Impact of an Antimicrobial Dressing in Reducing Surgical Site Infections in Cardiac Surgery Patients

Index: WOUNDS. 2013;25(7):178-185.

  Abstract: Background. In addition to prevention guidelines already in place, the effectiveness of an antimicrobial dressing on the occurrence of surgical site infections (SSIs) among adult patients undergoing cardiac surgery was evaluated. Methods. A house-wide replacement of the plain postoperative gauze dressing with a sterile dressing impregnated with 0.2% polyhexamethylene biguanide directly on the incision after closure in the operation room was performed. From May 2005 to March 2007, 1658 patients were enrolled in this study. Surgical site infections were identified using the Centers for Disease Control and Prevention standard criteria. Results. Of the 1658 patients enrolled, 1399 patients were included in the analysis, 692 with the plain dressing and 707 with antimicrobial dressing. The overall and leg site infection rate was significantly higher in the plain dressing group compared to the antimicrobial group but similar in the sternal site. Overall, the antimicrobial dressings significantly reduced infection (OR 0.58 [0.38-0.89]). Obesity was also a strong independent predictor of SSI regardless of the site of surgery. Increasing age at surgery and left ventricular ejection fraction of 30%-49% were also independent predictors of infection. Conclusion. The antimicrobial dressing had a positive effect by reducing the infection rate, especially for leg incisions using conventional open techniques, and could be a worthwhile addition in conjunction with a strategic program.

Introduction

  Surgical site infections (SSIs) remain a major contributor in cardiac surgery complications, leading to increased patient suffering, morbidity and mortality, and health care cost.^1-4 Despite the implementation of several interventional strategies in an effort to reduce the rate of SSIs among cardiac surgery patients, it seems necessary to investigate new ways to prevent complications and improve outcomes.^1,5   The incisions performed during cardiac surgery are classified as “clean” as there is, in most instances, no underlying inflammatory process and the respiratory, alimentary, genital, and urinal tracts are not entered.⁶ The risk of developing a postoperative surgical site infection is affected by the degree of microbial contamination that occurs at the incisional site at the time of surgery.⁷ Colonization of the surgical site with pathogenic microorganisms may occur via introduction of the subject’s endogenous microflora or via external sources such as a contaminated instrument, a breach in aseptic technique, or environmental contamination.^7-8 Infection of the surgical wound may then result if the colonizing microorganism is given appropriate conditions for multiplication.   Presently, some commonly used dressings offer a highly absorbent dressing with a porous structure that does not act as a barrier to bacterial penetration. Others offer permeability to water vapor loss from tissue, but impermeability to microorganisms and external contamination. Neither of these dressings provides any inherent antimicrobial activity that might reduce colonization and prevent postsurgical infection.   The use of dressings impregnated with antimicrobial agents has been studied in the context of chronic wound care and in surgical wounds where heavy microbial contamination has resulted in delayed wound healing and/or infection.^8-10 Studies have illustrated the efficacy of these dressings in reducing both preexisting microbial burden within the wound site, as well as preventing bacterial penetration through the dressing.^9-10   Although there is little information regarding the efficacy of dressings impregnated with polyhexamethylene biguanide (PHMB) on fresh postoperative wounds and incisions, evidence gleaned from its use in the treatment of chronic wounds, namely the reduction of preexisting microbial burden and decreased bacterial penetration suggests that the PHMB-impregnated dressing could contribute to a reduction in the incidence of SSIs in cardiac surgery patients.   The purpose of this study was to evaluate the impact of an antimicrobial dressing on SSI rates associated with cardiac surgery.

