Postoperative wound infection is a common healthcare problem. The process of wound infection is complex and involves an interplay between several biological pathways at the molecular levels. Wound infections account for high morbidity and mortality. Current data indicates that surgical site wound infections account for over two million nosocomial infections in patients who have been hospitalized in the United States.
Surgical wound infections are classified as follows by the Centers for Disease Control and Prevention (CDC):
To be classified as a surgical site infection, the wound must:
If the surgeon opens the wound for cleaning, it is considered a surgical wound infection. A wound is not considered to be infected if there is only a stitch abscess. The majority of surgical site wound infections are due to endogenous flora that is usually present on the mucous membranes, skin, or hollow viscera. In general, when the microbiological flora concentration is higher than 10,000 microorganisms per gram of tissue, there is a high risk for an infected wound.
The etiology of postoperative wound infection is complicated by the heterogeneous nature of these infections. They vary by geographical region, surgical subspeciality, and the wide array of procedures performed.
Risk factors can be divided up into patient factors and procedural factors.
Patient risk factors for wound infection include advanced age, malnutrition, hypovolemia, obesity, steroid use, diabetes, use of immunosuppressive agents, smoking, and coexistent infection at a remote site.
Procedure-related risk factors include the formation of a hematoma, the use of foreign material such as drains, leaving dead space, prior infection, duration of surgical scrub, preoperative shaving, poor skin preparation, long surgery, poor surgical technique, hypothermia, contamination from the operating room, and prolonged perioperative stay in hospital.
When looking at the theatre environment, appropriate planning, maintenance, and training are essential to minimize the rates of SSI. An appropriate theatre block should allow optimal patient flow and the separation of clean and contaminated areas. Furthermore, the operating room layout and item materials should be arranged in a way to maintain a clean and sterile environment. When considering ventilation, positive pressurization, filtering, laminar airflow systems, and the number of air exchanges are important factors. Appropriate reduction of patient flora in the form of chlorhexidine shower is widely used in some specialties the day before surgery. Hair removal should only be done when necessary, and with clippers just before surgery. Using iodine or chlorhexidine based agents for preparation of surgical area is a controversial topic, and certain subspecialties have their protocols. For the surgeon, appropriate scrubbing technique and double gloving have been shown to reduce rates of infection. The WHO surgical checklist has been designed to improve communication, prevent complications, and to improve safety overall, which includes surgical site infections as well.
The type of surgical procedure is also a significant risk factor. Surgical procedures and, therefore, wounds are further classified as clean, clean-contaminated, contaminated, and dirty-infected with very different rates of postoperative wound infection rates.
Classification is defined as follows from the CADTH report 2011:
The concept of asepsis or antisepsis was established in the 19th century by Semmelweis, who used carbolic acid as an antiseptic and showed that hand washing before a delivery reduced puerperal fever and surgical infection rates. He has seen a decrease in mortality due to puerperal sepsis from 12% to 2%. Before this concept was established, the risk of surgical infection was significantly higher. Acknowledgment of the aseptic approach made a significant impact on outcomes. In current times it is challenging to provide accurate data for postoperative wound infections as the term covers a wide variety of specialties, operations, patients, and geographies.
Also, identifying wound infections is more challenging due to increased prevalence of day case surgery, and shortened hospital stays.
CDC data from 2018 is to be read with the above precautions. The morbidity for 2018 in the US was 157,500 for surgical site infections (SSI), with an estimated mortality of 8,205. 11% of all deaths in intensive care units were associated with SSI. It is a burden for the patient with an additional 11 days of hospitalization for each SSI and a burden to the system with an overall cost of $3.2 billion per year.
SSI rates also depend on what type of surgery is performed:
Microbial contamination of the surgical wound is the first step in the development of an SSI, which may come from either endogenous or exogenous sources.
