Nonlinear heart rate variability measures during the oral glucose tolerance test

Heart rate variability (HRV) exhibits a different response to different external stimuli (emotional, auditory, etc). In this paper, HRV irregularity and correlations in short time scales were assessed after a sudden increase in plasma glucose concentration (a stimulus of glucose), using approximate entropy (ApEn) and short-term fluctuation (α1) of detrended fluctuation analysis, respectively, in people with metabolic syndrome and healthy control subjects. 5-min RR interval time series were extracted from ECG recordings acquired at five timed intervals (0, 30,..., 120 min) of an oral glucose tolerance test. We have found that, in fasting and 30 min after the glucose stimulus, people with metabolic syndrome showed statistically significantly greater values of ApEn than control, an indicative of a more irregular HRV In addition, 60 min after the glucose stimulus, when plasma glucose concentration reaches its maximum value, HRV irregularity and short-term HRV correlation showed a reduction in people with metabolic syndrome. Further studies, using other nonlinear measures and a larger dataset, must be carried to characterize the response of HRV to the glucose stimulus.