Effect of nimodipine on the autoregulation of cerebral blood flow studied by laser-Doppler flowmetry

The present work examines whether nimodipine impairs autoregulation of CBF during hypotension. The CBF of 16 anesthetized rabbits was measured with a laser-Doppler flowmetry probe placed on the external surface of a plexiglas window, chronically inserted in the skull. Autoregulation was triggered by aortic bleeding. First, the effects of three doses of nimodipine (1, 3 and 10 μg/kg) and the solvent were studied in 10 rabbits in which MABP was maintained at 50 mmHg for one minute. Second, 10 μg/kg i.v. nimodipine was administered to 6 rabbits in which MABP was kept at 30 mmHg for one minute. Before bleeding, the 10 μg/kg dose significantly decreased MABP (from 96 ± 11mmHg to 81 ± 11mmHg, P < 0.01) and increased CBF (from 104 ± 20%to147 ± 25%, P < 0.01) as compared to the solvent. In the first set of experiments, only the 10 μg/kg dose suppressed the autoregulatory vasodilation, but CBF was not different from control (84 ± 17%versus87 ± 12%), probably because of the previous induced vasodilation. In the second set of experiments, active vasodilation occurred and the CBF during hypotension was not different from control (72 ± 26%versus65 ± 11%). We conclude that under nimodipine the triggering of the active autoregulatory vasodilation is dependent on both the severity of hypotension and the previous nimodipine-induced vasodilation.