Cerebral autoregulation adjusts cerebrovascular resistance in the face of changing perfusion pressures to maintain relatively constant flow. Results from several studies suggest that cardiac output may also play a role. We tested the hypothesis that cerebral blood flow would autoregulate independent of changes in cardiac output. Transient systemic hypotension was induced by thigh-cuff deflation in 19 healthy volunteers (7 women) in both supine and seated positions. Mean arterial pressure (Finapres), cerebral blood flow (transcranial Doppler) in the anterior (ACA) and middle cerebral artery (MCA), beat-by-beat cardiac output (echocardiography), and end-tidal Pco(2) were measured. Autoregulation was assessed using the autoregulatory index (ARI) defined by Tiecks et al. (Tiecks FP, Lam AM, Aaslid R, Newell DW. Stroke 26: 1014-1019, 1995). Cerebral autoregulation was better in the supine position in both the ACA [supine ARI: 5.0 ± 0.21 (mean ± SE), seated ARI: 3.9 ± 0.4, P = 0.01] and MCA (supine ARI: 5.0 ± 0.2, seated ARI: 3.8 ± 0.3, P = 0.004). In contrast, cardiac output responses were not different between positions and did not correlate with cerebral blood flow ARIs. In addition, women had better autoregulation in the ACA (P = 0.046), but not the MCA, despite having the same cardiac output response. These data demonstrate cardiac output does not appear to affect the dynamic cerebral autoregulatory response to sudden hypotension in healthy controls, regardless of posture. These results also highlight the importance of considering sex when studying cerebral autoregulation.

Background and Purpose-Orthostatic intolerance and falls differ between sexes and change with age. However, it remains unclear what role cerebral autoregulation may play in this response. This study was designed to determine whether cerebral autoregulation, assessed using transcranial Doppler ultrasound, is more effective in elderly females than in males. Methods-We used transcranial Doppler ultrasound to evaluate cerebral autoregulation in 544 (236 male) subjects older than age 70 years recruited as part of the MOBILIZE Boston study. The MOBILIZE Boston study is a prospective cohort study of a unique set of risk factors for falls in seniors in the Boston area. We assessed CO 2 reactivity and transfer function gain, phase, and coherence during 5 minutes of quiet sitting and autoregulatory index during sit-to-stand tests. Pϭ0.002) in the autoregulatory band, implying less effective cerebral autoregulation. However, reduced autoregulation in males was not below the normal range, indicating autoregulation was intact but less effective. Conclusions-Female subjects were better able to maintain cerebral flow velocities during postural changes and demonstrated better cerebral autoregulation. The mechanisms of sex-based differences in autoregulation remain unclear but may partially explain the higher rates of orthostatic hypotension-related hospitalizations in elderly men. (Stroke.

The majority of cerebral autoregulation research has focused on the middle cerebral artery. However, many symptoms of presyncope indicate posterior cerebral hypoperfusion. To address this issue, we measured cerebrovascular reactivity, cerebral blood flow velocity and dynamic cerebral autoregulation in the middle cerebral artery and vertebral arteries during orthostatic stress to presyncope in 9 healthy subjects. There was no significant difference in either the decline in cerebral blood flow velocity or indices of dynamic cerebral autoregulation between the middle cerebral and vertebral arteries prior to or during presyncope. In conclusion, there is no significant difference in regulation of blood flow between the vertebral artery and middle cerebral artery in healthy subjects. Further study is required to determine whether or not a difference exists in syncopal patient populations.