Introduction
The left internal mammary or thoracic artery (LIMA or LITA) to left anterior descending (LAD) anastomosis has been demonstrated to improve survival and freedom from adverse cardiac events with superior patency rates as compared to saphenous vein grafts (SVGs) . The impaired graft patency rates of SVGs (50%–60% after 10 years) led to the utilization of the right IMA graft as a second conduit in coronary artery bypass grafting (CABG) procedures . Despite accumulating evidence from several large nonrandomized risk-adjusted observational studies and metaanalyses supporting the use of bilateral IMA (BIMA) grafts in CABG surgery, utilization remains fairly low worldwide as the merits of this technique are still debated. In addition to the added technical complexity of BIMA grafting, concerns of increased superficial or deep sternal wound infections (SSWIs or DSWIs) with associated increased morbidity and mortality contribute to surgeons’ reluctance to adopt more BIMA grafting .
This chapter will analyze patient populations at high risk for developing DSWIs and highlight perioperative techniques to avoid sternal complications with the utilization of BIMA grafting.
Sternal anatomy and blood supply
Knowledge of the anatomy of the sternum and its blood supply is crucial to understanding the etiology and prevention of sternal complications following ITA harvesting. The main source of sternal arterial blood supply derives from sternal branches in the intercostal spaces originating from the IMAs.
The IMA descends 1–2 cm from the lateral margin of the sternum vertically to terminate at the level of the sixth intercostal space as a bi- or trifurcation to the superior epigastric artery, musculophrenic artery, and/or a branch to the xiphoid process. The IMAs provide arterial blood supply to the chest wall and the sternum, abdominal wall, diaphragm, pericardium, phrenic nerve, pectoralis major muscle, and the mammary gland.
Branches of the IMAs and both anterior and posterior intercostal arteries are sources of blood supply to the sternum by feeding a complex periosteal vascular plexus system on the anterior and posterior surface of the sternum . The sternal branches of the IMAs are the major source of sternal blood supply and arise mainly into the upper intercostal spaces (topographically corresponding to the sternal ossification zones), while fewer sternal branches arise into the lower part of the sternum forming an arcade-like pattern of blood supply with one or more upper or lower sternal branches or with perforating branches of the IMAs . Berdajs et al. identified up to eight different patterns of arcade arrangement of sternal arteries based on the origin of the branches from the IMAs four morphological variants of collateral blood supply of the sternum .
In type I the sternal, perforating, and intercostal branches arise from the common main stem. In type II the sternal branches arise independently from the medial aspect of the IMA, and the perforating and intercostal branches arise from a common branch. In type III the sternal and perforating arteries arise from a common stem while the intercostal branches arise directly from the IMA. In type IV, all branches arise directly from the IMA ( Fig. 15.1 ).
The preservation of the branches of the IMAs during harvesting of IMA grafts is crucial for the development of collateral blood supply to the sternum. As such, it is of utmost importance to divide side branches of the IMAs as close as possible to the IMA trunk to avoid harm to the distally originating branches feeding the collateral blood supply of the sternum .
If the branches are divided close to the IMA trunk, anterior intercostal branches in type I and II patterns anastomosing with posterior intercostal arteries (originating from the descending thoracic aorta) may provide sufficient collateral blood supply to the sternum.
It has been shown in several studies that single or BIMA harvesting leads to impaired blood flow to the sternum and significantly influences sternal bone and wound healing . However, reduced sternal blood flow after IMA harvesting is not only dependent on the number of IMA side branches harvested but also on the technique utilized, the patient’s pattern of sternal blood supply as well as the techniques used to close the sternum. It is also important to remember that sternal wound complications are also dependent on patient comorbidities and perioperative factors.
Internal mammary artery harvesting techniques
Pedicled internal mammary artery harvest
With the traditional pedicled harvesting technique, the IMA is harvested along with its accompanying veins, the endothoracic fascia, and surrounding connective fatty and muscular tissue ( Fig. 15.2 ). Side branches are clipped and divided by electrocautery or sharp dissection.
