Heart rate dynamics assessed by different strategies of symbolization

The analysis of symbolic dynamics applied to the RR interval series is able to retrieve information about dynamical properties. Three different strategies of symbolic transformations were investigated: (1) symbolization based on the deviation from the average, (2) symbolization based on equidistant levels between the minimum and maximum, (3) binary symbolization of the first derivative. Each method was applied to the RR interval series and its difference of 17 healthy subjects obtained during head-up tilt testing. Dynamical properties were assessed by the occurrence of short sequences (`words') of length 3. The occurrence of words was grouped according to variations of the symbols in each word (0, 1, and 2 variations). Linear regression analysis showed that the occurrence of these words reflects changes of heart rate dynamics during graded head-up tilt. Hence, very short symbolic sequences are able to track changes of the cardiac autonomic regulation.