Dynamic Changes in Cerebral Blood Flow, O2 Tension, and Calculated Cerebral Metabolic Rate of O2 during Functional Activation Using Oxygen Phosphorescence Quenching

Abstract

Changes in cerebral blood flow (CBF) using laser–Doppler and microvascular O₂ oxygen tension using oxygen-dependent phosphorescence quenching in the rat somatosensory cortex were obtained during electrical forepaw stimulation. The signal-averaged CBF response resulting from electrical forepaw stimulation consisted of an initial peak (t = 3.1 ± 0.8 seconds after onset of stimulation), followed by a plateau phase that was maintained throughout the length of the stimulus. In contrast, microvascular O₂ tension changes were delayed, reached a plateau level (t = 23.5 ± 1.7 seconds after the onset of stimulation) that remained for the length of the stimulus and for several seconds after stimulus termination, and then returned to baseline. Using Fick's equation and these dynamic measurements, changes in the calculated cerebral metabolic rate of oxygen (CMRO₂) during functional stimulation were determined. The calculated CMRO₂ response initially was comparable with the CBF, but with protracted stimulation, CMRO₂ changes were approximately one-third that of CBF changes. These results suggest that a complex relation exists, with comparable changes in CBF and CMRO₂ initially occurring after stimulation but excessive changes in CBF compared with CMRO₂ arising with protracted stimulation.