Surgical Site Infections and Associated Operative Characteristics

Classification of Wound Type and SSI

The wound classification developed by the National Academy of Sciences in the 1960s distinguishes four levels of risk (clean, clean-contaminated, contaminated, infected or dirty) based on degree of colonization and/or infection. Clean wounds are defined as an uninfected operative wound in which no inflammation is encountered and the respiratory, alimentary, genital, or uninfected urinary tracts are not entered. Clean-contaminated wounds are operative wounds in which the respiratory, alimentary, genital, or urinary tracts are entered under controlled conditions and without unusual contamination. Specifically, operations involving the biliary tract, appendix, vagina, and oropharynx are included in this category, provided no evidence of infection or major break in technique is encountered.

Contaminated wounds are defined as open, fresh, accidental wounds. In addition, this category includes operations with major breaks in sterile technique (e.g., open cardiac massage) or gross spillage from the gastrointestinal tract, and incisions in which acute, non-purulent inflammation is encountered. This includes necrotic tissue without evidence of purulent drainage (e.g., dry gangrene). Dirty or infected wounds include old traumatic wounds with retained devitalized tissue and those that involve existing clinical infection or perforated viscera. This definition suggests that the organisms causing post-operative infection were present in the operative field before the operation.

In a retrospective analysis of a large infection surveillance data set, the SSI rate was 2.1%, 3.3%, 6.4%, and 7.1%, for clean, clean-contaminated, contaminated, and infected, respectively [3]. Another study of general and vascular procedures reported that wound class was an independent predictor of SSI; odds ratios (ORs) were 1, 1.04, 1.7, and 1.5 for clean, clean-contaminated, contaminated, and infected, respectively [4].

Sources of SSIs

The source of SSI can be considered as exogenous or intrinsic to the patient. Exogenous sources include surgical personnel, the operative physical environment, and tools, equipment, and materials brought to the surgical field. Surgical personnel can increase the risk of exogenous contamination through operative attire (scrubs, caps, etc.), breaks in technique, or poor hand hygiene. The method of ventilation and airflow for the operative suite may influence SSI. Intrinsic sources of pathogens obviously include the patient's skin, mucous membranes, and gastrointestinal tract. Ongoing infections in areas remote from the operative field increase risk of SSI to develop at the operative site.

Ongoing attention is being paid to reducing exogenous sources of infection. This includes vigorous discussions regarding type of surgical caps, etc. More meaningful attention and research has been focused on room cleaning and decontamination processes to reduce environmental transfer of pathogens to patients. This includes not only physical cleaning techniques and products, but also terminal room cleaning with decontamination techniques such as ultraviolet-C and hydrogen peroxide systems to reduce environmental contamination [5].

Significant effort has been put forth in investigating the patient as an intrinsic source of infection. Examples include pre-operative treatment to decontaminate the nares in patients positive for Staphylococcus aureus colonization and pre-operative chlorhexidine body washes and wipes [6,7]. Efforts have been made to minimize body hair removal and when necessary to use clippers as opposed to razors in the operating room immediately before surgery [8]. Skin preparation with chlorhexidine/ethanol preparations is known to reduce infections [9]. Of course, the use of antibiotic agents in the immediate pre-operative period clearly is associated with a decreased risk of SSI [10].

Operative Factors that Contribute to SSIs

Multiple operative characteristics have been associated with increased risk of development of SSI. These include blood loss, emergency/urgent cases, duration of the operation, type of anesthesia, and resident involvement.

Blood loss during an operative procedure has been associated with SSI across multiple different procedure types. For example, in caesarian section, an OR of 1.3 increased risk of infection has been demonstrated for every 100 mL of blood loss [11]. Blood transfusion for a hysterectomy was significantly associated with a higher risk of SSI (OR 3.58; 95% confidence interval [CI] 1.21–10.62) [12]. In spine surgery, an estimated blood loss of more than one liter is a risk for the development of SSI (p = 0.017) [13]. Hypothetical reasons that blood loss and transfusion may be associated with SSI are based on relative levels of antibiotic agents, impaired tissue perfusion, body temperature changes, or relative immunosuppression.

Longer operative times have also been identified as a risk factor for SSI. According to early National Surgical Quality Improvement Program (NSQIP) data, in hospitals that were high outliers for SSI versus low outliers, operations took significantly longer (128.3 ± 104.3 min vs. 102.7 ± 83.9 min, p < 0.001) [14]. This holds true across most individual disciplines. For example, in a study on neurosurgical procedures, there was a direct correlation between operative time and SSI, with shorter times having a decreased risk and longer times having an increased risk [15].

The type of anesthesia, usually general versus regional anesthesia, has been associated in some areas to increase the risk of SSI. This has been studied mostly in the orthopedic literature. One such study of total hip arthroplasty examining close to 13,000 patients with 39.5% undergoing regional anesthesia, the adjusted odds for deep SSI were significantly lower in the regional anesthesia group versus the general anesthesia group (OR] = 0.38; 95% CI = 0.20–0.72; p < 0.01) [16]. In another study looking at NSQIP data of 111,683 patients, 1,928 received local anesthesia; in 109,755 cases, patients underwent general anesthesia where local anesthesia potentially could have used. In the unmatched and propensity matched analyses, patients receiving local anesthesia had a significantly lower SSI incidence than patients receiving nonlocal anesthesia [17]. The risk of SSI and anesthesia is not fully understood; however, general anesthesia may be associated with more significant changes in tissue perfusion, temperature, and immune-mediating pharmacologic effects.

