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Surgical Site Infections (SSI): Causes, Prevention and Treatment

Definition

Superficial Incisional SSI:

Superficial incisional surgical site infection develops within 30 days near the skin incision; the infection is limited to tissue above the fascia. Defining criteria are pus discharge, organism detection in wound secretions, or iatrogenic opening of the incision.

Deep incisional SSI:

Deep incisional surgical site infection develops within 30 days (or within a year for implants), including tissues under the fascia. Defining criteria are pus discharge, organism detection in wound secretions, or iatrogenic opening of the incision, including deeper structures.

Organ or space infection:

Organ or space infection is an abscess or other signs of organ or body cavity infection. Defining criteria are purulent discharge after draining the cavity, detection with an appropriate examination (histology, radiology), or diagnosis during a reoperation.

Epidemiology: Risk of Surgical Wound Infections

The risk of surgical site infections depends on many factors (see the following sections). An important factor is the degree of contamination of the wound at the end of the operation (Wagenlehner et al., 2011a):

Clean Wound and Risk of SSI:

A clean wound is defined as an uninfected surgical wound without opening of the gastrointestinal or genitourinary tract and without encountering inflammation. The most common organisms of SSI are staphylococci, and the risk is (in patients without risk factors) less than 2%.

Clean-contaminated Wound and Risk of SSI:

The gastrointestinal or urogenital tract was opened in a controlled manner and without unusual contamination of the wound. The risk for wound infection is 2–4% for urinary tract interventions and 5–10% for colon surgery. In addition to staphylococci, the most common pathogens for SSI are enterobacteria and enterococci. Anaerobe pathogens are possible after bowel surgery.

Contaminated Wound and Risk of SSI:

Wounds with uncontrolled contamination with infectious urine or gastrointestinal content or fresh traumatic wounds. The risk for wound infection is 10–15%, and the most common pathogens are similar to the category clean-contaminated.

Dirty-infected Wound and Risk of SSI:

Dirty-infected wounds result from surgical interventions in body regions with massive bacterial contamination from existing infections or old traumatic wounds. The risk for wound infection is 15–40%, and the most common pathogens are similar to the category clean-contaminated.

Risk Factors, Etiology and Pathogenesis

Surgical site infection begins with contamination (bacterial are inoculated into the wound), continues with colonization (establishment and proliferation of the pathogen), and may end in infection (local and systemic reaction of the immune system of the body, depending on pathogen concentration and virulence factors). The pathogenesis explains the interval between surgery and wound infection from several days to weeks.

Bacterial concentration in the wound:

The higher the bacterial concentration in the wound at the end of the operation, the more likely the wound infection will be.

Airborne bacterial contamination:

Airborne bacterial contamination is the major contributor to SSI. Airborne particles carry microorganisms, mainly Staphylococcus aureus, which settle on the surgeon's hands and instruments or sediment directly into the wound.

Bacterial contamination from the patient:

Bacteria from the skin, gastrointestinal or urogenital tract may enter the wound. Hematogenous infection is rare and is associated with implants.

Impairment of lymphatic drainage:

Surgery with impairment of the lymphatic drainage of the surgical wound (transection of the lymphatic vessels or removal of lymph nodes) increases the risk of wound infection, e.g., in radical inguinal lymphadenectomy (SSI up to 70%).

Tissue hypoxia:

The bacterial destruction by phagocytes is oxygen-dependent. The bacteriocidal effect is due to the formation of superoxide radicals from molecular oxygen by the NADPH-coupled oxygenase. An oxygen deficiency of phagocytes inhibits their destructive function. Tissue hypoxia results from general hypoxia, volume depletion, pain, hypothermia, compression by retractors, traumatic preparation, and excessive coagulation. The resulting tissue hypoxia increases the incidence of wound infections (Hopf et al., 1997).

Bleeding:

Wound hematoma and bleeding leading to blood transfusion are risk factors for SSI (Houbiers et al., 1997).

Implants:

Suture material, mesh, or implants are risk factors for surgical site infections.

Comorbidity:

The following diseases increase the risk for SSI: cachexia, coagulopathies, shock and burns, polytrauma, diabetes mellitus, obesity, immunodeficiency, smoking, alcoholism, glucocorticoid therapy, existing infections in other organs (Pessaux et al., 2003).

Preoperative Measures for Prevention of SSI

Optimize patient risk factors:

Avoid malnutrition, reduce body weight in overweight, optimize diabetes, check heart and lung function, stop smoking 6 to 8 weeks before surgery, and avoid alcohol.

Nosocomial bacteria:

Minimizing the preoperative inpatient length of stay reduces the likelihood of nosocomial infection of the wound.

Bowel preparation:

unnecessary for surgery of the small bowel, often used for urologic surgery using the colon; see section bowel preparation.

Preoperative shower:

With antiseptic soap.

Hair removal:

Shaving the surgical site increases the risk of wound infection, and micro-injuries with colonizing hospital germs are blamed. If hair removal is necessary, use an electric clipper just before surgery in the operating room (Kjonniksen et al., 2002).

