External beam radiation for prevention of intimal hyperplasia in peripheral arterial bypasses.

PURPOSE: Neointimal hyperplasia is one of the limiting factors in infrainguinal prosthetic vascular grafting. We conducted a pilot study to assess the possible role of radiotherapy for the prevention of this proliferating cellular process. METHODS AND MATERIALS: Ten patients (7 men, 3 women; median age: 73 years) suffering from critical leg ischemia were treated by infrainguinal revascularization using 6-mm polytetrafluoroethylene. Postoperative radiation therapy of the anastomotic sites started within 24 h. A dose of 20.4 Gy was delivered in 12 fractions in 2.5 weeks by external beam radiation with electrons. The target volume was defined as the clip-marked anastomotic region plus a 1.5-cm safety margin in each direction. RESULTS: Radiation therapy was technically feasible in all cases without radiation-related side effects. In one patient, radiotherapy had to be terminated prematurely, because of a hematoma at the distal anastomotic site with consequent skin necrosis. One patient developed an occlusion of a suprainguinal graft without anastomotic stenosis after 9 months; 2 other grafts occluded at 12-month intervals. At a median follow-up of 16 months, the remaining 6 patients had no signs of restenosis. CONCLUSIONS: The restenosis rate of peripheral vascular grafts after external beam radiation was lower than expected without additional treatment. The value of this approach will now be determined in a randomized prospective trial.

Fluid dynamics, wall mechanics, and oxygen transfer in peripheral bypass anastomoses.

Intimal hyperplasia at vascular anastomoses seems to be promoted by altered flow conditions and stress distributions within the anastomotic region. In order to gain deeper insight into postoperative disease processes, and subsequently, to contribute to the development of improved vascular reconstructions, detailed studies, also on local flow dynamics and related mass transport and wall mechanical effects, are required. In context with in vivo studies, computer simulation based on casts of femoro-popliteal bypasses implanted into sheep were performed to analyze the flow dynamics, the oxygen transport, and the wall and suture mechanics in anatomically correct bypass configurations related to three established surgical techniques and resulting geometries (conventional type anastomosis, Taylor-patch and Miller-cuff anastomoses with venous interposition grafts of different modifications). The influence of geometry, compliance of the graft, the interponated vein patch and vein cuff, and of the artery was included. Time-dependent, three-dimensional Navier-Stokes equations describing the flow field, and a nonlinear shell structure for the vessel walls were coupled using finite element methods. The numerical results demonstrate nonphysiological flow patterns in the anastomotic region. Strongly skewed axial velocity profiles and secondary velocities occur in the junction region. In the Miller-cuff a vortex may induce a wash-out effect which protects the host artery. On the artery floor opposite the junction flow separation and zones of recirculation were found. The analysis of oxygen transport illustrates a correlation between zones of low wall shear stress and reduced oxygen flux into the wall. Wall mechanics show that increased compliance mismatch leads to increased and discontinuous intramural stresses. Comparison to histomorphological findings on intimal hyperplasia shows certain correlations, particularly increased compliance mismatch has a proliferate influence on suture line hyperplasia. The reduction of compliance mismatch using vein interposition results in decreased generation of intimal hyperplasia, and therefore, contributes to improvement of patency rates, while the geometrical modification and the resulting change of the flow pattern seems to be less important for the growth of anastomotic intimal hyperplasia.

Numerical study of hemodynamics and wall mechanics in distal end-to-side anastomoses of bypass grafts.

The development and progress of distal anastomotic intimal hyperplasia seems to be promoted by altered flow conditions and intramural stress distributions at the region of the artery-graft junction of vascular bypass configurations. From clinical observations, it is known that intimal hyperplasia preferentially occurs at outflow anastomoses of prosthetic bypass grafts. In order to gain a deeper insight into post-operative disease processes, and subsequently, to contribute to the development of improved vascular reconstructions with respect to long term patency rates, detailed studies are required. In context with in vivo experiments, this study was designed to analyze the flow dynamics and wall mechanics in anatomically correct bypass configurations related to two different surgical techniques and resulting geometries (conventional geometry and Miller-cuff). The influence of geometric conditions and of different compliance of synthetic graft, the host artery and the interposed venous cuff on the hemodynamic behavior and on the wall stresses are investigated. The flow studies apply the time-dependent, three-dimensional Navier-Stokes equations describing the motion of an incompressible Newtonian fluid. The vessel walls are described by a geometrically non-linear shell structure. In an iterative coupling procedure, the two problems are solved by means of the finite element method. The numerical results demonstrate non-physiological flow patterns in the anastomotic region. Strongly skewed axial velocity profiles and high secondary velocities occur downstream the artery-graft junction. On the artery floor opposite the junction, flow separation and zones of recirculation are found. The wall mechanical studies show that increased compliance mismatch leads to increased intramural stresses, and thus, may have a proliferative influence on suture line hyperplasia, as it is observed in the in vivo study.