Adrenergic and Vagal Baroreflex Sensitivity in Autonomic Failure

BACKGROUND: The baroreflex is responsible for maintaining a stable blood pressure (BP) despite changes in body positions and fails in many autonomic disorders. The baroreflex regulates BP by changing the heart rate (vagal component) and total peripheral resistance (adrenergic component). Baroreflex sensitivity is widely used to quantify the vagal component of the reflex, but the adrenergic component is not quantifiable in the autonomic laboratory. OBJECTIVES: To develop and validate an index of adrenergic baroreflex sensitivity. DESIGN: We validated this index with microneurographically recorded muscle sympathetic nerve discharges generated by the Valsalva maneuver and verified it against groups of patients with graded severities of adrenergic failure. RESULTS: Adrenergic baroreflex sensitivity relates BP recovery time to the preceding decrease in BP evoked by the Valsalva maneuver. This index showed a graded and highly significant impairment in 3 groups of patients, (1) those with orthostatic hypotension (n = 26), (2) those with borderline orthostatic hypotension (n = 34), and (3) those with impaired reflex vasoconstriction without orthostatic BP change (n = 24), when compared with an age- and sex-matched control group (n = 29). Adrenergic baroreflex sensitivity better tracked the severity of adrenergic failure than the vagal component of baroreflex sensitivity and provides a much needed index to quantify total peripheral resistance changes in patients with adrenergic failure. CONCLUSIONS: The 2 indices of baroreflex sensitivity separately evaluate the vagal and adrenergic components of the baroreflex. Combined, they provide an index of composite or global baroreflex function.