Local haemodynamics of arterial bypass graft anastomoses

Why Patencies of Femoropopliteal Bypass Grafts with Distal End-to-End Anastomosis are Comparable with End-to-Side Anastomosis

OBJECTIVE

Despite the theoretical favourable hemodynamic advantage of end-to-end anastomosis (ETE), femoropopliteal bypasses with distal ETE and end-to-side anastomosis (ETS) have comparable clinical patencies. We therefore studied the effects of different in vivo anastomotic configurations on hemodynamics in geometrically realistic ETE and ETS in vitro flow models to explain this phenomenon.

METHODS

Four ETE and two ETS models (30° and 60°) were constructed from in vivo computed tomography angiography data. With flow visualization physiological flow conditions were studied.

RESULTS

In ETS, a flow separation and recirculation zone was apparent at anastomotic edges with a shifting stagnation point between them during systole. Secondary flow patterns developed with flow deceleration and reversal. Slight out of axis geometry of all ETE resulted in flow separation and recirculation areas comparable to ETS. Vertical flow patterns were more stable in wider and longer bevelled ETE.

CONCLUSION

Primary flow disturbances in ETE are comparable to ETS and are related to the typical sites where myointimal hyperplasia develops. In ETS, reduction of anastomosis angle will diminish flow disturbances. To reduce flow disturbances in ETE, the creation of a bulbous spatulation with resulting axial displacement of graft in relation to recipient artery should be prevented.

Contemporary Management of Femoral Popliteal Revascularization

Symptomatic peripheral artery disease of the femoral popliteal segment can be treated by surgical and endovascular revascularization, but controversy exists about the best approach. Conventional approaches to revascularization have focused on lesion anatomy to decide on bypass versus endovascular treatment, but advances in endovascular therapy make an endovascular-first approach increasingly feasible-either as a single approach or as an adjunct to short-segment bypass (ie, hybrid procedure). In this review, we discuss the medical, endovascular, and surgical treatment of femoral popliteal revascularization with a special emphasis on advances in percutaneous therapy.

Mid- and long-term results of the treatment of infrainguinal arterial occlusive disease with precuffed expanded polytetrafluoroethylene grafts compared with vein grafts

BACKGROUND

Prosthetic grafts for lower-extremity bypass have limited patency compared with autologous vein grafts. Precuffed expanded polytetrafluoroethylene (ePTFE) grafts alter the geometry of the distal hood to improve patency. This study reports the authors' long-term results on the use of precuffed ePTFE grafts for infrainguinal bypasses in patients with arterial occlusive disease and compares these with results of reversed great saphenous vein grafts (rSVG).

METHODS

A retrospective review of billing codes identified 101 polytetrafluoroethylene (PTFE) and 47 rSVG bypasses performed over a 6-year period. Femoral to below-knee popliteal and femoral to tibial bypasses were analyzed. Data collected consisted of risk factors, Rutherford classification, bypass inflow and outflow, runoff vessels, patency, amputation, and death. Primary end points consisted of primary, assisted-primary, and secondary patency along with limb salvage.

RESULTS

Mean age of the patients was 76 years in the PTFE group and 69.8 years in the rSVG group. For femoral to below-knee popliteal bypasses, primary patency at 1, 3, and 5 years in the PTFE group was 76.9%, 48.7%, and 43.3%, respectively, compared with 77.1%, 77.1%, and 77.1%, respectively, in the rSVG group (P = 0.225). Secondary patency was 89.2%, 70.9%, and 50.6% in the PTFE group compared with 84.4%, 84.4%, and 84.4% in the rSVG group (P = 0.269). Limb salvage was similar in the PTFE compared with the rSVG group (97.7%, 90.5%, and 79.4% vs. 83.3%, 83.3%, and 83.3%; P = 0.653). For femoral to tibial bypasses, primary patency in the PTFE group at 1, 3, and 5 years was 57.1%, 40.4%, and 22.1%, respectively, compared with 67.4%, 67.4%, and 50.6%, respectively, for the rSVG group (P = 0.246). Secondary patency was 75.5%, 44.9%, and 22.7% in the PTFE group compared with 91.8%, 91.8%, and 52.5% in the rSVG group (P = 0.022). Limb salvage at 1, 3, and 5 years was 79.2%, 55.7%, and 55.7%, respectively, in the PTFE group compared with 96.4%, 96.4%, and 64.3%, respectively, in the rSVG group (P = 0.046).

