Risk Factors for and Epidemiology of Surgical Site Infections

Surgical Site Infections: Categories, Definitions, and Surveillance

As many as 5% of patients having surgery develop a SSI with an appreciable morbidity and mortality rate and cost to health care systems [5–8]. The theoretical categorization of surgical incisions into clean, clean-contaminated, contaminated, and dirty [9] has stood the test of time in research and audit; and there is good microbiological support for retaining this theoretical grouping [10]. The further classification of SSIs into superficial, deep, and organ/space remains the most widely used system [11]. However, although it is relatively easy to diagnose infection clinically [12], the U.S. Centers for Disease Control and Prevention (CDC) definitions record only the categorical data of whether infections are present or absent. One of their criteria—whether an SSI has been recorded by the attending physician/surgeon—has been questioned and is left out by some investigators [13]. The other criteria are a purulent discharge, isolation of organisms, the presence of the Celsian signs (calor, rubor, dolor, et tumor), and wound separation or the need for drainage of a collection. In research and audit, it is preferable to have interval or continuous data, and this can be provided by scoring systems for SSIs; the ASEPSIS score ( a dditional treatment, s erous discharge, e rythema, p urulent exudates, s eparation of deep tissues, i solation of bacteria, and hospital s tay for > 14 days) probably is the best-used example [14]. This allows differentiation of a minor, superficial SSI from one that might be life-threatening.

After clean wound surgery (no inflammation is encountered, no viscus is opened, and there is no breach of aseptic technique [9]), there is a paradoxical controversy. Traditionally, guidelines refer to the large study undertaken by Cruse and Foord [15] in which the SSI rate in this category was found to be 1.4%. However, SSI rates in these cases before and since this seminal paper have been reported to be between 1% to more than 15% [16–25]. The discrepancy probably lies in the accuracy of audit, whether it is based on telephonic or questionnaire surveillance, or if it extends into primary care with a trained, unbiased, and blinded observer, ideally collecting interval data. The costs of managing SSIs in this clean wound category are transferred to primary care, where there may be concern about the adequacy of recognition and management. Most clean wound SSIs occur in a group of patients increasingly having their surgery undertaken on a day-case basis. As most SSIs take approximately 7–10 days to manifest, the number of SSIs seen in primary care will increase absent resources for or education on their optimal recognition and care. If mandatory reporting of SSIs is to be introduced widely, it is clear that for accuracy, this will entail extra personnel and resourcing.

Guidelines for Prevention and Treatment: Antibiotic Prophylaxis

Guidelines for the prevention and treatment of SSIs have been published [26–28]. In the guideline from the National Institute of Health and Clinical Excellence (NICE) of the United Kingdom, the operative period was split into three phases (preoperative, intra-operative, and postoperative) with guidelines, for example, on hair removal, antibiotic prophylaxis, and warming for which there is Level I evidence of efficacy; other guidelines include advice on skin preparation, surgical dressings, and the use of tissue viability specialists for the management of open wounds after infection [29].

Lord Moynihan, in a classic description of the ritual of a surgical operation [30], was aware that bacteria needed to be excluded from the operative field or from the patient's environment to prevent infection. He regarded every operation as an experiment in bacteriology. Since then, the rationale for antibiotic prophylaxis was pioneered by Miles et al. [31]. They showed that an antibiotic given prior to an injection of staphylococci into the skin of rabbits prevented subsequent formation of an abscess. They described the 4-h “decisive period” during which host defenses are breached without the presence of the protective host inflammatory response and before complement, neutrophils, monocytes, and other factors begin to appear. This is the period when antibiotic prophylaxis is effective, as the first randomized controlled clinical trial of antibiotic prophylaxis was to show [32]. Since then, Class I evidence has been accumulated that antibiotic prophylaxis is effective in clean-contaminated and contaminated surgery. In general, prophylactic antibiotics are given empirically to cover the decisive period and the likely bacterial spectrum; once, intravenously, at induction of anesthesia, and repeated only if there is excessive blood loss or unexpected spillage of infective viscus contents [28,29]. In clean prosthetic surgery, including the placement of vascular grafts and joint prostheses, antibiotic prophylaxis is equally effective but often, and justifiably, given for two or three doses. The use of antibiotics in clean, non-prosthetic surgery remains controversial, although further research is being pursued in this field. With the background of increasing antibiotic resistance and emergence, there is pressure to be certain all other aspects of prevention and management are engaged [33]; and there are many national and local directives being introduced or available. In “dirty” surgery, prophylaxis should be considered as empirical treatment based on local antibiotic policies and resistance patterns and the spectra of the organisms likely to be encountered during the operation. The value of antibiotic prophylaxis and treatment in the surgical wound categories has been summarized, with particular reference to the reduction of SSIs [34].