Semiskeletonized internal mammary artery harvest
The semiskeletonized harvesting technique is carried out in a similar fashion as the pedicled harvesting technique. However, this pedicle comprises only the IMA and the accompanying veins ( Fig. 15.3 ).
Skeletonized internal mammary artery harvest
With the skeletonized harvesting technique, only the artery is harvested while preserving accompanying structures ( Fig. 15.4 ). The absence of the endothoracic fascia and surrounding connective tissue increases the length of the harvested mammary graft . However, the artery is at a somewhat higher risk for damage and injury, and the technique requires meticulous attention during harvest and is technically more demanding and time-consuming than the pedicled harvesting technique. Furthermore, the artery may be subjected to increased manipulation and electrocautery-induced thermal damage during dissection.
Deep sternal wound infection after pedicled versus skeletonized bilateral internal mammary artery harvesting
Several studies, mostly nonrandomized, retrospective, and observational, have sought to determine whether skeletonized BIMA harvesting is superior to pedicled grafting with regards to sternal wound infections. It is also noteworthy that glycemic control as proposed by the STS guidelines and the American Association for Thoracic Surgery (AATS) expert consensus review for the treatment and prevention of sternal wound infections were largely disregarded or only partially reported in the majority of studies .
Peterson and colleagues investigated the prevalence of DSWI in their subgroup of diabetic patients with skeletonized and pedicled BIMA harvesting . The occurrence of DSWIs was significantly lower in patients with skeletonized harvesting compared with patients who received pedicled BIMA grafting (1.3% vs 11.1%, P =.03). Furthermore, there was no significant difference in the prevalence of DSWI between diabetic patients receiving skeletonized IMAs and patients without diabetes with pedicled IMA harvesting ( n =578) (1.2% vs 1.6%, P =.08) . Benedetto et al. observed in a post hoc analysis of 2056 patients enrolled in the Arterial Revascularization Trial that pedicled BIMA [odds ratio (OR), 1.80; 95% confidence interval (CI), 1.23–2.63] but not skeletonized BIMA (OR, 1.00; 95% CI, 0.65–1.53) or skeletonized single IMA (OR, 0.89; 95% CI, 0.57–1.38) was associated with a significantly increased risk of any sternal wound complications compared with a pedicled single IMA . When restricted to severe sternal wound complications, no statistically significant difference was observed between pedicled and skeletonized BIMA harvesting. However, the risk of bias cannot be ruled out and no randomization was applied in this post hoc analysis. Furthermore, the study was underpowered to detect differences in severe sternal wound complications among the groups. De Paulis and colleagues demonstrated no increased risk of SSWI and DSWI in patients with pedicled BIMA harvesting compared to BIMA harvesting in a skeletonized fashion (4.7% vs 3.3%; P =.4) . However, in diabetic patients, the incidence of DSWI was significantly higher in patients with pedicled BIMA harvesting compared to skeletonized BIMA harvesting (12.5%; P <.01). Multivariable regression analyses revealed BIMA harvesting in a pedicled fashion (OR, 4.1; 95% CI, 1.4–12.1) as an independent risk factor for DSWI.
Several metaanalyses investigated the impact of the technique used in BIMA harvesting on the incidence of DSWI. Saso et al. observed fewer sternal wound infections in patients receiving skeletonized IMA grafts (OR, 0.41; 95% CI, 0.26–0.64) . The reduction of sternal wound infections was more pronounced in diabetic patients undergoing skeletonized BIMA harvesting (from 14.2% to 2.4%). Sá and colleagues confirmed these findings in their metaanalyses with a larger pooled sample size of 4817 and 2633 patients, respectively . In a large metaanalysis comparing 122,465 diabetic patients receiving single IMA grafting with 3770 diabetic patients receiving BIMA grafting, Deo et al. observed an increased incidence of DSWI with BIMA grafting compared to single IMA grafting (3.1% vs 1.6%; P <.05) . However, when restricted to skeletonized IMA harvesting techniques, no significant difference in DSWI was observed between patients receiving skeletonized BIMA versus skeletonized single IMA grafts (risk ratio 0.94, 95% CI: 0.42–2.09).
