Experimental evaluation of flow patterns in interposition vein cuff anastomosis. The influence of non-planarity of the inflow

Clinical evidence suggests that the development of myointimal hyperplasia in prosthetic femorodistal bypass grafts may be reduced by the interposition of a cuff of autologous vein between the graft and the recipient artery. Previous experimental work has shown that some of the benefits may be attributed to the geometry of the cuffed anastomosis. Since the distal anastomosis in vivo is often non-planar we have carried out a preliminary study in a model where the graft is at an angle of 45 degrees to the anterior-posterior plane of the anastomosis. This out-of-plane angulation produces highly asymmetric flow patterns in the anastomosis with significant flow separation on the ipsilateral side of the cuff. In the proximal and distal outflow, however, the velocity vectors show significant helical motion with temporal instability in the distal outflow.     

The effect of proximal artery flow on the hemodynamics at the distal anastomosis of a vascular bypass graft: computational study

The formation of distal anastomotic intimal hyperplasia (IH), one common mode of bypass graft failure, has been shown to occur in the areas of disturbed flow particular to this site. The nature of theflow in the segment of artery proximal to the distal anastomosis varies from case to case depending on the clinical situation presented. A partial stenosis of a bypassed arterial segment may allow residual prograde flow through the proximal artery entering the distal anastomosis of the graft. A complete stenosis may allow for zero flow in the proximal artery segment or retrograde flow due to the presence of small collateral vessels upstream. Although a number of investigations on the hemodynamics at the distal anastomosis of an end-to-side bypass graft have been conducted, there has not been a uniform treatment of the proximal artery flow condition. As a result, direct comparison of results from study to study may not be appropriate. The purpose of this work was to perform a three-dimensional computational investigation to study the effect of the proximal artery flow condition (i.e., prograde, zero, and retrograde flow) on the hemodynamics at the distal end-to-side anastomosis. We used the finite volume method to solve the full Navier-Stokes equations for steady flow through an idealized geometry of the distal anastomosis. We calculated the flow field and local wall shear stress (WSS) and WSS gradient (WSSG) everywhere in the domain. We also calculated the severity parameter (SP), a quantification of hemodynamic variation, at the anastomosis. Our model showed a marked difference in both the magnitude and spatial distribution of WSS and WSSG. For example, the maximum WSS magnitude on the floor of the artery proximal to the anastomosis for the prograde and zero flow cases is 1.8 and 3.9 dynes/cm2, respectively, while it is increased to 10.3 dynes/cm2 in the retrograde flow case. Similarly, the maximum value of WSSG magnitude on thefloor of the artery proximal to the anastomosis for the prograde flow case is 4.9 dynes/cm3, while it is increased to 13.6 and 24.2 dynes/cm3, respectively, in the zero and retrograde flow cases. The value of SP is highest for the retrograde flow case (13.7 dynes/cm3) and 8.1 and 12.1 percent lower than this for the prograde (12.6 dynes/cm3) and zero (12.0 dynes/cm3) flow cases, respectively. Our model results suggest that the flow condition in the proximal artery is an important determinant of the hemodynamics at the distal anastomosis of end-to-side vascular bypass grafts. Because hemodynamic forces affect the response of vascular endothelial cells, the flow situation in the proximal artery may affect IH formation and, therefore, long-term graft patency. Since surgeons have some control over the flow condition in the proximal artery, results from this study could help determine which flow condition is clinically optimal.     