Warming, Tissue Viability, and Surgical Site Infection

It has been recognized that low tissue perfusion increases the risk of subsequent infection and that warming, locally or systemically, improves perfusion and tissue oxygenation [35,36]. Following one of the first clinical trials of the value of systemic warming [37], there has been a meta-analysis of trials [38] providing Level I evidence that systemic warming reduces SSIs; reduces blood loss and the need for transfusion; reduces morbidity and mortality rates, including cardiac events; and is cost-effective. There also has been a NICE guideline that has promoted warming [39], but from virtually all the studies reviewed, only the value of intra-operative systemic warming using forced air warming (FAW) systems was recommended.

Local warming, using a radiant heat card, has been even more effective than systemic warming using FAW in clean, non-prosthetic surgery [40,41]. The SSI rates were 75% lower than for standard (at the time) unwarmed patients. Serendipitously, it was found, through in-depth surveillance, that warmed patients (6.5%) were given fewer antibiotics for complications of incision healing than those who were not warmed (15.9%). Similarly good results have been found, after using an alternative to systemic warming with FAW, with conductive polymers after elective open abdominal surgery [42] and after surgery for peritonitis [43]. The use of conductive polymers probably offers a cheaper, re-usable system that is more flexible and can be used for maintenance of normothermia throughout the whole of the perioperative period. A preliminary report has shown the benefit of warming with conductive polymers in accident and emergency departments [44]. This flexibility of use will extend to transport and other areas of medicine.

The use of supplemental oxygen, with a target FiO₂ of 0.8, in the recovery room also has an effect on SSI rates [45,46]. However, the NICE guideline [29] recommended that oxygen be given only at a rate to ensure 95% hemoglobin saturation and did not recommend supplemental oxygen, particularly as an FiO₂ of 0.8 was difficult to achieve without intubation.

Although tight control of blood sugar has been found to be related to SSIs after cardiac surgery and has been taken up by many hospitals for all major surgery in the United States [47], this practice has not yet found favor in the United Kingdom. Again, control of blood sugar for all non-diabetic patients was not recommended in the NICE guidelines [29].

Good Antibiotic Stewardship: Is It Time To Revisit Antiseptics?

More than 150 years ago, Ignaz Semmelweis showed that the simple action of washing the hands in chloride of lime between the postmortem room and the delivery suite could reduce puerperal infection by a factor of ten. Antiseptics, rather than disinfectants, have come a long way since then, and there is a compelling argument to revisit their wider use in the light of increasing antibiotic resistance and microbial emergence. This use of antiseptics has been recognized widely in chronic wound care, with increasing acceptance of the part played by biofilms and the control of bioburden in the contamination–colonization–critical colonization-infection concept [48–52]. The conventional antiseptics [53,54] have in the last few years been joined by silver and polyhexamethylene biguanide in the control of the bioburden in chronic wounds [55,56], although there are continuing studies. Level I-evidence-based proof of antiseptic effectiveness through meta-analysis of adequate randomized controlled trials is lacking, possibly because of the prohibitive cost of such trials.

Antiseptics have always had a role in surgical incision care, skin preparation, and hand washing, but were popularized by Lord Lister with his antiseptic principle [57]. Antiseptics have a huge advantage over antibiotics in having multiple sites of direct, non-specific action: They are toxic to bacterial cell walls, membrane proteins and efflux pumps, cytoplasmic organelles, and cell respiratory processes and can denature enzymes and nucleic acids. As a result, resistance has not yet been described clinically. Antibiotics, by contrast, act more specifically, and resistance to their nucleic acid activities, for example, can be plasmid-transmitted.

Another exciting development in the use of antiseptics is their incorporation into coated or impregnated antimicrobial sutures. Triclosan, a non-toxic antiseptic with no apparent risk of resistance [58,59], is the first to be used for this purpose, with good functional antimicrobial effects being found experimentally [60] and with no change in clinical handling characteristics from the parent reabsorbable braided polyglactin 910 (Vicryl^®, ETHICON, Somerville, NJ) suture [61]. The antimicrobial qualities of this triclosan-coated suture (Vicryl Plus^®, ETHICON) have been found to be long lasting against common pathogens such as staphylococci and Escherichia coli, even in the presence of biological fluid [62]. Its clinical efficacy for the prevention of SSIs also has been assessed in several challenging clinical areas:

Sternal SSIs after cardiac surgery are a cause of considerable morbidity as well as death, and standard polyglactin 910 suture has been compared with polyglactin 910 suture with triclosan for sternal closure [63]. Closure of the deep fascia, subcutaneous tissue, and skin with or without the antimicrobial suture showed a fall from 6.3% to 0, with each SSI costing $11,200 according to the authors. Although the trial was seriously flawed in being retrospective and not randomized, the followup was prolonged and used acceptable definitions and post-discharge surveillance techniques.

The use of antimicrobial suture has been claimed to be cost effective in these studies and appears to be associated with a lower SSI rate. More diverse prospective studies with adequate power, randomization, and excellence of post-discharge surveillance are expected. Nevertheless, the days of chromic catgut and silk have gone!

Author Disclosure Statement

No conflicting financial interests exist.