SSI acquisition depends on exposure to bacteria and the host’s ability to control the inevitable bacterial contamination of a surgical wound. The likelihood of developing an SSI is a complex interaction among several variables including overall host characteristics (ie, age, immunosuppression, obesity, diabetes), effectiveness of antimicrobial prophylaxis, surgical site tissue condition and presence of foreign material, and degree of wound contamination.
Several studies identify the extremes of age as a risk factor for SSI. The risk of SSI is higher in infants compared to older children92
and higher among older adults compared to younger cohorts.45,46
With increasing age, the skin’s dermis and basement membrane thins. In addition, the skin loses its supply of cutaneous nerves and blood vessels. These physiologic changes contribute to slow or impaired wound healing.93,94
However, the risk of SSI may only increase up until a certain age. Kaye et al. found that after age 65, the risk of SSI decreased by 1.2% for each additional year of life.95
Therefore, the risk of SSI may be due to comorbidities and immunosuppression not directly due to increasing age. Furthermore, this result may indicate a selection bias of “healthier” older patients for surgery.
History of Radiation
History of prior radiation therapy at the site of surgery increases SSI risk because of the risk of underlying tissue damage.47,96
Irradiated skin is hypovascular and easily injured with slight trauma. Given the damaged tissue and lack of perfusion, surgical incisions in locations with prior radiation treatment are more likely to develop a wound complication.
History of Prior Skin and Soft Tissue Infection
History of a prior skin and soft tissue infection (SSTI) is another risk factor for SSI development.49
While not fully understood, the increased SSI risk among these patients may reflect differences in inherent immunity and susceptibility to infection.
Patients with diabetes mellitus are more likely to develop SSIs. In a meta-analysis including 14 prospective studies, patients diagnosed with diabetes were twice as likely to develop an SSI compared to patients without a diagnosis of diabetes.50
The increased risk of SSI among patients with diabetes is consistent across multiple surgical procedures and is likely multifactorial.97
Patients with diabetes have a high incidence of small vessel disease, leading to impaired oxygen and nutrition delivery to peripheral tissues. Hypoxemia and lack of nutritional support reduce the systemic ability to prevent infection.98
Dronge et al. found that patients with a hemoglobin A1c level above 7% were significantly more likely to develop infectious complications compared to patients with a hemoglobin A1c level below 7%.99
Malnutrition is prevalent among surgical patients. One of the most commonly used markers of malnutrition is albumin, and hypoalbuminemia increases the risk of SSI.52,100
Hypoalbuminemia may lead to increased risk of SSI through several mechanisms. First, hypoalbuminemia can lead to poor tissue healing, decreased collagen synthesis, and granuloma formation in surgical wounds.101,102,103
These factors can impair wound healing and predispose the tissue to infection. Second, low albumin impairs macrophage activation and induces macrophage apoptosis, which decreases innate immunity response.103,104
Lastly, hypoalbuminemia can lead to tissue edema and leakage of interstitial fluid into the surgical wound.105
This fluid can serve as a medium for bacteria to proliferate and ultimately lead to infection.
TABLE 15-3 Summary of Known Surgical Site Infection Risk Factors
The skin’s dermis and basement membrane thin with increasing age, and the skin loses its supply of cutaneous nerves and blood vessels, which can lead to poor wound healing.45,46
History of radiation
Radiation therapy produces underlying tissue damage and contributes to poor wound healing.47,48
History of prior SSTI
A prior history of SSTIs may be related to differences in inherent immunity and susceptibility to infection.49
Hyperglycemia impairs innate immunity mechanism to fight bacteria. In addition, elevated glucose leads to glycosylation of proteins, which in turn slows wound healing.50,51
Poor nutrition leads to poor tissue healing, decreased collagen synthesis, and granuloma formation in surgical wounds. Low albumin impairs macrophage activation and induces macrophage apoptosis, which decreases innate immunity response. Hypoalbuminemia can lead to tissue edema and leakage of interstitial fluid into the surgical wound.52
Tobacco smoke impairs wound healing by vasoconstriction leading to relative ischemia, reduced inflammatory response, and alteration in collagen metabolism.53
Decreased blood flow in adipose tissue leads to less oxygen and antibiotic delivery.54,55,56
Immunosuppressive medications and conditions
Immunosuppressive medications or clinical conditions blunts the inflammatory phase of wound healing.57,58,59,60,61
Decreased tissue oxygenation
Decreased tissue oxygenation leads to diminished oxidative killing by neutrophils and impaired tissue healing caused by reduced collagen formation, neovascularization, and epithelialization. Low oxygen levels may decrease the efficacy of perioperative antibiotics62,63,64,65,66
Perioperative hypothermia impairs host defenses against surgical wound contamination: vasoconstriction causing reduced tissue perfusion to wounded tissue with reduced access for key immune cells, decreased motility of key immune cells, and reduced scar formation.67
Cellular functions of leukocyte adherence, chemotaxis, phagocytosis, and bactericidal activity are improved by insulin and better glycemic control, suggesting a direct relation between cellular function deficits and elevated blood glucose.68
Anticoagulants can cause of persistent oozing of the incision, slow wound healing.69,70,71,72
Blood transfusions impact the risk of infection by modulating the immune system.73
Wound contamination from patient
Shaving creates microscopic cuts in the skin that later serve as niduses for bacteria to multiply.27
Absence of appropriate barrier devices and drapes allows bacteria from deeper skin layers and hair follicles to recolonize the surgical site during the operation.
