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Matthew James Alan Rickard; Time-Related Reduction in Ahmed Valve Flow Resistance: A 6-Month Study using a Novel In Vitro Pulsatile Perfusion Apparatus. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3188.
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To explore the long-term effect of the natural ocular pulse repetition on the flow resistance (pressure drop) of the Ahmed Valve FP-7. Note: the ocular pressure pulse repeats approximately 40 million times a year.
The in vitro experimental apparatus consists of a constant flow syringe pump (New Era), piezoresistive pressure sensor (Honeywell), and a piezoelectric actuator and external amplifier (Physik Instrumente), which connect to a test sample via PVC tubing. When approximately 200 volts were applied to the actuator (via the external amplifier and a function generator), the outer surface of the tubing was compressed at a location between the syringe pump and the pressure sensor, which caused a momentary flow surge larger than the 3 microliters per minute setting of the syringe pump in the direction of the FP-7 valve. As the actuator and tube were relaxed (voltage off), the ocular pulse quickly decayed to a baseline pressure. This caused a short pressure spike ranging from 1.3 to 3.7 mmHg at a rate of 75 pulses per minute, approximating the expected in vivo ocular pulse. Distilled water was used to simulate aqueous humor. Every few days the pressure sensor was sampled at a rate of 10 Hz for one hour and this data set was analyzed to provide mean IOP (pressure drop across the valve), standard deviation of IOP, mean Ocular Pulse Amplitude (OPA) (difference between the maximum and mean pressures in a pulse), and standard deviation of OPA. The number of conditioning days (pulsed with flow) were 196, which simulated just over six months of post-operation time and spanned over 340 real-time days (August 2012 - July 2013).
The mean IOP, as shown in Fig. 1, dropped substantially within the first 50 days of pulsing, with standard deviation of IOP shown as error bars. The IOP remained between 4 and 7 mmHg for the remainder of the experiment.
A novel pulsatile in vitro experimental apparatus demonstrated that the resistance of the AGV FP-7 decreases to low pressure levels within 50 days and implies that the long-term resistance is significantly lower than the initial design resistance. This aids in understanding the small clinical differences observed in long-term IOP in studies comparing the AGV to the (valve-less) Baerveldt.
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