Local-scale analysis of cardiovascular signals by detrended fluctuations analysis: effects of posture and exercise

The fractal structure of heart rate is usually quantified by estimating a short-term (alpha(1)) and a long-term (alpha(2)) scaling exponent by Detrended Fluctuations Analysis (DFA). Evidence, however, has been provided that heart rate is a multifractal signal, better characterized by a large number of scaling exponents. Aim of this study is to verify whether two scaling exponents only from DFA provide a sufficiently accurate description of the possibly multifractal nature of cardiovascular signals. We measured ECG and finger arterial pressure in 33 volunteers for 10 minutes during each of 3 conditions: supine rest (SUP); sitting at rest (SIT); light physical exercise (EXE). DFA was applied on the beat-by-beat series of R-R interval (RRI) and mean arterial pressure (MAP). We then computed the local scaling exponent alpha(n), defined as the slope of the detrended fluctuation function F(n) around the beat scale n, in a log-log plot. If alpha(1) and alpha(2) correctly model the multiscale structure of blood pressure and heart rate, we should find that alpha(n) is constant over a short-term and a longterm range of beat scales. Results show that only the long-term alpha2 exponent provides a relatively good approximation of the multiscale structure of RRI and MAP. Moreover, posture and physical activity have important effects on local scaling exponents, and on the range of beat scales n where alpha(n) can be approximated by a constant alpha2 coefficient.