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L. Schmetterer, J. Kolodjaschna, F. Berisha, E. Polska; Effects Of Yohimbine On Dynamic Autoregulaion In The Ophthalmic And The Middle Cerebral Artey . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1795.
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Dynamic autoregulation assesses the full time course of the autoregulatory process by analyzing beat–to–beat changes in both perfusion pressure and blood flow parameters. We have previously shown that the time response in blood flow velocities after a step decrease in blood pressure differs between the ophthalmic and middle cerebral artery (OA; MCA) (Kolodjaschna et al. IOVS 2005;46(2):636–40). In the present study we hypothesized that the response of blood flow velocities in the OA may be altered by administration of the alpha–2 antagonist yohimbine.
16 healthy nonsmoking male subjects were studied in a randomized placebo controlled design on two study days. Ultrasound parameters and arterial blood pressure (ABP) were recorded in each subjects before, during, and after a step drop in blood pressure after administration of either yohimbine or placebo. A thigh cuff technique was used to induce the drop in ABP. Peak systolic flow velocity (PSV) and end diastolic flow velocity (EDV) were assessed with a color Doppler probe for the OA and the MCA. Systolic and diastolic blood pressures (SBP, DBP) were measured continuously using a Finapres system. Autoregulation index, as a measure of vascular resistance, was calculated for systole and diastole as AIS= SBP/PSV and AID=DBP/EDV, respectively.
The time course of blood flow velocity in response to the thigh cuff release was statistically significant for systole and for diastole versus baseline in the two arteries under study (p < 0.001 each). In addition, we found a statistically difference in the response of the time course of flow velocities and autoregulatory indices between the OA and the MCA (p < 0.001). Autoregulatory indices indicate an early vasodilatation distal to the MCA and a slightly retarded vasoconstriction distal to the OA. The latter effect was reduced by administration of yohimbine (p=0.33), whereas the response in the MCA was not modified by the alpha–2 antagonist (p = 0.83).
Our results confirm differences between the autoregulatory response of the MCA and the OA after a step decrease in perfusion pressure. Modification of the dynamic response of flow velocities in the OA by yohimbine indicate that vasoconstriction after a step decrease in blood pressure is related to a change in sympathetic drive mediated at least in part via alpha–2 receptors.
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