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W. A. Van Heuven, J. W. Kiel; Arterial Pressure at the Entrance to the Retinal and Choroidal Circulations in the Rat. Invest. Ophthalmol. Vis. Sci. 2009;50(13):406.
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Commonly used rat models of glaucoma and ocular ischemia-reperfusion involve elevation of intraocular pressure (IOP), but the resultant decrease in perfusion pressures is difficult to gauge because the arterial pressure just outside the eye is not measured. This study sought to determine the arterial pressure feeding the retinal and choroidal circulations in the rat with a method similar to sphyngomanometry.
In pentobarbital anesthetized rats (n=4), we measured femoral arterial pressure (MAP) and IOP by direct cannulation, carotid blood flow by transit time ultrasonography, heart rate by digital cardiotachometery, and blood flow in the retina and choroid by laser Doppler flowmetry. The femoral artery and eye were at the same elevation as the heart and the pupil dilated with topical atropine. The protocol entailed raising IOP above systolic blood pressure by intravitreal saline injection and allowing IOP to decline spontaneously to baseline. The IOP at which retinal and choroidal blood flow first began to increase and when IOP pulsations began were determined as the threshold opening pressure.
The MAP was 83.1 ± 3.8 mmHg. The retinal opening pressure was 77.2 ± 2.5 mmHg, significantly less than MAP and greater than for the choroid and IOP pulsations. The opening pressure for the choroid and IOP pulsations were similar at 71.0 ± 4.2 and 70.4 ± 3.7 mmHg.
Since IOP pulsations are due to cardiac synchronous changes in choroidal blood volume, the similar opening pressures for the choroid and IOP pulsation are expected. What is surprising is that the retinal opening pressure is higher than for the choroid, indicating a higher arterial pressure in the central retinal artery (CRA) than in the short posterior ciliary arteries (SPCA). The results indicate that to calculate the rat ocular perfusion pressure based on femoral MAP (or tail cuff MAP which should be similar), correction factors of 7% and 15% are needed for the MAP calculation of the CRA and SPCA, respectively.
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