Lack of or inappropriate administration of perioperative antibiotics will not prevent the inevitable burden of microorganisms at surgical site.74
Without appropriate surgical site preparation, soil and transient organisms will not be removed.75
Wound classification delineates the degree of contamination of a surgical wound at the time of the operation.76
Wound contamination from operating room personnel
Transition of skin flora on the hands of healthcare personnel to the patient and operating room from lack of appropriate hand-washing or gloving to surgical sites.77
Movement of microorganisms from surgical staff’s hair, mouths, body, or shoes to the operating room contaminates surgical wounds.77,78,79,80
Increasing the number of microorganisms in the operating room environment increases opportunity for SSI. Most of the airborne contamination comes from persons present in the operating room and their movements.81,82,83,84
Longer operative duration is associated with increased wound contamination, increased damage to wound cells, and the local environment.85,86
Not maintaining adequate blood supply, not gently handling tissue, inadvertent entry into hollow viscus, leaving behind devitalized tissue, inappropriate use of drains and sutures, and inappropriate postoperative wound management.27
Foreign material promotes inflammation at the surgical site and increases the risk of SSI.87,88
Intra-articular steroid injection
Infection may be introduced at the time of injection, especially if rigorous antisepsis is not applied. May decrease the host immune response to the introduction of such bacteria.38
Periarticular joint injections
Injections are often prepared without the use of a sterile hood; infections can be introduced when these injections are compounded or at the time of infusion, as catheters are commonly used to deliver the medications.89
Wounds that remain uncovered after surgery may be subject to environmental contamination or ongoing drainage that decreases the integrity of the surrounding skin.90,91
Tobacco use is associated with adverse outcomes following surgery, including SSI. Postoperative wound healing complications occur more often in smokers and former smokers compared to those who never smoked. A systematic review identified four randomized trials that assessed the effect of preoperative smoking cessation (4- to 8-week interval of abstinence) on postoperative wound healing. This study showed that current or past smokers had an increased risk for postoperative infection (OR 1.9, CI 1.0-3.5).53
The many compounds that constitute tobacco smoke impair wound healing and increase SSI risk through several mechanisms.106,107,108
The physiologic mechanisms include vasoconstriction, which causes relative ischemia of operated tissues. Tobacco smoke also leads to a reduced inflammatory response and impaired innate immune system response to bacteria.108
Lastly, the elements in tobacco smoke can alter collagen metabolism, which is essential for skin and tissue integrity.
Obesity is another risk factor associated with developing SSI. One meta-analysis included 20 studies that evaluated SSI outcomes in orthopedic surgeries. The authors found that the risk of SSI for patients with obesity was almost two times the SSI risk for patients without obesity (RR 1.915; 95% CI 1.53-2.40).54
Studies including colorectal surgery patients (OR 1.59; 95% CI 1.32-1.91)109
and coronary artery bypass surgery patients (OR, 1.8; 95% CI 1.4-2.3)55
have reported similar conclusions. An additional recent study also found a trend of increasing risk of SSI for almost all surgery types when body mass index (BMI) increased from normal to morbidly obese.56
Obese patients may be at increased risk for SSI because of depth of adipose tissue, creation of dead space, and decreased blood flow in adipose tissue. Without adequate blood flow reaching the tissues, surgical wounds are less likely to heal. Decreased blood flow may also reduce antibiotic delivery and increase wound tension.85,110
Immunosuppressive Medications and Conditions
Patients with suppressed immune systems are at increased risk of SSI because the inflammatory phase of wound healing may be blunted. The increase in SSI risk is seen in patients with various levels of immunosuppression, including transplant recipients, patients undergoing chemotherapy, and other patients taking immunosuppressing medications.57,58,59,60,111
Glucocorticoids, on the other hand, may not affect SSI risk as strongly as other immunosuppressive therapies.61
Some degree of anti-inflammation may prevent wounds from becoming chronically inflamed, while significant suppression of inflammation can prevent wound healing.112,113
Decreased Tissue Oxygenation
Low oxygenation also increases the risk of SSIs. Oxygen tension is often low in wounds and in colorectal anastomoses at the end of surgery. This may reduce bacterial eradication, the body’s defenses against bacteria, and tissue healing. Possible mechanisms include diminished oxidative killing by neutrophils and impaired tissue healing caused by reduced collagen formation, neovascularization, and epithelialization.62,63,64,65
Further, many of the antibiotics used perioperatively for SSI prophylaxis are oxygen-dependent in their effect,66
and low oxygen levels may decrease their effectiveness.
Maintaining normal body temperature is vital for the body to maintain its normal function. However, many factors that patients are exposed to in the operating room can cause hypothermia: anesthetic drugs, cold operating room, skin antisepsis, cold irrigation of a patient with the body uncovered, and the use of intravenous solutions. Most cellular functions are temperature-dependent, and hypothermia also provokes systemic responses.67
Several mechanisms help to explain why perioperative hypothermia impairs host defenses against surgical wound contamination, including vasoconstriction and subsequent diminished perfusion, decreased motility of key immune cells, and reduced scar formation, which is necessary to prevent wound dehiscence and recontamination.
Postoperative hyperglycemia may increase SSI risk more than a diagnosis of diabetes. Hyperglycemia impairs innate immunity mechanism to fight bacteria. In addition, elevated glucose leads to glycosylation of proteins, which in turn slows wound healing.51
Latham et al. found that hyperglycemia during the immediate postoperative period was an independent risk factor for developing SSI even among patients without a history of diabetes and the risk of infection correlated with the degree of glucose elevation.68
Patients with blood glucose of 200 mg/dL or higher within 48 hours after surgery had 2.5 times the odds of developing an SSI as patients with glucose <200 mg/dL.
While postoperative anticoagulation is an evidence-based practice to prevent deep vein thrombosis in the postoperative period, anticoagulants may increase the risk of SSIs. Several studies have implicated anticoagulation therapy as a cause of persistent oozing of the incision, slow wound healing, and subsequent SSIs.69,70,71,72
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