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Ann Thorac Surg 2004;77:676-683
© 2004 The Society of Thoracic Surgeons

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Original article: cardiovascular

Risk factors for mediastinitis after cardiac surgery

^a Instituto Dante Pazzanese de Cardiologia, São Paulo, Brazil
^b Universidade Federal de São Paulo, São Paulo, Brazil

Accepted for publication August 6, 2003.

^* Address reprint requests to Dr Abboud, Infection Control, Instituto Dante Pazzanese de Cardiologia, Rua Estela 287 Apt. 44, São Paulo SP 04011-001, Brazil
e-mail: csa{at}directnet.com.br

	Abstract

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BACKGROUND: Postoperative mediastinitis is one of the most feared complications in patients who undergo cardiac surgery because in addition to a high mortality rate (10% to 47%), there are increases in the length of hospital stay and in hospital costs. The purpose of the present study is to assess the risk factors for mediastinitis after cardiac surgery, the mediastinitis rate, and the mortality rate in our institution.

METHODS: To determine the risk factors, a matched case-control study was carried out, with 39 cases and 78 controls, among the patients who underwent cardiac surgery at the Dante Pazzanese Cardiology Institute, São Paulo, Brazil.

RESULTS: In the period of the study, 9,136 cardiac surgeries were performed and the mediastinitis rate was 0.5%. In the multivariate analysis, the independent risk factors found were obesity (odds ratio, 6.49; 95% confidence interval, 2.24 to 18.78), smoking (odds ratio, 3.27; 95% confidence interval, 1.04 to 10.20), intensive care unit stay more than 2 days (odds ratio, 4.50; 95% confidence interval, 1.57 to 12.90), and infection at another site (odds ratio, 8.86; 95% confidence interval, 1.86 to 42.27). The mortality rate was 23% among the patients with mediastinitis.

CONCLUSIONS: We observed two independent risk factors related to patients' antecedents (obesity and smoking) and two risk factors related to problems in the postoperative period (length of intensive care unit stay and infection at another site). Efforts should be concentrated so that patients lose weight and stop smoking before elective surgeries. There should also be a prevention program against hospital infection directed to, and intensified for, at-risk patients.

	Introduction

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Mediastinitis after thoracic surgery is defined as the infection of organs and spaces, which may occur in 0.4% to 2.4% of cases according to data taken from the literature published in the past decade [1–7]. When this complication occurs, it increases the length of hospital stay and hospital costs, besides being possibly lethal. Mortality can vary between 10% and 47% [1, 2, 8]. Risk factors that lead patients to develop mediastinitis have been widely studied, but the data in many studies conflict. The assessment of those factors is extremely important, for it allows control and preventive measures to be taken purposefully, to avoid the appearance of this infectious postoperative complication.

	Material and methods

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The institution
Dante Pazzanese Cardiology Institute, located in São Paulo, is a public university hospital, with capacity for 218 beds, destined for cardiovascular surgeries. Approximately 2,000 cardiac surgeries are performed there every year. The institution maintains a hospital infection control program, with active and prospective epidemiologic surveillance of hospital infections [9] according to the Centers for Disease Control and Prevention notification criteria [10].

1. Isolation of the microorganism in the culture of mediastinal tissue or of liquid obtained through fine-needle aspiration or during the surgery.
   
2. Evidence of mediastinitis seen during a surgery or through histopathologic evaluation.
   
3. Presence of one of the following clinical findings (other causes having been eliminated): fever (>38°C), chest pain, or sternal instability in addition to any of the following eventualities:
   

1. Draining of purulent secretion of the mediastinal area.
   
2. Isolation of a microorganism in blood culture or from the purulent secretion drained from the mediastinal area.
   
3. Widening of the mediastinum detected through radiologic evaluation.
   

Study design
During the period from January 1995 to January 2001, all mediastinitis cases identified were assessed in a matched case-control study.

Entry conditions for cases and controls
Patients older than 18 years of age, who underwent cardiac surgery (coronary arteries, mitral valve, congenital or with association of diseases), were entered into the study. Patients undergoing cardiac transplant were excluded, as well as those who had incomplete data or whose records were not found.

Matching criteria
The matching was based on the patient's age plus or minus 10 years, and the date of surgery plus or minus 1 month. Case patients were defined as those who underwent cardiac surgery and presented with mediastinitis according to the Centers for Disease Control and Prevention criteria [10]. Control patients were those who underwent cardiac surgery and did not have mediastinitis. These patients were selected by drawing lots, considering all who were liable to infection according to the matching criteria.

