The effect of transient hypercapnia on task-related changes in cerebral blood flow and blood oxygenation in awake normal humans: a functional magnetic resonance imaging study

It has recently been reported in a-chloralose anesthetized rats that the he modynamic response to somatosensory stimulation almost doubled following tr ansient hypercapnia (THC). In principle, this effect could be employed to e nhance the sensitivity of perfusion-based fMRI experiments. To investigate whether a comparable effect was detectable in awake normal humans, changes in cerebral blood flow (Delta CBF) and the effective transverse relaxation time (DeltaT(2)*) induced by a visual search task were measured in 10 healt hy volunteers before and after THC. Concerning DeltaT(2)*: no significant d ifferences were found, whereas in four subjects Delta CBF was significantly decreased (p < 0.01) following THC. These results demonstrate no increase in the CBF response following THC for awake humans. We conclude that the mo st likely explanation for this discrepancy with the earlier results obtaine d with animals is an as yet unknown mechanism of modulation of the choliner gic system by the anesthesia. Copyright (C) 2000 John Wiley & Sons, Ltd.