Synchronisation analysis of heart period variability signal based on corrected conditional entropy

Regularity and synchronisation between different rhythms mainly at low (LF) and high frequency (HF) is evaluated in heart rate variability through conditional entropy (CE). CE quantifies the degree of information left after regular patterns have been removed. In short (about 300 points) data sequences CE is evaluated with a correction which considers as random the patterns that appear only one time and would otherwise be considered as regular. The minimum of the corrected CE (CCE) with increasing pattern length is chosen as the most reliable CE estimate. Beat-to-beat heart period series were analysed in 8 normal subjects at rest (R), head up tilt (T) and controlled respiration (CR). A significant decrease in CCE was found in T (0.9/spl plusmn/0.052) compared to R (1.01/spl plusmn/0.037); during CR, CCE displayed a slight and not significant increase (1.07/spl plusmn/0.055). The enhanced LF activity present in T seems to have a greater regularising power than the periodical input provided by CR at HF.