Cerebrovascular reactivity and cerebral autoregulation in normal subjects

Background: Cerebrovascular reactivity (CVR) testing with CO2 challenge is used clinically as a measure of cerebrovascular reserve. However it is not known whether CVR measures the same physiological process as spontaneous cerebral autoregulation (CA). Purpose: To compare CVR with CA in healthy Volunteers, using continuous monitoring of cerebral blood flow velocity (CBFV) and arterial blood pressure (ABP). Methods: We prospectively studied CVR and CA in 18 healthy volunteers. CVR was assessed using mean CBFV changes after 5% CO2 inhalation. CA was determined by transfer function analysis to derive the phase shift between spontaneous ABP and CBFV fluctuations at 0.1 Hz. Results: CO2 inhalation produced a significant decrease in phase shift from 37.9 +/- 13.8 degrees to 21.0 +/- 7.2 degrees (p<0.001). In addition, there was a significant correlation between CVR and CA changes during CO2 inhalation (R = -0.50,p = 0.03), but not between CVR and baseline CA (R = 0.22,p = 0.4). Conclusion: We showed a decrease in spontaneous CA after vasodilatation. However, the lack of correlation between baseline CA and degree of CVR suggests that cerebrovascular reserve and CA are based in part on distinct physiological mechanisms. Further Studies are needed to determine which of these parameters is most useful to guide treatment decisions in pathological states. (c) 2009 Elsevier B.V. All rights reserved.