Smaller, stiffer coronary bypass can moderate or reverse the adverse effects of wave reflections

Comparison between mechanical properties of human saphenous vein and umbilical vein

Background

As a main cause of mortality in developed countries, Coronary Artery Disease (CAD) is known as silent killer with a considerable cost to be dedicated for its treatment. Coronary Artery Bypass Graft (CABG) is a common remedy for CAD for which different blood vessels are used as a detour. There is a lack of knowledge about mechanical properties of human blood vessels used for CABG, and while these properties have a great impact on long-term patency of a CABG. Thus, studying these properties, especially those of human umbilical veins which have not been considered yet, looks utterly necessary.

Methods

Umbilical vein, as well as human Saphenous vein, are respectively obtained after cesarean and CABG. First, histological tests were performed to investigate different fiber contents of the samples. Having prepared samples carefully, force-displacement results of samples were rendered to real stress–strain measurements and then a fourth-order polynomial was used to prove the non-linear behavior of these two vessels.

Results

Results were analyzed in two directions, i.e. circumferentially and longitudinally, which then were compared with each other. The comparison between stiffness and elasticity of these veins showed that Saphenous vein’s stiffness is much higher than that of umbilical vein and also, it is less stretchable. Furthermore, for both vessels, longitudinal stiffness was higher than that of circumferential and in stark contrast, stretch ratio in circumferential direction came much higher than longitudinal orientation.

Conclusion

Blood pressure is very high in the region of aorta, so there should be a stiff blood vessel in this area and previous investigations showed that stiffer vessels would have a better influence on the flow of bypass. To this end, the current study has made an attempt to compare these two blood vessels’ stiffness, finding that Saphenous vein is stiffer than umbilical vein which is somehow as stiff as rat aortic vessels. As blood vessel’s stiffness is directly related to elastin and mainly collagen content, results showed the lower amount of these two contents in umbilical vein regarding Saphenous vein.

FLOW VISUALIZATION IN RIGHT CORONARY ARTERY BYPASS MODELS

It is well known from fundamental fluid mechanics that separation regions occur at bends and bifurcations of blood vessels. In addition to this, a secondary flow is also created. This means that the flow is moving forward like a vortical plait. Vortices are created that move counterclockwise to each other perpendicular to the mainstream direction. Three-dimensional flow exists, which is totally different to the well-known parabolic flow in straight pipes under laminar flow conditions. Therefore, a short fundamental introduction to biofluid mechanics is presented in this paper. A coronary artery model with different bypasses is shown as an example. The coronary artery is a prominent cardiac vessel often affected by the atherosclerosis process, which can lead to its full blockage. Blood flow is restored by the construction of a bypass performed by implanting part of the saphenous vein. This bypass is subjected to varying flow conditions during the various phases of pulsatile blood flow. For precise location of the regions associated with flow abnormalities, flow visualization through the complete bypass, covering the arterial and bypass sections, is required. This forms the objective of the present work: to visualize flow changes in bypass models of the right coronary artery prior to its bifurcation under pulsatile flow conditions.

Analog electrical model of the coronary circulation in case of multiple revascularizations

In this work, we propose an analog electrical model of the coronary circulation for patients with obstructive disease undergoing revascularization. In this clinical situation, the collateral circulation to the occluded artery is difficult to ascertain via preoperative measurements and well-developed collaterals might induce long-term restenosis of the revascularized artery due to flow competition mechanisms. The proposed model allows an original biomechanical analysis of per-operative hemodynamic data in order to assess quantitative evaluation of pressures and flows inside the native stenosed arteries, the collateral network and the bypass grafts. Average cardiac cycle values are analysed. In the case of 3-vessel disease and chronic occlusion of the right coronary artery, the quantitative results confirm the protective effects of the collateral flows in the pathological situation, but also show that the revascularization of the occluded right artery is fully justified since the collateral flows remain low, even when the left territory is revascularized. The model thus provides a computational tool to evaluate therapeutic strategies for each patient.