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Eur J Cardiothorac Surg 2001;20:1168-1175
© 2001 Elsevier Science NL
a Department of Biomedical Engineering/Medical Informatics, Linköping University, Linköping, Sweden
b Department of Cardiothoracic Surgery and Anesthesia, Linköping Heart Center, University Hospital, S-581 85 Linköping, Sweden
Received 7 March 2001; received in revised form 17 August 2001; accepted 5 September 2001.
Corresponding author. Tel.: +46-13-224828; fax: +46-13-100246
e-mail: hans.rutberg{at}lio.se
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Abstract |
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 Top Abstract 1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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Key Words: Cardiac surgery • Surgical wound infection • Postoperative mediastinitis • Multivariate analysis • Risk factors
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1. Introduction |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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Several studies have examined and identified possible causes and risk factors associated with sternal infections, although with conflicting results. They include patient-related risk factors such as age, gender, obesity, diabetes mellitus, and chronic obstructive pulmonary disease (COPD), and procedure-related factors such as prolonged preoperative stay, duration of surgery, use of bilateral mammary grafts, reoperation for control of bleeding, and the need for repeated blood transfusions [1,2,6–12]. The pathogens causing postoperative infections have also been reported [10,11,13], the most common of which are Coagulase-negative staphylococci and Staphylococcus aureus. Since the onset of a sternal wound infection is often late (weeks to months) and starts after the patient has left the hospital, a long-term follow-up is necessary. The patients themselves and the referring physicians in the region need to get involved in the assessment of sternal wound complications to improve surveillance. Analysis of risk factors enables risk assessment of both individual patients and patient groups, and thus makes it possible to improve routines in order to take more effective preventive and control measures.
This study was aimed at investigating the incidence of superficial sternal wound complications and deep sternal infections/mediastinitis after cardiac surgery and identifying preoperative, intraoperative and postoperative factors that may influence the risk of surgical site infection (SSI) after cardiac surgery.
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2. Materials and methods |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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2.2. Antibiotic regimen and surgical preparation
The patients were admitted to the hospital either from home or via referral from another unit or hospital the day before surgery. Preoperative preparation of the patient included two antiseptic showers with hexachlorophene soap. Excessive hair was removed with a special hair-cutting machine the night before surgery. Standard perioperative antibiotic prophylaxis with cloxacillin (2 g) was given intravenously starting 30 min prior to incision in a total of three doses. Patients with a history of allergy to penicillin received three doses of clindamycin 600 mg over 1 day and patients with immunosuppression received cloxacillin in combination with an aminoglycoside. The patient's skin was disinfected with colored alcoholic 0.5% chlorhexidine (5 mg/ml). The operating rooms have laminar high-flow ventilation and HEPA-filtered air (ultraclean air).
2.3. Surgical treatment of postoperative sternal wound infection/mediastinitis
The aim of the surgical revision was to remove all infected or necrotic tissue by debridement and if possible perform a primary wound closure. It was also mandatory to stabilize any sternal instability. If the sternum to a great extent had to be surgically removed, muscle flaps or transfer of the greater omentum was used for reconstruction. Occasionally continuous mediastinal irrigation systems were used.
2.4. Surveillance and data collection
The sternal incision site was assessed on a daily basis during the patient's stay (5–8 days) in the Department of Cardiothoracic Surgery. Diagnosis of identified sternal infections was based on positive cultures, clear dehiscence of the sternotomy, fever, pain, redness, secretion, purulent drainage, and sternal instability. Operating room logs were reviewed to identify all surgical revisions and they were later classified according to the CDC criteria by a cardiac surgeon (H.G.).
All patients were followed up by phone 2 weeks postoperatively and all were given a form to be returned to the unit 6 weeks after surgery. The patients were asked to answer questions about whether they had experienced any infection after discharge and if they had received medical treatment. In connection with visits to the referring cardiologist 2 and 6 months, respectively, after surgery, further information was collected. The physician questionnaire form was designed to elicit information about signs of infection, localization, whether cultures were taken, care given by health care professionals, and whether antibiotics were prescribed for any infection. If a wound infection was present, it was classified and documented by the physician according to specified definitions for superficial infection, deep infection and need for surgical revision. Although these postdischarge data on infections collected by means of the questionnaire could not be classified by the CDC criteria as strictly as the surgical revisions, we were nevertheless able in this way to take into account the wound problems reported by an attending physician (one CDC criterion for SSI).
