Spectral characteristics of ventricular response to atrial fibrillation

To investigate the spectral characteristics of the fluctuation in ventricular response during atrial fibrillation (AF), R-R interval time series obtained from ambulatory electrocardiograms were analyzed in 45 patients with chronic AF and in 30 age-matched healthy subjects with normal sinus rhythm (SR). Although the 24-h R-R interval spectrum during SR showed a 1/f noise-like downsloping linear pattern when plotted as log power against log frequency, the spectrum during AF showed an angular shape with a breakpoint at a frequency of 0.005 ± 0.002 Hz, by which the spectrum was separated into long-term and short-term components with different spectral characteristics. The short-term component showed a white noise-like fiat spectrum with a spectral exponent (absolute value of the regression slope) of 0.05 ± 0.08 and an intercept at 10^-2 Hz of 4.9 ± 0.3 log(ms²/Hz). The long-term component had a 1/f noise-like spectrum with a spectral exponent of 1.26 ± 0.40 and an intercept at 10^-4 Hz of 7.0 ± 0.3 log(ms²/Hz), which did not differ significantly from those for the spectrum during SR in the same frequency range [spectral exponent, 1.36 ± 0.06; intercept at 10^-4 Hz, 7.1 ± 0.3 log(ms²/Hz)]. The R-R intervals during AF may be a sequence of uncorrelated values over the short term (within several minutes). Over the longer term, however, the R-R interval fluctuation shows the long-range negative correlation suggestive of underlying regulatory processes, and spectral characteristics indistinguishable from those for SR suggest that the long-term fluctuations during AF and SR may originate from similar dynamics of the cardiovascular regulatory systems.