Heterogeneous Autoregulatory Capacity in the Rat Cerebral Cortex as Observed by a Novel Two-Dimensional Flow Mapping Technique

Cerebral blood flow (CBF) autoregulation in a small area of the cerebral cortex (region of interest, ROI) was examined by a novel optical method recently developed by the authors. A two-dimensional (2-D) map of mean transit time (MTT) was created during hemorrhagic hypotension in nine rats. The total average MTT in the control group ranged from 1.43 to 2.30 s (mean ± SD 2.09 ± 0.43 s), increased to 3.29 ± 0.84 s at a mean blood pressure of 50 mmHg, and decreased to 1.53 ± 0.37 s after reinfusion of blood. When 47 average CBF values obtained during nine experiments were plotted against the mean arterial blood pressure, autoregulatory changes were broadly observed above about 70 mmHg. The 2-D relative flow maps, calculated as 2-D MTTcontrol/2-DMTThypotension revealed that the autoregulation in the ROI was not homogeneous when the blood pressure decreased to 75 mmHg: some microregional areas remained at the control flow level, whereas others showed a decrease in CBF at the same perfusion pressure. We hypothesize that the well-autoregulating areas might represent the functionally active parts or functional units of the cortex. One possibility is that blood flow is redistributed to the functionally active portion from other resting portions of the cortical capillary bed when the reduced perfusion pressure challenges the cerebral circulation.