Treatment of Sternal Wound Infection with Vacuum-Assisted Closure

  Abstract: Introduction. Previous work has demonstrated the efficacy of vacuum-assisted closure (VAC) in the treatment of poststernotomy local wound infections, compared to historical treatment protocol. The negative pressure has been found to protect wounds against contamination, prevent wound fluid retention, increase blood flow, and increase rates of granulation tissue formation. For this study, a retrospective analysis compared patients receiving VAC as definitive treatment versus bridging to delayed flap closure. Methods. Sixteen patients developed sternal wound infections after cardiac surgeries at the authors’ institution from 2006 to 2008. Data was gathered regarding patient comorbidities, treatment method, and outcome. Study objectives included assessment of risk factors that warranted secondary surgical closure and examination of long-term followup where VAC was the definitive treatment modality. Results. Group A (n = 12) had VAC as the final treatment modality. Group B (n = 4) required myocutaneous flap closure. One patient in Group B passed away prior to flap surgery. Both groups had similar risk factors, except Group B had a higher risk of body mass index (BMI) > 35 that was near statistically significant (P = 0.085; odds ratio = 0.0, 95% CI = [0.0 – 1.21]). Group A required a shorter hospital stay on average. Long-term follow-up showed the majority of Group A had completely healed sternal wounds 2-3 years from initial cardiac surgery. Conclusions. Vacuum-assisted closure as definitive treatment modality is a successful, first line therapy for local superficial sternal wound infections. When deep infections occur, however, VAC as bridge-to-flap coverage is recommended over attempted secondary healing with VAC.

Introduction

  Sternal wound infections after cardiac surgery is a concerning complication, increasing morbidity and mortality. Approximately 0.3% - 5.0% of median sternotomy surgical approaches result in infection.¹ Mortality rates range in the literature between 14% - 47%.² Preoperative risk factors for sternal wound infections include diabetes mellitus, chronic obstructive airway disease, obesity, and smoking.³ Postoperative risk factors include blood transfusions, surgical chest exploration, prolonged postoperative ventilation, and longer stay in the intensive care unit.^4-6   Microbiology of sternal wound infections is variable. Staphylococcus aureus is the most common pathogen (29%), followed by Staphylococcus epidermidis (22%), with a notable frequency of Pseudomonas aeruginosa, methicillin-resistant staphylococci and streptococci, facultative and aerobic gram-negative rods, and anaerobes.^7,8   Since the mid-1950s, when the median sternotomy became a common approach for intracardiac procedures, the poststernotomy wound infection has had few treatment solutions.⁹ Superficial infections were treated by irrigation, debridement, and open dressing changes. Deep infections were more difficult to treat. One of the first treatment options for deep sternal wound infections was a closed mediastinal antibiotic irrigation system.¹⁰ This was an improvement from prior treatment regimen of open wound healing, following initial debridement, with frequent dressing changes to promote granulation and secondary wound closure.¹¹ Later treatment options involved debridement of devitalized tissue, daily dressing changes, and eventual delayed definitive closure of the wound by vascularized flaps such as pectoralis muscle, rectus muscle, or omental transpositions.^12,13   In the 1990s, the advent of negative pressure devices improved management of pressure ulcers and chronic wounds.¹⁴ Since then, vacuum-assisted closure (VAC) devices have revolutionized wound management, improving skin grafts, enhancing reepithelialization of skin graft donor sites, and allowing safe temporary closure of the abdomen.^15-17 Negative pressure devices improve tissue healing through several proposed mechanisms, including an increase in local blood flow, reduction in tissue edema, removal of chronic wound fluid and necrotic tissue, reduction in bacterial colonization rates, and wound size contraction.¹⁸   Vacuum-assisted closure consists of a vacuum pump, polyurethane foam into which an evacuation tube is embedded, and a transparent adhesive dressing (KCI International, San Antonio, TX).¹⁹ The reticulated polyurethane foam has a 400 µm - 600 µm pore size. The foam is cut and contoured to fit the size of the tissue defect. The foam is covered with an adhesive drape and connected through the evacuation tube to the vacuum pump. The suction generates a continuous vacuum, equally distributed in the foam. The negative pressure ranges from 0 - 200 mm Hg with typical therapeutic range from 75 mm Hg - 125 mm Hg. The foam is changed every 2-3 days.   Nevertheless, the literature lacks a large prospective multicenter trial. Long-term outcome, in general, is lacking. Questions remain regarding sternal stability and need for further treatment. In an attempt to improve upon the existing methods, this study aimed to evaluate VAC as an effective short-term treatment and durable long-term treatment for sternal wound infections. Furthermore, this study sought to investigate the utilization of VAC to lessen the need for further invasive treatment options, such as myocutaneous and/or omental flap coverage. The authors expected patients who responded to VAC as definitive treatment to have fewer co-morbid risk factors than patients who required secondary surgical closure. In addition, the authors proposed that the VAC technique as definitive treatment would reduce treatment time, hospital stays, and outpatient followup. Finally, the authors expected long-term followup to show complete healing with VAC as definitive closure, as well as high patient satisfaction.

