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Year : 2003  |  Volume : 21  |  Issue : 4  |  Page : 246-251

Postoperative wound infection in patients undergoing coronary artery bypass graft surgery: A prospective study with evaluation of risk factors

Rheumatology, PD Hinduja National Hospital and Medical Research Centre, Mumbai - 400 016, India

Correspondence Address:
Rheumatology, PD Hinduja National Hospital and Medical Research Centre, Mumbai - 400 016, India

 ~ Abstract 

PURPOSE: Wound infection is an important cause of morbidity and occasional mortality after coronary artery bypass graft surgery (CABG). The aim of this study was to report postoperative wound infection in CABG surgery patients. METHODS: Consecutive patients undergoing CABG surgery between January 1998 and October 1999 have been studied. The exclusion criteria included, age less than 30 years, penicillin / cephalosporin allergy and associated other cardiac pathologies. The parameters studied were age, sex, obesity, hypertension, diabetes, myocardial infarction, chronic renal failure, previous surgeries, alcohol consumption, smoking, length of pre and postoperative hospital stay, antibiotic prophylaxis, MRSA screening, and duration of surgery. Wounds were classified as per modified CDC's NNIS criteria. Suspected sites of infection were cultured and antibiotic susceptibility of cultured organisms was tested. Postoperative follow up was for two months. RESULTS: Six hundred and fifteen patients were studied. Of these 116 (18.86%) developed SSI, involving sternum 75%, leg 21.3%, and forearm sites 3.44%. Organisms isolated at sternum site were MSSE, MRSA, and MRSE, at leg site E. coli and MSSE, and at forearm site MSSE and MSSA. Sternal site, obesity, diabetes mellitus and female sex were associated with significantly higher infection rates (p= 0.001). No antibiotic protocol proved more effective. SSI increased the postoperative hospital stay and the total treatment cost. CONCLUSIONS: Post CABG surgery SSI rate is high. Sternum and leg are the common infection sites. Obesity, uncontrolled diabetes mellitus and female sex are associated with higher infection rates. "Higher" antibiotics do not lower postoperative infection rates.

How to cite this article:
Bhatia J Y, Pandey K, Rodrigues C, Mehta A, Joshi V R. Postoperative wound infection in patients undergoing coronary artery bypass graft surgery: A prospective study with evaluation of risk factors. Indian J Med Microbiol 2003;21:246-51

How to cite this URL:
Bhatia J Y, Pandey K, Rodrigues C, Mehta A, Joshi V R. Postoperative wound infection in patients undergoing coronary artery bypass graft surgery: A prospective study with evaluation of risk factors. Indian J Med Microbiol [serial online] 2003 [cited 2021 Feb 28];21:246-51. Available from:

Despite improved understanding of the pathophysiology and improved methods of prevention and prophylaxis, infection remains the most common cause of postoperative morbidity and mortality.[1] Wound infection after CABG surgery is a major problem. Over the years the rate and pattern of infections in CABG patients in our hospital had not shown a major change. It was therefore felt necessary to study, and analyse the risk factors for the development of wound infection, role of antibiotic prophylaxis, as well as impact of infection on mortality, morbidity, and cost.

