Effects of exercise and passive head-up tilt on fractal and complexity properties of heart rate dynamics

tk;1Passive head-up tilt and exercise result in specific changes in the spe ctral characteristics of heart rate (HR) variability as a result of reduced vagal and enhanced sympathetic outflow. Recently analytic methods based on nonlinear system theory have been developed to characterize the nonlinear features in HR dynamics. This study was designed to assess the changes in t he fractal and complexity measures of HR behavior during the passive head-u p tilt and during exercise. Fractal exponent (alpha (1)) and approximate en tropy (ApEn), measures of short-term correlation properties and overall com plexity of HR, respectively, along with spectral components of HR variabili ty were analyzed during a passive head-up tilt test (n = 10) and a low-inte nsity steady-state exercise (n = 20) in healthy subjects. We observed that alpha (1) increased during the tilt test (from 0.85 +/- 0.22 to 1.48 +/- 0. 20; P < 0.001) and during the exercise (from 1.00 +/- 0.22 to 1.37 +/- 0.14 ; P < 0.001). ApEn also increased during the exercise (from 1.04 +/- 0.11 t o 1.11 +/- 0.08; P < 0.05), but it did not change during the tilt test. The normalized high-frequency spectral component decreased and the low-frequen cy component increased similarly during both the exercise and the tilt test (P < 0.001 for all). Exercise and passive tilt result in an increase of sh ort-term fractal correlation properties of HR dynamics, which is related to changes in the balance between the low- and high-frequency oscillations in controlled situations. Overall complexity of HR dynamics increases during exercise but not during passive tilt.