Asian Cardiovasc Thorac Ann 2004;12:227-232
© 2004 Asia Publishing EXchange Ltd
Semi-skeletonized Internal Mammary Artery Grafts and Sternal Wound Complications
Yongzhi Deng, MS,
Karen Byth, PhD1,
Hugh S Paterson, FRACS
Department of Cardiothoracic Surgery, Westmead Hospital
1 Westmead Millennium Institute, Westmead, Australia
For reprint information contact: Hugh S Paterson, FRACS Tel: 61 2 9845 7994 Fax: 61 2 9845 8314 Email: patersonH{at}aol.com Department of Cardiothoracic Surgery, Westmead Hospital, P.O. Box 533, Wentworthville, New South Wales 2145, Australia.
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ABSTRACT
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This study aimed to evaluate the risk factors for sternal wound complications in patients undergoing myocardial revascularization using bilateral semi-skeletonized internal mammary arteries. Prospectively collected data on 751 patients undergoing coronary artery surgery from September 1994 to August 2002 were analyzed. The mean age of the patients was 56 years, 633 (84%) were male, 44 (6%) were over 66 years of age, and 170 (23%) were diabetic. Forty-four (5.9%) patients developed sternal wound complications. Among these cases, sternal infection occurred in 22 (2.9%) patients, of which 15 (2.0%) had sternal infection with mediastinitis and 7 (0.9%) had sternal infection alone. Independent risk factors for any sternal wound complications were peripheral vascular disease, diet-controlled diabetes, and delayed sternal closure. The risk factors for sternal infection were diabetes, postoperative pulmonary complications, and postoperative stroke. The perioperative mortality rate was 1.5% (11 patients), including 2 patients who had sternal wound complications. The use of bilateral semi-skeletonized internal mammary artery conduits carries a comparable sternal wound complication rate as conduits harvested by other techniques.
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INTRODUCTION
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With increasing evidence that patients who received bilateral internal mammary artery (BIMA) grafts have better long-term outcomes than those receiving single internal mammary artery (SIMA) grafts,1,2 BIMA grafting is increasingly adopted as the routine. Since the technique employed for harvesting the internal mammary artery (IMA) varies, the incidence of sternal wound complications (SWCs) varies from 1.5% to 6.9%.1–8 With the conventional wide-pedicled dissection of the BIMAs, sternal infection rates of 1.7% to 6.9% have been reported.1–5 A full skeletonizing technique has been used in some institutions with reported sternal infection rates of 1.5% to 1.9%.6–9
To capitalize on the advantages of both the conventional and the full skeletonizing technique, a semi-skeletonizing technique has been developed.10 With this method, a maximum length of the IMA can be prepared with minimal trauma to the parasternal tissues and without major alteration of the conventional technique. This article reports the results of a consecutive series of 751 patients in an 8-year period who received bilateral semi-skeletonized IMAs during coronary artery bypass grafting (CABG). Because SWCs are the main concern when using BIMA grafts, we evaluate the incidence of these complications with this technique and attempt to define the perioperative risk factors for their occurrence.
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PATIENTS AND METHODS
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We analyzed prospectively collected data on 751 consecutive patients who underwent CABG using the BIMAs from September 1994 to August 2002 performed under the guidance of a single surgeon (HSP). The mean age of the patients was 56 years (range, 27 to 79 years), with 44 (6%) of them above 66 years. Among the patients, 633 (84%) were male, 170 (23%) had diabetes, and 57 (8%) underwent concomitant operations. The average number of grafts was 4.0 per patient.
