Prevention of Perioperative Surgical Site Infection

Classification of SSI

Classification of wounds can be based on the degree of bacterial load or contamination that they contain. Since the landmark 1964 National Academy of Sciences’ National Research Council study on the use of ultraviolet lights in the operating room (OR), wounds have been classified by the level of risk of contamination. The four categories are clean, clean/contaminated, contaminated, and dirty/infected. This classification of the degree of contamination in the surgical site during surgery has become the traditional system for predicting infection risk in surgical wounds.

SSIs are classified into three groups: (1) superficial incisional, (2) deep incisional, and (3) organ/space. A superficial incisional SSI involves only the skin or subcutaneous tissue, a deep incisional SSI involves the fascia or muscular layers, and an organ/space SSI involves any part of the body opened or manipulated during the operative procedure, excluding the previously mentioned layers. The criteria used for defining an SSI are listed in Box 28.1 . The identification of SSI involves interpretation of clinical and laboratory data. The Centers for Disease Control and Prevention (CDC) has derived standardized surveillance definitions and these criteria must be applied consistently when classifying an SSI.

Microbiology

The risk of infection increases once the site has been contaminated with greater than 10 ⁵ organisms per gram of tissue. The bacterial burden necessary to cause infection is significantly reduced when foreign material is in place (i.e., only 100 staphylococci per gram of tissue when introduced on silk sutures). Risk is also proportional to the toxins produced by the specific pathogen because these agents can facilitate host invasion, damage tissues, and interfere with host defenses.

The pathogens responsible for SSIs have not significantly changed over time. Table 28.1 lists the distribution of pathogens associated with SSIs. Staphylococcus aureus, Escherichia coli, coagulase-negative staphylococci, and Enterococcus faecalis represent nearly 50% of the pathogens associated with SSIs. S. aureus is the most common SSI pathogen for most types of surgery, but E. coli is more prevalent in abdominal surgery and Enterococcus species are most prevalent in transplant surgery.

Sources of Pathogens

The primary source of SSI pathogens is the endogenous flora of the patient’s skin, mucous membranes, or hollow viscera. When the mucous membrane or skin is incised, the exposed tissues are at risk of contamination with endogenous flora. These organisms are usually aerobic gram-positive cocci (e.g., staphylococci). When an organ system is entered, endogenous flora specific to it may be the source of the pathogen (e.g., enteric flora such as E. coli or other gram-negative rods when bowel surgery is performed).

Surgical pathogens can also be derived from exogenous sources. Examples include surgical personnel (especially members of the surgical team), the OR environment (including air), and materials on the sterile field (such as instruments, equipment, containers) during an operation. Exogenous flora are primarily aerobes, especially gram-positive organisms (e.g., staphylococci and streptococci). Although S. aureus is usually from an endogenous source, evidence suggests exogenous pathways as well. Fungal SSIs are infrequent but can be endogenously or exogenously derived. Nevertheless, there are over two dozen reports of Candida infections in prosthetic joints, and a growing number of studies report Candida infections after cardiac surgery. Although these infections are infrequent, they are associated with serious problems, including a greater than 50% mortality.

Risk Factors

A risk factor is a characteristic statistically associated with, although not necessarily causally related to, an increased risk for a particular outcome. Risk factors for developing an SSI after surgery can be broadly separated into preoperative, intraoperative, and postoperative settings and relate to patient, procedure, or facility factors ( Table 28.2 ). Nonmodifiable patient factors include increasing age, recent radiotherapy, and preexisting or history of skin or soft tissue infection. Potentially modifiable patient risk factors include glycemic control, alcohol, smoking status, malnutrition, obesity, and immunosuppression. Procedure-related factors include emergency surgery, surgical complexity, and wound classification. Facility risk factors include inadequate ventilation, increasing OR traffic, and inappropriate sterilization of equipment. In this section, preoperative, intraoperative, and postoperative preventative strategies for SSI will be examined in further detail.

Preoperative Setting