Transfer function analysis of dynamic cerebral autoregulation in humans

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Abstract
To test the hypothesis that spontaneous changes in cerebral blood flow are primarily induced by changes in arterial pressure and that cerebral autoregulation is a frequency-dependent phenomenon, we measured mean arterial pressure in the finger and mean blood flow velocity in the middle cerebral artery (V˙MCA) during supine rest and acute hypotension induced by thigh cuff deflation in 10 healthy subjects. Transfer function gain, phase, and coherence function between changes in arterial pressure andV˙MCA were estimated using the Welch method. The impulse response function, calculated as the inverse Fourier transform of this transfer function, enabled the calculation of transient changes inV˙MCA during acute hypotension, which was compared with the directly measured change in V˙MCA during thigh cuff deflation. Beat-to-beat changes inV˙MCA occurred simultaneously with changes in arterial pressure, and the autospectrum of V˙MCA showed characteristics similar to arterial pressure. Transfer gain increased substantially with increasing frequency from 0.07 to 0.20 Hz in association with a gradual decrease in phase. The coherence function was >0.5 in the frequency range of 0.07–0.30 Hz and MCA was similar to the measuredV˙MCA during thigh cuff deflation. These data suggest that spontaneous changes inV˙MCA that occur at the frequency range of 0.07–0.30 Hz are related strongly to changes in arterial pressure and, furthermore, that short-term regulation of cerebral blood flow in response to changes in arterial pressure can be modeled by a transfer function with the quality of a high-pass filter in the frequency range of 0.07–0.30 Hz.