Effect of CO₂ on dynamic cerebral autoregulation measurement

Arterial pCO₂ is known to influence cerebral autoregulation but its effect on the dynamic relationship between mean arterial blood pressure (ABP) and mean cerebral blood flow velocity (CBFV), obtained from spontaneous fluctuations in ABP, has not been established. In 16 normal subjects, ABP was measured non-invasively (Finapres), CBFV was estimated with Doppler ultrasound in the middle cerebral artery, and end-tidal CO₂ (EtCO₂) was measured with an infrared capnograph. Recordings were made before, during and after breathing a mixture of 5% CO₂ in air. The coherence function, amplitude and phase frequency responses, and impulse and step responses for the effects of ABP on CBFV were calculated by spectral analysis of beat-to-beat changes in mean ABP and CBFV before (mean CO₂ 5.55±0.38 kPa), during (6.43±0.31 kPa) and after 5% CO₂ (5.43±0.26 kPa). During 5% CO₂, the coherence function and the amplitude frequency response were significantly increased for frequencies below 0.05 Hz and the phase was reduced for the frequency range 0.02 - 0.1 Hz. The impulse and step responses indicated that 5% CO₂ reduces the efficiency of the autoregulatory mechanism. A 20.7% average increase in CBFV induced by a 14.4% increase in EtCO₂ was found to be mediated by a 25.9% reduction in critical closing pressure, while the change in resistance-area product was non-significant.