Higuchi fractal dimension and spectral entropy as measures of depth of sedation in intensive care unit

The ability of two easy-to-calculate nonlinear parameters, the Higuchi fractal dimension (HDf) and spectral entropy, to follow the depth of sedation in the intensive care unit is assessed. For comparison, the relative beta ratio is calculated. The results are evaluated using clinical assessment of the Ramsay score. The results show that the HD/sub f/ discriminates well between Ramsay scores 2-4 while beta ratio is superior for deeper levels of sedation. The value of the HD/sub f/ correlates highly with the cutoff frequency of the low-pass prefilter while spectral entropy is sensitive to the length of the analysis window.