A method for near-continuous determination of aqueous humor flow; Effects of anaesthetics, temperature and indomethacin

Abstract

A method is described for near-continuous determination of aqueous humor flow. The anterior chamber is perfused with push-pull coupled syringes at a low rate with a fluid containing labelled albumin. An external circuit is used to determine continuously the anterior chamber concentration of the labelled protein. The dilution data are analysed on-line by a minicomputer which permits rapid calculation of the anterior chamber volume and the rate of flow of aqueous humor. The technique and some experiments of technical interest are reported. Experiments in monkeys with different anaesthetics resulted in flow values of 0·99 ± 0·02, 1·47 ± 0·09 and 0·99 ± 0·04 μl min −1 for pentobarbital, urethane and ketamine anaesthesia, respectively. By using 125I-labelled albumin in one eye and 131I-labelled albumin in the other, it was possible to determine flow in both eyes. Highly significant correlation coefficients between the two sides were found for the rate of aqueous flow, intraocular pressure and anterior chamber volume. Rapid changes in inflow into the anterior chamber from the posterior chamber were produced by elevating and then lowering the intraocular pressure; the delay inherent in the method was about 6 min. Indomethacin, 3 mg kg −1 body wt., had no effect on aqueous humor flow in eyes cannulated with a minimum of trauma. In eyes with problematic cannulation indomethacin at this dose tended to delay an irritation response. Changes in temperature of the fluid perfused through the anterior chamber had no clear effect on the rate of aqueous flow. Warming the animals about 3–4°C above the normal temperature tended to increase the rate of aqueous flow. Cooling by 3–4°C had no clear effect. Cooling after an initial warming also had no clear effect. The rate of flow of aqueous humor from the anterior chamber to the general circulation was calculated from data for the accumulation of labelled albumin in the general circulation. The difference between the rate of aqueous flow determined from the dilution data and the flow into blood was assumed to represent uveoscleral flow. In 14 animals with an aqueous flow of 1·19 ± 0·08 μl min −1 the flow to the general circulation was 0·57 ± 0·055 and uveoscleral flow 0·61 ± 0·09 μl min −1. The procedure and mathematical treatment will be applicable to flow determinations with other large molecules and in other systems.