Computational model of blood flow in the aorto-coronary bypass graft

Background

Coronary artery bypass grafting surgery is an effective treatment modality for patients with severe coronary artery disease. The conduits used during the surgery include both the arterial and venous conduits. Long- term graft patency rate for the internal mammary arterial graft is superior, but the same is not true for the saphenous vein grafts. At 10 years, more than 50% of the vein grafts would have occluded and many of them are diseased. Why do the saphenous vein grafts fail the test of time? Many causes have been proposed for saphenous graft failure. Some are non-modifiable and the rest are modifiable. Non-modifiable causes include different histological structure of the vein compared to artery, size disparity between coronary artery and saphenous vein. However, researches are more interested in the modifiable causes, such as graft flow dynamics and wall shear stress distribution at the anastomotic sites. Formation of intimal hyperplasia at the anastomotic junction has been implicated as the root cause of long- term graft failure.Many researchers have analyzed the complex flow patterns in the distal sapheno-coronary anastomotic region, using various simulated model in an attempt to explain the site of preferential intimal hyperplasia based on the flow disturbances and differential wall stress distribution. In this paper, the geometrical bypass models (aorto-left coronary bypass graft model and aorto-right coronary bypass graft model) are based on real-life situations. In our models, the dimensions of the aorta, saphenous vein and the coronary artery simulate the actual dimensions at surgery. Both the proximal and distal anastomoses are considered at the same time, and we also take into the consideration the cross-sectional shape change of the venous conduit from circular to elliptical. Contrary to previous works, we have carried out computational fluid dynamics (CFD) study in the entire aorta-graft-perfused artery domain. The results reported here focus on (i) the complex flow patterns both at the proximal and distal anastomotic sites, and (ii) the wall shear stress distribution, which is an important factor that contributes to graft patency.

Methods

The three-dimensional coronary bypass models of the aorto-right coronary bypass and the aorto-left coronary bypass systems are constructed using computational fluid-dynamics software (Fluent 6.0.1). To have a better understanding of the flow dynamics at specific time instants of the cardiac cycle, quasi-steady flow simulations are performed, using a finite-volume approach. The data input to the models are the physiological measurements of flow-rates at (i) the aortic entrance, (ii) the ascending aorta, (iii) the left coronary artery, and (iv) the right coronary artery.

Results

The flow field and the wall shear stress are calculated throughout the cycle, but reported in this paper at two different instants of the cardiac cycle, one at the onset of ejection and the other during mid-diastole for both the right and left aorto-coronary bypass graft models. Plots of velocity-vector and the wall shear stress distributions are displayed in the aorto-graft-coronary arterial flow-field domain. We have shown (i) how the blocked coronary artery is being perfused in systole and diastole, (ii) the flow patterns at the two anastomotic junctions, proximal and distal anastomotic sites, and (iii) the shear stress distributions and their associations with arterial disease.

Conclusion

The computed results have revealed that (i) maximum perfusion of the occluded artery occurs during mid-diastole, and (ii) the maximum wall shear-stress variation is observed around the distal anastomotic region. These results can enable the clinicians to have a better understanding of vein graft disease, and hopefully we can offer a solution to alleviate or delay the occurrence of vein graft disease.

     

64层螺旋CTA在冠状动脉搭桥术后桥血管评价中的临床应用

目的：评价64层螺旋CTA在冠状动脉搭桥术后桥血管评价中的应用价值。方法：收集67例冠脉搭桥术后64层螺旋CTA资料,回顾性分析其成像条件、心电编辑软件对图像质量的影响;薄层容积图像及VR、CPP、MIP等后处理方法对桥血管的显示情况,并观察桥血管通畅状态。结果：67例CTA容积图像质量均满足诊断要求,其中15例经B受体阻滞剂控制心率;27例经过心电编辑软件后处理;检出正常桥血管107支,占74_31％（107／144）;异常静脉桥血管32支,22．22％（32／144）,包括近端吻合口尖角状闭塞13支,钙化斑块7支,软癍块8支,混合斑块1支;远端吻合口狭窄3支;正常动脉桥血管35支,24-31％（35／144）;异常动脉桥血管5支,3．47％（5／144）,包括闭塞内乳动脉桥血管4支,远端吻合口狭窄1支。上述多种后处理方法有助于多角度、多方位显示桥血管近、远端吻合口、桥血管行程及其与毗邻结构关系、有无斑块及管腔狭窄程度等。结论：64层螺旋CTA是冠脉搭桥术后评估桥血管状态准确、安全及简便的影像学检查方法。     

Characterisation of vortex shedding in vascular anastomosis models using pulsed Doppler ultrasound.