 ~ Materials and methods Top

In this prospective study, between January 1998 and October 1999 all patients who underwent CABG surgery were included. The exclusion criteria were age below 30 years, history of allergy to penicillin / cephalosporins and associated additional cardiac pathologies such as valvular damage, aortic aneurysm and congenital heart disease needing surgery. For each patient, demographic characteristics and data on risk factors and surgical site infection were recorded on a predetermined proforma. The risk factors studied were age, sex, obesity, diabetes, hypertension, chronic renal failure, organ failure, length of preoperative hospital stay, duration of surgery, wound class, MRSA carriage, length of postoperative stay, past history of surgery, myocardial infarction, smoking, alcohol consumption and antibiotic prophylaxis. Methicillin resistant Staphylococcus aureus (MRSA) screening was performed in 131 patients only. Postoperatively all patients were followed up for two months on an outpatient basis.
Swabs were collected from all clinically suspect sites of infection. A wound infection was suspected if any of the following were present - fever, inflammation at the wound site, oozing or bleeding from the incision sites, i.e., sternum, leg and forearm. Culture techniques followed the standard guidelines of National Committee for Clinical Laboratory Standards (NCCLS).[2] Wound swabs were processed for Gram stain and Ziehl Neilson stain and cultured on blood agar, MacConkey's agar, chocolate agar and potassium tellurite media and incubated at 370C in 5%-10% CO2 for 18 hours. Antimicrobial susceptibility tests were performed using NCCLS guidelines.[2]
Preoperative and postoperative MRSA screening of patients was done in 131 patients. Swabs were collected from nose, axillae and skin (sternal site) and plated on Mueller Hinton agar with 4% NaCl.[3] Antibiotic susceptibility test was performed by Kirby Bauer Disc Diffusion Method.[3]
During the hospital stay, patients were evaluated daily preoperatively and postoperatively from third day onwards. Data on infection was collected and classified according to the criteria laid down by the Centres for Disease Control and Prevention, National Nosocomial Infections Surveillance System (CDC's NNIS).[4]
The wound infection was classified using modified CDC's NNIS criteria[4] as follows.
Mild infection: minimal wound discharge, which settled with oral antibiotics and did not delay patient's recovery from surgery.
Moderate infection: minimal to moderate purulent discharge which required antibiotics, dressing and delayed recovery from surgery.
Severe infection: frank pus discharge from the wound requiring antibiotics and surgical intervention.
On the day prior to surgery and on the day of surgery patients were advised to take a bath with 4% chlorhexidine which was applied to the whole body excluding the face and retained for at least 5 minutes before taking bath. Patients underwent chin to toe shaving, the evening before surgery. Operative site was prepared using 10% povidone -iodine scrub solution (Betadine) for disinfection. Sternotomy site was covered with an iodine membrane (Ioban 3-M). Cautery was used to harvest internal mammary artery. Intra and postoperative blood transfusion was given as necessary.
Sternal wound closure was performed using monofilament undyed vicryl. Wound was washed with povidone iodine and sealed with dressing for three days, unless oozing was present. If oozing was present the wound was opened and dressing was done and checked for infection. No local antibiotic was applied to the wound site. Absorbable sutures were used. Drain site suture was removed on the seventh postoperative day. Uncomplicated cases were discharged by tenth postoperative day.
Postoperatively, patients were seen daily and checked for signs and symptoms of wound infection as detailed in material and methods. In the presence of any of these, wound swabs were immediately collected and processed for culture and sensitivity. Based on the results of culture and sensitivity, appropriate antibiotic(s) was administered and culture repeated after one week. Dose, route of administration and duration of antibiotic treatment were recorded. For mild and moderate infections antibiotics were started only after culture sensitivity results were available. For severe infections swabs were collected and antibiotic treatment was instituted subject to modification as per culture sensitivity report.
Statistical analysis
Data measured on continuous scale were expressed as mean SD and compared using Student's t-test. Categorical data were expressed as percentages and evaluated using Chi square or Fischer's Exact Probability Test (FEPT). P<0.05 was taken as statistically significant. Odds ratios were estimated. Multiple logistic regression analysis was carried out. The statistical package used was number cruncher statistical system (NCSS).

 ~ Results Top

Six hundred and twenty three patients had undergone coronary artery bypass surgery by a single surgeon between January 1998 and October 1999. Eight patients were excluded due to early postoperative deaths, not related to infection [low cardiac output (4), multiple organ failure (1) and acute renal failure (3)]. Therefore, data of 615 patients has been analysed. Of these 116 (18.86%) developed SSI. [Table - 1], [Table - 2] detail the data pertaining to risk factors, type of organisms and severity of infection.
Multiple antibiotic prophylaxis regimens were used during the study (details not given). Diabetes, female gender and obesity were significantly associated with the development of SSI. Severity of infection was related to diabetes (especially uncontrolled) and obesity. This was especially so for sternal and leg wounds. Sternum was the most common infection site. It also accounted for >70% cases of severe infection. The most commonly isolated organisms at the sternal site were methicillin sensitive Staphylococcus epidermidis (MSSE) followed by MRSA, MRSE, gram negative organisms and polymicrobial. At leg site  E.coli   was the most common isolate, followed by MSSE, other gram negative organisms and polymicrobial organisms. At forearm site MSSE and MSSA were the only organisms isolated.
Almost 50% of cases with SSI necessitated a change of antibiotic. The average increase in the cost of treatment was 3.8% for mild infections, 14.7% for moderate infections, and 29.4% for severe infections. The duration of postoperative hospital stay of patients who developed infection was significantly higher (mild infection 15 days, moderate infection 20 days, and severe infection 25 days). Other factors showed no statistically significant difference. There were no infection related deaths.

