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.

Background and Purpose-Dynamic methods of measuring cerebral autoregulation have become an accepted alternative to static evaluation. This article aims to describe a set of data collected from healthy volunteers by a dynamic method, the purpose being to qualify and quantify expected results for those who may be designing a study using this technique. Methods-Cerebral blood flow velocity (CBFV) (measured by transcranial Doppler) and arterial blood pressure (Finapres) were recorded in 16 normal subjects before, during, and after the induction of a blood pressure drop (release of bilateral thigh cuffs). This procedure was repeated 6 times for each subject. A mathematical model was applied to the data to generate an autoregulatory index (ARI) with values between 0 and 9. Results-The ARI values for this sample population follow a normal distribution, with a meanϮSD of 4.98Ϯ1.06 (nϭ15).Analysis of the cumulative mean ARI values of all subjects showed an exponential-type convergence of ARI toward the sample mean as the number of test iterations increased. The population average blood pressure drop on thigh cuff release was 26.4Ϯ7.1 mm Hg (nϭ16), occurring in 4.6Ϯ1.7 seconds. The corresponding population average drop for CBFV was 15.6Ϯ5.8 cm/s, taking 2.5Ϯ1.0 seconds. No significant trend was noted in the measurements as the number of test iterations increased. The correlation between the predicted and actual CBFV, having a mean value of 0.76Ϯ0.19, showed evidence of a nonlinear relationship to ARI values. Significant correlation was also found between ARI and (1) arterial blood pressure before cuff release and (2)

The contributions of beat-to-beat changes in mean arterial blood pressure (MABP) and breath-by-breath fluctuations in end-tidal CO2 (EtCO2) 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 EtCO2) 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 of EtCO2. The estimated mean CBFV step response to MABP displayed the characteristic return to baseline caused by the cerebral autoregulatory response. The corresponding response to EtCO2 showed a gradual rise taking approximately 10 s to reach a plateau of 2.5%/mmHg. This study demonstrated that spontaneous fluctuations in EtCO2 can help to explain the CBFV variability at rest if appropriate signal processing techniques are employed to address the limited power and bandwith of the breath-by-breath EtCO2 signal.

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