Vortex shedding at vascular anastomoses were investigated in vitro using a 20 MHz pulsed-wave Doppler velocimeter. Centreline velocity measurements were made at various axial distances in simplified polyurethane models of proximal and distal end-to-side anastomoses of angles 15, 30, 45, 60 and 80 degrees using pulsatile flow waveforms similar to those in femoropopliteal bypass grafts. The in-phase and quadrature Doppler signals were recorded and the maximum frequency waveform, averaged over 64 cycles, was obtained using short-time Fourier transform. A fourth-order Butterworth low-pass filter was employed to separate the vortex velocity signal from the convective velocity. The vortex signal envelope was calculated using a Hilbert transform method and the vortex amplitude was taken as the maximum of this envelope. The results show that higher vortex amplitude were found in the proximal anastomoses and under resting flow conditions. Although the vortex amplitudes generally increased with angles of anastomosis, they were found to be higher in the 60 degrees than in the 80 degrees proximal anastomosis. The vortex structures were investigated using spectrograms and these show prominent features at 40-50 Hz indicative of the short-duration oscillatory signals during the decelerative phase of systole expected from the passage of vortices. The study indicates that flow disturbances due to vortex shedding may be a common feature in femoropopliteal bypass grafts.     

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.     

Hemodynamic simulation in a novel design for femoral bypass grafts

The effectiveness of femoral bypass grafts is correlated with the geometric configuration and hemodynamics of the bypass and the arteries. As an attempt to develop a new design for femoral bypass grafts, we present a novel geometric configuration for a symmetrically implanted 2-way bypass graft. In order to investigate how the symmetric 2-way bypass grafts affect the flow patterns through the anastomosis, physiologic blood flows in 1-way and 2-way models for a fully stenosed femoral bypass were simulated with the finite element method, and the hemodynamic factors in these models were studied. The temporal and spatial distributions of flow patterns and wall shear stresses in the vicinity of distal anastomosis during the cardiac cycle were analyzed. The results computed showed that the 2-way model has more preferable hemodynamics than the 1-way model in the distribution of flow patterns and wall shear stresses, and it may improve the flow conditions and decrease the probability of restenosis. However, the limitations of the 2-way bypass model may counteract the positive effects. More detailed hemodynamic studies are necessary to fully assess the viability of the 2-way bypass graft.     

Local and global geometric influence on steady flow in distal anastomoses of peripheral bypass grafts

We consider the effect of geometrical configuration on the steady flow field of representative geometries from an in vivo anatomical data set of end-to-side distal anastomoses constructed as part of a peripheral bypass graft. Using a geometrical classification technique, we select the anastomoses of three representative patients according to the angle between the graft and proximal host vessels (GPA) and the planarity of the anastomotic configuration. The geometries considered include two surgically tunneled grafts with shallow GPAs which are relatively planar but have different lumen characteristics, one case exhibiting a local restriction at the perianastomotic graft and proximal host whilst the other case has a relatively uniform cross section. The third case is nonplanar and characterized by a wide GPA resulting from the graft being constructed superficially from an in situ vein. In all three models the same peripheral resistance was imposed at the computational outflows of the distal and proximal host vessels and this condition, combined with the effect of the anastomotic geometry, has been observed to reasonably reproduce the in vivo flow split. By analyzing the flow fields we demonstrate how the local and global geometric characteristics influences the distribution of wall shear stress and the steady transport of fluid particles. Specifically, in vessels that have a global geometric characteristic we observe that the wall shear stress depends on large scale geometrical factors, e.g., the curvature and planarity of blood vessels. In contrast, the wall shear stress distribution and local mixing is significantly influenced by morphology and location of restrictions, particular when there is a shallow GPA. A combination of local and global effects are also possible as demonstrated in our third study of an anastomosis with a larger GPA. These relatively simple observations highlight the need to distinguish between local and global geometric influences for a given reconstruction. We further present the geometrical evolution of the anastomoses over a series of follow-up studies and observe how the lumen progresses towards the faster bulk flow of the velocity in the original geometry. This mechanism is consistent with the luminal changes in recirculation regions that experience low wall shear stress. In the shallow GPA anastomoses the proximal part of the native host vessel occludes or stenoses earlier than in the case with wide GPA. A potential contribution to this behavior is suggested by the stronger mixing that characterizes anastomoses with large GPA.     

