Experience with Vancomycin Prophylaxis Prior to Cardiovascular Surgery (CVS) at the Minneapolis VA Medical Center (MVAMC): A Case-Control Study

Background:   Prophylactic antibiotics are given for CVS to reduce the incidence of surgical site infections (SSI). Emergence of resistant microorganisms is driving changes in antibiotic choices. Vancomycin was the preferred drug for CVS at MVAMC due to the prevalence of MRSA infections, however data on efficacy and dosing administration has been lacking. 

Objective: To identify risk factors associated with SSI occurring after CVS with vancomycin prophylaxis.

Methods:   We performed a case-control review of patients who underwent CVS at MVAMC from fiscal year 2006 to 2009. Patients with SSI were matched by age (+/- 2 yrs) and by time of surgery (+/- 1 mo) with 2 CVS patients without SSI. Charts were reviewed for surgical description and length, characteristics of prophylactic antibiotic administration, infecting organisms, and patient co-morbidities. T-tests were used to compare groups on continuous variables, and logistic regression models estimated the odds ratio of SSI between groups for categorical variables. Vancomycin levels were assessed pre and post-operatively on a subset of patients to assess appropriate drug dosing.

Results: 36 patients with SSI were matched with 72 patients without SSI.  All patients received vancomycin prophylaxis prior to surgery.  Cefuroxime was added to vancomycin beginning in 01/2008 for gram-negative coverage.  56% of SSI were caused by gram-positive organisms, compared to 55% due to gram-negative organisms before 2008 and 14% after the addition of cefuroxime. Statistical analysis did not uncover significant risk factors for SSI among the measured variables. An interim assessment showed that increasing vancomycin doses increased post-operative serum levels, however dose (P = .843), patient weight (P = .746), and body mass index (P = .981) were non-significant factors for SSI. 

Conclusions: We could not identify risk factors among patients receiving vancomycin prophylaxis for CVS that lead to higher rates of SSI. Interestingly, drug dosage and anthropometrics were not significant factors in predicting SSI. This suggests that weight based dosing may not be necessary for prophylaxis. Due to the design of the study we cannot judge the efficacy of vancomycin against gram-positive infections, however we did see a notable reduction of infections due to gram-negative bacilli after addition of cefuroxime prior to surgery.