Closed-loop baroreflex control of total peripheral resistance in the cat: identification of gains by aid of a model

The baroreflex regulation of total peripheral resistance was quantified in closed-loop conditions. To vary arterial pressure cardiac output was reduced by graded inferior caval vein occlusion or by arterial bleeding. In eight lightly anaesthetised cats the static relation between mean arteriovenous pressure gradient and mean flow could be described by a curve that was convex to the pressure axis and had zero intercept. The ratio of the change in resistance to a given change in arterial pressure was taken as resistance gain (GR). The value of this gain was estimated with the aid of a model which predicts pressure from flow when the right parameter value for gain is filled in. It consists of a non-linear negative-feedback control system with control pressure as reference point and with a constant gain. The estimation was carried out with the aid of an automatic identification procedure. GR varied from 0.002 to 0.010 min . ml-1 in different animals under light anaesthesia. With deeper anaesthesia gain decreased by 35 to 50% and became zero with very deep anaesthesia or barodenervation. Assuming that the reflex is a linear system about control pressure and flow we linearised our model and computed the static overall open-loop gain (GO). Within this narrow range GO varied from 0.64 to 2.30 for different cats under light anaesthesia and decreased by the same percentage as GR with deeper anaesthesia.