Additional data were collected from the patient records and cardiac surgery databases. Forty-two preoperative, intraoperative and postoperative variables were recorded. The microbiology registry was checked for cultures taken from all patients who underwent their first surgical revision from January 1996 through September 1999. Bacteria were identified from sternal or mediastinal tissue or fluid. Mortality data were obtained from the Swedish National Cause of Death Register.
2.5. Statistical analysis
Differences in absolute frequencies between patients with and without superficial sternal wound complications and deep sternal infections/mediastinitis with respect to sample size were analyzed using the Pearson 
2 statistic, and differences in mean values were analyzed using a two-tailed Student's t-test. All variables suggested by the univariate analysis (P<0.15) in any of the groups, or judged to be clinically important such as gender were entered into a backward stepwise multiple logistic regression analysis model [14]. Risk factors for ‘all sternal wound complications’ were evaluated first, and then ‘superficial sternal wound complications’ and ‘deep infections/mediastinitis’ were evaluated separately. P values of <0.05 were considered to indicate statistical significance on two-tailed testing. Statistical precision, the goodness of fit of the logistic model, was evaluated by the Hosmer and Lemeshow test. Statistical accuracy, or model discrimination, was assessed with the area under the ROC curve (c value), derived from the same set of patients and computed by means of non-parametric methods for each model. Statistical analysis was performed using the SPSS statistical package version 9.0.
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3. Results |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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One hundred thirty-two surgical revisions of sternal infections were performed. Of these 47 patients (1.6%) were classified as having deep sternal infections and 50 (1.7%) as having mediastinitis. The median interval between initial operation and surgical revision due to infection was 19 days (range 3–433 days). Seventy-six percent of the patients who underwent revisions had previously been discharged without symptoms. Twenty-eight of the infected patients needed two or more revisions and antibiotics before the infection was cured. The average hospital stay in the Department of Cardiothoracic Surgery was 9.1±3.9 days for patients without sternal infection and 16.3±11.7 days (range 5–52 days) for patients who underwent surgical revisions.
The 30 day mortality was 2.7% for patients without sternal wound complications and 2/291 (0.7%) for all patients with sternal wound complications, 1/194 (0.5%) for superficial sternal wound complications, and 1/97 (1.0%) for deep sternal infections/mediastinitis. The 1 year mortality rate from January 1996 through December 2000 was 4.8% for patients without sternal wound complications, and 11/291 (3.8%) for patients with sternal wound complications. The patients with superficial sternal wound complications had a 1 year mortality of 4/194 (2.1%), and those with deep sternal infections/mediastinitis had a 1 year mortality of 7/97 (7.2%).
3.3. Pathogenesis
Of the 97 patients who underwent surgical revisions, 91 were cultured. Twenty-two patients were culture-negative. The most commonly isolated pathogen from wound cultures taken at the first surgical revision was Coagulase-negative staphylococci, which caused 36/91 (39.6%) of the infections. The second most common pathogen, Staphylococcus aureus, caused 15/91 (16.5%) of the infections and was more common in patients with mediastinitis (12/15, 80%) than in patients with deep sternal infections (3/15, 20%). Other pathogens responsible for sternal infections and mediastinitis were Propioni, Acinetobacter, Enterobacter cloacae, Escherichia coli and Klebsiella.
3.4. Risk factors
The risk factor analysis was divided into preoperative, intraoperative and postoperative factors. By means of univariate analysis 23 of 42 variables were associated with increased risk of sternal wound complications in any of the three groups (Table 4). Twenty-four variables were included in a multivariate analysis; the following independent predictors of superficial sternal wound complications and deep sternal infections/mediastinitis were identified: an age of <75 years, obesity, insulin-dependent diabetes, smoking, peripheral vascular disease, a NYHA score of 
3, bilateral use of IMAs, and prolonged ventilator support. There were six preoperative patient-related risk factors, one intraoperative risk factor and one postoperative risk factor (Table 5). The addition of non-significant variables did not substantially improve the multivariate prediction model. The Hosmer and Lemeshow Goodness of Fit test showed that model fit was good for ‘the three groups’ (P=0.99, P=0.56, and P=0.83, respectively). The discriminating ability of the models by ROC area analysis was low for all sternal wound complications and superficial sternal wound complications (ROC, 0.64 and 0.62), and relatively strong for deep infections/mediastinitis (ROC, 0.82).
