Platelet emboli in rat brain cross when the contralateral carotid artery is occluded

The pathogenesis of embolic events ipsilateral to an occluded carotid artery is uncertain. To examine this question we combined occlusion of the left common carotid artery with embolism from the right common carotid artery in rats. Following ligation of the left carotid artery in 20 experimental rats, we irradiated the right carotid artery with a laser (632 nm, 200 mW/cm2, 12-15 minutes) following the intravenous injection of 12.5 mg/kg of the photosensitizing agent Photofrin II. Controls had left carotid artery occlusion with (n = 13) or without (n = 6) Photofrin II. Fifteen of the 20 experimental rats survived to be perfused at 24 hours; cerebral infarcts were identified in 12 rats, with bilateral infarcts in 10. There were 112 infarcts (101 small [less than 2.5 mm] and 11 large [greater than 2.5 mm] on the right and 103 (93 small and 10 large) on the left. Emboli were seen in association with some infarcts and were evenly distributed in the two hemispheres (37 emboli on the right and 40 on the left, with the midline azygous artery occluded in four animals). Left carotid artery occlusion did not produce infarcts or emboli in the controls. We conclude that cerebral infarcts in the distribution of an occluded common carotid artery may be caused by emboli from the contralateral carotid artery in rats.