Regional Cerebral Blood Flow and Focal Cortical Perfusion: A Comparative Study of 133 Xe,85 Kr,and Umbelliferone as Diffusible Indicators

We report that regional CBF determined by the initial slope technique using ¹³³Xe and ⁸⁵Kr in cats and rabbits can be significantly influenced by the size of the field of measurement. The clearance curves of umbelliferone, a lipid-soluble intracellular pH fluorescent indicator, from a visually avascular 80-fm field were used to cross-correlate rCBF with focal cortical perfusion. Our findings indicate that in the cat, as the gamma or beta detector's field of volume was reduced, regional CBF (rCBF) measured by intraarterially injected ¹³³Xe or ⁸⁵Kr decreased in value by 33% and 28%, respectively, and the slope of the rCBF-P_aco₂ response curve became less steep by 56% and 45%, respectively. Umbelliferone, measuring a much smaller volume of tissue, showed a lower normocapnic flow and a more oblique P_aco₂ response curve. In the rabbits studied, the normocarbic rCBFs and the rCBF-P_aco₂ response curves measured with the three techniques corresponded to those measured in the cat. These results suggest that large field/volume measurements assess a measure of flow that is a weighted average of several distinct flow compartments and that these compartments differ in their response to changes in P_aco₂.