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Ann Thorac Surg 2005;79:161-162
© 2005 The Society of Thoracic Surgeons
Department of Surgery, National University of Singapore, 5 Lower Kent Ridge Rd, Singapore 119074
One of the most contentious issues in surgery is the use of prophylactic topical antibiotics to prevent wound infection [1]; cardiac surgery is no exception. Despite regular use of prophylactic intravenous antibiotics, postoperative mediastinitis occurs in a significant number of patients undergoing open heart surgery through median sternototmy. This major complication plagues the most commonly used incision in cardiac surgery, and is associated with a mortality rate as high as 47%. Staphylococcus aureus or S epidermidis are isolated in as many as 70% of the cases. Therefore, prophylactic instillation of antigram-positive antimicrobials during wound closure may reduce the risk of mediastinitis. This concept has indeed been tested in a previous prospective, randomized, controlled study; Vander Salm and associates [2] found that topical vancomycin applied during wound closure after median sternotomy was associated with a significant reduction in the rate of sternal wound infection (from 3.6% in the control group to 0.45% in the treatment group). We found sub-MIC levels of vancomycin in patient's serum after topical application of 1 g vancomycin during sternotomy closure [3]. This, at least in theory, may encourage the emergence of vancomycin-resistant pathogens. Therefore finding a non–vancomycin antimicrobial that effectively prevents mediastinitis will be a welcome addition to the field.
In this issue of The Annals, Friberg and associates report the results of a prospective, randomized clinical trial of using prophylactic collagen-gentamicin sponges placed between the sternal edges at the time of wound closure after mediastinotomy in 2,000 cases undergoing open heart surgery. The incidence of surgical site infection was significantly reduced from 9% in the control group to 4.3% in the treatment group. These apparently higher rates of surgical wound infection were attributed to the fact that this is a prospective randomized study with longer and near complete follow-up. Friberg and colleagues have used their own modification of the Centers for Disease Control (CDC) definition of wound infection, where "...sternal wire involvement with infection was the cut-off between superficial and deep wound infection...." Based on this definition, the incidence of deep wound infection was also reduced from 3.3% in the control group to 2.3% in the treatment group.
The CDC definition for surveillance of surgical site infections [1] identifies the following three categories of surgical site infection: superficial incisional surgical site infection, deep incisional surgical site infection, and organ/space surgical site infection. This classification alone may be adequate for incisions with a reasonable demarcation between the superficial and the deep components of the wounds, as well as well-defined organ and space compartments, such as thoracotomy and laparotomy incisions. In cases of mediastinitis, however, the sternum is considered an organ in the CDC definition, yet it represents the deep layer of the wound and is always involved in the disease process. It is almost impossible to determine whether the infection is confined to the pectoral fascia, the sternal edges, the sternal wires, the retrosternal space, or the mediastinal spaces alone. Infections at the suprastenal notch and around the xiphoid cartilage are also difficult to define as deep-, organ-, or space-site infection.
We have previously devised a classification for postmedian sternotomy deep wound infection [4]. In this classification, all deep sternal wound infections were classified according to the time of first presentation, and according to whether the patient had any previous attempts of surgical treatment for mediastinitis. The classification also takes into account the factors that carry a proven risk of developing mediastinits such as diabetes, obesity, and the use of oral steroids or immunosuppressive agents.
Friberg and coworkers acknowledge that the application of other wound score systems was beyond their resources; they also highlight other limitations of their study including the lack of a collagen sponge only—without gentamicin—group, and the significantly higher incidence of reopening for bleeding in the treatment group (2.3% in the control group versus 4% in the treatment group). No explanation was given for the relatively higher number of patients on steroid therapy in the control group as compared with the treatment group (4.9% in the control group versus 2.2% in the treatment group), and whether preoperative nasal swab was performed routinely on these patients preoperatively. The use of systemic steroids, and a Staphylococcus-positive nasal swab are known risk factors for developing postoperative mediastinitis. Furthermore, the rationale for using gentamicin (which is not commonly used as an anti–gram-positive agent) to prevent a predominantly gram-positive infection has been, and to some extent, well argued. One remarkable finding in this study is that the 60-day mortality of all cases diagnosed with mediastinitis was 1.5% (2 patients, both belonging to the treatment group); that is less than the 1.9% 60-day mortality rate of the overall patient population! It remains to be seen whether these results, and the authors' reasoning, will persuade the readers of The Annals to adopt this approach into their daily practice.
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