[Assessment of the lower threshold in autoregulation of the blood supply of the optic nerve disk].

Existence of the autoregulation phenomenon in the blood supply system of the optic nerve has been established in the 1990; however, some of its quantitative and qualitative characteristics still require clarification. The goal of the present work was assessment of the lower margin in autoregulation of the blood supply of the optic nerve in the rabbit in conditions of attenuated level of perfusion pressure. Total of six adult rabbits weighing 2.5-3.0 kg, anesthetized with urethane (0.9-1.1 g/kg) were investigated in acute experiments. Intensity of the local blood flow in the optic nerve disk was evaluated quantitatively by means of the hydrogen clearance. The frontal chamber of an eye was punctured with syringe needle, which via silicon tube was attached to the reservoir filled with artificial aqueous humor. Adjustment of vertical position of the reservoir provided different levels of the intraocular pressure. Calculation of the perfusion pressure of the eye was made according to mean level of the systemic arterial pressure, intraocular pressure, and a constant, which depends on an animal species. The results obtained allow suggesting with a certain degree of precision that elevation of the intraocular pressure to about 40-50 mmHg does not alter the blood supply of the optic nerve disk. Its further increase however leads to almost linear dependence between elevation of the intraocular pressure and decrease of the blood supply of the optic nerve disk. Meanwhile, the systemic arterial pressure should be considered as well, because volume of the perfusion pressure in the eye is determined by difference between systemic arterial- and intraocular pressures.