Coupling of sympathetic nerve traffic and BP at very low frequencies is mediated by large-amplitude events

Abstract

This study explores the functional association between renal sympathetic nerve traffic (NT) and arterial blood pressure (BP) in the very-lowfrequency range (i.e., Ͻ0.1 Hz). NT and BP (n ϭ 6) or BP alone (n ϭ 17) was recorded in unanesthetized rats (n ϭ 6). Data were collected for 2-5 h, and wavelet transforms were calculated from data epochs of up to 1 h. From these transforms, we obtained probability distributions for fluctuation amplitudes over a range of time scales. We also computed the cross-wavelet power spectrum between NT and BP to detect the occurrence in time of large-amplitude transient events that may be important in the autonomic regulation of BP. Finally, we computed a time sequence of cross correlations between NT and BP to follow the relationship between NT and BP in time. We found that NT and BP follow comparable self-similar scaling relationships (i.e., NT and BP fluctuations exhibit a certain type of power law behavior). Scaling of this nature 1) points to underlying dynamics over a wide range of scales and 2) is related to large-amplitude events that contribute to the very-low-frequency variability of NT and BP. There is a strong correlation between NT and BP during many of these transient events. These strong correlations and the uniformity in scaling imply a functional connection between these two signals at frequencies where we previously found no connection using spectral coherence. wavelet analysis; sympathetic nervous activity; cross correlation; blood pressure fluctuations; transient events; selfsimilar invariance ARTERIAL BLOOD PRESSURE (BP) and sympathetic nerve traffic (NT) are well known to be functionally coupled, or highly coherent, to maintain normal homeostatic function. In the unanesthetized rat, for example, oscillations in sympathetic NT that repeat in ϳ2.5 s are clearly very tightly coupled with fluctuations in BP that have a similar period (3). This high coherence can be explained on the basis of the baroreflex (4). This 0.4-Hz rhythm appears to be a natural instability within this reflex, similar to a resonance in an electrical circuit or mechanical system produced by the time constants and delays in the constituent physical and neuronal phenomena. A similar phenomenon in the rabbit occurs at 0.3 Hz (13) and in the human at 0.1 Hz (7, 17). Conversely, the coherence between sympathetic activity and BP in the rat falls to very low values below a frequency of 0.1 Hz (3). In other words, the mathematical processes used to compute the coherence do not detect a close functional relationship between rhythmic changes in BP and corollary changes in sympathetic NT, or vice versa, that require more than ϳ10 s to repeat. A weak coupling between sympathetic activity and BP within the very low frequencies (i.e., Ͻ0.1 Hz) is unexpected, because the baroreflex is widely credited with minimizing BP fluctuations during, for example, postural changes that certainly fall at least within the upper limits of the low-frequency range.Stationarity of a signal, or "time series," is required in spe...

Blood pressure power within frequency range ∼0.4 Hz in rat conforms to self-similar scaling following spinal cord transection

“…More recently, our group (7) reported that arterial BP and sympathetic nerve activity (SNA) signals did not conform precisely to stationary 1/f noise, at least within the frequency range between 0.02 and 2.0 Hz in the unanesthetized rat. We found that intermittent "large-amplitude events" are important contributors to the very low-frequency variability in these signals (6). With this caveat, the dynamic behavior of HR, BP, and SNA generally displays self-similar behavior below 0.4 Hz.…”

“…Spectral power within the range from 0.35 to 0.45 Hz was computed with fast Fourier transform procedures described in detail elsewhere (4). Likewise, procedures for the wavelet computation have been described elsewhere (6,10). Briefly, the wavelet transform of a time series x(t) is defined as…”

Analysis of sympathetic neural discharge in rats and humans

“…those produced by myogenic responses originating in regional circulations (Létienne et al 1998). Therefore, slow RSNA fluctuations comprise both baroreflex-and non-baroreflex-mediated RSNA variations (Burgess et al 2003).…”

Wavelet-Based System Identification of Short-Term Dynamic Characteristics of Arterial Baroreflex

“…Burgess et al 2 showed that cross spectrum analysis using wavelet transform characterized strong coupling between sympathetic nerve traffic and AP at frequencies of <0.1 Hz. Davrath et al 4 reported that timevarying power obtained from wavelet transform of the spontaneous HR or AP fluctuation in humans are remarkably modulated at approximately 0.1 Hz under standing condition.…”