Effect of CO2 on dynamic cerebral autoregulation measurement

Arterial pCO(2) is known to influence cerebral autoregulation but its effec t on the dynamic relationship between mean arterial blood pressure (ABP) an d mean cerebral blood flow velocity (CBFV), obtained from spontaneous fluct uations in ABP, has not been established. In 16 normal subjects, ABP was me asured non-invasively (Finapres), CBFV was estimated with Doppler ultrasoun d in the middle cerebral artery, and end-tidal CO2 (EtCO2) was measured wit h an infrared capnograph. Recordings were made before, during and after bre athing a mixture of 5% CO2 in air. The coherence function, amplitude and ph ase 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 r ange 0.02-0.1 Hz. The impulse and step responses indicated that 5% COz redu ces the efficiency of the autoregulatory mechanism. A 20.7% average increas e 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 resista nce-area product was non-significant.