IN-VITRO AND IN-VIVO FLOW CHARACTERISTICS OF GLAUCOMA DRAINAGE IMPLANTS

Purpose: To determine pressure-flow characteristics at physiologic flo w rates in vitro and in vivo in rabbits for Ahmed, Baerveldt, Krupin d isk, and OptiMed glaucoma implants. The Molteno dual-chamber implant a lso was evaluated in vivo only. Methods: Five samples of each glaucoma implant were studied. Baerveldt implants were ligated partially for i n vitro testing. Opening and closing pressures in air or after immersi on in balanced salt solution or plasma were evaluated for the valved d evices (Ahmed and Krupin). Pressures were measured in vitro and in viv o in normal rabbits at flow rates preset at between 2 and 25 mu l/minu te after the tubes were connected to a closed manometric system. In vi vo measurements were made 24 hours after implantation. Resistance to f low was calculated using Poiseuille's equation after at least three se parate flow rate readings.Results: In air, the Ahmed and Krupin valves had opening pressures of 9.2 +/- 3.4 and 7.2 +/- 0.6 mmHg and closing pressures of 5.2 +/- 0.9 and 3.9 +/- 1 mmHg, respectively. Neither op ening nor closing pressures could be determined when Ahmed and Krupin valves were immersed. In vitro, the Ahmed and OptiMed devices had high er pressures than did other devices at a 2-mu l/minute flow rate of ba lanced salt solution. During perfusion with plasma, only the OptiMed d evice maintained higher pressures than with balanced salt. With all de vices, pressures fell rapidly to zero after flow was stopped. The Opti Med device demonstrated the highest resistance values. In vivo, the Ah med device provided pressures of 7.5 +/- 0.8 mmHg and the OptiMed devi ce gave pressures of 19.6 +/- 5.6 mmHg at a 2-mu l/minute flow rate. A fter 15 minutes of flow shutdown, the OptiMed implant maintained press ures of 7.1 +/- 1.1 mmHg. The Baerveldt (nonligatured), Krupin, and Mo lteno dual-chamber implants had similar resistances and pressures in v ivo. Pressures with all devices in vivo fell rapidly to zero after con junctival wound disruption. Conclusion: Neither the Ahmed nor Krupin d evices had demonstrable opening or closing pressures when tested in vi tro immersed in balanced salt solution or plasma. With all devices, pr essures were higher in vivo than in vitro due to tissue-induced resist ance around the explant. Both Ahmed and Krupin valves functioned as fl ow-restricting devices at the flow rates studied, but did not close af ter initial perfusion with fluid.