In vitro flow characteristics of the Ahmed and self-constructed anterior chamber shunts

To compare in vitro opening pressures (OP) and closing pressures (CP) of the Ahmed VS-1 and VS-2 glaucoma valves with those of several self-constructed valve 'prototypes,' and to assess their ability to maintain perfusion pressures between 6 and 21 mm of Hg.

Ahmed VS-1 (n = 6), 2 groups of Ahmed VS-2 (group 1: n = 12; group 2: n = 14), and self-constructed valves with linear incisions in the long axis of the tube wall (n = 6) or X-shaped incisions in the tube walls (n = 2).

Valves were perfused with deionized water, lactated Ringer's solution (LRS), Dulbecco's modified Eagle's medium (DMEM), DMEM plus 50% equine serum (ES), and 100% ES. Flow rates of 2.85, 4.2, 6.0, 9.0, and 12.0 microliter/min were used for each perfusate. Valves were tested 3 times for reproducibility, and OP/CP were compared for each system.

OP/CP of the VS-1, VS-2 (group 1), VS-2 (group 2), and linear 1.0-cm incisional valves with thick tubing consistently increased with increasing perfusion rate. Linear 0.5-cm (thick tubing) and 1.0-cm (thin tubing) incisional valves had increasing OP/CP with increasing perfusion rate in all but a few instances. Mean OP/CP decreased with increasing perfusate osmolarity for all perfusates except LRS, using the VS-1 and V-2 (group 2) valves. Mean OP/CP were consistently lower for VS-1 than VS-2 (group 1) valves at any given flow rate and for any given perfusate. Mean OP/CP were consistently lower for VS-2 (group 1) than VS-2 (group 2) valves at any given flow rate and for any given perfusate. The linear 0.5-cm incisional valves with thick and thin tubing induced the highest mean OP/CP, maximizing at > 30 mm of Hg.

Only the VS-2 (group 2) valves consistently had mean OP/CP between 6 and 21 mm of Hg for all perfusates and at all flow rates.

Anterior chamber shunts, although imperfect, appear to offer a physiologically sound alternative for glaucoma management.