Proximal placement of lateral thigh skin markers reduces soft tissue artefact during normal gait using the Conventional Gait Model

A primary source of measurement error in gait analysis is soft-tissue artefact. Hip and knee angle measurements, regularly used in clinical decision-making, are particularly prone to pervasive soft tissue on the femur. However, despite several studies of thigh marker artefact it remains unclear how lateral thigh marker height affects results using variants of the Conventional Gait Model. We compared Vicon Plug-in Gait hip and knee angle estimates during gait using a proximal and distal thigh marker placement for ten healthy subjects. Knee axes were estimated by optimizing thigh rotation offsets to minimize knee varus-valgus range during gait. Relative to the distal marker, the proximal marker produced 37% less varus-valgus range and 50% less hip rotation range (p < 0.001), suggesting that it produced less soft-tissue artefact in knee axis estimates. The thigh markers also produced different secondary effects on the knee centre estimate. Using whole gait cycle optimization, the distal marker showed greater minimum and maximum knee flexion (by 6° and 2° respectively) resulting in a 4° reduction in range. Mid-stance optimization reduced distal marker knee flexion by 5° throughout, but proximal marker results were negligibly affected. Based on an analysis of the Plug-in Gait knee axis definition, we show that the proximal marker reduced sensitivity to soft-tissue artefact by decreasing collinearity between the points defining the femoral frontal plane and reducing anteroposterior movement between the knee and thigh markers. This study suggests that a proximal thigh marker may be preferable when performing gait analysis using the Plug-in Gait model.     

Changes in lipid parameters after intestinal resection or bypass in the rat.

Intestinal resection, bypass and adaptative postoperative mechanisms developed as a consequence of that surgery, are considered good methods for improving knowledge of gastrointestinal physiology as well as possible effects that the intestine could have on the general metabolism. 50% jejunoileal bypass (BP), 50% proximal (PR) and distal (DR) intestinal resections were performed on rats to compare the influence of resected intestinal segments or bypassed loop localization could exert on different serum lipid parameters. One month after surgery significant increases in total serum cholesterol and cholesterol esters were found. There was no change in free cholesterol. A decrease in triglyceride was observed after distal and proximal resection but no changes after bypass. The cholesterol/phospholipid ratio was increased after resection and after bypass. It has been suggested that the changes in lipid metabolism produced after resections and bypass depend mainly on the loss of absorptive surface rather than on the position of the resected segment. The bypass loop may itself still exert some influence on lipoprotein metabolism, mainly on high density lipoprotein-cholesterol.     

Numerical modeling of simulated blood flow in idealized composite arterial coronary grafts: steady state simulations

This paper presents a comparative study of simulated blood flow in different configurations of simplified composite arterial coronary grafts (CACGs). Even though the composite arterial grafting is increasingly used in cardiac surgery, it is still questionable whether or not the blood flow in such grafts can adequately meet the demands of the native myocardial circulation. A computational fluid dynamics (CFD) model was developed to conduct computer-based studies of simulated blood flow in four different geometric configurations of CACGs, corresponding to routinely used networks in cardiac surgery coronary grafts (T, Y, Pi and sequential). The flow was assumed three-dimensional, laminar and steady and the fluid as Newtonian, while the vessel walls were considered as inelastic and impermeable. It was concluded that local haemodynamics, practically described by velocity, pressure drop, wall shear stress (WSS) and flow rates, may be strongly influenced by the local geometry, especially at the anastomotic sites. The computations were made at mean flow rates of 37.5, 75 and 150ml/min. The side-branch outflow rates, computed for each bypass graft, showed noticeable differences. The results, which were found both qualitatively and quantitatively consistent with other studies, indicate that the Pi-graft exhibits significantly less uniform distribution of outflow rates than the other geometric configurations. Moreover, prominent variations in WSS and velocity distribution among the assessed CACGs were predicted, showing remarkable flow interactions among the arterial branches. The lowest shear stress regions were found on the lateral walls of bifurcations, which are predominantly susceptible to the occurrence of coronary artery disease (CAD). In contrast, the highest WSS were observed at the turn of the arterial branches.     

