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- Physiologically-based testing system for the mechanical characterization of prosthetic vein valves

Results of the normal volunteer study were extremely valuable in defining both typical dimensions and anatomy of veins in the proximal lower extremity, but also in quantifying the velocities, flow rates, and cycle periods under various postural and physiologic conditions. Specifically, over the 11 subjects, the common femoral vein diameter was 1.04 ± 0.14 cm and peak flow rates (derived from peak velocities) were 0.6 L/min, 1.34 L/min, 0.78 L/min and 1.6 L/min for the cases SUBR, SUAF, STBR and STAF, respectively. Based on these data, a diameter of 1.25 cm was chosen for the prosthetic valve design to allow for some distention of the vein upon insertion. Representative values for peak flow rate under breathing and ankle flexion conditions were taken (0.69 L/min and 1.5 L/min, respectively) as the average values measured in the two postural positions (Supine and Standing), since the breathing and ankle flexion movements acted as the primary driving forces in each case. These peak values were then obtained in the 1:1 Pulsatile Flow System by setting the steady flow component to 50% of those values (0.35 L/min and 0.75 L/min, respectively) and allowing the roller pump to superimpose an additional unsteady component. Pumping frequencies were based upon typical breathing (~15 breaths/min = 15 rpm, or 0.25 Hz) and walking (~30 steps/limb/min = 30 rpm, or 0.5 Hz) cycles.

To illustrate the overall testing procedure, a preliminary vein valve design was developed (Fig. 3). This device has several novel features [18] and was conceived in order to allow for a normally open device that fully closes upon application of backpressure. A 2:1 scale device was built using SLA fabrication techniques and incorporating the threaded extension as described earlier. The device frame was then wrapped with a 5 mil (127 μm) thick sheet of Biospan® (Polymer Technology Group, Berkeley, CA) to serve as leaflets. These were cut to a shape such that the closed leaflet edges contacted along a straight line across the valve lumen.

a. 2:1 Mock-Up System

The prototype device was then installed in the 2:1 Mock-up system and evaluated as described earlier. Initial observations showed that the leaflet had a natural tendency to fold inwardly along a nearly diagonal line between the tip of the flange and a point halfway between the two flanges along the base. Additional observations also showed that, for the leaflets to close quickly and reliably, they needed to have some initial inward deflection. These two issues were resolved by modifications to the side of the frames and means of attachment of the leaflet to the base. A threaded extension was also added at this point to improve axial alignment of the device and to reduce leakage around connectors (Fig. 4).

A set of these devices was fabricated and a series of tests were then made on these designs in the 2:1 Mock-up System. Key results for the prototype device were that it had a forward steady flow pressure drop of 1.25 ± 0.153 mmHg and a resistance of 1.92 ± 0.578 mmHg*min/L. Under sudden hydrostatic backpressure conditions, the valve required 1.38 ± 0.0447 s to close and allowed 19.9 ± 4.40 cm3 of reflux followed by 4.36 ± 1.48 cm3 and 1.23 ± 1.02 cm3 of leakage during the 1st and 2nd one second post-closure intervals, respectively.

b. 1:1 Physiologic Flow System

After completion of tests on a series of such devices, 1:1 prototypes were similarly constructed and evaluated in the pulsatile flow system to see what differences dimensional scale and test conditions would make. Results of these tests are shown for the 'No Device' case and one valve prototype under supine and standing conditions in Tables 2 and 3, respectively. The purpose of the 'No Device' case was: 1) to quantify losses in the system (i.e. tubing, connectors, etc.) which were not related to the test device itself, and, 2) to serve as a baseline for worst case values that would be seen with a completely incompetent valve. [Note: For the EOA calculations, values for Cd under the 'No Device' case were 2.32 ± 0.03 Math and 6.38 ± 0.142 Math for the SUBR and SUAF conditions, respectively.] Finally, the results from repeatability testing of these variables over two runs on the same prototype valve design under supine and standing conditions are shown in Tables 4 and 5, respectively.

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