title: Computational model of blood flow in the aorto-coronary bypass graft.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: School of Mechanical and Production Engineering, Nanyang Technological University, 63 97 98, Singapore. msmeena@ntu.edu.sg <msmeena@ntu.edu.sg>
authors.forename: Meena
authors.initials: M
authors.lastname: Sankaranarayanan
authors.forename: Leok Poh
authors.initials: LP
authors.lastname: Chua
authors.forename: Dhanjoo N
authors.initials: DN
authors.lastname: Ghista
authors.forename: Yong Seng
authors.initials: YS
authors.lastname: Tan
mesh_terms: D000818:Animals; D001011:Aorta / Q000503:physiopathology* / Q000601:surgery*; D001783:Blood Flow Velocity; D001794:Blood Pressure; D003198:Computer Simulation; D003326:Coronary Circulation; D003331:Coronary Vessels / Q000503:physiopathology* / Q000601:surgery*; D006801:Humans; D008955:Models, Cardiovascular; D033081:Shear Strength; D013314:Stress, Mechanical; D025321:Surgery, Computer-Assisted / Q000379:methods; D014680:Veins / Q000503:physiopathology* / Q000637:transplantation*
references.citation: J Biomech. 2000 Aug;33(8):1011-22
references.pmid: 10828332
references.citation: J Biomech Eng. 1999 Jun;121(3):265-72
references.pmid: 10396691
references.citation: Surg Today. 2000;30(11):993-8
references.pmid: 11110393
references.citation: J Biomech Eng. 2001 Feb;123(1):80-7
references.pmid: 11277306
references.citation: Int J Artif Organs. 2001 Mar;24(3):145-51
references.pmid: 11314808
references.citation: J Biomech. 2001 Aug;34(8):1049-64
references.pmid: 11448697
references.citation: J Biomech Eng. 2001 Jun;123(3):277-83
references.pmid: 11476372
references.citation: Biorheology. 2002;39(3-4):525-31
references.pmid: 12122276
references.citation: Ann Biomed Eng. 2003 Apr;31(4):420-9
references.pmid: 12723683
references.citation: Chin Med J (Engl). 2003 Mar;116(3):436-9
references.pmid: 12781054
references.citation: J Thorac Cardiovasc Surg. 2003 Sep;126(3):645-50
references.pmid: 14502134
references.citation: J Thorac Cardiovasc Surg. 2003 Nov;126(5):1531-6
references.pmid: 14666029
references.citation: Surgery. 1972 Dec;72(6):1007-17
references.pmid: 5087270
references.citation: Surg Forum. 1976;27(62):208-10
references.pmid: 1019858
references.citation: Arch Surg. 1979 Dec;114(12):1369-73
references.pmid: 160777
references.citation: Ann Surg. 1983 Apr;197(4):479-83
references.pmid: 6219641
references.citation: J Vasc Surg. 1990 Dec;12(6):747-53
references.pmid: 2243410
references.citation: J Vasc Surg. 1992 Apr;15(4):708-16; discussion 716-7
references.pmid: 1560562
references.citation: Ann Med. 1994 Apr;26(2):95-100
references.pmid: 8024734
references.citation: J Biomech. 1996 Jul;29(7):855-72
references.pmid: 8809616
references.citation: J Biomech Eng. 1997 May;119(2):187-94
references.pmid: 9168395
references.citation: Yonsei Med J. 1998 Apr;39(2):166-74
references.pmid: 9587258
references.citation: Ann Thorac Surg. 2000 Oct;70(4):1350-4
references.pmid: 11081897