Coronary artery bypass grafting hemodynamics and anastomosis design: a biomedical engineering review

Abstract

In this paper, coronary arterial bypass grafting hemodynamics and anastomosis designs are reviewed. The paper specifically addresses the biomechanical factors for enhancement of the patency of coronary artery bypass grafts (CABGs). Stenosis of distal anastomosis, caused by thrombosis and intimal hyperplasia (IH), is the major cause of failure of CABGs. Strong correlations have been established between the hemodynamics and vessel wall biomechanical factors and the initiation and development of IH and thrombus formation. Accordingly, several investigations have been conducted and numerous anastomotic geometries and devices have been designed to better regulate the blood flow fields and distribution of hemodynamic parameters and biomechanical factors at the distal anastomosis, in order to enhance the patency of CABGs. Enhancement of longevity and patency rate of CABGs can eliminate the need for re-operation and can significantly lower morbidity, and thereby reduces medical costs for patients suffering from coronary stenosis. This invited review focuses on various endeavors made thus far to design a patency-enhancing optimized anastomotic configuration for the distal junction of CABGs.

Figure 1

Illustration of coronary arterial bypass grafting. A saphenous vein graft is anastomosed proximally to aorta and distally to downstream of the stenosis of the right coronary artery (RCA). The internal mammary artery (IMA) which branches from aorta is anastomosed to the left anterior descending (LAD) coronary artery.

Figure 2

Injury-induced intimal hyperplasia. A tissue section of a rat’s common carotid artery: Intimal hyperplasia 14 days after treatment with angioplasty balloon. The locations of the internal and external elastic lamina are indicated by an arrow and an arrowhead, respectively (taken from [60] with permission).

Figure 3

Volume-Pressure curves for vein and artery. Schematic volume-pressure curves for artery and vein (the slope of the curve indicates the compliance). Compliance of a vein is greater than arterial compliance at low pressures and smaller than that at high pressures.

Figure 4

Intramural stress distribution at a conventional ETS anastomosis. Contour of mean intramural stress (normalized by the inflating pressure) at the anastomosis of a conventional ETS graft-artery junction. The suture-induced stress concentrations range from 3 to 36 times the stress values along the distal host artery (taken from [15] with permission).

Figure 5

Geometric compliance matching. The graft constructed with an elliptic cross-section (b) develops peak stresses that are orders of magnitude lower than those developed with conventional ETE configurations (a). Circular grafts and vessels cut on a bias (bevelled end) provide an increased anastomotic junction perimeter which can reduce stress concentrations by distributing the loading force among a potentially greater number of sutures. The introduction of matching geometric compliance that dominates at the anastomotic junction minimizes the consequences of material mismatch between graft and vessel and has the potential to reduce the suture line stress (taken from [71] with permission).

Figure 6

Spatial distribution of intimal hyperplasia in an end-to-side distal anastomosis. Outline of the spatial distribution of IH in an ETS distal anastomosis: IH occurs preferentially around the suture-line (especially at the toe and heel of the anastomosis) and on the bed of the host artery (taken from [84] with permission).

Figure 7

In vivo evidence that geometry of ETS anastomosis causes IH. (a) ETS distal anastomosis with a vein patch (b) a section of distal ETS anastomosis; creation of an arbitrary stricture at the hood to simulate the toe (*) results in transferring the IH from the suture-line at the anastomotic toe to the stricture at the hood (physiologic toe); arrows indicate direction of blood flow (taken from [83]).

Figure 8

Typical flow patterns and HPs distribution in a distal ETS anastomosis. (a) Outline of the typical spatial distribution of HPs and IT, and (b) flow patterns in the distal ETS anastomosis of arterial bypass grafts. A stagnation point forms on the arterial bed due to the bifurcation of the graft flow into the proximal and distal outlet segments (POS and DOS) of the coronary artery after impinging on the arterial bed (adopted from [106] with permission).

Figure 9

Effect of anastomotic angle on the flow regime. Flow streamlines in the symmetry plane of distal ETS anastomoses with different anastomotic angles (a) 60°, (b) 45°, and (c) 30°. Flow separation at the toe and size of the reversed-flow region downstream of the anastomosis increases with anastomotic angle (taken from [27] with permission).

Figure 10

Helical graft. Flow mixing visualization by bolus injection into water flow (Re = 550) in U-tubes with (a) a conventional tube, and (b) a helical tube. Significantly greater mixing can be observed in the helical tube, which can enhance fluid-wall mass transport and render the spatial distribution of WSS relatively uniform in curved conduits. (c) angiogram of an arteriovenous access PTFE helical graft. Angiographic examinations have suggested that there exists reduction of helical geometry at or after implantation, which might be attributable to graft elongation under arterial pressure (taken from [136,137] with permission).

