**3. Definition and classification**

There is still no universally accepted method for treatment of DSWI or other sternal wound complication. One possible explanation for this is that until relatively recently, thorough classification schemes providing specific criteria for assignment as deep sternal infection, mediastinitis, superficial infection, or sterile sternal dehiscence were lacking. However, it is important to distinguish between these individual entities since each demands a different management strategy. Furthermore, results of reported series which include a variety of wound complications will differ from those that report a homogenous population. For example, sternal wound infections that are limited to the superficial soft tissues obviously demand less aggressive intervention for treatment and will generally respond more readily to treatment than deep sternal infection with higher likelihood for success. Therefore, when

RIGHT ATRIUM +++ ++ 0 RIGHT VENTRICLE +++ + + LEFT ATRIUM +++ + + LEFT VENTRICLE ++ 0 +++ SVC +++ ++ 0 IVC +++ ++ 0 ASC AORTA/root +++ + + RIGHT SCA ++ ++ 0 INNOMINATE +++ ++ 0 LEFT SCA + 0 +++ DESC AORTA 0 0 +++ Main PA +++ 0 ++ RIGHT PA ++ +++ 0 LEFT PA ++ 0 +++ PROX TRACHEA ++ + + ESOPHAGUS 0 +++ ++ HEMIDIAPHRAGM ++ + +

Table 1. Comparison of exposure of various intrathoracic anatomic structures through median sternotomy, right thoracotomy, or left thoracotomy. (+++ denotes excellent, reliable

thoracic diseases in multiple populations approached through this incision are factors which combine to make median sternotomy the most commonly performed osteotomy worldwide [20]. Recently, several alternatives to median sternotomy have been promoted, including thoracoscopic and robotic approaches to cardiac and thoracic procedures [21, 22]. These approaches are possible as a result of high fidelity instrumentation and video platforms but are limited by steep learning curves and expense. Therefore, for most hospital systems performing cardiac surgical procedures, median sternotomy remains the

There is still no universally accepted method for treatment of DSWI or other sternal wound complication. One possible explanation for this is that until relatively recently, thorough classification schemes providing specific criteria for assignment as deep sternal infection, mediastinitis, superficial infection, or sterile sternal dehiscence were lacking. However, it is important to distinguish between these individual entities since each demands a different management strategy. Furthermore, results of reported series which include a variety of wound complications will differ from those that report a homogenous population. For example, sternal wound infections that are limited to the superficial soft tissues obviously demand less aggressive intervention for treatment and will generally respond more readily to treatment than deep sternal infection with higher likelihood for success. Therefore, when

THORACOTOMY

0 +++ ++

LEFT THORACOTOMY

STRUCTURE STERNOTOMY RIGHT

MAIN STEM BRONCHI

mainstay incision.

exposure; 0 denotes no reliable exposure)

**3. Definition and classification** 

comparing different treatment modalities for sternal wound infection, it is important to be clear about the extent of infection, since heterogeneity could skew reported results.

Two prominent classification schemes have been proposed and are in use. Mediastinal dehiscence or the more chronic form, sternal nonunion, is defined as sternal wound disruption without any evidence for infection either clinically or pathologically [23, 24]. These entities will not be discussed in any detail. In contrast, mediastinitis, as characterized by the U.S. Centers for Disease Control and Prevention (CDC), is an infection of the mediastinum diagnosed by isolation of pathogenic organisms from the mediastinal fluid or tissue especially when there is obvious evidence of infection at the time of sternal exploration [25]. Alternatively, a combination of clinical features including chest pain, sternal drainage with bony instability, fevers, radiographic findings such as widened mediastinum, and bacterial isolation may also warrant a diagnosis of mediastinitis. Obviously, from the surgical perspective, these definitions are somewhat lacking since either superficial infections, confined to the soft tissues, or deep infections, involving the bone and/or retrosternal space could produce bacteremia and clinical signs of severe infection [23].

Fortunately, more descriptive classification schemes have been introduced and provide more specific insight into the pathologic involvement of the sternal tissues and the clinical consequences and course [23, 26, 27]. For example, the classification scheme introduced by El Oakley and Wright is based on the time at which the patient presents with mediatiastinitis relative to the initial surgical procedure [23]. Schulman et al have advocated a similar classification system [27]. In addition, the El Oakley description also accounts for relevant risk factors underlying the clinical scenario and whether or not previous attempts to treat the sternal wound infection have been made and failed [Table 2]. Therefore, five distinct categories of infection are described, each with important treatment implications. For example, the subtypes I and II appear to respond well to primary sternal closure with mediatinal irrigation, while subtypes III-V appear to require more aggressive sternal debridement and repair techniques [23].


