Symbolic analysis of short-term heart period variability during graded head-up tilt

Recently very simple parameters derived from symbolic analysis were proposed as a tool to non-invasively assess cardiac autonomic modulation from short (less than 300 samples) heart period variability series. They have been found capable to respond to physiological stimuli and pharmacological interventions inducing changes of the autonomic tone. However, it is not known if the same indexes are capable to follow the gradual shift of the sympatho-vagal balance. In this study we utilize a graded head-up tilt protocol to produce a gradual increase of the sympathetic modulation and we demonstrate that one of these indexes derived from symbolic dynamics and applied to short heart period variability series is capable to follow the gradual changes of the sympatho-vagal balance, thus suggesting that it might be a valid alternative to more standard linear indexes based on spectral analysis.