Intraoperative Measures for Prevention of SSI

Anesthesia:

All measures concentrate on avoiding tissue hypoxia, a decisive risk factor for wound infection (Hopf et al., 1997). In some studies, the prophylactic increase of the inspiratory oxygen concentration reduced the wound infection rate.

Prevention of hypothermia:

Hypothermia leads to hypoxia due to vasoconstriction and shivering. In addition, hypothermia leads to an impairment of the immune system.

Prevention of hypovolemia:

Hypovolemia leads to hypotension, hypoperfusion, and, thus, hypoxia.

Avoidance of unnecessary blood transfusions:

Blood transfusions increase the risk of wound infections (Houbiers et al., 1997). The risk has to be judged against the consequences of hypovolemia and hypoxia due to anemia.

Hypercapnia:

A mild increase in the carbon dioxide (CO2) level improves subcutaneous perfusion and may thus reduce the rate of wound infection (Akca et al., 2003). Hypocapnia should be avoided.

Perioperative antibiotic prophylaxis:

See section perioperative antibiotic prophylaxis.

Prevention of bacterial contamination via the surgeon:

The individual surgeon is a risk factor for wound infection in many studies (Cuthbertson et al., 1991): protective factors are surgical hand disinfection, wearing double gloves, glove changes after contamination or prolonged surgery, occlusive surgical clothing (textile laminate, polypropylene), surgical mask, adherence to hygiene recommendations.

Prevention of airborne bacterial contamination:

Clean dust-free operation room, vertical laminar flow with filter system, few people present in the operating theater, minimization of conversations, shortest possible duration of surgery, disposible occlusive surgical clothing.

Surgical approach:

Minimize subcutaneous scalpel incisions, use moist laparotomy pads to protect wound edges, and hemostasis limited to the most necessary.

Skin disinfection:

Available are alcoholic and aqueous disinfectant solutions with chlorhexidine, PVP-iodine, or octenidine. Skin disinfection (without mucous membranes) should be done with alcoholic solutions. Mucous membranes are disinfected using aqueous solutions. In abdominal surgery, using plastic adhesive drapes to protect the wound from dermal bacteria is likely ineffective. Before skin closure, disinfection of the wound edges should be repeated.

Avoid excessive bleeding:

Hematomas, blood transfusions, and intraoperative hypotension resulting from bleeding are risk factors for SSI (Jensen et al., 1990b) (Houbiers et al., 1997).

Wound irrigation:

Saline wound irrigation is a way to remove blood, sedimented bacteria, and particles in elective surgery with clean wounds. For contaminated wounds, additional use of an antiseptic is possible, most preferably polyhexanide or octenidine. The benefit of wound irrigation is controversial.

Subcutaneous wound closure:

The use of subcutaneous suction drainage or subcutaneous sutures is controversial. The drainage could be the entry port for bacteria, and subcutaneous sutures lead to ischemia. In lean patients, additional subcutaneous closure is unnecessary. Subcutaneous sutures, e.g., after a Caesarean section, proved beneficial in obese patients (Chelmow et al., 2004).

Skin suture:

Skin closure is possible using metal clips, skin suture with monofilament non-resorbable suture material or intracutaneous stitches with resorbable monofilament suture material without significant differences in surgical sites infections (Cochetti et al., 2020). Patients prefer intracutaneous suture techniques with cosmetically attractive results. Small trocar incisions after laparoscopic procedures can be closed quickly and equivalently without suturing with wound closure strips.

Postoperative Measures for Prevention of SSI

Wound dressing:

Wound dressings have a minor effect on the wound infection rate since SSI results from bacterial contamination during the operation and due to the patient's risk factors. The standard of care is dry gauze dressings; hydrocolloid dressings are more expensive and have no measurable clinical benefits. Randomized trials proved the early dispensability of the wound dressing (Merei et al., 2004). Early postoperative showering does not increase the incidence of wound infections (Neues et al., 2000).

Pain management:

Sufficient analgesia reduces vasoconstriction, increases tissue oxygen tension, and thus significantly reduces the rate of wound infection (Akca et al., 1999).

Blood glucose control:

Physiological glucose concentrations reduce the risk of wound infection (Lazar et al., 2004). In diabetic patients, blood glucose levels should stay below 200 mg/dl.

Treatment of Wound Infection

Local therapy:

Ubi pus, ibi evacua is the Latin aphorism of local therapy: reopen the wound, drain the pus, and irrigate the wound. Perform a wound swab to determine the resistance of bacteria. Smaller wounds should heal with secondary intention. Larger wounds are treated with secondary healing until clean granulating wound surfaces are reached; a secondary suture can shorten the wound healing time. Alternatively or additionally, vacuum therapy can accelerate wound healing.

Antibiotic treatment:

Start with a calculated antibiotic treatment depending on the expected bacteria. If necessary, change the antibiotic treatment after obtaining the results of the wound swab.

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