Although the majority of studies and metaanalyses investigating the effect of skeletonized versus pedicled BIMA harvesting on sternal wound infections seem to be in favor of the skeletonized technique, a considerable number of groups reported favorable outcomes with the pedicled technique. Using a modified pedicled BIMA harvesting technique with sparing of the communicating distal bifurcation of the IMA to the chest wall and preservation of the pericardiacophrenic artery branch, Sajja et al. reported a low incidence of DSWI involving 3072 diabetic and nondiabetic patients with single IMA and BIMA grafting . No statistically significant difference with regards to DSWI was observed in diabetic patients with BIMA and single IMA grafting as well as in nondiabetic patients with BIMA and single IMA grafting (0.55% vs 0.48% vs 0.62% vs 0.82%; P =.84) . Furthermore, multivariate analysis failed to show BIMA harvesting ( P =.89) and diabetes mellitus ( P =.96) as predictors of sternal wound infection.
Ura et al. reported on the incidence of mediastinitis in high-risk patients with diabetes, old age, and on dialysis in the setting of pedicled BIMA grafting . With a meticulous harvesting technique by avoiding excessive injury to the periosteum and cartilage along with a pinpoint hemostasis technique performed by an experienced team, an early mediastinitis rate as low as 1.3% was achieved in patients with diabetes. In summary, the factors responsible for DSWI following IMA harvesting are multifactorial and are not always related to the harvesting technique. Patients, who are obese, have chronic obstructive pulmonary disease (COPD), and poorly controlled diabetes, have the greatest risk for developing DSWI following single IMA and BIMA grafting, regardless of the harvesting technique. Nevertheless, in this group of high-risk patients, the current evidence would favor using a skeletonized technique for either IMA or BIMA grafting if an IMA graft is used.
Preoperative interventions to avoid sternal complications
Screening and treatment for nasal carriers of Staphylococcus , skin antiseptic preparation, and administration of prophylactic antibiotics
Expert consensus statements from both, the AATS and the European Association for Cardio-Thoracic Surgery (EACTS), recommend preoperative screening for nasal carriers of Staphylococcus species and that mupirocin should be administered in all detected carriers .
The patient’s skin flora represents the main source of endogenous pathogens for surgical site infections (SSIs). Preoperative skin antiseptic preparation by bathing with antiseptic agents such as chlorhexidine gluconate or povidone-iodine reduces the bacterial count of the skin thus reducing the risk of bacterial contamination of the wound . The preventive effect on SSIs, however, remains inconclusive and controversial .
Another intervention for the prevention of SSI for sternal wound infections is the prophylactic perioperative administration of antibiotics. Its benefits have been extensively proven in patients undergoing cardiac surgery (Class I, Level of Evidence A) . Beta-lactam antibiotics are the standard agents for prophylaxis in cardiac surgery patients; however, in patients with a known penicillin allergy, vancomycin with an additional antimicrobial agent with Gram-negative coverage should be used. Vancomycin should also be used for surgical prophylaxis in populations with a high incidence of MRSA (Class I, Level of Evidence A). The STS Practice Guidelines on antibiotic prophylaxis recommend postoperative prophylactic antibiotics to be given for 48 hours or less (Class IIa, Level of Evidence B) . In general, whenever possible, all remote infections should be treated prior to cardiac surgery .
Preoperative hair removal of the surgical site is often performed the night before the operation or immediately before sterile draping of the patient. Controversy also exists as to which method is preferable—clipping, shaving, or chemical depilation—and as to whether to perform depilation at all. A network metaanalysis of 19 randomized controlled trials confirmed the absence of any benefit of depilation to prevent SSI, and the higher risk of SSI when shaving is used for depilation .
Smoking and chronic pulmonary disease
Chronic pulmonary disease and tobacco use are known risk factors in cardiac surgery and are associated with an increased risk for the development of DSWI .
