Differentiation of periodic from nonperiodic low-frequency heart rate fluctuations.

Periodic low-frequency (LF) fluctuations of heart rate (HR) may be of diagnostic and prognostic value in diverse pathologic cardiopulmonary conditions. Two principal components of LF HR rate variations may be distinguished: periodic fluctuations and nonperiodic, nonstationary changes. The frequency content of these two components may overlap considerably. In order to avoid a tedious work-intensive visual analysis an efficient computer-based method for detection, differentiation, and quantitation of these signals is required. Two methods for separating periodic from nonperiodic HR changes are presented, namely, detrending and bandwidth (BW) calculation. A group of healthy individuals was evaluated in order to assess these methods in individuals with significant LF periodic episodes (15 patients) contrasted to those without LF periodic episodes (94 patients). The commonly used method of detrending consists of a fitted polynomial which by subtraction removes low frequencies originating from nonstationary changes without affecting periodic fluctuations. We found, however, that the frequencies involved in nonstationary and periodic fluctuations often overlap and thus the detrending method may not be highly efficient. In a second method we postulated different shapes for power spectrum curves of periodic and nonperiodic episodes. This latter method is based on BW calculation of the LF component of the R-R power spectrum and proved to be more efficient in detecting periodic episodes. It showed higher significance levels for the difference between the periodic and nonperiodic groups when the BW or the ratio between peak power and BW in the LF range was used. This new, alternative detection method may be employed in further studies which seek to elucidate the clinical relevance of the LF range and, in particular, the mechanisms for such long-wavelength periodic fluctuations.