A computational fluid–structure interaction analysis of coronary Y-grafts
Introduction
Coronary artery bypass graft (CABG) surgery, the standard procedure to treat advanced coronary artery disease, consists in bypassing a blocked portion of a coronary artery in order to restore the proper blood flow to the heart. The incidence of CABG operation is significant in the Western countries. For example, in the USA the number of CABG operations was about 250,000 in 2006 (0.09% of the population) and it is expected to increase by over 50% between 2006 and 2025 [1]. The bypass used for the procedure is typically autologous, i.e., harvested from the patient’s own body. In case of patients with isolated left anterior descending (LAD) coronary artery disease, the recommended treatment is the use of the left internal mammary artery (LIMA) bypass, due to its excellent long-term patency rate [2]. For patients with multivessel coronary disease, together with LIMA, other types of grafts are used to bypass the remaining coronary occlusions [3]. In particular, arterial or venous grafts may be used either as a conventional free (i.e., connected proximally to the aorta and distally to the diseased coronary vessel) or composite (i.e., connected proximally to the first graft, usually the LIMA, and distally to the diseased coronary artery) bypasses. In the latter case, two of the most common choices for the second graft are the radial artery (RA) and the saphenous vein (SV), which are typically attached with a Y anastomosis to the LIMA graft to form a composite Y-graft [4], [5]. RA and SV grafts have both advantages and disadvantages (see [5] for a review) and today there is still controversial evidence regarding the best choice. As a matter of fact, several clinical trials have demonstrated better long-term patency rates for arterial conduits (see e.g. [6], [7], [8]), however SV remains one of the most widely used graft because of its accessibility, sufficient length, ease of use, and in those cases where the radial artery cannot be used (e.g. when the Allen’s test1 shows inadequate collateral hand perfusion or in presence of a non-severe coronary artery stenosis) [10], [11], [12].
The main factor affecting graft patency is the development of intimal hyperplasia (IH) at the anastomosis between the graft and the coronary artery. IH is the progressive intimal thickening due to abnormal proliferation of smooth muscle cells in the tunica intima of the vessel wall, which results in the reduction of the lumen of the graft and may eventually lead to restenosis and graft failure [13], [14], [15]. Although the underlying mechanisms of IH development have not been completely elucidated yet, suggested hypotheses are the presence of hemodynamic disturbances (e.g., stagnation and recirculation regions) and localized stress concentrations in the graft wall, especially in the region of the anastomosis [16], [17], [18], [19]. In particular, extensive IH formation typically occurs when a compliance mismatch between the graft and the host coronary artery is present.
Many computational studies have addressed the problem of finding a possible correlation between hemodynamics in CABGs and IH development (see [20] for a review of recent numerical investigations). Most of them focused on the quantification of hemodynamic parameters to explore the theory of IH development due to the disturbed fluid-dynamics (see e.g. [21], [22]). Only a few authors have attempted to study the influence of mechanical factors like internal wall stresses on IH. In these studies, accurate models of the vessel wall mechanics were considered. For instance, some authors prescribed a given internal pressure to surrogate the blood dynamics [23].
Within this context, the fluid–structure interaction (FSI) between blood and the vessel walls should provide a better quantification of the internal wall stresses and at the same time would allow to assess also the fluid-dynamic factors. However, no numerical FSI simulations in patient-specific CABG geometries have been made so far. Furthermore, despite the ever-increasing clinical interest in the use of composite Y-grafts and the search for the best graft to be used (RA vs SV), no numerical investigations have been made to compare the effects of the graft choice on the resulting fluid-dynamic and mechanical factors that may contribute to the failure of the graft.
The purpose of this work is to investigate the possible causes of graft failure in patient-specific multiple Y-CABGs, with particular attention to the fluid-dynamics and wall mechanics resulting from different choices (arterial vs venous). With this aim, we perform a computational FSI analysis in three patients with multivessel coronary disease and treated with a Y-graft. In particular, we characterize different grafts by changing their mechanical properties (Young’s modulus) and geometric characteristics (diameter). Since the risk of graft failure is related to the degree of coronary stenosis [24], [25], for each case we consider three possible degrees (50%, 70%, 90%) to assess the fluid-mechanic and mechanical factors in relation to the severity of the stenosis.
Section snippets
Patients dataset
In this study, we consider three patients (P1, P2 and P3 in the following) with severe multivessel coronary artery disease who underwent off-pump CABG surgery with a composite Y-graft. In particular, these patients featured two stenoses, one in LAD and another one in a second coronary vessel, see Table 1 for details. In what follows, we refer to these as LAD stenosis and second stenosis. The patients were treated with a LIMA bypass to restore LAD perfusion and with a second graft (radial artery
Results
For each patient and for each stenosis degree, we report in Fig. 6 the peak diastolic ( s) velocity field for the RA graft case. In particular, we plot the streamlines color-coded with the velocity magnitude in a portion of the fluid domain comprising the anastomosis related to the second stenosis and the two stenoses. We do not report here the results corresponding to the SV graft cases because they are very similar. From this figure, we observe a more disturbed fluid-dynamics at the
State of the art and choice of the computational model
Most of the computational studies that have analyzed intimal hyperplasia formation in coronary artery bypass graft anastomoses focused on hemodynamics (see e.g. [22], [49], [50]). The first attempts to include also mechanical analyses in the vessel wall were done by Ballik et al. [23], which performed in idealized anastomoses a structural analysis with different Young’s moduli for the native arteries and the graft by prescribing a given constant-in-space internal pressure to surrogate the
Conclusions and limitations
In this work, at the best of our knowledge, we have addressed for the first time a fluid–structure interaction computational study in realistic coronary artery by-pass grafts. In particular, we have focused on multiple by-passes (Y-grafts) of three patients and study the different fluid-dynamic and mechanical answers of the radial artery and saphenous vein grafts. Both fluid-dynamic (RTT) and wall mechanic (Von Mises stress) quantities usually considered to identify the regions more prone to
Conflict of interest
None declared.
Funding
C.V. was partially supported by INDAM-GNCS and by the Italian MIUR PRIN12 project no. 201289A4LX “Modelli matematici e numerici del sistema cardiocircolatorio e loro applicazione in ambito clinico”.
Ethical approval
Approval of the ethical committee of Ospedale Sacco, Milan, Italy, and informed consent from the patients were obtained. The data were handled according to the relevant policies and procedures of Ospedale Sacco, Milan, Italy.
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