Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 81

The vascular pressure promoting flow, the resistance of blood vessels, and the viscosity of the blood all influence the blood flow through all tissues, including the eye. The pressure head for blood flow (i.e., perfusion pressure) in most tissues is the difference in pressure between the arteries and the veins. However, in the eye, the IOP approximates the venous pressure, so the perfusion pressure is the difference in pressure between the small arteries entering the eye and the IOP. Of clinical importance is that the perfusion pressure to the eye is reduced by lowering the blood pressure or raising the IOP, as occurs in glaucoma. Studies of hemodynamics in the rabbit ophthalmic artery demonstrate that autoregulation maintains normal blood velocity and resistance when the IOP is below 40 mmHg. However, at higher pressures the autoregulatory capacity is limited. As a result, an IOP of about 15–17 mmHg is related to episcleral venous pressure of about 10–12 mmHg and about 5–7 mmHg of resistance from passage through the AH pathways!