Figure 11

Miller cuff construction. (a) Vein cuff is sewn longitudinally around arteriotomy (b) graft is then sutured end-to-cuff. Using a cuff has adverse effects on hemodynamic factors around the anastomosis (e.g., large variations in shear stress on the artery floor, low-momentum recirculation within the cuff, and prominent separation at the cuff toe), and any improved patency rates achieved with cuffed anastomoses have been attributed to increased anastomotic volume and the consequent ability to accommodate IH, before it causes significant stenosis, rather than any decrease in IH (taken from [139] with permission).

Figure 12

Taylor vein patch. (a) Schematic drawing of Taylor-patched anastomosis. (b) Intraoperative photograph of distal Taylor vein patch (6 mm PTFE graft bypass to below knee popliteal artery). Patched grafts have not shown significant improvement in primary patency rates as compared to non-patched grafts (taken from [150,151] with permission).

Figure 13

Linton patch. Schematic drawing of a Linton-patched anastomosis. The flow patterns of patched grafts are similar to those of conventional ETS anastomosis. The clinical patency of this technique has been reported to be 65-74% at 12-48 months post-operative.

Figure 14

Lei’s improved anastomotic geometry. Improved anastomotic geometry with S-shaped gradual transition in wall curvature and cross-sectional area at the toe region results in significant reduction of WSSG at the toe and on the floor as compared with standard ETS and Taylor patched configurations (adopted from [29] with permission).

Figure 15

Tyrell vein collar. Diagram of a Tyrell vein collar (a) without and (b) with a PTFE graft. Trials of Tyrrell collar venous anastomosis in AVGs have not shown any improvement in graft patency (taken from [154] with permission).

Figure 16

Numerical studies of three ETS anastomotic configurations. Geometric models of a conventional ETS anastomosis (base case), Venaflo™ graft, and modified graft-end design. Venaflo™ graft provides larger junction area and less disturbed flow patterns than the conventional ETS anastomosis, and the modified graft-end design further reduces the WSSG by elimination of the graft bulges. Results of clinical trials of the Venaflo™ graft are controversial; some studies showed promising graft patency rates in the Venaflo™ grafts (58% versus 21% in the conventional standard grafts at 24 months), while other investigations demonstrated inferior 1-year patency rates of the Venaflo™ grafts (43% versus 47% for non-cuffed ePTFE grafts) (adopted from [31] with permission).

Figure 17

Streamlined anastomotic configuration. Streamlined arterial bypass graft configuration: Although this geometric design can reduce the peak WSSG at the toe of the anastomosis, particle-wall interaction remains significant, which can result in platelet activation and may lead to IH (adopted from [159] with permission).

Figure 18

Prosthetic bifurcating graft-end configuration. Schematic drawing of the prosthetic graft-end configuration designed to reduce flow stagnation and flow separation zones. This prosthetic graft can be connected in an ETE fashion with the POS and DOS (adopted from [160] with permission).

Figure 19

Cuff-like sleeve. Schematic view of a distal ETS anastomosis with an incorporated sleeve. Sleeve models with higher necks were deemed preferred in terms of hemodynamics at the distal anastomosis (taken from [161] with permission).

Figure 20

Flow-splitter. Schematic wire-frame view of a graft-artery junction and implanted flow-splitter. This flow-splitter can divert the flow from the arterial bed to avoid flow impingement on the bed, mitigate the flow separation at the toe, and reduce the size of flow recirculation areas. However, it causes flow impingements on the arterial side-walls and increases the WSS on both sides of the artery bed centerline which can result in high values of time-averaged WSSG (taken from [162] with permission).

Figure 21

Prosthetic bifurcating vascular grafting device. (a): (1) Flow from the proximal anastomosis (2,3) suture-lines of the end-to-end anastomoses of the distal section (4) the host artery. (b) The bifurcated flows impinge upon each other at the central lumen of the distal anastomosis and avoid arterial bed impingement to reduce the possibility of IH formation. Also, flow separation at the toe is eliminated and the opposing self-correcting flows rapidly return to normal hemodynamic behavior. (c) Intraoperative photograph of the graft implanted into a porcine aorta (adopted from [163,164] with permission).

Figure 22

Coupled STS-ETS sequential anastomoses bypass graft design. Flow streamlines through the coupled STS-ETS sequential anastomoses bypass graft design. Part of the graft flow, which is diverted into the coronary artery at the STS anastomosis, lifts up the flow coming from the graft at the ETS anastomosis and directs it smoothly into the coronary artery; this prevents arterial bed impingement and eliminates the stagnation point and flow recirculation at the ETS anastomosis. This design provides a spare route for the blood flow to the coronary artery to avoid re-operation in case of re-stenosis in either of the anastomoses (adopted from [167] with permission).