Modelling and measuring the viscoelastic properties of the in vivo rat eye

Abstract

Purpose : The Goldmann Equation models the eye as a purely resistive element, which is adequate understeady-state conditions but insufficient to quantify the dynamic properties of the eye. We propose a model with both resistive and viscoelastic components that represent the trabecular meshwork and scleral shell, respectively. This study aims to use the model to measure viscoelastic parameters of rat eyes.

Methods : Figure 1 provides a schematic of the viscoelastic eye model and experimental setup, which was simulated in MATLAB. Two 33-gauge needles were advanced into the anterior chamber of anesthetized Brown-Norway rat eyes. The first needle was connected to a pressure transducer to record true intraocular pressure (IOP). The second needle was attached to a stopcock connected with 25-gauge tubing to anotherpressure transducer, a flow meter, and a controllable pump. IOP and flow data were collected at 50Hz. Outflow facility (1/RT) was measured by recording steady-state flow at multiple IOP setpoints. Ocular compliance (CW) was measured by recording IOP responses to bolus injections of varying volumes. To determine corneoscleral resistance (RW), the time constant of the eye was measured by fitting an exponential function to the decay in IOP after a pressure step. RW was calculated from the measured facility, compliance, and decay time as follows: t = CW (RW + RT). To help minimize error, a third needle of small and known resistance RSH was inserted in the eye as a shunt, eliminating uncertainties in RT.

Results : Viscoelastic data were successfully collected from 5 animals. Model simulations accurately described measured pressure and flow dynamics to step changes in IOP of 5 mmHg. 1/R_T, C_W, and t were measured to be 0.017 ± 0.01 ml*min^-1*mmHg^-1, 0.08 ± 0.05 ml*mmHg^-1, and 0.18 ± 0.05 min, respectively. Based on these results, R_W averaged 2.22 ± 1.27 mmHg*min*ul^-1 across animals.

Conclusions : The viscoelastic model captures ocular fluid dynamics of anesthetized rats to pressure and flow steps. The “shunt method” provides a quick and accurate means of estimating Rw. RW is relatively small but may cause overestimation of RT if measurements have not reached steady state.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.

 

Figure 1. Electrical circuit equivalent of the viscoelastic eye (red) and experimental setup (black).