Heterogeneous response of cerebral blood flow to hypotension demonstrated by laser speckle imaging flowmetry in rats.

The response of cerebral blood flow (CBF) to mild hypotension shows great variability. CBF changes in different cortical regions at a mean arterial blood pressure (MABP) of 70 mmHg (near the lower limit of CBF-pressure autoregulation) were analyzed using laser speckle imaging flowmetry (LSIF). We hypothesize that variations in CBF autoregulation might be due to differences between regions within the cortex of each animal, regional heterogeneity, as opposed to differences between animals. Different responses of local CBF to hypotension in each rat were demonstrated as %CBF(70), the ratio of CBF at a MABP of 70 mmHg to CBF at a MABP of 100 mmHg. Regional heterogeneity ranged from 12 to 51%, expressed as the coefficient of variation of %CBF(70). At the same time LSIF revealed distinctly different patterns of CBF-pressure autoregulation between animals. Between-animal heterogeneity represents a continuous distribution, spread between animals with predominantly decreased CBF (%CBF(70) < 85%, rat #3) and animals with a high proportion of increased flow responses (%CBF(70) > 115%, rat #2). There are approximately equal contributions to the heterogeneity of %CBF(70) from within-animal (57 +/- 5%, percent variance +/- standard error) and between-animal (43 +/- 26%) variations. Within-animal variations could be due to heterogeneity of vascular anatomy, while the diversity of vascular control mechanisms might contribute to between-animal variations.