Are Surgical Site Infections an Anesthesiologist's Problem?

SSIs led to a higher incidence of readmissions and reoperation and increased mortality. Mitigating a patient's SSI risk can significantly decrease perioperative patient complications and hospital costs and potentially increase hospital bed capacity by freeing hospital beds and increasing patient throughput.

SSIs are defined as an infection related to an operative procedure that occurs at or near a surgical incision within 30 days of a procedure or 90 days of implanted prosthetic material. There are many modifiable and nonmodifiable risk factors that increase a patient's risk for developing an SSI. Nonmodifiable risk factors include gender and age, whereas modifiable risk factors, like smoking, obesity, diabetes management, nutrition status, and frailty, increase risk. High comorbidity index, as reflected in a high American Society of Anesthesiologists score, has also been associated with an increased odds ratio (OR) for SSI. Surgical factors that contribute to increased SSI risk include the surgical complexity and duration with longer surgeries increasing the SSI. Furthermore, a preoperative hospital length of stay greater than 8 days can result in a 10- fold increase in SSI.

Modifiable risk factors under the purview of the Anesthesiologist include initial antibiotic timing, antibiotic redosing during surgery, intraoperative glycemic control, avoidance of hypothermia, goal-directed fluid therapy, and minimizing allogenic transfusions. In this article, the authors Leisy, Barnes, & Weavind aim to show which interventions matter, and which do not. With careful attention paid to the most impactful interventions, anesthesiologists can provide a critical role in SSI prevention, reducing patient complications and hospital costs.

PERIOPERATIVE ANTIBIOTICS

Appropriate surgical prophylaxis and SSI prevention involve antibiotic delivery, usually administered intravenously, to the operative site above the minimum inhibitory concentration (MIC) against the targeted organisms for the duration of the operation. This process can be divided into 3 major categories: antibiotic timing, antibiotic delivery, and antibiotic concentration.

ANTIBIOTIC TIMING

Antibiotic timing is an important factor in preventing SSIs and likely the most important variable that can be controlled by anesthesiologists.

For prophylactic antibiotics to be effective, the drug concentration at surgical tissue must reach MIC by the time of incision. This timing varies with the pharmacokinetic profile of a given antibiotic as well as the targeted tissue involved in the procedure.

Studies investigating the tissue concentration of antibiotics have shown it takes anywhere from 20 to 60 minutes following administration of cephalosporins to reach MIC in target tissue.

Vancomycin administration, in cardiac surgery where methicillin-resistant Staphylococcus aureus is a target organism, has a different pharmacokinetic profile. Consensus guidelines state that vancomycin infusion should be administered slowly over 1 hour to prevent histamine release and completed within 1 hour of skin incision.

Cesarean delivery guidelines recommend that antimicrobial prophylaxis be administered before skin incision, not after cord clamping. The correct timing of administration of prophylactic antibiotics is a complex clinical question related to an antibiotic's pharmacokinetic profile. There is evidence, as outlined above, suggesting that administering an appropriate antibiotic within 60 minutes of incision is associated with lower rates of SSI compared with 60 to 120 minutes before incision. Some data further suggest antibiotic administration earlier in the 60-minute window is superior to immediately before incision.

Equally important to initial dose timing is intraoperative redosing. Antibiotic redosing is recommended if surgical duration exceeds 2 half-lives for the given antibiotic. Antibiotic redosing compliance for long surgeries according to an antibiotic's pharmacokinetic profile is essential for SSI risk reduction.

ANTIBIOTIC DELIVERY

The second step in antibiotic prophylaxis is antibiotic delivery to the surgical site for the duration of surgery. Antibiotic delivery relies on adequate macrovascular and microvascular perfusion at the surgical site.

One factor limiting antibiotic delivery is perioperative hyperglycemia. Clinically significant hyperglycemia is defined as a blood glucose greater than 200 mg/dL. Regardless of perioperative glucose levels, diabetes alone increases SSI risk. Long-standing hyperglycemia contributes to macrovascular and microvascular occlusive disease, which significantly contributes to SSI and wound dehiscence in the plastic surgery population. Chronic microvascular disease and ischemia also prevent antibiotic delivery to the surgical site.

Interestingly, hyperglycemia alone is an independent predictor of SSI irrespective of diabetic status. The degree of hyperglycemia has been shown to impair phagocytic function and can be reversed by better blood glucose control. Perioperative, short-term hyperglycemia also affects innate immunity and contributes to an immune compromised state, thus increasing the risk for SSI.

Immediate preoperative glucose greater than 200 mg/dL should be treated with insulin irrespective of diabetic status. Some common goals for intraoperative glucose management include targeted blood glucose levels less than 180 mg/dL, hourly blood glucose checks if using an insulin infusion, every 2 hours for subcutaneous treatment, and every 4 hours for monitoring without treatment. Postoperatively, insulin therapy should be initiated for persistent hyperglycemia (>180 mg/dL) for a target glucose range 140 to 180 mg/dL in both critically ill and non critically ill patients.

Another factor affecting antibiotic delivery is hypothermia. Intraoperative hypothermia is caused by a rapid reduction in core temperature as blood flow is redistributed to the periphery where heat is continuously lost by convection and radiation in the operating room. Further heat loss leads to thermoregulatory vasoconstriction. Furthermore, hypothermia leads to impaired neutrophil function by prohibiting T-cell–mediated antibody production and decreasing oxidative bacterial killing.

