Blade Can Convert Ahmed Valve Flow Characteristics

A specially designed blade successfully destroyed the functionality of valve leaflets in a glaucoma device, turning it from a flow-restrictive device into non–flow-restrictive device — and following the procedure, intraocular pressures (IOP) were similar to a non-flow restrictive device, according to a study published in the Journal of Glaucoma.

The in vitro study tested 5 flow-restrictive devices, the Ahmed glaucoma valve (AGV), against a control group of 2 non–flow-restrictive devices, the Baerveldt/Aurolab aqueous drainage implant (AADI). Researchers used a connected 27 g cannula, open manometer, digital manometer, and automated infusion pump. They logged data with a digital manometer at 4 Hz that used computerized software. Once the AGV device’s flow characterization reached the steady phase, valve functionality was destroyed with a newly designed, special blade that sliced the device’s bi-membrane valve leaflets without affecting tube stability. Then, the device’s flow characteristics were compared to the AADI device.

They found that, following initial resistance to flow (5, 8 mmHg) in the AADI device for 1 hour to 3 hours, it dropped to 1 mm Hg in both implants. 

“The flow-restrictive AGV showed 3 distinct phases in the flow characterization,” investigators report. “The first phase included the transient phase followed by

the steady phase wherein the pressure was approximately 11.2 mm Hg. The pressure resistance of the AGV dropped significantly (P <.001) to a mean of 0.4 mm Hg (± 0.54 mm Hg, 1 mm Hg in 2 devices and 0 mm Hg in 3 devices) after the valve functionality was destroyed.” 

The average time for this pressure resistance drop following the valve slicing? About 10.2 ± 3.0 minutes (min 7, max 15), according to researchers. 

“The probable reason behind the time delay in loss of resistance even after the membrane slicing is as follows: the initial high resistance between the 2 membranous valve leaflets is reduced during the priming and flow characterization process, however, the resistance is not eliminated,” the study explains. “That is why it functions as a ‘valve.’ The resistance is offered by the leaflets of the valves that are prestressed to maintain apposition against each other. During the process of slicing the membranous valve, the pressure is applied by the blade on the leaflets that are pressed toward the casing and are sliced.”

Reference

Senthil S, Kumar GM, Chittajallu SNSH, et al. Conversion of flow-restrictive ahmed glaucoma valve to a nonrestrictive drainage implant by slicing the valve leaflets: an in vitro study. J Glaucoma. 2021;30(1):5–9. doi: 10.1097/IJG.0000000000001671.