Short-term sympathetic baroreflex sensitivity increases at lower blood pressures

Abstract

Objective Sympathetic baroreflex sensitivity (symBRS) can be defined as the maximum sensitivity of muscle sympathetic nerve activity (MSNA) to changes in arterial blood pressure. This sensitivity is the slope of the linear middle part of the sigmoid curve that relates blood pressure to MSNA. SymBRS is known to vary with conditions, for instance during cold pressor testing. We investigated whether symBRS is affected by infusions of phenylephrine and nitroprusside. Methods In 10 healthy subjects, vasoactive infusions were varied in slow steps, as customary in protocols to determine ‘graded infusion symBRS’ (symBRS inf). During each step, symBRS was estimated from spontaneous beat-to-beat fluctuations (symBRS sp). As a secondary goal, symBRS inf was compared to the symBRS sp without infusions. Results The symBRS sp for MSNA burst area varied with infusions, augmenting with decreasing blood pressure, however the symBRS sp for burst occurrence was not affected. There were large differences between symBRS inf and symBRS sp at rest. Conclusions symBRS sp varies systematically with infusions during a symBRS inf protocol. This denotes a fundamental difference between these methods. Significance The relationship between ‘slow’ infusion effects (symBRS inf) and changes in symBRS sp is elucidated. The mathematical model that describes this relationship can also explain the increase of symBRS found with other sympathoexcitatory stimuli.