CONCLUSIONS

Precuffed ePTFE grafts demonstrate similar 1-year patency to that of rSVG. However, mid- and long-term patency is reduced compared with saphenous vein grafts (SVG), especially to tibial targets. PTFE grafts to the popliteal demonstrate limb salvage rates similar to those of SVG. In the tibial vessels, limb salvage rates for PTFE grafts are significantly worse compared with SVG.

Early Results for Below-Knee Bypasses Using Distaflo

In patients who require lower extremity revascularization, prosthetic graft is a reasonable alternative in the absence of a suitable autologous vein conduit. However, prosthetic bypass grafts have limited patency, especially for infrageniculate reconstruction. Polytetrafluoroethylene grafts were geometrically modified at the distal end to increase their patency. The authors reviewed their experience with the Distaflo graft in patients who required lower extremity below-knee popliteal and tibial bypasses when no suitable autologous vein conduit was available. Chart review was conducted of the 57 patients who underwent 60 lower extremity bypasses over a 3-year period between June 2003 and April 2006. Twenty-four revascularizations were constructed to the tibial outflow sites, whereas the remaining grafts were placed to the below-knee (28) and above-knee (8) popliteal artery, respectively. Study endpoints were primary, assisted primary, secondary patency, and limb salvage at the time of follow-up. Distaflo bypass was performed at the infrageniculate level in 86.7% of cases (28 below-knee popliteal, 24 tibial). Mean follow-up time was 12 months (range, 0.5-37.5 months). At 1 year, primary, assisted primary, and secondary patencies and limb salvage rates for below-knee popliteal bypasses were 83.5%, 89.5%, 94.7%, and 94.4%, respectively. Primary, assisted primary, and secondary patencies and limb salvage rates for tibial bypasses were 44.4%, 44.4%, 63.2%, and 74.9%, respectively. Distaflo precuffed graft is a good alternative conduit for below-knee popliteal and tibial lower extremity reconstructions in the absence of an autologous vein and appears to have promising early patency and limb salvage rates even when used for tibial bypasses.

Swirling flow pattern in a non-planar model of an interposition vein cuff anastomosis

One of the main causes of long-term failure of ePTFE grafts is the development of anastomotic intimal hyperplasia which leads to graft thrombosis. Experimental studies with bypass grafts have shown an inverse relationship between mean wall shear stress and intimal hyperplasia. The geometry of the anastomosis has a strong influence on the flow pattern and wall shear stress distribution. The aim of this in vitro study was to investigate the influence of non-planarity in a model of a distal anastomosis with interposition vein cuff, an anastomosis configuration that is increasingly being used because of improved clinical results. Laser Doppler anemometer measurements were carried out in silicone rubber models of interposition vein cuff anastomoses with planar and non-planar inflow. The pulsatile flow waveforms were typical of those found in femoro-infrapopliteal bypass. Axial and radial velocities were measured in the proximal and distal outflow segments. As expected a symmetrical helical flow pattern (Dean flow) was evident in the planar model. The model with non-planar inflow, however, gave rise to swirling flow in both the distal and proximal artery outflow segments for during the systolic phase. In patients, the anastomosis is usually non-planar. Since the configuration depends in part upon the tunnelling of the graft, this may be altered to some extent. Non-planar anastomotic configurations induce a swirling flow pattern, which may normalise wall shear stress, thereby potentially reducing intimal hyperplasia.

Measurement of anastomosis geometry in lower extremity bypass grafts with 3-D ultrasound imaging

The attachment sites of lower extremity bypass grafts are known to exhibit a wide range of geometries. Factors that determine the geometry of a given anastomosis include graft material, graft site, native vessel size, graft size and individual patient anatomy. Therefore, it is difficult to specify a standard anastomosis geometry before surgery and difficult to predict the effect of the geometry on long-term graft patency. We have used 3-D ultrasound imaging to study 46 proximal anastomoses of lower limb bypass grafts. We have developed methods to characterize the 3-D geometry of the anastomosis in terms of component sizes and angles. These detailed geometric measurements describe a range of anastomosis geometries and establish standardized parameters across cases that can be used to relate anastomosis geometry to outcome.

