Nonstationarity and 1/f noise characteristics in heart rate

1 Centre for Nonlinear Dynamics, McGill University, Montreal, Quebec, Canada H3G 1Y6; 2 Institut für Mathematik, Universität Graz, A-8010 Graz, Austria; and 3 Department of Mathematics and Computer Science, Macalester College, Saint Paul, Minnesota 55105 The power spectrum of human heart rate (HR) measured over 24 h exhibits "power-law" 1/ f -type spectral behavior with 1. We investigate possible nonstationarity in time of the exponent using maximum likelihood estimation, which allows relatively short data segments to be used. Examination of 24-h HR records from ambulatory normal and congestive heart failure (CHF) subjects indicates that the power-law structure of HR is nonstationary. In addition, varies with time scale and is different for normal ( 1) and CHF ( 1.5) subjects. Simulations suggest that a possible mechanism underlying the observed power-law spectrum may be a switching between values of near zero (white noise) and near two (Brownian motion). This mechanism generates power-law forms quantitatively similar to CHF subjects when the switching occurs very rapidly and similar to normal subjects when the switching is less rapid. power law; control; self-similarity; model; surrogate data