That hemodynamics and not material mismatch is of primary concern in bypass graft failure: an experimental argument

The long term patency of end-to-side peripheral artery bypasses are low due to failure of the graft generally at the distal end of the bypass. Both material mismatch between the graft and the host artery and junction hemodynamics are cited as being major factors in disease formation at the junction. This study uses experimental methods to investigate the major differences in fluid dynamics and wall mechanics at the proximal and distal ends for rigid and compliant bypass grafts. Injection moulding was used to produce idealized transparent and compliant models of the graft/ artery junction configuration. An ePTFE graft was then used to stiffen one of the models. These models were then investigated using two-dimensional video extensometry and one-dimensional laser Doppler anemometry to determine the junction deformations and fluid velocity profiles for the rigid and complaint graft anastomotic junctions. Junction strains were evaluated and generally found to be under 5% with a peak stain measured in the stiff graft model junction of 8.3% at 100 mmHg applied pressure. Hemodynamic results were found to yield up to 40% difference in fluid velocities for the stiff/compliant comparison but up to 80% for the proximal/distal end comparisons. Similar strain conditions were assumed for the proximal and distal models while significant differences were noted in their associated hemodynamic changes. In contrasting the fluid dynamics and wall mechanics for the proximal and distal anastomoses, it is evident from the results of this study, that junction hemodynamics are the more variable factor.     

The effect of graft caliber upon wall shear within in vivo distal vascular anastomoses

Wall shear has been widely implicated as a contributing factor in the development of intimal hyperplasia in the anastomoses of chronic arterial bypass grafts. Earlier studies have been restricted to either: (1) in vitro or computer simulation models detailing the complex hemodynamics within an anastomosis without corresponding biological responses, or (2) in vivo models that document biological effects with only approximate wall shear information. Recently, a specially designed pulse ultrasonic Doppler wall shear rate (PUDWSR) measuring device has made it possible to obtain three near-wall velocity measurements nonintrusively within 1.05 mm of the vessel luminal surface from which wall shear rates (WSRs) were derived. It was the purpose of this study to evaluate the effect of graft caliber, a surgically controllable variable, upon local hemodynamics, which, in turn, play an important role in the eventual development of anastomotic hyperplasia. Tapered (4-7 mm I.D.) 6-cm-long grafts were implanted bilaterally in an end-to-side fashion with 30 deg proximal and distal anastomoses to bypass occluded common carotid arteries of 16 canines. The bypass grafts were randomly paired in contralateral vessels and placed such that the graft-to-artery diameter ratio, DR, at the distal anastomosis was either 1.0 or 1.5. For all grafts, the average Re was 432 +/- 112 and the average Womersley parameter, alpha, was 3.59 +/- 0.39 based on artery diameter. There was a sharp skewing of flow toward the artery floor with the development of a stagnation point whose position varied with time (up to two artery diameters) and DR (generally more downstream for DR = 1.0). Mean WSRs along the artery floor for DR = 1.0 and 1.5 were found to range sharply from moderate to high retrograde values (589 s-1 and 1558 s-1, respectively) upstream to high antegrade values (2704 s-1 and 2302 s-1, respectively) immediately downstream of the stagnation point. Although there were no overall differences in mean and peak WSRs between groups, there were significant differences (p < 0.05) in oscillatory WSRs as well as in the absolute normalized mean and peak WSRs between groups. There were also significant differences (p < 0.05) in mean and peak WSRs with respect to axial position along the artery floor for both DR cases. In conclusion, WSR varies widely (1558 s-1 retrograde to 2704 s-1 antegrade) within end-to-side distal graft anastomoses, particularly along the artery floor, and may play a role in the development of intimal hyperplasia through local alteration of mass transport and mechano-signal transduction within the endothelium.     

Is there a haemodynamic advantage associated with cuffed arterial anastomoses?