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4. Discussion |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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The rates of deep sternal infection and mediastinitis that we found were somewhat higher than in other studies [4–7,15]. There is no other hospital in our region providing cardiac surgical services, and patients in need of surgical revisions must therefore return to our center. Due to a low response rate from the referring physicians (40%), the true risk of superficial sternal wound complications may nevertheless still be underestimated. Early diagnosis and accurate classification of the depth of sternal wound infections remains difficult and requires increased clinical suspicion and specified parameters. Because of the increased pressure for early discharge, it has become increasingly important to instruct referring physicians about assessment and documentation of sternal wound complications and advise the patient about whom to contact in order to report any problems. Intensified co-operation between the Departments of Infectious Diseases in the region and the Department of Cardiothoracic Surgery at the University Hospital has lead to earlier recognition of sternal infections and more aggressive surgical management when needed, as well as reduced antibiotic consumption and a shorter postoperative stay [13].
All patients with a sternal infection are now evaluated by a cardiac surgeon, and revisions are more frequently done at an early stage. Although this approach increases the number of revisions, we believe that it contributes to the relatively low mortality rate due to infection that is seen in this study: an early mortality rate of 1.0%, and a late mortality rate of 7.2% for deep sternal infections/mediastinitis, compared to 9.8–14% reported by others [2,6,7]. Our finding that prognosis of deep sternal infections can be influenced by adequacy and quickness of treatment are supported by other authors [15]. Coagulase-negative staphylococci, known to be important pathogens in cardiac surgery [4,13], were the most frequent pathogens isolated from deep sternal infections at the first surgical revision, even in the mixed cultures in this material. In this investigation we focused on infections in the chest area, although we recognize that leg wounds have a substantial impact on morbidity related to cardiac surgery, and an estimated rate as high as 20% in the follow-up.
In this study we evaluated the whole population of cardiac surgical patients using a multivariate analysis to identify high-risk patients or procedures. In this kind of study the aspects of statistical significance versus clinical importance need to be considered. Most earlier studies have focused on deep infections or mediastinitis [4–8], whereas others have examined superficial infections [9,11] or exclusively studied risk factors for SSIs in specific cardiac procedures [2,12,16–19]. By analyzing risk factors for superficial sternal wound complications as well as deep sternal infections/mediastinitis, a pattern of different risk factors was shown. Obesity was independently associated with an increased risk of sternal infections in all groups. The patients with severe infections needing surgical revisions were older and showed signs of more advanced atherosclerotic disease as indicated by more peripheral vascular disease and a higher NYHA score.
Obesity, insulin-dependent diabetes, and a high NYHA score have also been identified previously as risk factors for deep sternal infections [1,6,9,12]. Prolonged time on a ventilator has been shown to increase the likelihood of sternal infections, and may contribute to extended exposure to risk factors in the ICU environment due to a prolonged stay and subsequent microbial colonization of patients [1,2]. Other studies have also shown that the incidence might be higher when bilateral IMAs are used [6,9], whereas others have failed to show such a relationship in properly selected patients [18]. According to a review of published studies, patients receiving internal mammary grafts tend to be younger and are more often male [1]; this agrees with our findings. Bilateral use of the IMA deprives the sternum of blood supply, and places the patient at high risk of sternal infection development [18–20]. Use of bilateral IMA should be restrictive in patients with obesity, insulin-dependent diabetes, COPD, and peripheral vascular disease [7,18,19]. All risk factors are not modifiable, but a proper prophylactic antibiotic dose with ultimate timing and careful skin preparation in obese patients, as well as maintaining controlled blood glucose levels in diabetic patients before and after surgery, may reduce major infections [10,17,21]. In our study over 50% of the patients with sternal infections were current smokers. Cigarette smoking compromises the immune system, has devastating effects on the respiratory system, decreases circulation to the skin, and delays primary wound healing and may increase the risk of SSI [10,20].
Patients who underwent coronary artery bypass grafting accounted for the highest rate of infections in all groups, followed by combined procedures for deep sternal infections/mediastinitis and valve surgery for superficial sternal wound complications. The risk factors we found in this study are mainly related to the patient, although one intraoperative risk factor and one postoperative risk factor were found. Risk factors identified in one institution may not be relevant in another, mainly due to changes in the process of care. Larger and more specific databases may be necessary in order to develop a risk model to identify the high-risk patient in different institutions and thereby modify treatment and reduce risk.
Actions to reduce infections and continuously control the effect of all preventive measures require early recognition and aggressive management, adequate registration, and established criteria for defining superficial and deep sternal infections. For comparisons to be valid over time, the same surveillance methods and classifications of sternal infections must be used [4,10]. The most appropriate use of collected SSI data is timely feedback to practicing surgeons and healthcare workers, as this leads to increased awareness and knowledge about these patients, and a more structured management in all affiliated centers.
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Acknowledgments |
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References |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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