ABSTRACT
Most studies of the effect of acute elevation of intraocular pressure (IOP) on ocular blood-flow have utilized optical coherence tomography (OCT) to characterize retinal and choroidal flow and vascular density. This study investigates the effect of acute IOP elevation on blood flow velocity in the retrobulbar arteries and veins supplying and draining the eye, which, unlike the retinal and choroidal vasculature, are not directly compressed as IOP is increased. By cannulation of the anterior chamber of 20 Sprague-Dawley rats, we increased IOP in 10â¯mmHg steps from 10 to 60â¯mmHg and returned to 10â¯mmHg. After 1â¯min at each IOP (and 3â¯min after return to 10â¯mmHg), we acquired 18â¯MHz plane-wave ultrasound data at 3000 compound images/sec for 1.5â¯s. We produced color-flow Doppler images by digital signal processing of the ultrasound data, identified retrobulbar arteries and veins, generated spectrograms depicting flow velocity over the cardiac cycle and characterized changes of vascular density and perfusion in the orbit overall. Systolic, diastolic and mean velocities and resistive and pulsatile indices were determined from arterial spectrograms at each IOP level. Baseline mean arterial and mean venous velocities averaged 30.9⯱â¯10.8 and 8.5⯱â¯3.3â¯mm/s, respectively. Arterial velocity progressively decreased and resistance indices increased at and above an IOP of 30â¯mmHg. Mean arterial velocity at 60â¯mmHg dropped by 55% with respect to baseline, while venous velocity decreased by 20%. Arterial and venous velocities and resistance returned to near baseline after IOP was restored to 10â¯mmHg. Both vascular density and orbital perfusion decreased with IOP, but while perfusion returned to near normal when IOP returned to 10â¯mmHg, density remained reduced. Our findings are consistent with OCT-based studies showing reduced perfusion of the retina at levels comparable to retrobulbar arterial flow velocity change with increased IOP. The lesser effect on venous flow is possibly attributable to partial collapse of the venous lumen as volumetric venous outflow decreased at high IOP. The continued reduction in orbital vascular density 3â¯min after restoration of IOP to 10â¯mmHg might be attributable to persisting narrowing of capillaries, but this needs to be verified in future studies.