Endogenous flora includes the patient’s skin, mucous membranes, and hollow viscera. The most common endogenous causative organisms are S. aureus, coagulase-negative staphylococci, Enterococcus, and Escherichia coli. However, this depends greatly on the procedure performed. In cardiac, breast, ophthalmic, orthopedic, and vascular surgeries, the most common causative organism is S. aureus and coagulase-negative staphylococci, while in abdominal surgeries, Gram-negative bacilli and anaerobes are more common.
Exogenous flora may come from the theatre room, including air, instruments, materials, and staff members. The most common exogenous organisms are staphylococci and streptococci. Also, the number and the virulence of the organism are major risk factors. There is an increasing rate of high virulence organisms isolated from post-operative wound infections such as MRSA, possibly due to the widespread use of broad-spectrum antibiotics.
In a study of community hospitals in the southeastern United States, the incidence of MRSA-associated SSI increased from 12% in 2000 to 23% in 2005. In the 2010 NHSN update, the proportion of SSI due to MRSA was 43.7%.
The typical timeframe for the appearance of SSI symptoms is 3-7 days post-procedure, and as per definition, they have to occur within 30 days post-surgery (or one year in cases with implant). Patients with metabolic syndrome, especially diabetics, smokers, elderly, and immunosuppressed people, are at higher risk. People with difficult/long or contaminated surgery are at higher risk as well. Most patients complain of a gradual onset of pain and discharge, and they may feel generally unwell.
Clinical features of surgical site infections are similar to the classical five signs of inflammation, but some small details set them apart. These include erythema, localized pain, unexplained persistent pyrexia, discharge from the wound (often purulent), wound dehiscence, and problems with wound healing.
When suspecting wound infection, dressings should be removed. The wound blisters, tight closure, grey, or black tissue will alert the clinician that there is ischemia and/or necrosis, therefore, increased risk of wound infection. If there is discharge, a microbiology sample at this point needs to be taken, and treatment commenced if the suspected diagnosis is wound infection. A serous or sanguineous discharge does not indicate infection, but purulent discharge does.
The examination would include assessing patients for sepsis from the wound, also from other causes, and starting appropriate therapy.
A study measuring the effect of introducing wound photography for scenarios where face to face review is not possible showed improvements in diagnostic accuracy and helped prevent overtreatment.
The diagnosis depends on clinical examination. However, microbiological swabs need to be taken to identify causative organisms and sensitivities.
If there is suspicion of deep-seated infection, ultrasound or CT/MRI imaging might be of benefit.
For preoperative risk assessment for SSI, various tools can predict the likelihood of developing an infection based on the risk factors. There are traditional systems such as the national nosocomial infection surveillance system, the Australian Clinical Risk Index, and the European System for Cardiac Operative Risk Evaluation, which are all internationally recognized. However, their value is somewhat limited because many risk factors are missing from their calculations. Some have weak discriminatory abilities or do not risk-stratify for specific surgeries. Due to the need for individualized treatment, more specialty and even operation-specific scoring systems are emerging, such as the Infection Risk Index in cardiac surgery or Surgical Site Infection Risk Score.
As surgical site infections take a massive toll on resources and cause morbidity and mortality, there are many general rules and guidelines in place to prevent them in the first place. These include preoperative skin preparation, using films on the skin, theatre sterility rules, perioperative and postoperative prophylactic antibiotics, and dressings. Also, improving the patient's natural defense mechanisms such as early mobilization and improving nutritional status.
For prophylaxis considerations, a safe, narrow-spectrum agent should be used with coverage for the expected microorganisms and should be prescribed for the shortest effective period. The antibiotic should be given 30 to 60 minutes before knife to skin time to allow tissue concentrations to reach therapeutic levels at the time of operation. For clean procedures, the antibiotics need to cover Staphylococci. For clean-contaminated procedures, similar coverage for Staphylococci is required, with additional coverage as required depending on the surgery and geography. This is usually cefazolin 2g (weight-adjusted) or vancomycin 15mg/kg plus metronidazole, cefoxitin, or ertapenem. For contaminated and dirty procedures, prophylaxis is generally not indicated, because therapeutic antibiotic management is required.
