Spontaneous fluctuations in cerebral blood flow: insights from extended-duration recordings in humans

To determine the dependence of cerebral blood flow (CBF) on arterial pressu re over prolonged time periods, we measured beat-to-beat changes in mean CB F velocity in the middle cerebral artery (transcranial Doppler) and mean ar terial pressure (Finapres) continuously for 2 h in six healthy subjects (5 men and 1 woman, 18-40 yr old) during supine rest. Fluctuations in velocity and pressure were quantified by the range [(peak 2 trough)/mean] and coeff icients of variation (SD/mean) in the time domain and by spectral analysis in the frequency domain. Mean velocity and pressure over the 2-h recordings were 60 +/- 7 cm/s and 83 +/- 8 mmHg, associated with ranges of 77 +/- 8 a nd 89 +/- 10% and coefficients of variation of 9.3 +/- 2.2 and 7.9 +/- 2.3% , respectively. Spectral power of the velocity and pressure was predominant ly distributed in the frequency range of 0.00014-0.1 Hz and increased inver sely with frequency, indicating characteristics of an inverse power law (1/ f(alpha)). However, linear regression on a log-log scale revealed that the slope of spectral power of pressure and velocity was steeper in the high-fr equency (0.02-0.5 Hz) than in the low-frequency range (0.002-0.02 Hz), sugg esting different regulatory mechanisms in these two frequency ranges. Furth ermore, the spectral slope of pressure was significantly steeper than that of velocity in the low-frequency range, consistent with the low transfer fu nction gain and low coherence estimated at these frequencies. We conclude t hat 1) long-term fluctuations in CBF velocity are prominent and similar to those observed in arterial pressure, 2) spectral power of CBF velocity reve als characteristics of 1/f(alpha), and 3) cerebral attenuation of oscillati ons in CBF velocity in response to changes in pressure may be more effectiv e at low than that at high frequencies, emphasizing the frequency dependenc e of cerebral autoregulation.