Short-term sympathetic baroreflex sensitivity increases at lower blood pressures

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