 ~ Discussion Top

Cardiothoracic surgery is associated with higher postoperative infection rates. It increases the length of hospital stay by 57% and cost of treatment by 42%.[5] It is therefore imperative to implement adequate measures to reduce the incidence of postoperative infection. To this end, it is necessary to identify the epidemiology of postoperative infections. The cost, impact, and choice of antibiotic prophylaxis need to be defined.
The overall rate of surgical site infection in this study was 18.86%. The reported incidence of SSI has ranged between 2% to 20%, the acceptable range being 11%-15%.[6] The high rate in the present study is a matter of concern and justifies such a study.
Sternal site infection accounted for nearly 75% of the cases of SSI. Many factors are linked with SSI at the sternum. Bone wax is frequently used as a haemostatic agent to prevent oozing from cancellous bone. Although bacterial infection in this setting is infrequent, its occurrence is associated with increased morbidity and mortality. Robicsek et al [9] have commented on the infection promoting role of bone wax. Another study has suggested that bone wax augments infection.[10] Bone wax was used in all our patients. Blood transfusion has been identified as an independent risk factor. Blood transfusion is associated with impaired immunocompetence.[11] In the present study all patients had received blood transfusion. Reexploration for haemorrhage has been reported as an important risk factor.[12] Reexploration means longer exposure of the surgical field and greater trauma to tissues. The rate of reexploration in the present study was 2.1%.
The internal mammary artery (IMA) is isolated from the chest wall as a pedicle, together with the vein, muscle, fat and accompanying endothoracic fascia. Cautery is used to separate the pedicle from the chest wall. Cauterization damages the blood supply to the sternum, impedes sternal wound healing and exposes the sternum to the risks of early dehiscence and infection, especially, if both IMAs are harvested. IMA is the primary source of blood supply to the sternum. There is frequently a substantial degree of ischaemic necrosis of the bone on the side of the IMA graft. The risk is particularly great in the elderly and the diabetic patients.[13] In the present study, single or bilateral IMA was used in almost all the patients.
The major pathogens in this study were  S.epidermidis   (42.24%) and S.aureus (15.5%). The remaining infections were accounted for by gram negative organisms (12.06%) and mixed infections (8.6%). In a survey from an English hospital,[14] S.aureus accounted for almost 40% of the pathogens identified. In the present study [Table - 2] staphylococci as a whole [MSSE (26.72 %), MRSE (15.5%), MRSA (12.06%) and MSSA (3.2%)] accounted for over 50% cases and 22.4% of severe infections. 19.8% of wounds clinically suspected to be infected were culture negative. Most of these wounds had mild infection, needed no specific intervention and did not adversely affect the cost of treatment or the hospital stay.
Polymicrobial infections (12.4%) frequently involved both gram positive and gram negative organisms, findings similar to those reported by Baskett,[15] and Palmer et al.[16] All the isolates of MSSE were sensitive to the antibiotic used for prophylaxis and it accounted for one third of the cases. Propensity of MSSE to colonise sutures is the possible explanation.
Among patient characteristics, female gender, diabetes and obesity showed a statistically significant correlation with SSI. These have also been identified as risk factors by others.[17]