Smoking is known for its detrimental effects on primary wound healing mechanisms. A large systematic review and metaanalysis of studies, including 479,150 patients, revealed that 4–8 weeks of preoperative smoking cessation significantly reduced SSIs in all surgical specialties . Sustained smoking postoperatively has been identified in a multivariate analysis as a risk factor for the development of DSWI and/or sternal dehiscence .
The risk of coughing and subsequent development of sternal instability in patients with severe COPD and smokers increases the risk of DSWI. Preoperative optimization of pulmonary function and aggressive pulmonary toilet can mitigate these effects.
In a study conducted by Hegazy et al. the outcomes of BIMA grafting in patients with COPD were analyzed . No significant differences were observed in the incidence of DSWI in patients with COPD compared to patients without COPD. The decision to perform BIMA grafting in patients with COPD should be made not only on the basis of the severity of the COPD but should also take into account other comorbidities such as obesity and diabetes, as discussed in the following section.
Obesity and glycemic control
Obese or morbidly obese patients are commonly considered at high risk for sternal wound complications . However, several studies failed to link obesity to an increased incidence of DSWI in the setting of BIMA grafting . Itagaki et al. investigated the impact of BIMA grafting on early outcomes of 1,526,360 patients from the Nationwide Inpatient Sample database who underwent isolated coronary artery bypass grafting with at least one internal mammary artery . While BIMA grafting was not identified as an independent predictor of DSWI (OR 1.03; 95% CI, 0.96–1.10), a higher incidence of DSWI in patients with diabetes mellitus (OR 1.90; 95% CI, 1.51–2.41) was observed with BIMA grafting. Diabetes mellitus has been shown to be an independent risk factor associated with reduced short- and long-term survival and a two- to fivefold increase in postoperative surgical wound infections in patients undergoing coronary artery bypass grafting surgery . Patients without diagnosed diabetes but perioperative high blood glucose levels (>250 mg/dL) also experience increased morbidity and mortality . Accordingly, the risk of DSWI is increased in these patient populations, and preoperative optimization of HbA1c levels and perioperative blood glucose levels have decreased the incidence of DSWIs in these patients . In a prospective study of 2467 consecutive diabetic patients undergoing open-heart surgery; Furnary et al. investigated the effect of sliding scale–guided intermittent subcutaneous insulin injections versus a continuous intravenous insulin infusion targeted to achieve blood glucose levels of less than 200 mg/dL on the risk of developing DSWIs . Continuous intravenous insulin infusion significantly reduced blood glucose levels and the incidence of DSWI as compared to intermittent subcutaneous injections (0.8% vs 2.0%; P =.01). Current STS and EACTS guidelines on perioperative medication in adult cardiac surgery state that tight glycemic control (<180 mg/dL) should be maintained intraoperatively during the first 24 hours postop and longer if extended periods of ICU care are required (Class I, Level of Evidence B) .
In their report on the incidence of DSWI in a cohort of 1001 consecutive CABG patients (73% with BIMA grafting), Kieser et al. demonstrated that the implementation of sequential infection prevention measures reduced their incidence of DSWI to 0% (irrespective of skeletonized or pedicled harvesting) . Obese, diabetic females had a 10-fold greater risk for DSWI than all other patients (21.4% vs 2.0%). Excluding these high-risk patients from BIMA grafting resulted in a zero incidence of DSWIs. Therefore caution should be exercised before subjecting obese, diabetic female patients to BIMA grafting.
Malnutrition
Malnourished and cachectic patients undergoing cardiac surgery have higher morbidity and mortality and are more likely to develop DSWI . Serum albumin is considered as a surrogate marker for the nutritional status of patients .
Engelman and colleagues identified a preoperative serum albumin level <2.5 g/mL to be associated independently with both increased mortality and incidence of sternal wound infections .
Preoperative optimization of nutritional status, as well as weight loss, in morbidly obese patients prior to undergoing cardiac surgery is desirable and should be instituted whenever possible in elective cases.
