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Dynamic cerebral autoregulation has been shown to be fast and effective, but it is not well known if the mechanism is symmetric, that is to say, it acts with equal compensatory action to upward as compared with downward abrupt changes in arterial blood pressure (ABP).Fourteen patients with head injuries and 10 normal subjects had bilateral transcranial Doppler and continuous ABP recording. Cyclic ABP stimuli were generated by large thigh cuffs, which were rapidly inflated above systolic pressure for 15 seconds alternating with 15 seconds of deflation. At least 8 such cycles were ensemble-averaged and the dynamic autoregulatory gain (AGup and AGdn) was estimated separately for upward and downward changes in ABP. The results were compared with the autoregulation index using conventional leg cuff releases.In normal subjects, AGdn was 0.74±0.18 and AGup was 0.77±0.17 (mean±SD); the difference was insignificant. The correlation between AGdn and AGup, however, was weak (r=0.24). In the patients with head injury, AGdn was 0.30±0.21 and AGup was 1.27±0.76, the difference being highly significant (P<0.001). There was a negative relationship between AGdn and AGup (r=−0.33). Autoregulation index correlated well with AGdn (r=0.79) and weakly negatively with AGup (r=−0.47).A strongly asymmetric dynamic response of the cerebral autoregulation was seen the majority of patients with head injury. It might also have been present, albeit to a lesser degree, in the normal subjects. The findings suggest that nonlinear effects may be present in the operation of the cerebral autoregulation mechanism.