Increased Glycemic Variability in Patients with Elevated Preoperative HbA1C Predicts Adverse Outcomes Following Coronary Artery Bypass Grafting Surgery

BACKGROUND: In the setting of protocolized glycemic control, the relationship between postoperative glycemic variability on major adverse events (MAEs) after cardiac surgery is unknown for patients with increased preoperative hemoglobin A1C (HbA1C >6.5%). In this study, we sought to establish (a) whether postoperative glycemic variability is associated with MAEs after CABG surgery and (b) whether preoperative HbA1C could identify patients at increased risk of postoperative glycemic variability. METHODS: Patients undergoing coronary artery bypass grafting with or without valvular surgery from January 2008 to May 2011 were enrolled in this prospective, single-center, observational cohort study. Demographic, intraoperative, and postoperative outcome data were obtained from institutional data collected for the Society of Thoracic Surgery (STS) database. The primary outcome, MAE was a composite of in-hospital death, myocardial infarction (MI), reoperations, sternal infection, cardiac tamponade, pneumonia, stroke, or renal failure. Glycemic variability in the postoperative period was assessed by the coefficient of variation (CV). CV was used as quartiles for the multivariate logistic regression. Variable selection in multivariable modeling was based on clinical and statistical significance and was performed in a hierarchical fashion. RESULTS: Of the 1461 patients enrolled, 9.8% had an MAE. Based on the established target of HbA1C <6.5% for the diagnosis of diabetes mellitus, we considered HbA1C as a binary variable (<6.5% and 6.5%) in our primary analysis. Multivariate logistic regression analyses for the preoperative variables only revealed that preoperative HbA1C (odds ratio [OR], 1.6; 95% confidence interval [CI], 1.1-2.3; P = 0.02), history of MI (OR, 1.9; 95% CI, 1.3-2.8; P = 0.001), and STS risk score per quartile (OR, 1.7; 95% CI, 1.4-2.1; P < 0.001) were associated with MAEs. When postoperative variables were included in the analyses, postoperative glycemic variability (CV per quartile) in the intensive care unit (OR, 1.3; 95% CI, 1.1-1.5; P = 0.03), mean glucose levels averaged over the first 4 postoperative hours (OR, 1.2; 95% CI, 1.0-1.4; P = 0.03), history of MI (OR, 1.8; 95% CI, 1.2-2.6; P = 0.004), and STS risk score per quartile (OR, 1.6; 95% CI, 1.3-2.0; P < 0.001) were associated with MAEs. Glycemic variability as assessed by CV was increased postoperatively in patients with preoperative HbA1C 6.5% (0.20 0.09 vs 0.16 +/- 0.07, P < 0.001). CONCLUSIONS: Postoperative glycemic variability is associated with MAEs after cardiac surgery. Glycemic variability is only measured when the patient leaves the intensive care unit, and there is no opportunity to intervene earlier. Preoperative HbA1C identifies risk for postoperative glycemic variability and may provide a more rational guide for targeting measures to reduce variability.