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GLOSSARY OF TERMS USED IN TIME SERIES ANALYSIS OF CARDIOVASCULAR DATA

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Trend in which the power of the spectrum is inversely proportional to the frequency f according to a 1/fpower law.

This trend characterizes the spectra of most biological signals. When the spectrum is plotted in a log-log scale, the 1/f trend appears as a straight line with slope -. Typically, blood pressure and heart rate log-log spectra show such a linear trend at frequencies lower than 0.02 Hz, and the value of can be easily evaluated by computing the regression line over a low frequency band. The exponent may change during physiological or pathological conditions such as during physical exercise (Nakamura et al, 1993), of after acute myocardial infarction or heart transplant (Bigger et al, 1996).
For fractal time series there is a simple relation between the slope and the Hurst exponent:

References:
Task force of the european society of cardiology and the north american society of pacing and electrophysiology. Hearth rate variability Standard of mesurement, physilogical interpretation, and clinical use. Circulation 93: 1043-1065, 1996.
Nakamura et al. (1993) Autonomic control of heart rate during physical exercise and fractal dimension of heart rate variability. J Appl Physiol.
Butler GC et al. (1994) Fractal nature of short-term systolic BP and HR variability during lower body negative pressure. Am J Physiol.
Bigger JT et al. (1996) Power law behavior of RR-Interval variability in healthy middle-aged persons, patients with recent acute myocardial infarction, and patients with heart transplants. Circulation

Links:
1/f noise in heart beat, a bibliography by the Rockfeller University