Transfer function analysis of dynamic cerebral autoregulation in humans

Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas 75231; and The University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75235 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 ( 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 and 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 in MCA during acute hypotension, which was compared with the directly measured change in MCA during thigh cuff deflation. Beat-to-beat changes in MCA occurred simultaneously with changes in arterial pressure, and the autospectrum of 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