Materials and Methods

  Approval from the Institutional Review Board at University of California Davis Medical Center was obtained for this study with individual consent waived. Institutional Review Board dates of approval are from April 18, 2007 to May 31, 2009.   Between 2006 and 2008, 590 coronary artery bypass graft procedures and 376 heart valve repair/replacement procedures were performed at the authors’ institution (UC Davis Medical Center). A total of 16 patients developed sternal wound infections and were treated with the VAC technique either as definitive therapy (Group A) or followed by a second procedure (Group B). Initially, the intended treatment for all patients was VAC only. In 4 cases, however, the wound defect was unresponsive to definitive VAC alone. These patients required myocutaneous and/or omental flap closure. Sternal wound infections were defined by positive results of microbial culture, persistent erythema and/or drainage from wound, or persistent pyrexia with neutrophilia. The VAC wound management protocol was initiated in 2006.   Patient characteristics and wound culture data were obtained and compared to assess differences between groups. Superficial infections were defined as infection having depth of skin to subcutaneous tissue. Deep infections were defined, for the purpose of this study, as infection having a depth of muscle to bone.   An attempt was made to follow up with patients by phone to obtain further information. Patients were asked about current wound size, further interventions at outside hospitals, satisfaction with the VAC device and overall sternal wound infection experience using a scale of 1 to 10, in which 1 = worst experience, and 10 = best experience.

Statistics

  Two-sided Fisher’s exact test was used to assess association between patient group, patient characteristics, and wound culture. Odds ratios (ORs) with 95% exact confidence intervals (CIs) were reported. Summary statistics are expressed as mean ± standard deviation (SD) (median; range). The two-sided exact Wilcoxon rank-sum test was used to compare groups A and B for VAC duration, hospital days, follow-up time, and satisfaction with the VAC device. A P-value < 0.05 was considered statistically significant.

