Cardiovascular parameters exhibit spontaneous oscillations at the respiratory frequency, and in the low frequency range (LF < 0.20 Hz). Although LF is attributed to the sympathetic control, the mechanism responsible for the oscillation, whether instability of the baroreflex loop, or activity of a central nervous system pattern generator, is controversial. To answer this question, time series of arterial blood pressure, heart period and left external iliac blood flow from chloralose-anaesthetised dogs were examined by standard statistics as well as by autoregressive spectral and cross-spectral analysis. The circulation to the left hind-limb was isolated and connected to a constant-pressure perfusing system, to obtain mechanical uncoupling from the central circulation. Three steady-state conditions were studied. A device inserted into the common carotid arteries allowed the carotid sinus region to be in continuity with the animal's arterial system (control) or perfused at constant pressure by an external source (carotid buffer); bilateral cervical vagotomy was also performed (vagi cut). Intra-individual (beat-to-beat) variability of each parameter was evaluated by standard deviation (SD) of time series in the three conditions. The average SD of heart period was reduced in carotid buffer and in vagi cut; the SDs of arterial pressure and iliac flow were not changed by these interventions. Autospectra of iliac flow time series in control showed a prominent peak at 0.05 ± 0.04 Hz (mean ± SD of all experiments), accounting for 90 ± 11% of the total variance. Arterial pressure exhibited a LF peak of similar frequency, with high coherence (0.83 ± 0.23) and complete phase opposition, indicating a strong reciprocal relationship between the two variables. Thus, iliac vascular resistance was behaving like total peripheral resistance. Therefore, the oscillation was attributed to an oscillation of the vasomotor sympathetic outflow. The spectral characteristics were never changed either in carotid buffer or in vagi cut. Since functional exclusion of the baroreceptor control system did not abolish LF, we favour the hypothesis of the central origin of the sympathetic oscillation. © 1995.
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