The rate of arterial blood pressure change as a factor in the hysteresis of the baroreceptor reflex.

We induced sinusoidal arterial blood pressure oscillations in anaesthetized dogs, and measured the resulting reflex changes in cardiac cycle length. The graphs of cardiac cycle length as a function of systolic arterial pressure displayed considerable hysteresis. The extent of the hysteresis and the direction around the loop varied with the frequency of the pressure oscillations. At relatively low frequencies (0.02 and 0.04 Hz) the loops were predominantly anticlockwise, whereas at higher frequencies (0.08 and 0.16 Hz) the loops were mainly clockwise in direction. In a second group of dogs we stimulated the transected vagus nerves with sequential ramp-like increases and decreases in voltage. The graph of the chronotropic response displayed a prominent anticlockwise hysteresis. Hence the anticlockwise hysteresis which was induced reflexly by cyclic oscillations of systemic arterial pressure is at least partly ascribable to an efferent vagal mechanism.