TRANSFER-FUNCTION ANALYSIS OF DYNAMIC CEREBRAL AUTOREGULATION IN HUMANS

To test the hypothesis that spontaneous changes in cerebral blood flow are primarily induced by changes in arterial pressure and that cerebr al autoregulation is a frequency-dependent phenomenon, we measured mea n arterial pressure in the finger and mean blood flow velocity in the middle cerebral artery (V-MCA) during supine rest and acute hypotensio n induced by thigh cuff deflation in 10 healthy subjects. Transfer fun ction gain, phase, and coherence function between changes in arterial pressure and V-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 V- MCA during acute hypotension, which was compared with the directly mea sured change in V-MCA during thigh cuff deflation. Beat-to-beat change s in V-MCA occurred simultaneously with changes in arterial pressure, and the autospectrum of V-MCA showed characteristics similar to arteri al pressure. Transfer gain increased substantially with increasing fre quency 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 <0.5 at <0.07 Hz. Furthermore, the predicted change in V-M CA was similar to the measured V-MCA during thigh cuff deflation. Thes e data suggest that spontaneous changes in V-MCA that occur at the fre quency range of 0.07-0.30 Hz are related strongly to changes in arteri al pressure and, furthermore, that short-term regulation of cerebral b lood flow in response to changes in arterial pressure can be modeled b y a transfer function with the quality of a high-pass filter in the fr equency range of 0.07-0.30 Hz.