Time-dependent changes in open-loop gains of baroreflex systems after massive hemorrhage

To quantify the effects of massive hemorrhage (5 ml/kg body weight) on the individual arterial baroreflex systems in the dog, changes in the open-loop gains of the intact arterial pressure control system (Gintact), the carotid sinus baroreflex system (GCS), and the vagally mediated and aortic arch baroreflex systems (GV) were measured repeatedly from the response to quick mild hemorrhage (2 ml/kg body weight) before and after massive hemorrhage. Fifteen mongrel adult dogs were divided into 3 groups. i.e., the control, vago-aortic nerve-severed, and carotid sinus-denervated groups. The dogs anesthetized with Nembutal (35 mg/kg body weight) were bled by 2 ml/kg body weight within 2 sec through a catheter inserted into the abdominal aorta. The arterial pressure change after mild hemorrhage was monitored via a catheter placed in the aortic arch. The open-loop gain of the baroreflex system was assessed as (delta API/delta APS--1), where delta API and delta APS are the immediate and steady-state falls in arterial pressure following mild hemorrhage. The mean values of Gintact, GCS, and GV before massive hemorrhage were 7.8, 2.0, and 1.8, respectively. Since Gintact is not a simple summation of GCS and GV, it is speculated that the carotid sinus baroreflex system interacts in a facilitatory way with the vagally mediated and aortic arch baroreflex systems. After massive hemorrhage, Gintact changed along a time course of parabolic form, whereas GCS did not change and GV decreased. These results suggest that the time-dependent change of Gintact after massive hemorrhage depends on the change in the open-loop gain of the baroreflex system making a facilitatory interaction.