Obesity has been reported as a weak association with SSI and that too only in relation to sternotomy.[18] The possible reasons for obesity being a risk factor include ineffective dose of prophylactic antibiotic, difficulty of proper skin preparation, adipose tissue providing a good substrate for infection and difficulties in vascular graft harvesting.
Diabetes, especially uncontrolled diabetes, was a significant risk factor for the development of SSI in the present study. Both diabetes and obesity are important associations of ischaemic heart disease and are commonly present in patients undergoing CABG surgery. Diabetics accounted for 43.7% of all cases in this study.
Many other risk factors have been implicated in wound infection. It has long been assumed that wound infections are common amongst patients with multiple preexisting diseases.[19] However age, organ failure, hypertension, chronic renal failure, smoking, male sex, past history of surgery and myocardial infarction were not found to be significant risk factors in this study.
The age groups, even when stratified decadewise, did not show a significant correlation with infection. Cruse and Foord[20] have emphasized that in patients over the age of 66 years the chances of developing wound infection are twice as great as in patients between 21-50 years of age. In the present study, 13.4% patients were over the age of 66 years. The infection rate in them was surprisingly only 3.7%.
There are conflicting reports on gender as a risk factor for SSI with some reporting increased risk and some decreased risk in males.[21] There are also reports of no significant association.[22] The association of female gender with SSI at leg site may possibly be due to the practice of shaving legs.[10] The reason for male sex as an independent predictor is unclear. It may be that males tend to have increased tension on their sternal incision, which in turn leads to increased risk of sternal instability.[21]
Duration of surgery can be a risk factor. Operations lasting for more than 2 hours are associated with increased infection rates.[23] The average duration of surgery in the present series was 4-5 hours. Length of preoperative hospital stay has been considered a risk factor. Though we noted a trend towards increasing incidence of infection with longer preoperative hospital stay the difference did not reach statistical significance (data not shown). The natural and main niche for S.aureus is the human nose, but other skin sites also can be colonized.[3] We studied preoperative nasal carriage in 131 patients and found no significant correlation with SSI (data not shown) and therefore did not continue study of nasal carriage in rest of the patients. Ridgway et al have made similar observations.[24] As already stated, a variety of antibiotic regimens based on cephalosporins with or with out aminoglycosides were used with no significant difference in the infection rates (data not shown).
Postoperative SSI increased the length of hospital stay and costs in proportion to the severity of infection. The cost increased by 3.8%, 14.7% and 29.4% in mild, moderate and severe infections respectively. The corresponding figures for increase in length of hospital stay were 5, 9 and 15 days respectively. There were no infection related deaths.
In summary, post-operative infection is an important problem in patients undergoing CABG surgery at our institution. Female sex, diabetes (especially uncontrolled) and obesity were the important risk factors identified. Smoking and preoperative hospital stay, though not statistically significant, appeared to be important. The use of newer antibiotics or longer antibiotic prophylaxis did not affect the postoperative infection rates.  

