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GS Hafner, DV Luu, R Mutharasan, SP Srinivas; Measurement Of Outflow Facility In Bovine Eyes Mounted In Vitro With A Microinjector Based Pressure Clamp . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1037.
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Purpose: Increased resistance to outflow of aqueous humor across the trabecular meshwork (TM) into the Schlemm’s canal (i.e., inverse of outflow facility) is implicated in the pathophysiology of primary open angle glaucoma. This study has implemented an apparatus for the measurement of the outflow facility based on pressure clamp approach suitable for bovine eyes in vitro. Methods: Intracameral infusion rate through a hypodermic needle (23G) is controlled to maintain a specified IOP (set point) by a syringe pump (Harvard 2400) held in a negative feedback control loop. IOP is sensed upstream of the pump at 3 Hz by a piezoelectric pressure sensor (resolution=0.02 mm Hg) mounted inline with the infusion capillary (1 mm diameter; stainless steel). A digital proportional integral (PI) control algorithm was implemented in the PC (using LabView 6i) to generate command outputs to the pump via RS232 port to adjust the injection rate (the manipulated variable) to maintain the at the IOP set point. The ratio of flow rate to IOP is the outflow facility. Experiments were conducted to characterize the dynamics of the feedback loop with a simulated outflow resistance and then outflow facility was characterized in intact bovine eyes at 37°C. Results: In response to a simulated step increase in backpressure by 10%, transition to a new flow rate required to maintain the IOP was attained in 2 min without an offset in IOP. Similarly, for a step change in the set point by 25 mm H20 (∼ 2 mm Hg), a new steady state in flow rate was achieved within 2 min without oscillations in IOP. The noise in IOP measurement was less than 0.1 mm Hg at a sampling rate of 3 Hz. The dead volume of the infusion line is 0.2 mL. In addition, the small diameter of the infusion capillary allows plug flow. At an outflow pressure gradient of 6 mm Hg, the facility in the bovine TM was 0.30.1 ml/min/mm Hg. This outflow increased continuously with time after 20 min of perfusion in PBS containing 1.4 mM CaCl2. Conclusion: This study demonstrates a computer controlled outflow apparatus with a sensitive pressure transducer, and is suitable for pharmacodynamic studies of the outflow facility. The flow into the a/c is adjusted precisely based on a specified IOP. When step changes are made in IOP, use of PI control algorithm facilitates a rapid and smooth approach to a new steady state flow. Increase in the outflow facility in the bovine eye after prolonged flow could be attributed to «washout» phenomenon reported previously.
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