The development of intimal hyperplasia at arterial bypass graft anastomoses is a major factor responsible for graft failure. A revised surgical technique, involving the incorporation of a small section of vein (vein cuff) into the distal anastomosis of PTFE grafts, results in an altered distribution of intimal hyperplasia and improved graft patency rates, especially for below-knee grafts. Numerical simulations have been conducted under physiological conditions to identify the flow behaviour in a typical cuffed bypass model and to determine whether the improved performance of the cuffed system can be accounted for by haemodynamic factors. The flow patterns at the cuffed anastomosis are significantly different to those at the conventional end-to-side anastomosis. In the former case, the flow is characterised by an expansive, low momentum recirculation within the cuff. Separation occurs at the graft heel, and at the cuff toe as the blood enters the recipient artery. Wall shear stresses in the vicinity of the cuff heel are low, but high shear stresses and large spatial gradients in the shearing force act on the artery floor during systole. In contrast, a less disturbed flow prevails and the floor shear stress distribution is less adverse in the conventional model. In conclusion, aspects of the anastomotic haemodynamics are worsened when the cuff is employed. The benefits associated with the cuffed grafts may be related primarily to the presence of venous material at the anastomosis. Therefore, caution is advised with regard to the use of PTFE grafts, pre-shaped to resemble a cuffed geometry.     

Attenuation of flow disturbances in tapered arterial grafts

Flow disturbances in tapered arterial grafts of angles of taper between 0.5 and 1.0 deg were measured in vitro using a pulsed ultrasound Doppler velocimeter. The increase in transition Reynolds numbers with angle of taper and axial distance was determined for steady flow. The instantaneous centerline velocities were measured distal to a 50 percent area stenosis (as a model of a proximal anastomosis), in steady and pulsatile flow, from which the disturbance intensities were calculated. A significant reduction in post-stenotic disturbance intensity was recorded in the tapered grafts, relative to a conventional cylindrical graft. In pulsatile flow with a large backflow component, however, there was an increase in disturbance intensity due to diverging flow during flow reversal. This was observed only in the 1.0 deg tapered graft. These findings indicate that taper is an important consideration in the design of vascular prostheses.     

Prospectively versus Retrospectively ECG-Gated 256-Slice CT Angiography to Assess Coronary Artery Bypass Grafts — Comparison of Image Quality and Radiation Dose

Objective

In this retrospective non-randomized cohort study, the image quality and radiation dose were compared between prospectively electrocardiogram (ECG)-gated axial (PGA) and retrospectively ECG-gated helical (RGH) techniques for the assessment of coronary artery bypass grafts using 256-slice CT.

Methods

We studied 124 grafts with 577 segments in 64 patients with a heart rate (HR) <85 bpm who underwent CT coronary angiography (CTCA); 34 patients with RGH-CTCA and 30 patients with PGA-CTCA. The image quality of the bypass grafts was assessed by a 5-point scale (1 = excellent to 5 = non-diagnostic) for each segment (proximal anastomosis, proximal, middle, distal course of graft body, and distal anastomosis). Other objective image quality indices such as noise, signal-to-noise ratio (SNR) and contrast-to-noise ratios (CNR) were assessed. Radiation doses were also compared.

Results

Patient characteristics of the two groups were well matched except HR. The HR of the PGA group was lower than that of the RGH group (62.0±5.0 vs. 65.7±7.4). For both groups, over 90% of segments received excellent or good image quality scores and none was non-evaluative. The image quality generally degraded as graft segment approached to distal anastomosis regardless of techniques and graft types. Image quality scores of the PGA group were better than those of the RGH group (1.51±0.53 vs. 1.73±0.62; p<0.001). There was no significantly difference of objective image quality between two techniques, and the effective radiation dose was significantly lower in the PGA group (7.0±1.2 mSv) than that of the RGH group (20.0±4.6 mSv) (p<0.001), with a 65.0% dose reduction.

Conclusions

Following bypass surgery, 256-slice PGA-CTCA is superior to RGH-CTCA in limiting the radiation dose and obtaining better image quality for bypass grafts.     

