.g., gentamicin) is appropriate in such cases, given vancomycin’s poor coverage of such organisms.¹⁵ Topical vancomycin has been shown effective in decreasing the incidence of sternal infections, and although used routinely in some practices, the development of resistant staphylococci is a genuine concern.¹⁶ An evidence-based guideline from the Society of Thoracic Surgeons recommends gram-positive prophylaxis for no more than 48 hours, in addition to preoperative nasal mupirocin.^12,15

Diagnosis

Patients with mediastinitis after sternotomy generally have clinical signs of wound drainage and sternal instability, but neither may be present initially. A spiking fever and an acutely elevated leukocyte count are common. Some patients manifest signs of sepsis with mental status changes and hemodynamic compromise. Mediastinitis can very rarely appear as early as 1 day after operation or as remotely as months after an operation. Rarely, those patients who have an indolent course presenting many months after operation may have isolated deep involvement, tracking down to the aorta and/or involving some artificial material such as a pledget or a braided suture.

The gravity of the diagnosis and variability of clinical signs has encouraged the use of imaging techniques to confirm or refute the possibility of deep sternal or mediastinal infection. Unfortunately, the variable diagnostic accuracy of most of these techniques permits them to be supportive¹⁷ but rarely if ever definitive in the diagnosis of deep infection. This is especially true in 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, both being nonspecific for infection.¹⁸ Nuclear studies (^99mTc) have been used in late-presenting indolent cases in an attempt to separate sternal involvement from infections superficial to it.¹⁹ All these imaging studies are, of course, confounded by changes that one can expect following the operative procedure itself. A profile of abnormal cytokine levels has been characterized,²⁰ with terminal SC5b-9 complement complex concentration being substantially higher in patients with mediastinitis, and having no overlap with values in non-mediastinitis, post–cardiac surgery controls. In difficult to diagnose cases, blind retrosternal, subxiphoid needle aspiration and culture has been variably employed, and aspiration with ultrasound guidance has been reported after cardiac transplantation.²¹ 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.²²

Treatment

While the need for operative treatment in anterior mediastinitis is firmly established, the techniques successfully employed vary greatly. The varied technical approaches are related to the timing of diagnosis (interval since antecedent operation), the depth-extent of infection, and the acuity of the patient. The experience and choice of the treating surgeon is also a factor in the technique used to manage a deep infection. In patients with suspected infection in whom there is drainage and some sternal instability, expeditious reexploration with débridement of the sternal edges and surrounding soft tissues, accompanied by irrigation and drainage may permit sternal re-wiring.^2,23 In patients who are septic, and/or in those with gross retrosternal purulence, a staged approach or immediate tissue coverage may be employed.

If sternal re-closure is elected after débridement an alternative wiring technique, either a variation of the Robicsek²⁴ weave or a commercially available plate fixation device, is generally used. Cultures obtained at operation dictate the systemic antibiotics ultimately used, but initial coverage may include a second-generation cephalosporin and gram negative coverage until gram stain or culture results are definitive. A variety of irrigation solutions and protocols have been employed in these patients. Diluted antibiotic, povidone iodine, and aqueous acid solutions have been reported.^23,25 The duration of irrigation has varied from three days to a week, while systemic antibiotics are continued, as would be the case for other adult bone infections. Unfortunately, this attempt at primary sternal closure has been reported to require secondary procedures in 20% to 40% of the patients.²⁶ More recently, one small series²³ employing the single-stage débridement, closure, and irrigation technique had success in 95% of patients so treated.

A two-stage approach involves an interval during which the sternum and skin are left open and a wound vacuum device placed.^27,28 Open management of sternal wounds is associated with a risk of sudden, sometimes fatal, cardiac hemorrhage from exposed grafts, the aorta, or (most frequently) the right ventricle. Needless to say, the risk of death in such patients is very high (>50%).^29,30,31 Given these risks, it has been recommended that close attention be paid to the proximity of the right sternal edge and the right ventricle or grafts. Decreased abrasive contact may be afforded by judicious use of sedation and mechanical ventilation until coverage can be achieved.

Whether used as an initial single-stage procedure or as a secondary procedure, tissue transposition into the anterior mediastinum has dramatically changed the prognosis of this once often fatal complication.³² Well-vascularized omentum or muscle can be used. Muscle options include rollover of the pectoralis major (detached from its humeral insertion, leaving intact the muscle’s origin and blood supply) or rotation and advancement of the pectoralis major (detached from its costal origins and thereby maintaining its lateral blood supply). The rectus abdominis muscle may, depending on the prior use of the ipsilateral mammary artery, be detached distally and rotated on its cephalad attachment into the anterior mediastinal space. The omentum may be based on the right gastroepiploic or mobilized, leaving the gastroepiploic intact. The sternum may be left open with the tissue flap between the remnant edges, or rarely it may be closed over the flap. Either way, closed suction drains are required for the large, mobilized skin flaps and sometimes beneath transposed tissue flap.

The use of omentum versus any specific muscle flap may be dictated by availability (e.g., in patients with prior laparotomies), but when the option exists, the use of omentum has been touted as advantageous over muscle flaps,³³ although it has also been cited as being associated with poorer survival outcome,³⁴ perhaps related to patient selection. It has also been successfully employed in infections after ascending aortic replacement.³⁵ Skin coverage over a transposed flap may be accomplished by primary presternal skin reapproximation, split-thickness skin grafting, or, with the rectus muscle, a skin paddle may be transposed as well.

As noted earlier, débrided sternal wounds may be prepared for flap coverage by the use of a closed high-pressure vacuum system in which a polyurethane foam (400-600 µm pore size) is cut to fit the anterior mediastinal space and sealed to the skin permitting a vacuum (negative 75 mm Hg) to be generated over the entire wound surface. The device is changed regularly to avoid tissue ingrowth. Particularly with smaller wounds, the vacuum treatment may obviate the use of flap coverage as the wound heals secondarily, with obliteration of the space over a period of weeks.^36,37