Materials and Methods

  The study was conducted at McGill University Health Centre (Montreal, Quebec) where approximately 1050 adult cardiac operations are performed annually. A nonrandomized prospective cohort study design was used for patients undergoing cardiac surgical procedures, namely coronary artery bypass grafting (CABG) and⁄or valve replacement and/or repair via median sternotomy, between May 28, 2005 and March 31, 2007. All patients ≥ 18 years who were scheduled for the above procedures were eligible for inclusion. All potential participants, regardless of the dressing used, were informed about the study and had the opportunity to opt out. Exclusion criteria included the presence of a concurrent infection and any systemic antibiotic other than those used for the purpose of perioperative prophylaxis. Subjects who died without an infection within 30 days following CABG surgery and within 1 year following valve surgery were excluded as well.^11-12   The following were also excluded from the study: subjects who had undergone another procedure through the same incision(s) within a year after enrollment in this study; subjects with a known allergy to chlorhexidine or other biguanides and/or allergy to tape; and patients participating in another clinical trial involving an investigational drug or device that would interfere with the study. The study was approved by the Internal Review Board of the McGill University Health Centre.   The preoperative protocol included the following for all patients: preoperative chlorhexidine showers; nasal decolonization with mupirocin; clipping of planned surgical incision sites; rigorous glycemia control; and antimicrobial prophylaxis administered in the operating theater within 1 hour of the surgical incisions (as per guidelines).^13-15   Antibiotic prophylaxis consisted of cephazolin 2 g IV preoperatively and 2 g IV at completion of surgery, followed by 3 subsequent similar doses administered every 8 hours for patients undergoing isolated CABG. All patients undergoing valvular surgery (isolated, multiple valve, or combination with CABG) received vancomycin 1 g IV within 1 hour of the surgical incision, 500 mg at the completion of the intervention, and 500 mg IV every 8 hours for 3 doses. All CABG patients who were allergic to penicillin received the vancomycin regimen.^16-18 All saphenectomies were performed using conventional open techniques.   Intervention consisted of a replacement of the sterile plain postoperative gauze dressing (Primapore, Smith and Nephew, St. Petersburg, FL; or Tegaderm, 3M Health Care, St. Paul, MN) with the sterile antimicrobial-impregnated dressing (Telfa AMD, Covidien, Mansfield, MA) on 3 units: the operating room, intensive care unit, and cardiac unit. The dressing was applied directly on the incision after closure had been performed with sutures and/or staples. Dressings were changed at 48 hours postsurgery (or sooner if oversaturated), and then changed whenever the dressing became saturated. Application of the dressing was discontinued when the incision had epithelialized. Due to the similarities between the dressings (ie, their identical appearance) and the potential for confusion, the plain dressing was evaluated from May 28, 2005 to March 31, 2006, and was completely replaced by the antimicrobial dressing from May 28, 2006 to March 31, 2007. A 2-month interval between study periods was necessary to insure adequate and complete replacement with the new dressing. There were no changes in techniques or patient care algorithms, and no other changes of any kind (including surgeons) during the entire study period.   Throughout the entire period of hospitalization, patients were monitored for signs and symptoms of infection (as per Center for Disease Control [CDC] guidelines) as well as for any adverse effects (ie, dermatitis) as per routine protocol/practice.   The criteria established by the CDC and Prevention Guidelines were used for the purposes of determining the presence of surgical site infection (superficial, deep, organ space infection). Subjects were followed for signs and symptoms of SSIs up to 1-year following surgical intervention. Surgical site infections were identified through chart reviews, clinic visits, readmission to a hospital, laboratory culture surveillance, and telephone interviews at 6 months and 1 year.^5,6,15 Data collection included, but was not limited to, risk factors; severity of illness using Parsonnet score, which is a validated, additive scoring system for predicting operative mortality risk in cardiac surgery¹⁹; antimicrobial prophylaxis; and preoperative preparation. The study was completed by June 2009 after the last patient telephone interview.   Descriptive statistics were computed for patients receiving the plain dressing and patients receiving the PHMB dressing. Comparisons between the 2 groups were based on the chi-square statistic for categorical variables and on t test for continuous variables. All P values reported are 2-tailed.   Odds ratio and 95% confidence intervals were obtained using logistic regression to compare the risk of infection according to type of dressing and adjusting for patients’ characteristics. To avoid collinearity in the modeling procedure, the Parsonnet score was not included as an adjustment factor when evaluating individual characteristics incorporated in its calculation.   All analyses were conducted with SAS version 9.2 (SAS Institute, Cary, NC).