Risk factors
Preoperative risk factors (data obtained before the surgery) were as follows: weight, height, body mass index (weight in kilograms divided by the height in meters squared), obesity, diabetes mellitus, smoking (up to 3 months before the surgery), hypertension, dyslipidemia, chronic obstructive pulmonary disease, previous use of corticoid or antibiotics, use of a permanent pacemaker, chronic or acute renal insufficiency, secondary diagnoses (endocarditis, rheumatic fever, collagen diseases, and so forth), acute myocardial infarction up to 1 month before the surgery, unstable angina, stable angina, presence of coronary stent, ejection fraction, functional class, previous sternotomy, type of surgery performed, use of ß-adrenergic drugs, and the length of hospital stay before the surgery.

Intraoperative risk factors were emergency surgery, duration of the surgery, perfusion time, aorta cross-clamping time, use of internal mammary arteries and their number, and receiving blood transfusion and the volume received.

Postoperative risk factors were reoperation (mainly for purposes of hemostasis review), use of intraaortic balloon, length of intensive care unit (ICU) stay, tracheostomy, duration of mechanical ventilation, use of inotropic drugs, and presence of infection at another site.

Risk factors not studied were the prophylactic antibiotic (because it is standardized at the institution), trichotomy, antisepsis, and surgical team (as it is a university hospital, there is rotation among the team members).

Statistical analysis
For the determination of risk factors for postoperative mediastinitis, a case-control study was performed. The effects of each variable were examined in a univariate approach with comparison of averages and application of the Student's t test, as well as with the analysis of contingency tables with ² tests or Fisher's exact test when necessary. Multivariate analysis was performed through the model of logistic regression with the odds ratio (OR) for each risk factor being identified with a 95% confidence interval (CI). The program used was SSPS for Windows, version 10.0 (SPSS Inc Headquarters, Chicago, IL).

	Results

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Risk factor analysis
During the period of the study, 46 cases of patients with mediastinitis were identified, of a total of 9,136 cardiac surgeries, yielding a mediastinitis rate of 0.5%. Of the 46 cases, 7 patients were excluded from the analysis: 3 had undergone transplant, 3 were younger than 18 years of age, and 1 patient's record was not located. We paired 78 controls to the 39 cases, yielding a total of 117 patients studied.

In the univariate analysis, the risk factors with p < 0.05 were obesity, diabetes, dyslipidemia, acute myocardial infarction, unstable angina, reoperation, infection at another site after the surgery, duration of the surgery, length of stay in the ICU longer than 2 days, and mechanical ventilation for more than 1 day. Table 1 shows the data of the descriptive statistical analysis. Smoking, the use of inotropic drugs, and coronary surgery yielded p < 0.10, but owing to their importance in other studies, they were included in the multivariate analysis with logistic regression. The independent risk factors found were obesity (OR, 6.495; 95% CI, 2.24 to 18.78); smoking (OR, 3.27; 95% CI, 1.04 to 10.20); infection at another site (OR, 8.86; 95% CI, 1.86 to 42.27); and length of ICU stay (OR, 4.50; 95% CI, 1.57 to 12.90; Table 2).

Thirty-six patients underwent surgical cleaning, and the bone culture was positive in 23 patients (59%; Table 3). Hemoculture was positive in 7 patients (18%), Staphylococcus aureus having been isolated in all cases, 6 (85.7%) of which were methicillin-resistant. Surgical wound culture was positive in all cases, with more than one agent isolated in some cases. Ten (25%) patients had infection in other sites after the surgery before the appearance of mediastinitis (4 in saphenous vein incisions, 4 pneumonias, 1 catheter, and 1 urinary infection). We found no correlation between the etiologic agents of those infections and mediastinitis, although 1 patient had infection in a bypass graft caused by methicillin-resistant S aureus as well as in the culture of mediastinal secretion with growth of the same agent, but molecular biology typing was not performed.

	Comment

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In our study, we found a mediastinitis rate of 0.5%, similar to that reported in the literature [3, 11, 12]. The exact mechanism by which mediastinitis develops is unknown and certainly multifactorial, so much so that the several studies performed do not agree among themselves. Table 4 shows risk factors found in the literature, case history, kinds of study, and the institution where the study was performed.

