The mechanism of embolic watershed infarction: experimental studies.

The mechanism for the preferential distribution of emboli to cerebral arterial borderzone regions, known to cause some watershed infarcts was studied. We hypothesized that emboli of a specific size range are selectively directed to the arterial borderzones due to the tendency of emboli to bypass the small arterial branches which emerge proximal to major borderzones. To test this hypothesis we perfused the brains of cadavers with suspensions of 90-210 microns glass microspheres and chemically extracted the particles from various arterial territories and a watershed zone. Particles in the 150-210 microns size range were found to be preferentially distributed to the watershed zone whereas particles less than 150 microns in size were randomly dispersed in leptomeningeal arteries of all vascular regions. To assess the role of branch size on the concentration of emboli at bifurcations, we perfused artificial analogs of evenly and unevenly branching bifurcations with suspensions of 90-150 microns and 150-210 microns particles. Branching cylinders with symmetrical branches contained the same concentration of particles, independent of particle size. In contrast, when one branch was one-quarter the size of the other, the concentration of 150-210 microns particles in the asymmetric branch was approximately 65% of the main trunk. Particles 90-150 microns in size were evenly distributed despite variation in branch size. These results indicate that emboli, of a limited range of size, may be selectively propagated to the distal ramifications of subarachnoid arteries located in the watershed zone rather than diverging into small calibre branches which arise along the way.