Resident involvement in cases has also been implicated as a risk factor for the development of SSI. The involvement of two or more residents in orthopedic procedures increased the risk of SSI [18]. In a study of laparoscopic gastric bypass operations with more than 21,000 operative cases in the resident involved versus no resident involved arms, resident involvement significantly increased the risk of superficial SSI, urinary tract infection, or sepsis. Of note, resident involvement also correlated with operative duration in this study, potentially confounding results [19]. Obviously the training of residents is necessary; however attention should be paid to modifiable factors such as technique, duration, skin preparation, and operating room traffic.

These are some of the recurring risk factors across multiple studies from different surgical disciplines. Interestingly, in a study by Campbell et al. [14] in 2008 using NSQIP data to identify 20 low outlier and 13 high outlier hospitals for SSI, high outliers had higher trainee-to-bed ratios and longer operative times. Low outlier hospital patients were less likely to present to the operating room anemic and less likely to receive a blood transfusion. Best practice bundles aimed at targeting these associated risk factors have proven to be of some benefit for reducing SSI and should be widely integrated [20]. Recent recommendations have been published by the World Health Organization specifically addressing aspects of pre-operative, intra-operative, and post-operative care in the prevention of SSI [21].

High-Risk Procedures for SSIs

Certain procedures are higher risk for SSI. Operative procedures that enter highly colonized spaces, such as colorectal procedures and gynecologic procedures, are understandably of increased risk for associated SSI (see wound classification above). Other high-risk surgeries, such as spine surgeries, may have higher rates of infection secondary to differences in skin flora, thickness of skin, or relative tissue perfusion, but this is hypothetical.

Colorectal surgical procedures count for a relatively high volume of gastrointestinal operations and clearly pose a higher risk for SSI. Hedrick et al. [22] demonstrated in 18,403 patients using NSQIP data a superficial SSI rate of 7.86% and a deep incisional SSI rate of 1.51%. Identified risk factors for SSI were determined to include age, alcohol abuse, American Society of Anesthesiologists (ASA) classification, stoma closure, open approach, body mass index, and hematocrit. Similar risk factors were found in another study by Hubner et al. [23] and included operative duration, emergency/urgent operations, open procedures, and appropriate timing of pre-operative antibiotic agents.

The rate of SSI in colorectal procedures is seemingly higher than that caught by NSQIP and other large databases. This is supported by the SSI rate found in other studies, such as the study by Itani et al. [24] comparing ertapenem versus cefotetan as a pre-operative antibiotic regimen for colorectal surgery. The SSI rates in this study ranged from 17.1% (ertapenem) to 26.2% (cefotetan). Hedrick et al. [25] also examined cases from their own institution where NSQIP captured SSI rates were 8.4%. Individual chart review by three independent surgeons, however, determined SSI rates ranged from 23% to 38%. Lack of documentation and subjective differences in chart interpretation accounted for most discrepancies.

Predicting SSIs

There have been several proposed scoring systems to predict the risk of SSI for a procedure. One such system, the National Nosocomial Infectious Surveillance System (NNIS) proposed a risk equation based on the dose of bacterial contamination, the virulence of the organism, and the resistance of the patient to infection [3]. The NNIS also created a risk index assigning a score of 0–3: One point for contaminated or dirty operations, one point for an ASA score of 3–5, and one point if an operation exceeds the 75% for duration. Mean SSI rates range from 1.5% for scores of 0% to 13% for scores of 3. Rates vary among surgical procedures, however, with high-risk operations for SSI such as colon surgery having rates of 3.2%, 8.5%, 16%, or 22% for scores or 0, 1, 2, or 3. The NNIS risk index has not been particularly impactful in changing the clinical treatment of patients.

Other scores have been proposed, including the SSI Risk Score that takes into account patient factors (smoking, increased body mass index, peripheral vascular disease, metastatic cancer, chronic steroid use, recent sepsis) [26]. In addition, operative characteristics are considered including surgical urgency, increased ASA class, longer operative duration, infected surgical sites, general anesthesia, performance of more than one procedure, and current procedural terminology (CPT) score.

Reducing SSI

We will never eliminate SSI. Through increased understanding of risk factors, we can strive to reduce risk, however. This includes educating all staff in and out of the operating room regarding process and guidelines. Blood loss and transfusion should be minimized. Advocate for minimally invasive approaches and avoid general anesthesia when possible. Assure the proper training and awareness of residents regarding SSI related issues.

Care bundles are being developed as a strategy to reduce SSI. Although some data from individual series have been conflicting, it appears that this is a useful strategy to decrease SSI rates. A recent meta-analysis of care bundles for colorectal surgery demonstrated a clear benefit in the reduction of SSI [20].

Conclusion

SSI represents a significant healthcare burden with impact on morbidity, death, and cost of care. Continued research is necessary to understand modifiable risk factors and preventive measures to minimize the impact of SSI on surgical care.