Vascular pressure-flow and pressure-resistance curves were determined by direct ophthalmic artery perfusion studies in enucleated eyes. Flow through the intraocular vascular bed ceases when the pressure perfusing the ophthalmic artery is reduced to 6 mm Hg above intraocular pressure. This critical closing pressure is not affected by the level of intraocular pressure but is markedly increased by adding levarterenol bitartrate (Levophed Bitartrate) to the perfusing fluid. Vascular resistance remains relatively constant at perfusion pressures in excess of the critical closing pressure. Vascular flow is directly related to perfusion pressure and is markedly affected by changes in intraocular pressure, especially at high perfusion pressures. The physiologic basis of critical closure is discussed.
Best M, Blumenthal M, Futterman HA, Galin MA. Critical Closure of Intraocular Blood Vessels. Arch Ophthalmol. 1969;82(3):385-392. doi:10.1001/archopht.1969.00990020387018