Validation of pressure measuring setup for microscopic tube flow: glaucoma drainage implant application

Abstract

Current glaucoma drainage implants (GDIs) are unable to provide consistent and long-term post-operative (post-op) intraocular pressure (IOP) control. Valved glaucoma drainage implants (VGDIs) and intra-operative (intra-op) non-valved glaucoma drainage implant (NVGDI) surgical interventions, which were developed in efforts to ensure immediate post-op IOP control, have not been able to provide consistent and long-term post-op IOP control. A novel method was developed by Dr. Paul Munden, involving the insertion of a degradable cylindrical insert into the tube of the GDI. The idea is that this degradable insert would provide sufficient temporary aqueous humor (AH) flow resistance through the tube in the immediate post-op stage. The flow resistance created by the insert would reduce over time as the insert degrades while scar tissue grows around the tube and provides natural flow resistance. With the success of this modification, the NVGDI would be able to provide both consistent and long-term IOP control. The primary hypothesis of this research is that an insert placed in a NVGDI tube, will provide flow resistance that is similar to that predicted by Hagen-Poiseuille (H-P) theory. Prior to testing the hypothesis, thorough validation experimental trials were performed by this author, demonstrating that two duplicate setups, which were developed for this research, corresponded well with each other. These validation trials also showed that the pumps and pressure transducers in the setups could accurately measure flow and pressure drop in a consistent manner. Additionally, an anti-microbial cleaning protocol was developed and implemented, ensuring long-term, accurate and consistent pressure measurements through microtubes. With the verified setups, an experimental study was performed to test the primary hypothesis. This study compared the experimental pressure loss (∆P) through GDI-like tubes with theory. To the author’s knowledge, there has not been any published research comparing experimental GDI results with relevant theoretical data, neither has there been published research measuring long-term (i.e., 5-30 days) in vitro ∆P through GDI tubes. The main objectives of the experimental study were to determine the effect of tube inner diameter (ID) on single tube flow resistance, the effect of annulus size on annular tube flow resistance, and the ability of the setups to measure and maintain consistent annular flow resistance over time. Single open tube flow trials were performed using microtubes with inner diameters (IDs) of 50 µm and 75 µm in order to determine the effect of tube ID on single tube flow resistance. There was a 1% - 30% difference between the average theoretical and experimental ∆Ps for the 50 μm tube trials and a 57% difference for the 75 μm tube trial. Accounting for flow rate and/or length variations in comparison with their respective nominal values, the theory-to-experiment differences were as low as -0.4% and 37% for the 50 and 75 µm tubes, respectively. Annular flow trials were performed using stainless steel hypodermic tubes of different IDs and nichrome wire inserts of different outer diameters (ODs) in order to determine the effect of annulus size on annular tube flow resistance. It was found experimentally and theoretically that an increase in effective diameter (Deff), which is a function of the tube’s inner diameter and the insert’s outer diameter for annular flow, causes a decrease in ∆P. The differences between theoretical and experimental ∆Ps for the annular flow samples were small when Deff was small enough to provide clinically relevant experimental ∆P values (i.e., 5-20 mm Hg). The results of the experimental study also showed that the developed test system can accurately maintain constant flow rate and measure ∆P across GDI-like tubes for more than 20 days. This time period matches the immediate post-op period wherein IOP needs to be controlled until scar tissue has grown sufficiently over the GDI to maintain a reasonable IOP level. Many glaucoma patients have previously undergone, or will undergo, cataract surgery and/or retinal reattachment surgery. These patients, who have had both VGDI surgery and cataract and/or retinal reattachment surgery, will have residual ocular viscosurgical devices (OVDs) and/or silicone oil left in the eye, which can flow through the implanted VGDI. Therefore, VGDI flow trials were performed using these setups to determine the ∆P effects on the VGDI when OVDs and silicone oil flow through the VGDI. From these trials, it was observed that the flow of OVDs and silicone oil in a VGDI caused a significant spike in ∆P. It was also observed that emulsified silicone oil caused a much higher ∆P spike as compared to the ∆P spike caused by silicone oil flowing through the GDI valve, even though the pure silicone oil took longer to pass through the valve. For future work, it is recommended that more thorough research be performed to develop the best mathematical model for predicting ∆P through the annulus of a GDI with a degradable insert. Annular flow trials with stiff tubes and inserts should be performed for periods longer than 30 days as a benchmark for comparison when Baerveldt Glaucoma Implant (BGI) tubes and degradable inserts are tested in the future. It is also recommended that the influence of barometric pressure on the flow rate and pressure within the tubes be further investigated. Future work should be done to examine the use, impact and designability of degradable inserts, and investigate the possibility of drug eluding inserts.