Ocular Physiology Clinical Trial
Official title:
The Effect of an α2-Adrenoceptor Antagonist (Yohimbine) on Dynamic Autoregulation in the Human Middle Cerebral Artery and Ophthalmic Artery
Blood flow autoregulation is defined as the ability of a tissue to maintain a relatively
constant flow, despite moderate alterations in perfusion pressure. Similar to the cerebral,
renal, coronary and skeletal muscle circulations, the ocular vascular bed shows the property
of flow autoregulation. This homeostatic mechanism allows blood supply to the eye to match
metabolic demand during daily activities, such as changes in posture, or in more critical
conditions.
Autoregulation has been found to be a complex phenomenon, showing heterogeneity in its site
and time course of action. Since metabolic, myogenic, neurogenic and possibly
endothelium-related mechanisms may be involved, several factors may vary depending on the
challenging stimulus, the vessel tone, or the degree of impairment of autoregulation.
To study the dynamics of ocular autoregulation, it is necessary to introduce a step
disturbance (stimulus) in ocular perfusion pressure and to record the responses of ocular
blood flow continuously before and after this step disturbance. The investigators have
employed a mechanical noninvasive technique to induce an ocular perfusion pressure step
disturbance without drugs or changes in the concentration of vasoactive substances in the
blood by using the thigh cuff technique inducing a small step decrease in ocular perfusion
pressure.
With this technique the investigators could show significant differences in the time
response of blood velocities in the ophthalmic and middle cerebral artery. This clearly
indicates different mechanisms to be responsible for autoregulatory mechanisms distal to the
vessels.
Interestingly our results indicate that in the ophthalmic artery a late vasoconstriction
occurs. Many previous investigations have demonstrated that sympathetic nerve stimulation
causes vasoconstriction in the ocular circulation. Accordingly, the present study tests the
hypothesis that α2-adrenoceptors are involved in the dynamic regulation of blood flow in the
ophthalmic and middle cerebral artery after a step decrease in perfusion pressure.
n/a
Allocation: Randomized, Endpoint Classification: Safety/Efficacy Study, Intervention Model: Crossover Assignment, Masking: Double-Blind, Primary Purpose: Treatment
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