Outflow distribution at the distal anastomosis of infrainguinal bypass grafts

Outflow distribution at the distal anastomosis of infrainguinal bypass grafts remains unquantified in vivo, but is likely to influence flow patterns and haemodynamics, thereby impacting upon graft patency. This study measured the ratio of distal to proximal outflow in 30 patients undergoing infrainguinal bypass for lower limb ischaemia, using a flow probe and a transit-time ultrasonic flow meter. The mean outflow distribution was approximately 75% distal to 25% proximal, with above knee anastomoses having a greater proportion of distal flow (84%) compared to below knee grafts (73%). These in vivo flow characteristics differ significantly from those used in theoretical models studying flow phenomena (50:50 and/or 100:0), and should be incorporated into future research.

Correlation of Intimal Hyperplasia Development and Shear Stress Distribution at the Distal End-side-anastomosis, in vitro Study Using Particle Image Velocimetry

Low shear areas at the distal anastomosis of peripheral bypasses are thought to promote neointimal hyperplasia. In this study we evaluated the fluid dynamic environment at the distal anastomosis of peripheral bypasses by means of a new method for in vitro flow visualization and quantitative velocity field measurement. A silastic model of a distal end-side anastomosis was attached to a mock circulation loop driven by an artificial heart. High resolution velocity fields were measured by means of particle image velocimetry (PIV). The velocity vector data were used to calculate vorticity omega, strain rates ex, shear rates h and shear stresses tau. Two separations and a stagnation zone were identified by means of flow visualization. Measured velocities inside the three zones were significantly lower than in the high velocity mainstream. Calculated shear rates and shear stresses inside the zones were significantly lower than human wall shear rates. At the transition between the effective mainstream and the boundary layers high vorticity and compressive strain fields existed, indicating the presence of high shear forces. The locations of these areas corresponded to the well known zones of intimal hyperplasia. The high resolution shear stress analysis supports the low shear theory of intimal hyperplasia development. A wall diversion angle greater than 6 degrees leads to flow separation and presumed IH promotion until high shear transition areas are reached.

Simulation of flow through a Miller cuff bypass graft

Unnatural temporal and spatial distributions of wall shear stress in the anastomosis of distal bypass grafts have been identified as possible factors in the development of anastomotic intimal hyperplasia in these grafts. Distal bypass graft anastomoses with an autologus vein cuff (a Miller cuff) interposed between the graft and artery have been shown to alleviate the effects of intimal hyperplasia. In this study, pulsatile flow through models of a standard end-to-side anastomosis and a Miller cuff anastomosis are computed and the resulting wall shear stress and pressure distributions analysed. The results are inconclusive, and could be taken to suggest that the unnatural distributions of shear stress that do occur along the anastomosis floor may not be particularly important in the development of intimal hyperplasia. However, it seems more likely that the positive effects of the biological and material properties of the vein cuff, which are not considered in this study, somehow outweigh the negative effects of the shear stress distributions predicted to occur on the floor of the Miller-cuff graft.

Optimising Miller Cuff Dimensions. The Influence of Geometry on Anastomotic Flow Patterns

OBJECTIVES

since cuff dimensions are variable, we studied the influence of cuff geometry on flow mechanics, in an attempt to identify the optimum configuration.

MATERIALS AND METHODS

bench studies involved the manufacture of anatomically accurate models of varying cuff dimensions, perfused in a specifically designed flow rig, simulating physiological conditions. Flow visualisation studies incorporating laser illumination of tracer particles enabled accurate analysis of flow patterns.

RESULTS

the vortex created within the proximal cuff of each model during the deceleration phase of the cardiac cycle was strongly influenced by the aspect ratio (AR=cuff height:length). The standard and high cuffs (AR=1.63 and 1.18, respectively) demonstrated cohesive vortices and stable flow patterns. Low and long cuffs (AR=2.6 and 2.25, respectively) created more complex vortices with large areas of flow separation and low velocities.

CONCLUSIONS

aspect ratio has an important influence on flow within the distal anastomosis, with cuff dimensions of 13 mm long and 8-11 mm high (standard and high cuffs) creating beneficial flow patterns anticipated to optimise wall shear stress and inhibit myointimal hyperplasia.