Results

  From May 2005 to March 2007 a total of 1658 patients were considered eligible and enrolled into the study; no patients opted out of the study. Of these eligible subjects, 219 were excluded because of death without an SSI within a year postsurgery, depending on the procedure. This was due to the patient’s potential of developing an SSI had the patient lived; postoperative complications; infectious complications other than SSI; wrong dressing applied; or lack of compliance to the pre- and postoperative protocol. An additional 40 patients were lost to follow up.   Of the 1399 patients included, 692 were treated with a plain dressing and 707 received the antimicrobial dressing. The 2 groups were comparable with respect to age, gender, hypertension, peripheral vascular disease, renal failure, dialysis dependency, and duration of surgery (Table 1).   Compliance to the preoperative preparation given to all patients (chlorhexidine showers, nasal decolonization with mupirocin, clipping of planned surgical incision sites, rigorous glycemic control, and antimicrobial prophylaxis) was also comparable in both groups (86.1% in the plain dressing group vs 88.4% in the antimicrobial dressing group) (Table 1).   Patient risk factors such as obesity, diabetes, and severe neurological disorder were significantly higher in the plain dressing group compared to the antimicrobial group.   The Parsonnet score was significantly higher among the group treated with the antimicrobial dressing. The proportion of patients undergoing CABG only, or with left ventricular ejection fraction (LVEF) > 50%, was higher in the plain dressing group.   The overall and leg site infection rate was significantly higher in the plain dressing group compared to the antimicrobial group. Sternal infection rate was similar in both groups (1.7% vs 1.6%) (Table 2). In the plain dressing group, 8 patients developed a double infection (sternal + leg) compared to 2 patients in the antimicrobial group.   For CABG alone, the overall SSI rate was 11.1% in the plain dressing group (61 SSIs for 548 procedures) vs 7.1% in the antimicrobial dressing group (31 SSIs for 436 procedures, P = 0.03). For valves alone, the SSI rate was similar in both groups.   In this study, exposure to the antimicrobial dressing significantly reduced infection after adjusting for differences in all other characteristics in the comparison groups (OR 0.58 [0.38-0.89]). Obesity was also a strong independent predictor of SSI regardless of the site of surgery (Table 3). Increased age at surgery and LVEF of 30%-49% were also independent predictors of infection. When stratifying per surgical site, age, obesity, and impregnated dressings remained independent predictors of infection in the leg site, while no significant predictors other than obesity could be found for the chest site.   Dressings were changed at 48 hours postsurgery or sooner if oversaturated, and then changed whenever the dressing became saturated. Application of the dressing was discontinued when the incision had epithelialized. In terms of number of days a dressing was necessary to be kept in place among noninfected sites—wounds diagnosed with infection were not counted—there were no statistical differences between the dressings for sternal incisions (4.3 days vs 4.4 days), but the number of days was significantly shorter for leg incisions receiving the PHMB-impregnated dressing when compared to the plain dressing, with 6.3 days vs 6.9 days, respectively (P = 0.04) (Table 4).

Discussion

  Monitoring of compliance with infection control practices has been in place for many years in this health care centre, as well as other known ways to prevent SSIs such as preoperative preparation and hair removal, including chlorhexidine gluconate showers and nasal decolonization with mupirocin; antibiotic prophylaxis; and glucose control.^13-15   Although the implementation of these measures has decreased the rate of serious infections associated with this type of surgery, such as mediastinitis and endocarditis, the overall rate has not reached a 0 benchmark with the vast majority of infections being leg infections.   The main objective of this study was to investigate if SSI rates associated with cardiac surgery could be reduced by the use of an antimicrobial dressing. In this study, the antimicrobial dressing had a better overall efficacy, particularly for leg infections, to prevent SSIs compared to the plain dressings used.   The sterile antimicrobial dressing is a cotton pad impregnated with 0.2% PHMB bonded on both sides with perforated nonadherent film. As antimicrobial agents, polymeric biguanides function by disrupting the membranes of both gram-positive and gram-negative bacteria resulting in the irreversible loss of essential cellular components and cell death. The development of bacterial resistance is minimized as the target of these antimicrobial agents is physical rather than molecular, and therefore microorganisms are unable to adapt. The bisbiguanide chlorhexidine has been widely used for almost 50 years in surgical handwashes, as an antiseptic in oral hygiene, and as a topical treatment in wound care. Chlorhexidine is an effective broad-spectrum antimicrobial agent with proven activity against gram-negative and gram-positive bacteria, as well as enveloped viruses and fungi.²⁰   Although PHMB has been recognized as having a superior antimicrobial activity at lower concentrations than chlorhexidine, attempts to formulate PHMB mixtures were unsuccessful and prevented their use in pharmaceutical products. As a result, PHMB has been marketed for use as a preservative in cosmetics, contact lens solutions, and baby wipes, and used as an ingredient in swimming pool sanitizers. Polyhexamethylene biguanide is poorly absorbed by human skin and has low systemic toxicity via both oral and dermal routes. Recent studies have illustrated that fibers impregnated with PHMB have the ability to suppress microbial growth, prompting its incorporation into a new generation of antimicrobial wound dressings.²¹   Risk factors such as diabetes, obesity, and age are major contributors to the increased risk of SSI and are well documented in the literature.²² After adjustment for patient characteristics and severity of illness, obesity was confirmed in this study as a strong independent predictor of infection regardless of the incision site or type of dressing used. Obesity remains a concern as its prevalence has been significantly on the rise within the past 3 decades.²³ Obesity results in increased amounts of adipose tissue, increased tissue trauma related to retraction, and increased operating time.²³   There were no significant differences in terms of how long the dressing was kept in place among noninfected sternal incisions but there was statistical difference (significantly shorter) for the antimicrobial dressing used on leg incisions. Considering that the study protocol called for the initial dressing to be removed 48 hours postsurgery, the remaining time to achieve wound healing was approximately 2 days for the sternal incisions and 4 days for leg incisions. Unfortunately, the authors were not able to determine the number of dressing changes during this specific period of time, and thus were not able to determine an accurate assessment of cost, as the information was not clearly indicated in the charts.

