Is there a glycemic threshold for impaired autonomic control?

OBJECTIVE Although hyperglycemia has been recognized as a predictor of cardiovascular autonomic neuropathy in diabetic patients, the glucose threshold at which autonomic control begins to become impaired has not been evaluated. This study examined whether fasting plasma glucose (FPG) or fasting plasma insulin (FPI) is associated with reductions in baroreflex sensitivity (BRS) in healthy volunteers. RESEARCH DESIGN AND METHODS FPG and FPI were measured after an overnight fast in 162 healthy volunteers (91 men, 71 women) who were 25-44 years of age. BRS was measured with power spectral analysis. RESULTS Univariate analyses showed that FPG was negatively correlated with BRS (r = -0.25, P < or = 0.001) with significant reductions observed in volunteers with FPG in the upper 2 quintiles (i.e., 93-124 mg/dl). However, after adjustment for other predictors of BRS (e.g., age, blood pressure, and BMI), the relationship between FPG and BRS was no longer significant. In contrast, FPI was negatively correlated with BRS in univariate analyses (r = -0.32, P < 0.0001) as well as after covariate adjustment, with close to a 50% reduction in BRS observed in the volunteers with insulin values in the highest quintile (i.e., 16-36 microU/ml). CONCLUSIONS These findings suggest that high normal levels of FPG are associated with reduced autonomic control secondary to the effects of aging, obesity, and elevated blood pressure on FPG levels and that elevations in FPI are associated with substantial reductions in autonomic cardiac control independent of other covariates.