犬不同血管搭桥方法及搭桥血管内流场的计算机数值模拟

目的血管搭桥术后的内膜增生往往导致手术失败,而内膜增生与搭桥血管内的流场密切相关,为改善搭桥血管中的流场结构,作者设计了偏心搭桥手术方法,利用计算机数值模拟技术,探索偏心搭桥和传统搭桥血管中流场的变化,为血管搭桥方法提供优化设计方案。方法16只犬随机分为偏心搭桥组和传统搭桥组进行血管搭桥,测定搭桥前后血管几何数据,搭桥后近心端及远心端吻合口血流量和血压。按测定的血管几何数据,FLUENT 6.2模拟搭桥血管内的流场。结果偏心搭桥近心端和远心端吻合口不在同一平面。传统搭桥中,主体动脉远心端吻合口对应面处存在一个较低壁面剪切应力（WSS）区域及流体停滞点,离脚跟较近的一部分流体会形成涡漩,血流进入主体动脉后,还会表现出迪恩涡二次流;偏心搭桥中,主体动脉吻合口对应面上的低WSS区域和流体停滞点消失,血流接触到吻合口底面后,以切向旋转的方式改变其流动方向,不会形成涡漩,且当血流进入主体动脉后,立即发生螺旋流态且能持续很长一段。结论偏心搭桥能够产生血液旋动流,显著增加远心端血流量、提高WSS。     

Arterial steal phenomenon in femoro-tibial bypass with arterio-venous shunt

Arterio-venous shunts are sometimes constructed at the distal anastomosis of femoro-tibial bypass grafts in order to increase blood flow velocity within the graft. However, the use of such a shunt may "steal' blood from an already ischaemic distal arterial bed. The aim of this study was to determine the conditions under which this might happen. Experiments were carried out on an in vitro model of the femoro-tibial bypass under steady flow conditions. The simple resistance model of Hyman and Brewer (J. Biomechanics 13, 469-675, 1980), modified to take into account the nonlinear pressure flow relationship through a stenosis, was used to interpret experimental data. Good agreement was obtained between measured and calculated steal.     

冠状动脉支架植入术后行冠脉旁路移植术

目的:总结15例冠状动脉支架植入术后行冠状动脉旁路移植术的临床经验。方法:回顾分析行冠状动脉支架植入术后行冠状动脉旁路移植术15例患者的资料,男10例,女5例,平均年龄(61±5)岁。行冠状动脉支架植入术后再行冠状动脉旁路移植术时间间隔(24±4)月,冠状动脉内置入支架3-6枚,左室射血分数为43%-64%,其中50%为3例。全组行体外循环下冠状动脉旁路移植术3例,行非体外循环心脏跳动下冠状动脉旁路移植术12例。结果:全组共行动脉桥吻合13支,静脉桥33支;围术期并发低心排综合征3例,肺部感染4例,胸腔内出血行胸腔闭式引流术2例,本组患者无死亡病例。术后平均住院日(13±4)天。结论:对冠状动脉内支架植入术后再狭窄或(和)冠状动脉再血管化不足的病例进行冠状动脉旁路移植治疗,可使冠状动脉达到充分再血管化,提高冠心病患者生活质量及预后。     

Numerical Simulation of Physiological Blood Flow in 2-way Coronary Artery Bypass Grafts

The Coronary Artery Bypass Graft (CABG) yields excellent results and remains the modern standard of care for treatment of occlusive disease in the cardiovascular system. However, the development of anastomotic Intimal Hyperplasia (IH) and restenosis can compromise the medium-and-long term effects of the CABG. This problem can be correlated with the geometric configuration and hemodynamics of the bypass graft. A novel geometric configuration was proposed for the CABG with two symmetrically implanted grafts for the purpose of improving the hemodynamics. Physiological blood flows in two models of bypass grafts were simulated using numerical methods. One model was for the conventional bypass configuration with a single graft (1-way model); the other model was for the proposed bypass configuration with two grafts (2-way model). The temporal and spatial distributions of hemodynamics, such as flow patterns and Wall Shear Stress (WSS) in the vicinity of the distal anastomoses, were analyzed and compared. Calculation results showed that the 2-way model possessed favorable hemodynamics with uniform longitudinal flow patterns and WSS distributions, which could decrease the probability of restenosis and improve the effect of the surgical treatment. Concerning the limitations of the 2-way bypass grafts, it is necessary to perform animal experiments to verify the viability of this novel idea for the CABG.