Bypass conduits and configurations were selected according to the following protocol. First, it was decided to use 2 conduits for the management of multivessel disease. Second, total arterial coronary revascularization was decided for patients below 66 years of age, for those with unsuitable long saphenous veins, or for those requesting arterial grafting. For arterial grafting, the left IMA was the primary conduit and the right IMA the secondary conduit when it was of sufficient length. This protocol has been previously described.11 The majority of the selected patients underwent BIMA T-graft construction for triple-vessel disease. The right IMA was used as a free graft in all but 8 patients, with proximal anastomosis to the aorta for right coronary revascularization or to the left IMA for circumflex revascularization, usually by sequential grafting from the right coronary system. Comorbidity was not considered a contraindication, except with the use of systemic steroids or hemodynamic instability during emergency procedures.
Surgery was performed through a median sternotomy with cardiopulmonary bypass established through right atrial and ascending aortic cannulation. Intermittent cold cardioplegia was used in the initial 335 patients and tepid blood cardioplegia thereafter. An intravenous second-generation cephalosporin antibiotic was administered intraoperatively and for 48 hours postoperatively for prophylaxis against infection.
Following sternotomy, the IMAs were exposed using a sternal retractor, and the right IMA was harvested first. The parietal pleura was brushed off the endothoracic fascia over the IMAs. A single diathermy incision in the endothoracic fascia was made immediately medial to the vascular bundle and the fascia reflected to expose the IMAs and venae comitantes. Using the veins for retraction, the vascular bundle was detached from the chest wall by applying diathermy in the intercostal spaces. The intercostal fat pads were harvested with the IMAs as this is the natural plane of dissection, reducing the need for dissection and diathermy. Following full mobilization, the IMAs were divided distally, and also proximally in the case of the right IMA. This technique has been described in detail by Horii and Suma.10 Following IMA harvesting, a left extrapericardial fat pad was mobilized for positioning behind the sternum at the end of the procedure.12
SWCs included the following types. Sternal infection (suppurative sternitis) was diagnosed when there was purulent fluid associated with the sternal halves and all the sternal wires and when a second surgical procedure was performed, involving removal of the wires with debridement and closure. This category was further divided into 2 groups: sternal infection with mediastinitis, and sternal infection alone. In patients without mediastinitis, there was minimal sternal dehiscence and the retrosternal fat pad was adherent to the back of the sternum. Mechanical dehiscence was considered present when sternal instability developed without evidence of infection. Superficial sternal wound infection was defined as the presence of subcutaneous inflammation that prompted antibiotic use, with or without superficial wound dehiscence.
Sternal infection with mediastinitis was treated with debridement followed by immediate closure using pectoralis major muscles and the greater omentum. Sternal infection present alone was treated with debridement followed by immediate closure using only pectoralis major muscles. The goal of reoperation for sternal infection was to debride all infected and avascular tissue and to obliterate dead space. Early mechanical dehiscence and late sternal non-union were treated with direct rewiring. Superficial sternal wound infection was treated conservatively with antibiotics. Patients were routinely followed-up 5 weeks postoperatively.
We chose 29 definable demographic, clinical, and operative variables for univariate and multivariate analyses with SWCs as the end point. Preoperative variables tested included the following: age (< or 
65 years), sex, diabetes (type II), symptom class, angina status, hypertension, cholesterol 6.5 mmol·L–1, history of smoking, chronic obstructive pulmonary disease (COPD), previous stroke, peripheral vascular disease (PVD, including carotid disease), chronic renal failure (abnormally high serum creatinine level), prior myocardial infarction (< 1 week earlier), left ventricular ejection fraction ( or < 30%), and redo operation. Operative variables included the following: surgeon (consultant or registrar), type of operation (isolated CABG or combined procedure), urgency of surgery, and number of anastomoses. Postoperative variables included the following: inotropic support, intraaortic balloon pumping (IABP), open sternotomy in the intensive care unit (delayed sternal closure), sternal reopening in the intensive care unit, reexploration for bleeding, reexploration for low cardiac output, myocardial infarction, stroke, dialysis, and pulmonary complications.
All statistical analyses were performed using SPSS 10 software (SPSS, Inc., Chicago, IL, USA). The degree of association between the presence of SWCs and the possible explanatory covariates was assessed by logistic regression analysis. Both univariate and multivariate models were fitted. The independent predictors of SWCs were identified by backward conditional elimination. Odds ratios and 95% confidence intervals are given. A significance level of 5% was used throughout.