Table 2. Classification scheme of mediastinitis introduced by El Oakley and Wright based on the time at which the patient presents with mediatiastinitis relative to the initial surgical procedure (modified from El Oakley and Wright [23]).

Sternal Wound Complications Following Cardiac Surgery 287

endocarditis

vessel CAD

Table 4. Compiled analyses for underlying risk factors associated with sternal wound infection. Obesity and diabetes mellitus are consistently shown to be independent predictors

wound, and ineffective or inadequate dosing of perioperative antibiotics [6, 9].

during surgery owing to the patient's size, increased bleeding, increased deadspace in the

Although multiple mechanisms for sternal wound complications are proposed, it is widely accepted that reduced sternal perfusion, often by virtue of internal mammary artery (IMA) harvesting for use as a vascular conduit in coronary artery revascularization, is one of the most important causes of sternal nonhealing and infection [1-6, 8, 26], especially when both IMAs are harvested for bypass graft surgery [5]. Therefore, more cases of DSWI appear to occur after coronary artery bypass grafting or after combined procedures that include coronary artery surgery [7, 35]. Other viable explanations for DSWI etiology include poor bone stock from osteoporosis, malnutrition, and other factors; poorly performed sternotomy leading to sternal fractures and/or costosternal disassociation; and other patient related factors including peripheral vascular disease and lung disease [8, 36]. Several other factors have been implicated in the development of DSWI but may not be manifest in the context of a retrospective review or randomized trial because the numbers are too low. For example, it is generally accepted that postoperative steroids or chronic immunosuppression increases risk for DSWI, but this has been difficult to demonstrate in even large database reviews [13, 37]. Finally, Risnes et al have demonstrated in a review of over 18,000 consecutive patients undergoing coronary artery surgery in Norway, the major preventable risk factor associated with the development of DSWI was the amount of blood product transfusion

559,004 Obesity

6,459 Obesity, NYHA class, redo surgery, CPB duration

3,760 Diabetes, preoperative hemodynamic instability, renal failure, bilateral IMA, sepsis and/or

5,690 Obesity, diabetes mellitus, COPD, bilateral IMA

300,000 Obesity (BMI of 30-40 kg/m2), diabetes, previous MI, urgent operative status, hypertension

4,826 Diabetes, obesity, reduced EF, renal failure, multi-

1,980 Obesity (BMI>30 kg/m2), diabetes, > 4 units RBC transfusion, IABP, current smoking

operation, recent smoking; h/o CVA, total HLOS,

transfusion, advanced age, male sex, diabetes

7,978 Obesity (BMI>30 kg/m2), diabetes, urgent

sepsis/endocarditis postop

18,352 COPD, obesity (BMI > 30 kg/m2), blood

Study # Patients Identified Risk Factors

Milano CA, et al. Circulation 1995 [9]

Toumpoulis IK, et al. Chest 2005 [55]

Immer FF, et al. Ann Thorac Surg 2005 [94]

Prabhakar G, et al. Ann Thorac Surg 2002 [32]

Olsen MA, et al. J Thorac Cardiovasc Surg 2002

Cayci C,et al. Ann Plast

of poststernotomy mediastinitis.

Surg 2008 [52]

Risnes I, et al. Ann Thorac Surg 2010 [12]

perioperatively [12].

Sjorgren J, et al. Ann Thorac Surg 2005 [90]

Fowler G, et al. Circulation 2005 [13]

[10]

The classification by Jones et al differs from that of El Oakley and Wright in that it is more descriptive anatomically and physiologically [26, TABLE 3]. Three different "types" of sternal infection are described, encompassing both superficial and deep infections, and based on the degree of underlying tissue involvement with infection. We have preferred the use of this classification system as it is simpler to use since it based strictly on features observed or encountered at the time of initial sternal exploration. In addition, Type 3b is physiologically meaningful since it denotes the patient who is systemically ill from the sternal wound process. In our own institutional experience of 222 adult cardiac surgical patients treated for postoperative DSWI, approximately 50% of patients exhibited septicemia (Jones 3b) upon initial presentation [14].


Table 3. Mediastinal wound classification system modified from Jones et al [26]. Although anatomic involvement by infection in distinguished, the presence of septicemia is the most important feature clinically.