Intraoperative strategies to avoid sternal complications
Strict adherence to routine hygiene protocols and compliance with sterility principles are mandatory to avoid the risk of wound contamination with exogenous pathogens during an operation. Maintenance of laminar airflow ventilation throughout the procedure with little to no interruptions is necessary to reduce airborne microbial contamination of the surgical field.
Midline sternotomy
Attention to detail should be paid during the sternotomy to avoid any shift from the midline of the sternum. Paramedian sternotomies are prone to develop dehiscence and, ultimately, result in sternal wound infections .
Bone wax and topical antibiotics
Routine application of bone wax to the cut edges of the sternum as a means of hemostasis should be avoided (Class III, Level of Evidence B) . By creating a physical barrier, bone wax interferes with subsequent bone healing, induces chronic inflammation, and increases the risk of infection .
The recent AATS guidelines on prevention and management of sternal wound infections recommend the application of topical antibiotics to the cut edges of the sternum on opening and before closing all cardiac surgical procedures involving a sternotomy (Class I, Level of Evidence B) . Lazar and colleagues applied 2.5 g powdered vancomycin diluted in 2 mL of normal saline as a slurry to both edges of the sternum in 1075 consecutive patients from December 2007 to August 2013 and compared these patients with a cohort of patients ( n =2190) from December 2003 to November 2007 without topical vancomycin application . Along with perioperative antibiotics and tight glycemic control, this resulted in the elimination of any type of sternal wound infections in both nondiabetic and all types of diabetic patients. A recent metaanalysis confirmed these favorable outcomes of topical vancomycin .
The application of gentamicin–collagen sponges between the sternal halves has been investigated in a large, multicenter, randomized trial after previous studies reported a significant decrease in the incidence of sternal wound infections . The primary analysis revealed no significant difference in sternal wound infection in patients randomized to the gentamicin–collagen sponge group (8.4%) compared with patients randomized to the control group (8.7%) ( P =.83) . However, this observation was attributed to failure to follow the manufacturers-advised sponge implantation protocol in which the longer exposure of the gentamicin sponge to saline solution resulted in a washout of the active ingredient. Two other metaanalyses of randomized trials observed a beneficial effect of gentamicin–collagen sponge application on the incidence of DSWI .
Electrosurgical devices
Traditionally, low-strength, high-frequency conventional electrocautery is applied during pedicled and skeletonized mammary harvesting. A meticulous harvesting technique is a prerequisite to avoid damage and injury to adjacent structures. An alternative energy device, the ultrasonic scalpel (Harmonic Scalpel; Ethicon Endo-Surgery, CVD, Cincinnati, Ohio), has been the subject of several observational studies involving IMA harvesting . Kieser et al. observed less damage to the IMA with faster IMA harvesting using skeletonized techniques . The incidence of DSWI was not increased when compared to patients with conventional electrocautery IMA harvesting. Recently, the initial experience with a new electrosurgical device, the PEAK PlasmaBlade (Medtronic Advanced Energy, Portsmouth, New Hampshire, United States), for mammary harvesting was reported. The PlasmaBlade is a monopolar electrosurgical device that uses pulsed radiofrequency energy to generate a plasma-mediated discharge along an insulated electrode, creating a cutting edge while the blade stays near body temperature . Histological analysis demonstrated a more intact endothelial layer and a tendency to better wall integrity in IMAs with the use of PlasmaBlade. Whether this new electrosurgical device with lower operating temperatures and less thermal damage translates to decreased sternal wound infections remains to be seen.
Sternal closure technique
Achieving sternal stability is a prerequisite for adequate bone healing and avoiding the development of sternal wound infections. Traditionally, the parasternal or transsternal single wiring and figure-of-eight wiring techniques have been utilized as the primary sternal closure techniques after cardiac surgery with a sternotomy. Numerous studies evaluated the superiority of either closure techniques, with mixed results . However, the figure-of-eight technique seems to be favorable in high-risk patients (i.e., chronic pulmonary disease, obesity, BIMA harvesting, diabetes, and paramedian sternotomy) with fragile sternums . In patients with multiple fractures the Robicsek weave technique decreases the incidence of sternal dehiscence and infections ( Fig. 15.5 ).