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RESULTS
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SWCs occurred in 44 (5.9%) patients. Sternal infection occurred in 22 (2.9%) patients, of which 15 (2.0%) had sternal infection with mediastinitis (5 following initial rewiring for mechanical dehiscence) and 7 (0.9%) had sternal infection alone. Twelve (1.6%) patients had mechanical sternal dehiscence, 13 (1.7%) had superficial sternal wound infection, and 2 (0.3%) were detected with late sternal non-union at the 5th-week follow-up.
The early (perioperative) mortality rate was 1.5% (11 of 751 patients). Mortality among patients with SWCs was 4.5% (2 of 44 patients), compared to 1.3% (9 of 707 patients) among those without SWCs ( p = 0.08). Other perioperative complications included stroke in 10 (1.3%) patients, myocardial infarction in 13 (1.7%) patients, and reopening for bleeding in 24 (3.2%) patients. In addition, 14 (1.9%) patients required IABP and 75 (10%) received inotropic support. Out of 34 patients with COPD, 4 developed pulmonary complications postoperatively and 1 had sternal infection.
Multivariate analysis indicates that PVD, diabetes, delayed sternal closure, postoperative stroke, and postoperative pulmonary complications are independent predictors of SWCs (Table 1
). Diet-controlled diabetes was a risk factor for any SWC, but diabetes treated by oral hypoglycemics or insulin failed to reach significance. On the other hand, diabetes was an independent risk factor for sternal infection with no significant difference in the adjusted odds of infection between these 3 treatment groups (odds ratios: insulin, 3.3; oral hypoglycemics, 1.9; diet-controlled, 6.0). There were 20 diabetic patients who were over the age of 65 years; none of them developed SWCs.
The incidence of SWCs and sternal infection in the patients who required IABP was 28% and 14%, respectively, and in those who needed inotropes it was 10% and 5%. However, when subjected to multivariate analysis, these factors did not reach statistical significance.
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DISCUSSION
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One of the major concerns regarding the use of BIMA conduits in CABG operations is the suspected higher incidence of SWCs and associated increased postoperative morbidity and mortality. However, Lytle and colleagues1 showed that perioperative morbidity and mortality were essentially similar using either SIMA or BIMA grafts and that in-hospital mortality was 0.7% for both groups, but survival for the BIMA group was higher than for the SIMA group at 5, 10, and 15 years. Buxton and colleagues2 demonstrated that increasing the use of BIMA conduits reduced overall postoperative complications.
The surgical techniques for harvesting the IMA can be divided into 3 types (Figure 1). In most institutions, the IMA is isolated from the chest wall as a wide pedicle, together with accompanying veins, adjacent endothoracic fascia, and some distal transversus thoracis muscle. Harvesting is relatively quick because electrocautery is usually employed for dividing branches. However, diathermy may further damage the blood supply to the sternum, impeding sternal healing and exposing the sternum to the risk of early dehiscence and infection, particularly when both IMAs are harvested. It has been reported that this risk is higher in elderly patients and patients with diabetes.3,5
Some centers have harvested the IMA as a fully skeletonized conduit. Vineberg13 used the fully skeletonized IMA in the 1960s; others followed and even used it routinely.7,8,14 Although the full skeletonization technique has advantages, it has not been widely used in daily practice because it is more time-consuming and technically more complex than the pedicle technique. In particular, caution is needed to avoid trauma to the naked IMA.10
The semi-skeletonizing technique has several advantages over both the above techniques. The semi-skeletonized IMA is longer than the pedicled IMA, thus providing greater versatility in arterial myocardial revascularization without the use of vein grafts, and it has a lower risk of damage than the fully skeletonized IMA. Minimizing the trauma associated with harvesting may speed up healing and reduce SWCs. In our series, the occurrence of sternal infection was 2.9%, with 2.0% having associated mediastinal infection. This is comparable to the results of Galbut and coworkers,7 who harvested the BIMAs in a fully skeletonized fashion and reported an incidence of sternal infection of only 1.5% (in 1,087 patients), and to the results of Lytle and colleagues1 at 2.5% using pedicled IMAs. It is of note that Calafiore and colleagues6 observed a reduction in sternal infection from 4.6% to 1.7% after changing from pedicle to fully skeletonized harvesting (Table 2).
