Non linear HRV analysis The symbolic analysis is conducted on the same sequences of 300 consecutive heart beats 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 are considered. Thus each sequence of 300 heart beats has 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 at least 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.