Nonstationarity and 1/f noise characteristics in heart rate

Abstract

The power spectrum of human heart rate (HR) measured over 24 h exhibits “power-law” 1/ f α-type spectral behavior with α ≈ 1. We investigate possible nonstationarity in time of the exponent α 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, α varies with time scale and is different for normal (α ≈ 1) and CHF (α ≈ 1.5) subjects. Simulations suggest that a possible mechanism underlying the observed power-law spectrum may be a switching between values of α 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.