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Table 2. Comparison of Reported Rates of Sternal Wound Complications After Bilateral Internal Mammary Artery Grafting and Independent Risk Factors
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Apart from the surgical technique, other factors such as diabetes mellitus, obesity, COPD, emergency procedure, prolonged operative time, aortic crossclamp time, prolonged mechanical ventilation, and the use of SIMA or BIMA grafts have been reported as independent risk factors for SWCs (Table 2
). However, among the variables tested, we found only diabetes, PVD, delayed sternal closure, postoperative pulmonary complications, and postoperative stroke increasing the risk of SWCs.
Adult-onset diabetes is classified according to treatment in an attempt to indicate severity. It is generally considered that insulin-treated patients are at greater risk of complications because of the implied severity of the disease. However, in this study, diet-controlled diabetics had the highest incidence of both SWCs and sternal infection. Aggressive management of diabetes in the postoperative period has been shown to reduce the incidence of sternal infection.15 It is likely that the diet-controlled diabetic patients in this study received less aggressive management, despite applying to all patients the routine protocol for the management of hyperglycemia. The age of onset of diabetes is also an indicator of severity, and it is of note that no SWCs developed in the diabetic patients in this study who were over 65 years of age. Despite the greater risk of SWCs in diabetic patients, it is felt that the benefits provided by the use of BIMA grafts outweigh this risk.16
PVD is a marker of the severity of vascular disease and is associated with worse postoperative outcomes with respect to morbidity and early and late survival. Patients with PVD may also have impaired sternal blood supply and thus carry a higher risk for postoperative SWCs.
Pulmonary complications were defined in this study as conditions that require prolonged or reinstituted mechanical ventilatory support for pulmonary dysfunction, or the use of antibiotics to treat established bronchial or pneumonic infection. It is possible that the increased subclinical sternal mobility that these patients experience in association with respiratory complications and additional physiotherapy contributes to the risk of SWCs. COPD has been reported to be an independent risk factor for SWCs.8 Among the 34 patients with COPD in this study, only 1 developed sternal infection and only 4 developed pulmonary complications. Many of these patients received additional oral antibiotic prophylaxis, which possibly reduced the incidence and severity of pulmonary complications with a resultant reduction in sternal infection. Nasopharyngeal cultures have been correlated with organisms from the infected mediastinum.17
The number of patients who had delayed sternal closure or who suffered postoperative stroke in this study was small. Although both factors are found to be significant predictors of SWCs, it is likely that confounding factors, such as an aberrant sternotomy or a neurological deficit due to low cardiac output, skewed the results. Prolonged low cardiac output has been shown to be a risk factor for SWCs.18 However, we did not observe significant association between the use of IABP or inotropic support and SWCs.
In conclusion, our experience with the use of bilateral semi-skeletonized IMA conduits suggests that the rate of SWCs is comparable to those of other harvesting techniques, but this technique provides the advantage of efficiency of harvesting along with the versatility of extra length. Diabetes, PVD, and pulmonary complications raise the risk of SWCs.
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REFERENCES
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- Loop FD, Lytle BW, Cosgrove DM, Mahfood S, McHenry MC, Goormastic M, et al. J. Maxwell Chamberlain memorial paper. Sternal wound complications after isolated coronary artery bypass grafting: early and late mortality, morbidity, and cost of care. Ann Thorac Surg
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ABSTRACT
INTRODUCTION