To combat intraoperative hypothermia and its adverse effects, some studies have evaluated the effects of perioperative warming. Preoperative warming is a low-risk intervention that may offer some benefit against SSI.

Intraoperative hypotension (systolic blood pressure [SBP] < 80 mm Hg) limits local tissue perfusion leading to decreased oxygen tension and potential lower antibiotic delivery.

ANTIBIOTIC CONCENTRATION

Beyond delivery of the antibiotic to the surgical site at the appropriate time is the effective concentration of antibiotic at the tissue level. This effective antibiotic concentration must be above the MIC for organisms at the targeted tissue bed for effective antimicrobial prophylaxis to occur.

One variable affecting antibiotic concentration is weight-based antibiotic dosing. A patient's blood volume is estimated based on weight, age, and sex. Some perioperative antibiotics (vancomycin and gentamicin) are weight based, but many are fixed doses used for all patients.

Intraoperative volume of distribution states can also change rapidly. Another important factor affecting antibiotic concentration is intraoperative fluid administration. Large volumes of crystalloid independently contribute to increased risk of SSIs, and goal-directed fluid therapy has repeatedly been shown to decrease SSI risk.

Another common clinical scenario in the operating room is that of elevated intraoperative blood loss. The clinical practice guidelines for antibiotics for antimicrobial prophylaxis in surgery recommends redosing antibiotics after 1.5 L EBL. There is, however, a paucity of evidence to support this recommendation.

Intraoperative blood loss results in lost circulating serum antibiotics, which is then diluted by volume resuscitation and blood transfusions. Significant blood loss also leads to increased blood transfusions. Blood transfusions have independently been associated with increased SSI.

Allogenic blood transfusions have important immunomodulatory effects on the recipient's immune system, which may contribute to increased SSIs. These immunologic effects might be avoided when autologous transfusions are used instead.

More information is needed to determine if autologous transfusions can decrease the risk of SSIs; however, it should be noted that allogenic transfusions, if clinically indicated, should not be withheld to prevent SSI.

OTHER CONSIDERATIONS

A few other recommendations have been derived from the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) guidelines on SSI prevention. Surgical wounds are often poorly oxygenated, and this low oxygen tension has been associated with SSIs. Wound tissue oxygen tension predicts the risk of wound infection in surgical patients. Hyperoxia, achieved by increasing the fraction of inspired O2 (FiO2) to 0.80, is thought to overcome this relative tissue hypoxia in the surgical wound bed.

Supplemental oxygen is ubiquitous and seemingly well tolerated during anesthesia. A recent risk-benefit analysis review of the current data supporting or negating the use of 0.80 FiO2 in noncritically ill patients concluded that hyperoxia was well tolerated and had many beneficial effects with few indications of harm, so it may be used in noncritically ill, intubated patients undergoing surgery.

Current WHO guidelines recommend (but with a degraded from strong to conditional advice) the use of FiO2 of 0.8 to reduce SSI for patients undergoing general anesthesia for surgery. Similarly, the CDC released guidelines that ''strongly recommend'' the use of increased fraction of inspired oxygen (FiO2) both intraoperatively and in the immediate postoperative period for patients with normal pulmonary function undergoing general anesthesia with endotracheal intubation. The guidelines do not support the use of supplemental oxygenation for patients undergoing general anesthesia without endotracheal intubation or for patients undergoing regional anesthesia.

Similarly, mild under ventilation and hypercapnia have been shown to increase both subcutaneous and colonic oxygen tension through increased oxygen delivery and availability. Despite the increased oxygen tension, there has been no benefit shown to reduce SSI.

With increased length of stay, readmission risk, reoperation, and mortality, SSIs are a major postoperative complication. An anesthesiologist's role mitigating SSI risk is vital. Appropriate SSI prevention begins with timely antibiotic administration to achieve MIC against the targeted organisms for the duration of the operation. As evidence suggests, antibiotics should be administered 0 to 60 minutes before incision with some evidence showing added benefit when administered 15 to 60 minutes before incision. Beyond antibiotic timing, antibiotic delivery should also be considered. Avoiding intraoperative hyperglycemia and hypothermia has reliably reduced SSI, whereas hypotension has failed to show increased SSI risk. Appropriate weight-based dosing and practicing goal-directed fluid therapy can both reduce SSI. Blood loss, widely known to increase SSIs, increases blood transfusions and often requires large-volume resuscitation. Last, although data on hyperoxia are in equipoise, it is well tolerated and recommended by the WHO and CDC to prevent SSIs. Reducing SSIs in the perioperative setting is widely researched. The importance of anesthesia in the prevention and management of SSIs should not be overlooked. As new data emerge, continuing education to support the implementation of new best practice standards is of upmost importance.

• Take-home points for the anesthesiologist limiting surgical site infection risk
• 1.Preoperative antibiotics administered within 0 to 60 minutes before surgical incision
• 2.Redose antibiotics if greater than 2 half-lives have passed during surgery
• 3.Perioperative glucose control for targeting glucose less than 200 mg/dL
• 4.Avoid hypothermia (temperature < 36C) and consider preoperative warming
• 5.Use goal-directed fluid resuscitation Redose antibiotics if greater than 1.5 L estimated blood loss
• 6.Practice restrictive transfusion criteria and consider autologous transfusion when appropriate

Source: Leisy, Barnes, & Weavind; Advances in Anesthesia 39 (2021) 1–15

https://doi.org/10.1016/j.aan.2021.07.00