 Nonlinear analysis Non-linear analysis was conducted by means of symbolic analysis. The symbolic analysis is conducted on the same sequences of 300 consecutive heart beats that are used for the autoregressive analysis. The whole range of the R-to-R interval into each series is uniformly divided in 6 slices symbols and pattern of 3 consecutive heart beat intervals were considered. Thus each sequence of 300 heart beats had its own R-to-R range and 298 consecutive triplets of symbols. The Shannon entropy of the distribution of the patterns is calculated to provide a quantification of the complexity of the pattern distribution. All triplets of symbols are grouped into 3 possible patterns of variation: i no variation 0V, all 3 symbols were equal, ii 1 variation 1V, 2 consequent symbols were equal and the remaining symbol was different, iii patterns with 2 variations 2V, all symbols were different from the previous one. Previously, the percentage of 0V patterns was found to increase and 2V decrease in response to sympathetic stimuli, whereas 2V patterns increased and 0V decreased in response to vagal stimuli. The percentage of the patterns 0V and 2V was calculated, and the 0V2V ratio was calculated to estimate the balance between sympathetic and vagal modulation. 