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L. Schmetterer, G. Weigert, H. Resch, G. Garhoefer; Effects of Intravenous Clonidine on Ocular Blood Flow . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4723.
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Purpose: Clonidine, an alpha 2 agonist reduces intraocular pressure after systemic and topical administration. The current study was performed to assess the effect of intravenously administered clonidine on ocular blood flow. Methods: A randomized double masked placebo controlled two–way cross over study was performed in 10 healthy volunteers. Clonidine (0.2µg/kg) or placebo was infused intravenously over 10 minutes. Hemodynamic measurements were performed at baseline and 30 minutes after infusion start. Chorodidal blood flow was assessed using laser Doppler flowmetry, retinal vessel diameters were measured using a Retinal Vessel Analyzer. Red blood cell velocity was determined by using bi–directional laser Doppler velocimetry. Retinal blood flow was calculated based on retinal vessel diameters and red blood cell velocity. Results: Intravenous administration of clonidine decreased mean arterial pressure from 76±7 mmHg to 64±5 mmHg(p<0.01). As expected, clonidine decreased IOP from 12±3 mmHg to 8±3 mmHg. Retinal arterial diameters increased after administration of clonidine by +3±3% (p=0.014), whereas retinal venous diameters tended to increase by +1.1±2.8%.(p=0.6) Red blood cell velocity decreased by –14±16% (p=0.028). Thus, calculated retinal blood flow decreased by –12±13% (p=0.46). Subfoveal chorodial blood flow increased after drug administration by +16±13% (p<0.05). Conclusions: In the present study intravenously administration of clonidine reduced MAP and intraocular pressure. The effects of clonidine on retinal and choroidal blood flow were different. The increase in choroidal blood flow indicates vasodilatation within the choroid. The decrease in retinal blood flow indicates clonidine–induced vasoconstriction in retinal resistance vessels, because the decrease in ocular perfusion pressure is well within the autoregulatory range of the retina.
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