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Joseph Nelly Sugu Sugira, Matthew Rickard; An Engineering Model for Predicting the Aqueous Humor Flow Rate through the Natural Pathway Following Glaucoma Surgeries. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4769.
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© ARVO (1962-2015); The Authors (2016-present)
To predict the aqueous humor flow rate through the natural pathway following glaucoma surgeries for various pre and post-operative conditions.
An engineering model has been developed for predicting the post-operative aqueous humor flow through the natural and surgically created pathways, where the latter can be a subconjuctival route initiated in glaucoma surgery such as a tube procedure (valve or valveless), a trabeculectomy, or a scleral mini-shunt operation. The model solves for the fluid flow parameters via analogy by solving equations commonly used in electrical circuit analysis. With the exception of the Ahmed Valve, the model assumes all resistances to be independent of the pressure differential across the resistor and all resistances are assumed unchanged with time. A steady outflow is assumed, which is driven by IOP (ignoring special behavior of the uveoscleral outflow). Resistance through the natural pathway, which is assumed to be the same both pre- and post-operative, is calculated using pre-operative IOP and by assuming all flow to pass through the natural pathway prior to surgery. To approximate the resistance of the Ahmed Valve (New World Medical, Model FP7), data was collected from a gravity-driven in vitro experiment using a low-flow liquid flow meter (Sensirion Inc.) and a pressure sensor (Honeywell).
For a pre- and post-operative IOP of 30 mmHg and 10 mmHg, respectively, 13% of the total flow is predicted through the natural pathway post-operation for any external path procedure (valve/valveless tube, trabeculectomy, scleral mini-shunt, etc.), based on the assumptions of an episcleral venous pressure and aqueous humor flow rate of 7 mmHg and 2.5 µL/min, respectively. For a post-operative IOP of 15 mmHg (all other conditions unchanged), 34% is predicted through the natural pathway. For the Ahmed Valve in the above 10 and 15 mmHg cases, the pressure downstream of the valve is predicted to be 1.6 and 7.1 mmHg, respectively. Since our model assumes the resistance in the natural pathway (TM, Schlemm’s canal, collector channels) to be unchanged pre- and post-operation, our prediction of flow maintained in the natural path is further increased if that resistance is reduced during surgery.
Our engineering model predicts that the aqueous humor outflow through the natural pathway following Glaucoma surgery can be significant.
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