Fig. 4. Relationship of gray matter cerebral blood flow (CBF) with drug concentration and condition (auditory stimulus rate). All CBF values have bee corrected to a Pco²of 40, thus any differences are not related to changes in Pco². CBF decreases as drug concentration increases (  P < 0.001 by analysis of variance, F^(2,89)= 10.3). CBF decreases approximately 15% from baseline to sedation (  P < 0.05) and 27% from baseline to hypnotic drug concentration (  P < 0.001). There is no difference in CBF with stimulus rate.