Local holder exponent analysis of heart rate variability in preterm infants.

Heart rate variability (HRV) displays scale-invariant fractal properties. Recent studies have revealed multifractal properties in the healthy human HRV, which could be characterized by singularities with various strength of local Hölder exponents embedded in HRV. In this paper, HRV time series from preterm infants, whose autonomic nervous system undergoes dramatic development, were collected longitudinally. Changes in fractality/multifractality of those HRV time series as the postmenstrual age were examined in order to see if they could quantify development of the autonomic nervous system. Temporal structure of the singularities at several representative time scales was also analyzed to show that intersingular event intervals could be well described by "power law distribution," and the singular events appeared with age-dependent long-range correlation in its strength. Detailed analyses suggested that fractality and multifractality of HRV, respectively, could quantify the development of the respiratory center and the parasympathetic nervous system in the preterm infants. The results obtained in this study might be beneficial for detecting occurrences of life threatening singular events such as big apnea in preterm infants.