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Paul Munden, Ajay Ramani, Ronald Dougherty, Sarah Kieweg, Michelle Boyce; An Experimental Model of Pressure Modulation in Baerveldt Implants to Reduce Post-Operative Hypotony. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2005.
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Baerveldt implants are an established surgical option for the control of intraocular pressure in some patients with glaucoma. Two comparative studies with 3 year follow-up showed Baerveldt implants to have a lower failure rate and require fewer medicines for IOP control compared to Ahmed implants. However, Baerveldt implants were shown to have equal or worse complication rates than Ahmed implants, most often associated with hypotony.We hypothesize that the IOP control properties of the Baerveldt implant can be experimentally modeled and predictably modified to prevent post-operative hypotony by introducing inserts of various material and diameters into the tubing outflow system. We tested the hypothesis using an experimental setup based on theoretical analysis of the Baerveldt implant tubing flow.
We used Poiseuille’s equation to calculate the reduction in internal diameter of the Baerveld implant tubing necessary to yield a target pressure of 5 - 15 mmHg in the tubing outflow system. An experimental set-up consisting of a micro-flow pump, pressure transducer and Baerveldt implant tubing was designed to model physiologic flow rates and measure resultant pressures through the system. Balanced salt solution was infused through the system at a constant physiologic 2.5 microliters per minute. Baseline flow resistance and pressure were measured over a 24 hours with no occlusion of the implant tubing and then with 4 mm length tubing inserts of various materials and diameters.
See Table 1.
Our experimental setup provides an accurate and reproducible platform for evaluating the pressure modulating effects of Baerveldt tubing inserts of varying materials and diameters. Experimental data will direct the design of flow restrictive inserts for use in human clinical studies assessing their effectiveness in preventing early post-operative hypotony after Baerveldt implant surgery.
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