Coherent averaging of pseudorandom binary stimuli: Is the dynamic cerebral autoregulatory response symmetrical?

Cerebral autoregulation acts to buffer changes in cerebral perfusion pressure. Previous studies have demonstrated that autoregulatory responses display hemispheric symmetry, which may be altered in stroke and traumatic brain injury. There is a paucity of data however on whether the response is symmetrical between those disturbances that cause cerebral hyperperfusion, to those that cause hypoperfusion. Using a previously described assessment method, we employed coherent averaging of the cerebral blood flow velocity (CBFV) responses to thigh cuff inflation and deflation, as driven by pseudorandom binary sequences, whilst simultaneously altering the inspired CO2. The symmetry of the autoregulatory response was assessed with regards to two parameters, its speed and gain. Using the first harmonic method, critical closing pressure (CrCP) and resistance area product (RAP) were estimated, and the gain of the autoregulatory response was calculated by performing linear regression between arterial blood pressure (ABP) and CBFV, ABP and CrCP, and finally ABP and RAP. A two-way repeated measures ANOVA was used to assess the effect of the direction of change in ABP and the method of CO2 administration. Our results suggest that whilst the direction of ABP does not have a significant effect, the effect of CO2 administration method is highly significant (p<10-4). We conclude that the autoregulatory response is symmetric, and that coherent averaging can be combined effectively with our novel assessment method to elucidate the role of CrCP and RAP in the autoregulatory response