Low-frequency renal sympathetic nerve activity, arterial BP, stationary “1/f noise,” and the baroreflex

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Abstract
The object of this study is to quantify the very low frequency (i.e., −β. The peak magnitude squared of the coherence near 0.4 Hz was 0.82 ± 0.08, but the apparent linear coherence fell off quickly at lower frequencies so that it was close to zero for frequencies <0.1 Hz. Moreover, at these low frequencies β, as computed by a coarse grain spectral analysis, was significantly ( P < 0.01) different for SNA (0.66 ± 0.12) and ABP (1.12 ± 0.14). Assuming that SNA and ABP are stationary time series, the results of our classical spectral analysis would indicate that SNA and ABP are not linearly correlated at frequencies with a period more than ∼10 s. Accordingly, we tested for stationarity by computing the spectral coherence and found that SNA and ABP are not stationary “1/ f noise” within the frequency range from 0.02 to 2.0 Hz. Rather the SNA exerts control over the cardiovascular system through intermittent bursts of activity. Such intermittent behavior can be modeled by nonlinear dynamics.