Noninvasive laser Doppler monitoring of compliance-related intimal wall disturbed flow in blood vessel grafts
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Compliance changes in natural and substitute blood vessels have been associated with sites of stenosis, suture lines, and stent devices, while compliance mis-matches between host vessels and vascular grafts have been implicated in early loss of patency of artery replacements and "disturbed flow" at intimal walls. "Disturbed flow" sites are frequently the locations of thrombotic deposits or aneurysm formation. This investigation employed the noninvasive optical technique of Laser Doppler Flowmetry (LDF) to monitor near-wall particle velocity changes in stiff and compliant vascular grafts, unconstrained, stenosed, or stented, during increasing flows of red-blood-cell-sized latex particles serving as light scattering objects. A sudden-tubular-expansion, in vitro test flow loop was used with 2mm-diameter-nozzle delivery Reynold's numbers (Re) of 1600-2800 producing 400-1200 Re flows in 20-cm-long, 5mm- and 7mm- vascular grafts at mean flow rates of about 200 ml/minute, relevant to human femoral artery replacement procedures with stiff, expanded polytetrafluoroethylene (Goretex) or compliant preserved umbilical cord vein (Biograft) substitutes. Computational Fluid Dynamics (CFD) simulations of the flow through the stiffer Goretex vessel, modeled as a rigid tube in fully open and mid-point suture-constricted diameters, illustrated the locations of recirculation flows and re-attachment points that correlated well with the measured LDF values in actual flow experiments. Experimental LDF results with compliant Biografts illustrated location-dependent variations that increased with mean graft flow rates and showed "disturbed flow"-defined here, experimentally, as persistent LDF fluctuations greater than 0.2, correlating with near-wall random theoretical particle velocities exceeding 2 mm/second--associated with central region suture-constrained, rigid-thin-tape constrained, catheter-entry, and stent-deployment compliance changing procedures. Additional experiments showed that an exterior polyester mesh support sleeve, usually supplied with clinical Biografts, could diminish flow fluctuations at apparently weaker Biograft wall sites. Differing from the results of CFD models of a rigid wall graft, LDF experimental data for compliant Biografts showed both pre-stenotic "disturbed flow" and post-stenotic stagnation, re-attachment points, while CFD results for near-wall flow suppression in stenotic and stented zones were confirmed with low LDF readings at those sites. The combined LDF and CFD findings favor further use of the LDF technique for in vitro studies of the intimal wall-flow consequences of potential new endovascular intervention procedures or devices. The equipment utilized in this investigation included a right-angle probe applying <2mW, 780 nm red light from a 50-micrometer diameter optical fiber through each graft wall to an internal depth of about 1mm, for reflection by moving particles to two 100-micrometer diameter light-collecting optical fibers located within 0.5mm of the light source. Good resolution of graft positional differences was obtained by moving the probe at 10mm increments along each graft's length, with the graft resting underwater on an aluminum platform and moderately compressed by 1.25 gram force from the vertically applied LDF probe. Experimentally, the use of a 5-second averaging time per location inspected gave highly reproducible results as shown in triplicate measurements at each of 6 experimental flow rates.