The influence of elevated intraocular pressure on vascular pressures in the cat retina.

PURPOSE Elevated intraocular pressure is known to reduce retinal blood flow, although the effect of intraocular pressure on retinal vascular pressures is unknown. Direct measurements of intravascular pressures were taken in the cat retina at various intraocular pressures. METHODS Micropipettes of 2- to 3-microns tip diameter were used in conjunction with a servonull pressure-measuring system to determine retinal intravascular pressures in supine anesthetized cats. Pressures in large (80 to 120 microns diameter) vessels near the optic disc were measured over a wide range of intraocular pressures. RESULTS Measurements show that retinal artery pressure depends on both intraocular pressure and mean systemic blood pressure, and that retinal vein pressure is determined by, but generally is different from, intraocular pressure, with no significant correlation to mean systemic blood pressure. Empirical equations are presented that predict statistically significant retinal artery, vein, and microvascular perfusion pressures. CONCLUSIONS Intraocular pressure is an important determinant of the microvascular perfusion pressure in the retina of the cat, particularly at low mean systemic blood pressure. It is also apparent that retinal vein pressure is always greater than intraocular pressure, which implies the existence of a high-resistance extraretinal segment of the retinal vein. The results suggest mechanisms for the loss of visual function in glaucoma and other retinal circulatory disorders.