Results

  Between 2006 and 2008, 16 patients developed superficial and deep sternal wound infections out of 835 median sternotomy procedures. The 3-year incidence was 1.9% (95% CI = 1.1% - 3.1%). Of these 16 patients, 4 required flap coverage. Table 1 summarizes the treatment and follow-up methods of each patient with early mortality. Table 2 summarizes the role of VAC in the 2 study groups. Group A consists of 12 patients who received VAC as definitive therapy. Of these, 6 received formal operating room irrigation and debridement. The other 6 received bedside or clinic irrigation and debridement. None of these patients had delayed primary surgical closure of their wound, nor did they receive flap coverage.   Figure 1 presents a patient from Group A with VAC for a draining sternal wound. Group B consists of 4 patients who required flap coverage of their sternal wounds. One of these patients died due to cardiovascular failure prior to delayed flap closure. The 3 remaining patients received VAC until their myocutaneous and/or omental flap coverage. Figure 2 presents a patient from Group B who received VAC as a bridge to eventual flap closure.   The following data is presented as the mean ± standard deviation; median; range. Group A (43.8 ± 32.6; 35.5; 10-136) had fewer days of VAC usage than Group B (175.8 ± 161.8; 176; 21 - 330), which was not statistically significant (P = 0.162).   Group A (43.3 ± 35.7; 32; 14 - 40) had fewer hospital days for their cardiac condition or sternal wounds than Group B (74 ± 47.9; 56.5; 40 - 143), but was not statistically significant (P = 0.074). As expected, Group A (3.7 ± 2.0; 3; 0 - 8) had fewer months of outpatient followup than Group B (16±13.9; 12; 4-36), which was statistically significant (P = 0.012). There were no complications as a result of the VAC device. All 16 patients survived 90 days after initial infection diagnosis.   Table 3 compares patient characteristics between Group A and Group B. As expected, both groups had similar preoperative risk factors. The one risk factor that reached near statistical significance was BMI > 35, found in 4 out of 10 patients in Group A, and all 4 patients in Group B (P-value = 0.085; OR = 0.0, 95% CI = [0.0 – 1.21]).   Table 4 compares the wound cultures between Groups A and B. As expected, both groups had similar microbial pathogens. The most common isolates were methicillin-resistant Staphylococcus aureus (6/16, 38%), coagulase negative Staphylococcus aureus (5/16, 31%), and methicillin sensitive Staphylococcus aureus (4/16, 25%). Half the wounds had polymicrobial cultures (8/16, 50%).   Telephone surveys were conducted with results shown in table 5. Ten of the 14 (71%) surviving patients were reached. The 4 patients lost to follow-up were from Group A. One patient from Group A and 1 patient from Group B had passed away at the time of phone questionnaire. All treatment and follow-up were performed at the University of California Davis Medical Center (UCDMC). None of the patients reached had received additional treatment for their sternal wounds outside UCDMC. One patient from Group A received 4 weeks of followup by a local cardiothoracic surgeon to assess healing after discharge from UCDMC, as the patient lived more than 1 hour away from UCDMC. All patients from Group A reached by telephone had healed sternal wounds. One patient in Group B still has a 1 cm draining wound for whom surgery was pending at the time of publication. Patients’ satisfaction with the wound VAC was diverse (6.4 ± 3.0; 7.5; 2 - 10). Group A (7.1 ± 2.8; 7.5; 2 - 10) had higher satisfaction with VAC than Group B (4.7 ± 3.1; 4; 2 - 8), but it was not statistically significant (P = 0.333). Patients who had a low satisfaction with VAC cited inability to sleep due to device noise, interference with mobility, and difficulty with maintaining a vacuum seal.

Discussion

  This study examines patient characteristics for those whom VAC was the sole treatment modality compared to those for whom VAC was used as a bridge-to-flap coverage. The results suggest that VAC has been successful as both a sole and bridging therapy in patients with sternal wound infections. This reduced the need for additional surgery for a significant number of patients in this study. There were no VAC-related deaths or complications. The 2 patients who died had significant comorbid conditions. Early mortality was 0 for this cohort.   These results are consistent with the literature. Superficial infections did not require delayed flap surgery, and patients did well with IV antibiotics, VAC therapy, and irrigation and debridement. The patients who did not respond to VAC therapy as sole therapy were more likely to have deeper infections, such as mediastinitis. These patients, for the most part, did well after surgical intervention. However, 1 patient in the surgical treatment group died, while another still requires additional surgical intervention over 3 years after initial sternal wound infection.   The VAC therapy was effective in that it obviated the need for flap coverage. While operative debridement is an additional procedure, the morbidity associated with the procedure is minimal compared to flap coverage. Requiring a debridement in the operating room is not considered a failure of VAC treatment.   On patient follow-up, the satisfaction with the VAC was not as high as expected. Perhaps this is a function of poor patient understanding of sternal wound infections. When patients were informed of the significant morbidity and mortality associated with these infections, their appreciation grew. It should be noted that all 4 patients lost to follow-up were from the group receiving VAC as definitive treatment.   Compared to the literature, both groups have many of the known independent risk factors for mediastinitis, which include obesity (OR 1.27 – 6.49),^20,21 diabetes mellitus (OR 2.6 – 5.82),^22,23 smoking (OR 1.8 – 3.27),^20,24 congestive heart failure (OR 1.33 – 3.36),^23,25 renal failure (OR 6.93),²⁶ peripheral vascular disease (OR 2.11 – 3.7),^23,27 coronary artery disease (OR 2.67 – 6.85),^26.28 and post-cardiac surgery re-exploration (OR 3.3 – 9.2).^28,29   Vacuum-assisted closure has proven effective in the management of a spectrum of sternal wounds. With superficial wounds, it has obviated the need for delayed flap closure, as well as reducing the discomfort of multiple daily dressing changes to 1-3 VAC changes a week. For deep sternal infections, VAC plays a role as dressing as the wound progresses toward healing, or as bridge-to-flap coverage.   This study is consistent with numerous studies in the literature describing improved outcomes with VAC, either as a stand-alone treatment or compared to other treatment modalities^30-38 Vacuum-assisted closure has been found to be more efficacious than continuous irrigation.³² All patients in this study survived 90 days after initial infection diagnosis. Domkowski et al³³ also presented a low early mortality (3.4%), and similar to this study, included both superficial and deep infections. A quarter of the patients (4/16, 25%) used VAC as a bridge-to-tissue flap treatment; this is consistent with Doss et al,³⁴ who bridged 20% of their patients with sternal wound infections using VAC. Sjogren et al³⁷ noted a significantly improved outcomes of sternal wounds treated with VAC compared to conventional treatment in terms of mortality rate and first line treatment failure rate. This study had 4 transitions from VAC to eventual flap coverage. Song et al¹² and Hersh et al³⁹ validate the use of VAC as bridge-to-flap coverage successfully.   It should be noted that a comparison group with conventional treatment, consisting of wet-to-dry dressing changes, was not feasible as it is no longer part of the treatment algorithm at the corresponding author’s institution. Furthermore, the authors do not believe in the application of skin graft or delayed primary closure in the setting of infection. A better understanding of VAC requires a larger multicenter randomized trial with an agreed-upon treatment protocol. These protocols, although present in the literature, lack significant consensus.