 ~ References Top

1.Lauwers S, Smet de F. Surgical site infections. Acta Clinica Belgica 1998:53-55.  Back to cited text no. 1    
2.Acra JF, Goldstein FW. Disk susceptibility test in Antibiotics in Laboratory Medicine eds Lorain V 3rd Edn William and Wilkins USA 1991:17-52.  Back to cited text no. 2    
3.Kluytmans JAJW, Mouton JW, vanden Bergh MFQ, Manders MJAAJ, Maat APWM, Wagenvoort JHT, Michel MF, Verburgh HA. Reduction of surgical site infections in cardiothoracic surgery by elimination of nasal carriage of Staphylococcus aureus. Infect Control Hosp Epidemiol 1996;17:780-785.  Back to cited text no. 3    
4.Crowe MJ, Cooke EM. Review of case definitions for nosocomial infections-towards a consensus. J Hosp Infect 1998;39:3-11.  Back to cited text no. 4  [PUBMED]  
5.Porcu A, Noya G, Dessanti A, Nilou P, Cottu P, Castiglia P, Dettori G. A new approach to the problem of surgical wound infcetions in clean operations. Minerva Chir 1996;51:691-696.  Back to cited text no. 5    
6.Simchen E, Shapiro M, Marin G, Sacks T, Michel J. Risk factors for postoperative wound infection in cardiac surgery. Infect Control 1983;4:215-220.  Back to cited text no. 6    
7.Loop FD, Lytle BW, Cosgrove DM, Mahfood S, Mc Goormastic M, et al. Sternal wound complications after isolated coronary artery bypass grafting : Early and late morbidity and cost of care. Ann Thorac Surg 1990;49:179-187.  Back to cited text no. 7    
8.Roy MC. Surgical site infections after coronary artery bypass graft surgery : Discriminating site -specific risk factors to improve prevention efforts. Infect Control Hosp Epidemiol 1998;19:229-233.  Back to cited text no. 8    
9.Robicsek F, Masters RN, Littman L, Born GV. The embolization of bone wax from sternotomy incisions. Ann Thorac Surg 1981;31:357-359.  Back to cited text no. 9    
10.Johnson P, Fromm D. Effects of bone wax on bacterial clearance. Surgery 1981; 89:206-209.  Back to cited text no. 10    
11.Raudat CW, Pagel J, Woodhall D, Wojtanowski M, Bergen RV. Early intervention and aggressive management of infected median sternotomy incision: A review of 2242 open-heart procedures. The American Surgeon 1997;63:238-242.  Back to cited text no. 11    
12.Ottino G, De Paulis R, Pansini S, Rocca G, Tallone MV, Comoglio C, et al. Major sternal wound infection after open heart surgery: A multivariate analysis of risk factors in 2,579 consecutive operative procedures. Ann Thorac Surg 1987; 44:173-179.  Back to cited text no. 12    
13.Rand RP, Cochran RP, Aziz S, Bradley HO, Allen DM, Verrier ED, et al. Prospective trial of catheter irrigation and muscle flaps for sternal wound infection. Ann Thorac Surg 1998;65:1046-1049.  Back to cited text no. 13    
14.Vandenbroucke-Grauls CMJE, Kluytmans JA. Prevention of postoperative wound infections: To cover up? Infect Control Hosp Epidemiol 2001;22:335-337.  Back to cited text no. 14    
15.Baskett RJF, Mac Dougall CE, Ross BD. Is mediastinitis a preventable complication? A 10-year review. Ann Thorac Surg 1999;67:462-465.  Back to cited text no. 15    
16.Palmer DL, Pett SB, Akl BF. Bacterial wound colonization after Broad-spectrum versus narrow spectrum antibiotics. Ann Thorac Surg 1995;59:526-531.  Back to cited text no. 16    
17.Vuorisalo S, Haukipuro K, Pokela R, Syrjala H. Risk features for surgical -site infections in coronary artery bypass surgery. Infect Control Hosp Epidemiol 1998; 19:240-247.  Back to cited text no. 17    
18.Lilienfeld DE, Vlahov D, Tenney JH, Mc Laughlin JS. Obesity and diabetes as risk factors for postoperative wound infections after cardiac surgery. Am J Infect Control 1988;16:3-6.  Back to cited text no. 18    
19.Mangram AJ, Horan TC, Pearson ML, Silver LC, Jarvis WR. The Hospital Infection Control Practices Advisory Committee Guidelines for the prevention of surgical site of infection. Infect Control Hosp Epidemeol 1999;20: 247-280.  Back to cited text no. 19    
20.Cruse PJ, Foord RRN. The epidemiology of wound infection. A ten year prospective study of 62,939 wounds. Surg Clin North Am 1980;60: 27-40.  Back to cited text no. 20    
21.Borger MA, Rao V, Weisel RD, Ivanov J, Cohen G, Scully HE, David TE. Deep sternal wound infection: risk factors and outcomes. Ann Thorac Surg 1998;65:1050-1056.  Back to cited text no. 21    
22.Thourani VH, Weintraub WS, Stein B, Gebhart SSP, Craver MJ, Jones EL, Guyton RA. Influence of diabetes mellitus on early and late outcome after coronary artery bypass grafting. Ann Thorac Surg 1999;67:1045-1052.   Back to cited text no. 22    
23.Garibaldi RA, Cushing D, Lerer T. Risk factors for postoperative infection. Am J Med 1991;91(Suppl 3B): 158S-162S.  Back to cited text no. 23    
24.Ridgway GL, Wilson AP, Kelsey MC. Preoperative screening cultures in the identification of staphylococci causing wound and valvular infections in cardiac surgery. J Hosp Infect 1990;15:55-63.  Back to cited text no. 24    
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2004 - Indian Journal of Medical Microbiology
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