Priming the Ahmed Glaucoma Valve: pressure required and effect of overpriming.

PURPOSE

To determine the pressure required to prime an Ahmed Glaucoma Valve (AGV) and determine whether the valve can be damaged by "over-priming pressure."

METHODS

Three AGVs, a syringe pump, and a manometer were used to assess priming pressure. Balanced salt solution was pumped through the AGV tube at increasing pressures until a jet of fluid was seen to eject from the AGV, as per manufacturer instructions. This was repeated 3 times for 3 different virgin AGVs giving the "priming pressure." A second experiment used the same experimental set up to determine the "over-priming pressure" on 3 other AGVs. Fluid was pumped through the AGV at increasing pressures until evidence of damage was seen. The valve function was assessed before and after the "over-priming" stress test. Valve function was determined by the closing pressure, which is the pressure at which the valve closes and fluid was no longer seen passing through the valve.

RESULTS

The priming pressure in the 3 AGVs was 2844, 3154, and 3051 mm Hg (mean, 3017±158 mm Hg). The maximum pressure generated using the syringe pump was 10,860, 10,343, and 10,860 mm Hg (mean, 10,688±299 mm Hg). No damage was observed in the valve mechanism. AGV closing pressure before the "over-priming" stress test was 8, 6, and 13 mm Hg and after the stress test was 6, 7, and 13 mm Hg.

CONCLUSION

This study demonstrates that the priming pressure is consistent at around 3000 mm Hg. In addition, over-priming is not likely to damage or disturb the closing pressure.