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D. O. Zamora, J. W. Kiel; The Nitric Oxide Donor, Sodium Nitroprusside, Increases Episcleral Venous Pressure. Invest. Ophthalmol. Vis. Sci. 2009;50(13):811.
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The pressure gradient between the intraocular pressure (IOP) and the episcleral venous pressure (EVP) is what drives aqueous outflow through the trabecular pathway. Anatomic examination and physiologic studies indicate that changes in episcleral blood flow can change EVP and so modulate IOP. Nitrovasodilators are commonly used in humans to increase peripheral blood flow and lower arterial blood pressure. We hypothesized that applying the nitric oxide donor, sodium nitroprusside (SNP), to the eye would vasodilate the episcleral circulation and increase EVP.
In anesthetized rabbits (n=13), we measured the EVP using a servonull micropressure system. Glass micropipette needles were pulled and bevelled to a 2-3 micron tip and used to cannulate episcleral veins in the superior quadrant of the eye. Additional measurements included mean arterial pressure, IOP, orbital venous pressure, carotid blood flow and heart rate.
Baseline EVP, without topical anaesthesia, was measured at 9.08 ± 0.62 mmHg. EVP increased significantly (p<0.01) to 11.59 ± 0.82 mmHg within minutes after locally applying SNP (50 µl; 10mg/ml) to the episcleral region. IOP also increased within minutes from a baseline reading of 19.37 ± 1.43 mmHg to 24.29 ± 1.52 mmHg. Systemic blood pressure, carotid blood flow, heart rate and orbital venous pressure all remained unchanged after SNP treatment.
The greater response of IOP than EVP indicates that SNP may alter other aspects of aqueous dynamics (i.e., decreased facility or uveoscleral outflow, or increased aqueous production). Alternatively, it may be that the EVP response is heterogeneous and greater changes are occurring within the unmeasured quadrants of the episcleral plexus. Nonetheless, the EVP response to SNP indicates that the episcleral circulation is sensitive to nitric oxide.
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