Background-The effect of orthostatic stress on dynamic cerebral autoregulation (CA) in normal subjects and patients with recurrent vasovagal syncope (VVS) is unclear. This study assessed the dynamic CA responses of both groups to head-up tilt. Methods and Results-Seventeen patients with recurrent VVS and 17 pair-matched control subjects underwent 70°h ead-up tilt for up to 30 minutes. Bilateral middle cerebral artery blood flow velocities (CBFV) were measured with transcranial Doppler ultrasound along with noninvasive beat-to-beat blood pressure (BP), heart rate, and transcutaneous and end-tidal CO 2 concentrations. Indices of dynamic CA were derived for periods before, during, and after tilt. Eight normal subjects who developed VVS in an identical protocol but who had no previous clinical history of syncope were also studied. CBFV and transcutaneous and end-tidal CO 2 levels declined significantly during head-up tilt in all groups (PϽ0.0001). Dynamic CA indices were unchanged throughout tilt in nonsyncopal control subjects and were initially unchanged in patients but deteriorated significantly in patients and syncopal control subjects in the minutes before (Pϭ0.027 and Pϭ0.012, respectively) and after (Pϭ0.002 and Pϭ0.007, respectively) syncope. Conclusions-Dynamic CA is preserved in patients and control subjects initially after head-up tilt. Autoregulatory function remains intact in nonsyncopal control subjects during prolonged orthostasis but deteriorates in patients and syncopal control subjects immediately before and after VVS.

Background and Purpose-Normal aging is associated with marked changes in the cardiovascular and cerebrovascular systems. Although cerebral autoregulation (CA) is impaired in certain disease states, the effect of age per se on dynamic CA in humans is unknown and the focus of this study. Methods-Twenty-seven young subjects (Յ40 years) and 27 older subjects (Ն55 years), matched for sex and systolic blood pressure (BP), underwent measurement of cerebral blood flow velocity by transcranial Doppler ultrasound and noninvasive beat-to-beat arterial BP measurement during induced and spontaneous dynamic BP stimuli. A standard dynamic autoregulatory index (ARI) was derived for each spontaneous and induced dynamic BP stimulus to include the step response, as well as cardiac baroreceptor sensitivity (BRS), for the 2 groups. Results-The mean age of the young group was 29Ϯ5 years, and that of the older group was 68Ϯ5 years. Cardiac BRS was reduced in the older group (8.6Ϯ4.5 versus 16.9Ϯ8.8 ms/mm Hg; PϽ0.0001). However, no age-related differences were demonstrated in step response plots or in ARI values for any pressor or depressor dynamic BP stimulus (Pϭ0.62), with mean ARI values for all stimuli combined being 4.9Ϯ1.8 for the young group and 5.0Ϯ2.3 for the older group. Conclusions-Although increasing age is associated with a decrease in cardiac BRS, dynamic CA, as assessed by step response analysis as well as cerebral blood flow responses to transient and induced BP stimuli, is unaffected by aging.