Role of Coronary Artery Bypass Surgery in Diabetes and Perioperative Glucose Management

Risk Profiles and Comorbidities

Patients with diabetes undergoing CABG surgery have an increased incidence of associated comorbidities, including chronic renal failure, peripheral vascular disease, reduced ejection fraction (EF), congestive heart failure (CHF), cardiomyopathy, hypertension, and previous myocardial infarctions (MIs), compared with nondiabetic patients. It is therefore imperative that a thorough preoperative evaluation be performed before CABG surgery in these patients in an attempt to minimize postoperative morbidity and mortality.

In the presence of stable symptoms, surgery should be delayed for 3 to 5 days following cardiac catheterization to avoid renal dysfunction caused by contrast nephropathy. Preoperative assessment with carotid ultrasound and ankle brachial indices helps to detect critical peripheral vascular lesions that may lead to strokes and lower-extremity ischemia and assists in determining whether patients are candidates for intra-aortic balloon pump (IABP) placement. Transthoracic echocardiography helps to detect global and regional wall motion abnormalities and underlying valvular disease that may need to be addressed at the time of surgery. In patients with a smoking history, pulmonary function studies are helpful to determine the need for preoperative bronchodilators and to optimize pulmonary toilet to avoid prolonged postoperative ventilation. However, despite adjustments for related comorbidities, diabetes is still a major independent risk factor for increased early and late mortality after CABG surgery.

Early Outcomes

Compared with nondiabetic patients, patients with diabetes who undergo CABG surgery have a higher perioperative mortality (3.2% to 3.7% versus 2.2% to 2.5%) and increased morbidity. They also have an increased incidence of sternal wound infections and mediastinitis, renal dysfunction necessitating replacement therapy, strokes, low cardiac output, and need for inotropic and IABP support, ^, all of which result in prolonged intensive care unit (ICU) and hospital stays.

Late Outcomes

Patients with diabetes also have poor long-term (5- to 10-year) survival compared with nondiabetic patients. This is especially true for diabetic patients who require insulin treatment. ^, In fact, some studies suggest that in contrast to diabetic patients receiving only oral agents, only diabetic patients requiring insulin have significantly worse long-term survival compared with nondiabetic patients. ^{, ,} Certain groups of diabetic patients appear to have worse long-term survival. Leavitt and coworkers found that patients with diabetes and concomitant peripheral vascular disease and renal failure had significantly worse 10-year survival compared with nondiabetic patients and with diabetic patients without these comorbidities.

Long-term outcomes for diabetic patients with reduced left ventricular (LV) function (EF below 40%) has varied. Trachiotis and coworkers in their review of 11,830 CABG patients found that in patients with an EF below 35%, diabetic patients had a 59% increase in the risk of long-term mortality ( P < 0.0001). Whang and coworkers found that in diabetic patients with an EF below 36% there was a 44% higher risk for rehospitalization for any cause ( P = 0.0001) and a 24% higher risk of readmission for cardiac issues ( P < 0.05) over a 6-year period. Others have found no difference in long-term mortality in diabetic patients with reduced EF. ^, This difference in outcomes may be a result of differences in myocardial viability in the patients who were studied. Patients with diabetic cardiomyopathy and reduced EF from longstanding hypertension and poor glycemic control may ultimately develop hypertrophy and fibrosis that leads to diastolic and systolic dysfunction. Diabetic patients with reduced but viable myocardium may derive a much larger benefit from CABG surgery than patients with cardiomyopathies with fibrotic and nonviable muscle.

Graft Patency

As noted in the preceding sections, patients with diabetes have less freedom from recurrent angina and the need for recurrent revascularization procedures. A major determinant for recurrent ischemic events after CABG surgery is graft patency. Diabetes was shown in earlier studies such as the Coronary Artery Surgery Study (CASS) to be an independent predictor of decreased graft patency. ^, In 2008, Singh and coworkers reported the impact of diabetes on graft patency after CABG surgery in 440 patients (115 with diabetes) followed for 1 year. Multivariable regression analyses found that diabetes was an independent predictor of 1-year graft occlusion (14.4% versus 9.7%; P = 0.03). The large majority of these conduits were saphenous veins. Radial artery grafts have also been found to have more spasm and a higher incidence of short-term occlusion in diabetic patients.

The internal mammary artery (IMA) has the highest graft patency of any CABG conduit and is especially important for diabetic patients. Hirotani and coworkers found that diabetic patients who had at least one patent IMA graft had significantly better overall survival and improved cardiac event–free survival compared with saphenous vein grafts alone. In an attempt to improve overall graft patency in diabetic CABG patients, it has been suggested that bilateral IMAs be routinely used for revascularization. Concerns have been raised regarding the possibility of higher rates of sternal dehiscence and mediastinal infections when bilateral IMAs are used in diabetic patients. In an attempt to define the benefits and risks of bilateral IMAs, Endo and coworkers studied outcomes of bilateral versus unilateral IMA revascularization in patients with diabetes. The cohort consisted of 1131 patients, 467 (41.3%) of whom had type 2 diabetes. In this group, 277 received a single IMA and 190 patients had bilateral IMAs. The hospital mortality and rate of deep sternal infections was similar between single and bilateral IMAs. There was no difference in long-term survival between the groups. However, in patients with preserved EF, 10-year survival (87.8 versus 75.2%; P = 0.04) as well as freedom from repeat CABG or MI (86.6% versus 69.05%; P = 0.0086) were better with bilateral IMAs. There was, however, no survival benefit in those patients with a reduced EF (below 40%). This study implies that diabetic patients with reduced EF and those with comorbidities that accompany reduced EF (e.g., peripheral vascular disease, renal failure) may not benefit from bilateral IMAs. Dissecting only the artery (skeletonization) of the IMA pedicle may decrease the incidence of sternal complications associated with bilateral IMA harvesting. However, there are concerns that skeletonization of the IMA may result in altered endothelial function that may compromise graft patency. So far, this has not been reported, but more long-term follow-up is necessary before the benefits of this technique are established. Nevertheless, as noted from these studies, the use of at least one IMA is crucial to better long-term survival and freedom from recurrent angina in the diabetic CABG patient.