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.

In vivo micropuncture of retinal vessels.

Micropuncture has proven to be a valuable tool for the local study of vascular parameters in many organ systems; however, it has not been applied to the study of the circulation of the retina. We report here our extension of micropuncture techniques [4] to use in the intact retina of the anesthetized cat. We use extremely sharp micropipettes with tip sizes much smaller than the diameter of erythrocytes to avoid hemorrhage. The micropipette is held by a microdrive which in turn is mounted on a precision goniometric micromanipulator. We micropuncture retinal arteries and veins with diameters ranging from 20 to 130 microns with no apparent damage to the vessel wall and no observed hemorrhage. During micropuncture we routinely inject nanoliter quantities of dyed saline, which we observe flowing in a plume from the micropipette tip within the lumen of the vessel. Micropuncture techniques may be used in the laboratory to study retinal autoregulatory mechanisms by microinfusion of vasoactive substances and by measuring blood pressure in retinal microvessels. In the clinic micropuncture may be useful for treating disorders such as retinal vascular occlusion.