Limitations

  This study compares patients who received VAC for sternal wound infections after cardiac surgery. The main limitations of the study include the small number of patients, the lack of randomization, and uneven treatment groups. However, this is expected in a relatively rare condition. Furthermore, all patients began VAC therapy with the intention of it being the sole treatment modality.

Conclusion

  Vacuum-assisted closure has improved the morbidity and mortality associated with sternal wound infections. Patients require fewer surgical interventions, resulting in improved recovery. Vacuum-assisted closure as definitive treatment modality is a successful, first line therapy for local superficial sternal wound infections. When deep infections occur, however, VAC as bridge-to-flap coverage is recommended over attempted secondary healing with VAC.

Acknowledgements

  The authors would like to acknowledge the Division of Cardiothoracic Surgery and Keanna Jordan from the Health Information Management Department at UCDMC for their help with data collection. This publication was made possible by Grant Number UL1 RR024146 from the National Center for Research Resources, a component of the National Institutes of Health and NIH Roadmap for Medical Research.

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Clinical outcome after poststernotomy mediastinitis: vacuum-assisted closure versus conventional treatment. Ann Thorac Surg. 2005;79(6):2049-2055. 38. Domkowski PW, Smith ML, Gonyon Jr. DL, et al. Evaluation of vacuum-assisted closure in the treatment of poststernotomy mediastinitis. J Thorac Cardiovasc Surg. 2003;126(2):386–390. 39. Hersh RE, Jack JM, Dahman MI, Morgan RF, Drake DB. The vacuum-assisted closure device as a bridge to sternal wound closure. Ann Plast Surg. 2001;46(3):250-254. Bobby Dezfuli, MD is from the Department of Orthopaedic Surgery, University of Arizona Medical Center, Tucson, Arizona. Chin-Shang Li, PhD is from the Department of Public Health Sciences Division of Biostatistics, University of California (UC) Davis, Davis, CA. J. Nilas Young, MD is from the Department of Surgery, Division of Cardiothoracic Surgery, UC Davis School of Medicine, Sacramento, CA. Michael S. Wong, MD is from the Department of Surgery Division of Plastic and Reconstructive Surgery, UC Davis School of Medicine, Sacramento, CA. Address correspondence to: Bobby Dezfuli, MD Department of Orthopaedic Surgery University of Arizona Medical Center 1609 N. Warren Avenue, Suite 110 PO Box 245064 Tucson, AZ 85719 dezfuli@email.arizona.edu Disclosures: None of the authors has a commercial or financial interest in VAC or KCI International.