Sources of error in monitoring high speed testing of vascular grafts

Mechanical compliance issues are increasingly important in the design, testing, and manufacture of vascular grafts. This has to do with the observed relationship between long-term patency of implanted grafts and accurate compliance matching of those devices with the recipient natural vessels. Another important concern in this type of investigation has to do with the use of mock arteries during the testing of implantable medical products such as intravascular stents, stent/grafts, etc. At issue here are not simply the techniques used to monitor the static compliance of the vessels, but the dynamic properties which are in effect during the in vivo utilization of these devices. Perhaps of even more importance is understanding the high speed dynamic properties of these vessels so that a proper and reliable high-speed durability experiment might be designed. There is a natural tendency to adopt procedures that monitor outside dimensions of the graft or artery. The theoretical problem with this approach is the fact that compliance matching is a phenomenon associated with the inner lumen of the tube, whether it is hydrodynamic considerations in vascular graft testing or loading considerations in stent testing. Optical techniques such as lasers and ultrasound are encumbered by two physical motion phenomena unique to this approach. The first is simple jumping or movement of the tube due to vibrations. This problem can be overcome by high sampling rates. The second is more problematic and results in lengthening of the tube that has longitudinal as well as radial compliance. Both quantitative and qualitative examples will be examined. A comprehensive understanding of the sources of error encountered in various monitoring techniques is reviewed. These techniques will include lasers, dynamic internal compliance, ultrasound, and cantilevered beams.