Retinal Blood Flow Response to Ocular Perfusion Pressure Modification

Abstract

Purpose: To quantify the effect of ocular perfusion pressure (OPP) lowering on absolute blood velocity in major retinal vessels using non-invasive high-speed imaging.

Methods: Anaesthetised adult male Long-Evans rats were used for all experiments (60:5mg/kg ketamine:xylazine). Blood pressure (BP) was lowered (n=6) via intravenous infusion of 2.5% sodium nitroprusside (100ul/min). Intraocular pressure (IOP) was increased (n=6) non-invasively by applying force around the equator of the eye with a calibrated variable aperture. Blood velocity was recorded during BP lowering (120 to 40 mmHg) and IOP elevation (10 to 40 mmHg). High-speed imaging (500 frames acquired at 1700fps, Metamorph 7.7) was achieved using an optical bench ophthalmoscope coupled to an Andor Neo sCMOS camera and a Polychrome V light source (577nm, bandwidth 10nm). Video frames were image registered and 25 sequential frames (starting at the peak of systole) were analysed using Particle Image Velocimetry (PIV) to calculate velocity. Data expressed as mean ± SEM and t-test and ANOVA were used to compare mean velocities.

Results: At basal conditions (systolic BP 99.6±1.9mmHg, IOP 10.3±0.3mmHg) mean blood velocity in arteries and veins at 1 disc diameter from the optic nerve were 92.1±15.1 mm/s and 88.6±8.9mm/s respectively. With reduction of systolic BP to 62.3±1.9mmHg, arterial velocity reduced to 22.7±1.8mm/s (p=0.03) and venous velocity to 32.2±5.7mm/s (p=0.01). At very low systolic BP (44.7±1.2 mmHg) arterial and venous velocities were 14.4±3.0mm/s (p=0.003) and 20.7±4.3mm/s (p=0.001 ), respectively. Blood velocity also reduced with IOP elevation. At an elevated IOP of 41.0±2.8mmHg, arterial and venous velocities were 29.9±6.3mm/s (p=0.015) and 21.9±10.4mm/s (p=0.002) respectively. For an equivalent OPP reduction (-40mmHg), IOP elevation and BP reduction produced a similar decrease in blood velocity in both arteries (-71.4±7.9, 2-way ANOVA, p=0.11) and venules (-69.60±8.7, p=0.69).

Conclusions: Intraocular pressure elevation and blood pressure lowering have similar effects on ocular blood velocity in large arteries and veins.

Keywords: 551 imaging/image analysis: non-clinical • 436 blood supply • 568 intraocular pressure