The effect of simulated episcleral venous pressure on total outflow facility in an ex-vivo porcine model for aqueous humor dynamics

Abstract

Purpose : Ex-vivo models to study aqueous humor dynamics typically lack episcleral venous pressure, even though converging evidence suggests that it is an important component of distal outflow resistance. In this study, we set out to simulate episcleral venous pressure by perfusing the ophthalmic artery in porcine eyes and to study the effect on total outflow facility.

Methods : In porcine post-mortem eyes (n=4), the ophthalmic artery was cannulated (PE tubing, o.d. 0.76mm). The vascular system was flushed with saline containing heparin (500I.E./mL) and trypan blue. The perfusion of the episcleral vasculature was confirmed by visual inspection of trypan blue appearance. The anterior chamber was cannulated with two 25G needles to simulate aqueous humor production and measure intraocular pressure, respectively. The anterior chamber was perfused at 4.5µL/min until a stable baseline was reached. Then, the ophthalmic artery was perfused using a gravity-based system at 60 mmHg to simulate mean arterial blood pressure. Intraocular pressure was measured continuously and outflow facility was calculated.

Results : Perfusion of the episcleral circulation could be visualized in all eyes. At baseline, the outflow facility was calculated as 0.349 ± 0.085µL/mmHg/min. After perfusion of the ophthalmic artery at 60 mmHg perfusion pressure, outflow facility dropped to 0.215 ± 0.0546 µL/mmHg/min. This decrease was statistically significant (p = 0.0068).

Conclusions : The present study shows that a commonly used model for aqueous humor dynamics can be expanded to simulate distal outflow resistance. More work is needed to characterize the model in detail and especially measure the induced level of episcleral venous pressure.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.