Mechanical and neural contributions to hysteresis in the cardiac vagal limb of the arterial baroreflex

Abstract

According to conventional wisdom, hysteresis in cardiac vagal baroreflex function exhibits a specific pattern: pressure falls are associated with longer heart periods and a smaller linear gain. A similar pattern occurs in the pressure-diameter relationship of barosensory vessels, and therefore it has been suggested that baroreflex hysteresis derives solely from vascular behaviour. However, we hypothesized that mechanical and neural baroreflex components contribute equally to baroreflex hysteresis. Blood pressure, carotid diameter and the electrocardiogram were recorded continuously during two trials of sequential bolus injections of nitroprusside and phenylephrine in 14 young healthy subjects. Baroreflex gain and its mechanical and neural components were estimated for falls and rises in pressure and diameter. The position or set point of the relations was quantified at the mean pressure and mean diameter. Gains were determined via piecewise linear regression. Set points and gains for falls versus rises in pressure and diameter were compared with the Chow test. Hysteresis was observed in all individuals, but not in every trial. In most, but not all, trials pressure falls were associated with longer heart periods and smaller linear gain, as conventional wisdom would predict. However, the pattern of hysteresis derived from the interaction of both mechanical and neural components. The two components most often acted in opposition to determine differences in set point, but in conjunction to determine differences in baroreflex gain. Therefore, we conclude that hysteresis is not solely determined by barosensory vessel behaviour but by the complex interaction of mechanical and neural aspects of the arterial baroreflex.

Figure 1

Typical baroreflex hysteresis pattern observed after sequential bolus injections of nitroprusside and phenylephrine Filled circles and continuous arrow pointing downwards represent the linear relation in the pressure–RR interval relationship for pressure fall. Open triangles and dashed arrow represent baroreflex resetting between pressure fall and rise. Open circles and continuous arrow pointing upwards represent the linear relation in the pressure–RR interval relationship for pressure rise.

Figure 2

Contribution of differences in baroreflex set point and gain to baroreflex hysteresis •, trials with differences in both set point and gain. ○;, trials with a difference either in set point or in gain. Open triangles represent the trial with no difference in set point and gain, yet exhibiting hysteresis.

Figure 3

Hysteresis in integrated baroreflex function (top panel) and the contribution of mechanical (middle panel) and neural (bottom panel) components of baroreflex in a representative trial •, falls after bolus injection of nitroprusside (NTP); ○, rises after bolus injection of phenylephrine (PE). Dashed lines indicate the position of set point.