Rapid adaptation of vagal baroreflexes in humans

We evaluated the time course of carotid baroreflex adaptation to arterial pressure elevations induced by phenylephrine infusion (1 microgram/kg per min) in 13 normotensive volunteers. Vagally-mediated carotid baroreceptor-cardiac reflex responses were assessed serially with a staircase pressure/suction sequence delivered to a neck chamber. Carotid baroreflex stimulus-response relations shifted up the response (R-R interval) axis and to the right on the stimulus (pressure) axis (determined as the carotid distending pressure at saturation) within 90-120 s (time between measurements) during the first 10 min. The shifts in position of the reflex relation were sustained throughout the infusion. The position of operational point pressure, relative to carotid distending pressure at saturation, was unchanged throughout the infusion period. These data suggest that human carotid baroreceptor reflex relations change within minutes to retain beat-to-beat pressure regulation as the prevailing pressure increases.