Smoking patients in our study had a 3.3 times higher chance of having infection when compared with nonsmokers. This risk factor was also reported by Nagachinta and colleagues [12] and Fariñas and associates [15]. Other studies have shown the correlation of chronic obstructive pulmonary disease and mediastinitis [14, 16]. Chronic obstructive pulmonary disease causes mechanical problems, leading to sternum instability, which in those cases may suggest the harmful effect of smoking in the pathophysiology of mediastinal infection as well. Apart from that, it is more often possible for patients with chronic obstructive pulmonary disease to be colonized [2].

The length of ICU stay for longer than 48 hours, in our group of patients, increased the risk of infection by 4.8 times. This risk factor was also reported by other authors [15, 17, 18]. The increase in the length of ICU stay denotes the clinical instability of the patient, who very often needs vasoactive drugs. This may, in turn, lead to tissue hypoperfusion, thus favoring the appearance of infections [2, 7, 13].

The presence of infection at another site after the surgery made it 8.8 times more likely for infected patients to have mediastinitis. In those cases, it is probable that bacteria reach the bloodstream and later the site of the surgery [2, 19]. In our study, agents found in remote infection sites did not present any correlation with the agents isolated for mediastinitis, except for one case of methicillin-resistant S aureus in the saphenous vein incision infection and in the secretion of the mediastinum. However, no specific tests were performed to try to confirm that hypothesis. We believe the onset of mediastinitis in patients with infection at another site is directly related not to the etiologic agent but to the patient's clinical state.

Diabetes is always a feared risk factor and is seen with caution by clinicians and surgeons because, as a result of its pathophysiology, microvascular alterations and high blood sugar may interfere negatively in the cicatrization process [20, 21]. It was observed in our case history in the univariate analysis, but it was not confirmed as an independent risk factor.

The use of two internal mammary arteries was another risk factor found [2, 3, 13]. In our analysis, those data were not observed, one of the reasons for that being that only 4 (3.8%) of the 91 (78.4%) patients who underwent coronary surgery used two mammary arteries (two cases and two controls).

Other risk factors may be involved, but they are difficult to be measured. The aspect of the bone, which can sometimes show signs of osteoporosis, ischemia, the surgeon's ability, failure to follow the antisepsis procedures, errors in the sternotomy and in the sternum rewiring, and excessive use of an electric scalpel are factors that are very often not mentioned but can be important factors in the pathophysiology of mediastinitis.

As for the etiologic agents isolated in the secretion of surgical wounds, 48.3% were gram-positive cocci, and the remaining were gram-negative organisms, which demonstrates an equal frequency of gram-positive and gram-negative cocci. When only hemocultures are assessed, however, we observe that S aureus was isolated in all cases, 85.7% of which were resistant to methicillin. These findings were different from those observed by Wang and Chang [22], who isolated 93% of Staphylococcus spp in their samples, most of which (80%) were also methicillin-resistant.

The death rate we found (23%) is similar to that observed in other studies [1, 2, 8].

As occurs with any case-control study, this one has its limitations, inasmuch as the data were collected retrospectively, and the number of mediastinitis cases is small. However, based on the data found in this study and in other works mentioned, we believe the prevention of mediastinitis must be a top priority in cardiovascular surgery services, because without it there may be an important increase in patients' morbidity and mortality rates.

In our sample, case patients had two preoperative risk factors, obesity and smoking. We believe patients who have some cardiac condition and might have to undergo surgery must be urged to lose weight, possibly even through a more intense program within the institution, which provides outpatient assistance to patients with cardiac conditions and at the basic health units. The same reasoning applies with regard to smoking, a situation that calls for more-effective actions. In our analysis, patients who had smoked until up to 3 months before the surgery were considered smokers, so the very common recommendation for patients to stop smoking during the month immediately before the surgery may not be completely beneficial for the patient. Therefore, antismoking programs should be initiated during the primary attention to patients with cardiac conditions. These measures are difficult to put into practice to the extent that they concern the patients' habits, but they should be publicized.

Two other risk factors found in our analysis concern postoperative complications: ICU stay longer than 2 days and infection at another site. These two factors are hard to counter, because the increased length of ICU stay may be multifactorial (peripheral vascular accident, cardiac, pulmonary or renal insufficiency, and so forth) and hard to control. As for infections at other sites, they occur most often with frequently manipulated patients, and there may be a correlation with the increased length of ICU stay, which links the two risk factors found.

We suggest that "alert" systems be created for patients who are closer to risk characteristics for the development of mediastinitis and that extra measures be discussed among infection control teams, clinicians, intensivists, and surgeons so that the appearence of infection in those patients is avoided.

	References

Top Abstract Introduction Material and methods Results Comment References

 

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