Background: Measurement of short-term fractal-like correlation properties of heart rate dynamics has been shown to be a useful prognostic indicator of adverse events in cardiac patients. Complexity measurements of heart rate variability (HRV) have already provided important information in many cardiac conditions. However, data on the physiological background of these newer nonlinear measures of HRV are limited.
Methods: Nine healthy subjects (aged from 22 to 35 years, 6 males, 3 females) had an electrocardiographic (ECG) recording during controlled breathing in supine position. HRV was analyzed for 5 min periods before and after intravenous injection of 0.6 mg of atropine using conventional HRV measures and newer nonlinear HRV measures including the short-term scaling exponent (alpha(1)) and approximate entropy (ApEn).
Results: The short-term scaling exponent alpha(1) increased significantly after atropine injection (1.01 +/- 0.23 vs 1.43 +/- 0.19, P = 0.001). There was no significant difference between ApEn values before and after atropine injection (0.87 +/- 0.17 vs 0.70 +/- 0.31, respectively, P = 0.27). At baseline before atropine administration, alpha(1) had a significant negative correlation with SDNN, RMSSD, and HF (r = -0.70, -0.76, -0.67, respectively, P < 0.05 for all), and a significant positive correlation with heart rate (r = 0.76, P < 0.05). After atropine injection, alpha(1) did not have significant correlation with any of the HRV parameters or heart rate. There were no significant correlations between ApEn and any of the HRV measures or heart rate either before or after atropine administration.
Conclusions: Vagal tone has an important influence on the values of the short-term scaling exponent alpha(1). However, vagal modulation is not a major determinant of the values of ApEn.