Scale-specific and scale-independent measures of heart rate variability as risk indicators

We study the correlation properties of heartbeat fluctuations using scale-specific variance (root-mean-square fluctuation) and scaling (correlation) exponents as measures of healthy and cardiac impaired individuals. Our results show that the variance and the scaling exponent, are uncorrelated. We find that the variance measure at certain scales is well suited to separate healthy subjects from heart patients. However, for mortality prediction the scaling exponents outperform the variance measure. Our study is based on a database containing recordings from 428 individuals after myocardial infarct (MI) and on a database containing 105 healthy subjects and 11 heart patients. The results have been obtained by applying two recently developed methods (DFA - Detrended Fluctuation Analysis and WAV - Multiresolution Wavelet Analysis) which are shown to be highly correlated.