Intraventricular dexmedetomidine decreases cerebral blood flow during normoxia and hypoxia in dogs

We tested the hypothesis that a centrally administered alpha 2-receptor agonist could alter the cerebrovascular response to hypoxia, without evidence of systemic absorption of the drug. Beagle dogs were anesthetized with 1.4% isoflurane and exposed to hypoxic hypoxia (Pao2 approximately 22 mm Hg) before and after ventricular-cisternal perfusion with mock cerebrospinal fluid (CSF group, n = 5) or dexmedetomidine (100 micrograms/mL; total dose 300 micrograms; DEX group, n = 6). Cerebral perfusion pressure, Paco2 and arterial oxygen content were controlled and regional cerebral blood flow (CBF; microspheres) and global cerebral metabolic rate for oxygen consumption (CMRO2) were measured. In another group (n = 5), drug distribution under the experimental conditions was assessed by 3H-clonidine administered by ventricular-cisternal perfusion. In the mock CSF group, flow to the cerebral hemispheres increased during hypoxia under baseline conditions and after CSF infusion: 66 +/- 8 to 170 +/- 15 mL.min-1.100 g-1 (265% +/- 24% of baseline value), 83 +/- 9 to 154 +/- 14 mL.min-1.100 g-1 (201% +/- 54% of post-CSF infusion value). DEX decreased normoxic flow in the cerebral hemispheres from 76 +/- 6 to 44 +/- 4 ml.min-1.100 g-1 with decreases in other regions of similar magnitude. After DEX, the absolute flow in all regions during hypoxia was 52%-55% of that prior to DEX (P < 0.05). However, because DEX also decreased normoxic CBF, the percent increase in flow during hypoxia was similar before and after DEX. CMRO2 was not affected by hypoxia prior to DEX. However, after DEX, hypoxia caused a marked reduction in cerebral oxygen delivery (5.2 +/- 1.0 vs 13.7 +/- 2.3 ml.min-1.100 g-1 for the CSF group) and CMRO2 (2.5 +/- 0.6 vs 3.9 +/- 0.6 ml.min-1.100 g-1). Regional accumulation of intraventricularly administered 3H-clonidine was greatest in periventricular brain structures (e.g., caudate nucleus, dorsal brainstem), and the concentration in the cerebral cortex was approximately 1% of the concentration in the ipsilateral caudate nucleus. We conclude that centrally administered DEX reduces CBF during normoxia and prevents adequate oxygen delivery during hypoxia. The mechanism of DEX-induced CBF reduction is not metabolically mediated, since CMRO2 is maintained at control values during normoxia despite the significant blood flow reduction. We believe that the reduction in CMRO2 during hypoxia in DEX-treated dogs is the result of a reduction of oxygen delivery rather than the underlying mechanism for the observed reduction in CBF during hypoxia.