Nonstationary multivariate modeling of cerebral autoregulation during free-breathing and hypercapnic conditions

We investigated the nonstationary properties of cerebral hemodynamics by examining the relationship between mean arterial blood pressure (MABP), end-tidal CO₂ tension (P_ETCO2) and middle cerebral artery blood flow velocity (CBFV) during resting and hypercapnic conditions. To this end, we employed univariate and multivariate time-varying Laguerre-Volterra models as well as a Recursive Least Squares estimation scheme with multiple adaptive forgetting factors. The results reveal the presence of nonstationarities mainly in the very low frequency range, which are more pronounced and irregular for the CO₂ reactivity component.