Linear modelling analysis of baroreflex control of arterial pressure variability in rats

The objective of the present study was to examine whether a simple linear feedback model of arterial pressure (AP) control by the sympathetic nervous system would be able to reproduce the characteristic features of normal AP variability by using AP and renal sympathetic nerve activity (RSNA) data collected in conscious sinoaortic baroreceptor denervated (SAD) rats. As compared with baroreceptor-intact rats (n = 8), SAD rats (n = 10) had increased spectral power (+ 680%) of AP in the low frequency range (LF, 0.0003-0.14 Hz) and reduced power (-19%) in the mid-frequency range (MF, 0.14-0.8 Hz) containing Mayer waves. In individual SAD rats, RSNA data were translated into 'sympathetic' AP time series by using the RSNA-AP transfer function that had been previously characterized in anaesthetized rats. AP 'perturbation' time series were then calculated by subtracting 'sympathetic' from actual AP time series. Actual RSNA and AP 'perturbation' time series were introduced in a reflex loop that was closed by using the previously identified baroreflex transfer function (from baroreceptor afferent activity to RSNA). By progressively increasing the open-loop static gain, it was possible to compute virtual AP power spectra that increasingly deviated from their progenitor spectra, with spectral power decreasing in the LF range (as a result of baroreflex buffering of haemodynamic perturbations), and increasing in the MF band (as a result of increasing transients at the resonance frequency of the loop). The most accurate reproduction of actual AP and RSNA spectra observed in baroreceptor-intact rats was obtained at 20-30% of the baroreflex critical gain (open-loop static gain resulting in self-sustained oscillations at the resonance frequency). In conclusion, while the gain of the sympathetic component of the arterial baroreceptor reflex largely determines its ability to provide an efficient correction of slow haemodynamic perturbations, this is achieved at the cost of increasing transients at higher frequencies (Mayer waves). However, the system remains fundamentally stable.