For treatment, each surgical specialty, body region, and operation type needs to have its strategy to treat the already developed SSI as the microbial spectrum is often different. Also, foreign bodies (mesh, implant, metalwork) have to be considered for removal due to biofilm formation. Source control remains important, with the appropriate selection of antibiotics based on the type of surgical procedure performed, and the expected microbial causes for the infection.
Often early surgical debridement is the preferred option to resolve infection successfully. However, in complex surgery, re-opening the surgical site can cause significant morbidity. If there is sufficient information that the infection is superficial, a decision can be made to treat conservatively.
If patients are found to be septic, a timely set of measurements and interventions can save patients' lives.
The patient can develop cellulitis in the area that is not connected to the surgical site. However, on the balance of probabilities, these infections are often treated as SSI as treatment is similar, and the need to determine the cause does not change management.
There is a life-threatening condition that needs to be ruled out when diagnosing surgical site infections. Necrotizing fasciitis is a rapidly spreading infection of the fascial planes. If rapidly spreading, a clinical diagnosis of necrotizing fasciitis is made, and the patient should be rushed for emergency surgery. Aggressive, multi-stage debridement of skin fascia and other ischaemic/dead tissue is the current strategy of dealing with the condition in addition to using appropriate antibiotics.
Many models were set up in different surgical specialties to aid in predicting prognosis and helping to identify high-risk patients to prevent surgical site infections.
One study looking at colorectal cancer patients found different physiological factors, tumor characteristics, and specifics of surgery to be good prognostic factors for the chance of SSI. Surgical factors like the procedure type, emergency surgery, dirty- infected wound class, presence of surgical drains, experienced surgeon, prolonged operating time, and some postoperative factors such as prolonged hospital stay and intraoperative transfusion were determined as independent risk factors.
Surgical wound infection complications can be categorized into local and systemic ones. Local complications include delayed and non-healing of the wound, cellulitis, abscess formation, osteomyelitis as well as further wound breakdown. Systemic complications include bacteremia with the possibility of distant hematogenous spread and sepsis.
In certain specialties such as cardiac surgery, there is a great emphasis on glycaemic control both in the preoperative and postoperative periods, with some studies finding a link between tighter blood glucose monitoring and reduced complication rates.
There are some identifiable and modifiable patient factors (BMI, diabetic control) that can help prevent postoperative wound infections. When surgery is offered and time allows, it is essential to optimize patients for surgery. This optimization is done to reduce the overall complication rate rather than to just focus on wound infections.
Most surgeries are a risk-benefit discussion between the healthcare team and the patient, especially when it comes to elective surgery. It is possible that due to patient risk factors, the risk to surgery is higher than the benefit of performing it, and the patient may be asked to lose weight, adhere to, or modify antidiabetic medications alongside a healthy diet and regular exercise. Smoking cessation is another essential step for optimization. A detailed look into immunosuppressive medications, including steroids, can help.
Measures taken by different healthcare providers vary largely both geographically and between specialties. A study on spinal surgery recommends HBA1C levels <8%, males over 60 to take alpha-blockers, serum albumin level over 3.5 g/dL, cardiac stress tests, and smoking cessation. On the other hand, a colorectal study on perioperative factors does not mention modifiable patient-related factors at all.
During the perioperative period, patients encounter a large spectrum of health professionals who are directly or indirectly involved in modifying factors responsible for developing postoperative wound infections.
Preoperatively it is essential to identify and address modifiable risk factors and appropriately counsel the patient regarding anticipated risk. While the screening process is often done by nursing staff and anesthetists, the discussion of risks is usually discussed with the surgeon during the consenting process.
Intraoperatively all theatre staff is responsible for maintaining sterility and ensuring optimal theatre time, temperature, available instruments.
While designing and building theatres and hospitals, theatre size and airflow are important factors.
Postoperatively, ward doctors, nurses, dietitians, physiotherapists, and assistants are all going to influence recovery and, therefore, postoperative wound infection rates.
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