Abstract
Purpose :
Most studies of the effect of acute elevation of intraocular pressure (IOP) on ocular blood-flow have focused on retinal and choroidal flow. This study determined the effect of acute changes in IOP on blood flow velocity in the retrobulbar arteries and veins supplying and draining the eye in a rat model.
Methods :
We inserted a 30-guage needle into the anterior chamber of Sprague-Dawley rats and increased IOP in 10 mm steps to 60 mmHg and then returned to 10 mmHg. After 1 minute at each pressure level (and 3 minutes after return to 10 mmHg), we used a 128-element, 18 MHz linear array probe to acquire plane wave ultrasound data at 3000 images/sec for 1.5 sec. Each image was formed by compounding transmits at 6 angles over ±6 degrees. After applying a signular value decomposition filter to suppress echo data from slow-moting or stationary tissues, we generated color-flow Doppler images, identified retrobulbar veins and arteries and produced spectrograms from which systolic, diastolic and mean flow velocity and resistance indices were determined (Fig 1). Twenty eyes (1 eye per rat) were examined.
Results :
Baseline mean arterial and venous velocities averaged 30.9±10.8 and 8.5 ±3.3 mm/sec, respectively. Arterial velocity progressively decreased at and above an IOP of 30 mmHg (Fig 2). At 60 mmHg, mean arterial velocity dropped by 55% with respect to baseline, venous velocity decreased by 20% and pulsatility index increased by 75%. Arterial and venous velocities and resistance indices returned to near baseline after IOP was restored to 10 mmHg.
Conclusions :
Ocular hypertension directly compresses the retinal and choroidal vasculature, but only indirectly affects the retrobulbar circulation. Our results show progressively decreasing retrobulbar arterial velocities an increasing pusatility at and above an IOP of 30 mmHg. Venous flow velocity decreased as well, but not as profoundly. The increase in retrobulbar arterial pulsatility index is consistent with compression of retinal vessels. Although the more moderate decrease in venous velocity could be artifactual due to reduced sensitivity to slow-flow, it may be attributable to a decrease in retrobulbar venous lumen diameter as venous blood pressure dropped with decreased volumetric outflow.
This is a 2021 ARVO Annual Meeting abstract.