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The aim of the present study was to investigate the effect of hypotensive tachycardias on cerebral blood flow (CBF) in the presence of significant carotid stenosis. The experiments were performed in 57 spontaneously breathing rats during arterial normoxia and normocapnia anesthetized with thiobarbital. CBF was determined with radiolabeled microspheres during control conditions (normofrequent sinus rhythm, normotension; group A; n = 15), during high-rate left ventricular pacing (660-840 ppm) at normotension (group B1; n = 13), borderline hypotension (group B2; n = 15) and severe hypotension (group B3; n = 7). In addition, CBF measurements were performed during borderline hypotension induced by hemorrhage (group C; n = 7). Global CBF was 1.09 +/- 0.29 ml g-1 min-1 in group A, 0.93 +/- 0.40 in group B1, 0.68 +/- 0.31 in group B2 (P < 0.05 vs. A), 0.42 +/- 0.16 in group B3 (P < 0.05 vs. A) and 0.83 +/- 0.2 in group C. The highest CBF values were found in the cerebellum (A; 1.43 +/- 0.5 ml g-1 min-1) and the lowest in the postocclusive tissue of the ipsilateral hemisphere (A; 0.74 +/- 0.2 ml g-1 min-1). In all groups a 15% mean CBF reduction in the right hemispherical cerebrum in comparison to the left hemisphere was observed (P < 0.01). In contrast, hemispherical CBF of the cerebellum did not differ. The CBF blood pressure relationship shifted to lower CBF values, the threshold of CBF regulation shifted to higher blood pressure values in the tissue regions distal to the occluded vessel during hypotensive tachycardias.(ABSTRACT TRUNCATED AT 250 WORDS)