Limitations

  Although a randomized study would have been ideal, having 2 dressings, identical in appearance, on 3 different units (operating room, intensive care unit, and cardiac unit) would have created too many opportunities for confusion and errors. This is why a noncontemporaneous study design was used to introduce the 2 dressings. Regardless of this limitation in design, the authors are confident that the differences observed in the outcomes were not due to different techniques or patient care algorithm, both having remained the same throughout the study period. The authors also adjusted for any potential differences in patient characteristics that may have occurred during the study period as patients treated with the plain dressing had more risk factors for wound infections than those treated with the PHMB dressing. The study period of 10 months was identical for both dressings and encompassed any potential seasonality of infections that may have occurred throughout the year. After the study period, when the plain dressing was reintroduced, the rate of infection began to increase to reach the previous baseline.

Conclusion

  The use of an antimicrobial dressing seems to reduce the infection rate among cardiac surgery patients, especially for leg incisions using conventional open techniques. The antimicrobial dressing could be a valuable addition to a complete program including preoperative preparation, postoperative standardized wound management protocols, and a surveillance program if shown to be cost-effective.

Acknowledgments

  The authors wish to thank Michael Libman, MD, for his advice on the study protocol; Danielle Vigeant, clinical nurse specialist, and the operating room material management advisors for their assistance; as well as the medical staff, nursing staff, and secretaries of the operating rooms, intensive care units, and cardiac surgery units of the Montreal General Hospital and Royal Victoria Hospital, without whom this study would not have been possible.   The authors also would like to thank the medical records staff of the McGill University Health Centre for their efficiency in providing patient records, and Marc-Olivier Chrétien, sales representative, patient care and safety products, at Covidien (Mansfield, MA), for his assistance throughout the study.

References

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Paul Brassard, MD, MSc, FRCPC; Tarik Alam, RN, BScN, ET; Catherine Robineau, RN, BSc; Christine Page, RN, MSc; and Benoit de Varennes, MD, MSc, FRCS are from McGill University Health Centre, Montreal, Quebec. Paul Brassard, MD, MSc, FRCPC; and Sophie Dell’Aniello, MSc are from Jewish General Hospital, Montreal, Quebec. Address correspondence to: Paul Brassard, MD, MSc, FRCPC Division of Clinical Epidemiology McGill University Health Centre and Centre for Clinical Epidemiology Lady Davis Research Institute Jewish General Hospital 3755 Cote Ste-Catherine, Suite H424.1 Montreal, Quebec H3T 1E2 paul.brassard@mcgill.ca Disclosure: Paul Brassard was supported by a clinician scientist career award by the Fonds de la recherche en santé du Quebec (FRSQ). The authors report no financial interest in, or affiliation with, the manufacturer of the dressings used in this study, and that Covidien provided no financial assistance related to this study.