Multivariate dynamic analysis of cerebral blood flow regulation in humans

The contributions of beat-to-beat changes in mean arterial blood pressure (MABP) and breath-by-breath fluctuations in end-tidal CO ₂ (EtCO ₂) as determinants of the spontaneous variability of cerebral blood flow velocity (CBFV) were studied in 16 normal subjects at rest. The two input variables (MABP and EtCO ₂) had significant cross-correlations with CBFV but not between them. Transfer functions were estimated as the multivariate least mean square finite impulse response causal filters. MABP showed a very significant effect in explaining CBFV variability (p<10 ^-11, Fisher´s aggregated-p test) and the model mean square error was significantly reduced (p<0.001) by also including the contribution EtCO ₂. The estimated mean CBFV step response to MABP displayed the characteristic return to baseline caused by the cerebral autoregulatory response. The corresponding response to EtCO ₂ showed a gradual rise taking approximately 10 s to reach a plateau of 2.5%/mmHg. This study demonstrated that spontaneous fluctuations in EtCO ₂ can help to explain the CBFV variability at rest if appropriate signal processing techniques are employed to address the limited power and bandwidth of the breath-by-breath EtCO ₂ signal.