Optimisation Of A Coronary Artery Bypass Graft Using CFD Simulation For Patients With Coronary Artery Disease

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Why doctors use Coronary Artery Bypass Graft

Coronary artery disease (CAD) is called as coronary heart attack disease. The large blood vessel called coronary arteries which help the heart in getting oxygen. Basically, it supplies blood to the heart and sometimes it becomes extremely narrow. As it becomes narrow the less amount of blood can pass through the arteries and the heard does not get enough blood or oxygen which can lead to chest pain, the problem in breathing while doing any physical activity. If these blood vessels get completely blocked, it may cause a heart attack. It also makes the heart muscle weak and can lead to failure of heart or heart rhythm problems (UCSF, 2018). It is determined that when the heart not able to pump blood properly to the rest of the body then it results in failure of the heart. A rhythm problem means any change in the normal beating pattern of the heart.CAD starts from having chest pain and breathing problems like shortness in breathing. This type of pain is also known as angina. It is related to having a feeling of becoming narrower or nervousness and may expand to the back of the neck, upper abdomen, back, the arms or jaw. All these symptoms can be noticed while doing any physical activity as heart muscle requires more oxygen during this time.  It happens because not enough blood and oxygen reaching to the heart through the coronary arteries (Dauerman, 2013). This can be referred to the "angina threshold" when the symptoms start occurring after reaching to the defined level of the physical activity. Arteriosclerosis is the reason for coronary artery disease. Arteriosclerosis starts developing from slight swelling in the wall of the blood vessel. It gets narrowed or blocked because fats, cells and other material get attached to the walls of the blood vessel and it is called plaques. In the beginning, it's hard to notice the deposits of the coronary arteries. And if it continues to get increasing then it might start influencing the flow of blood through coronary arteries, till the part of the heart not able to get enough oxygen. Chest pain and discomfort might be arising during physical activity or emotional stress (Major Satisha T S et al., 2012).

The Coronary Artery Bypass Graft (CABG) is considered to be a surgical process carried out by the doctors for restoring the flow of blood for delivering nutrients and oxygen to the heart muscle. The cardiac surgeon in the CABG develops new route by bridging stenosed artery with a vessel. The bypass graft is affected because of the poor hemodynamics in intimal hyperplasia. Blood’s hemodynamics changes the atherosclerosis condition such as wall shear stress, WSS spatial gradient and oscillatory shear index (Do, 2012). The hemodynamic parameters are influenced powerfully by conditions of flow that are dependent mostly on the geometry of the artery. The parameters are being used for examining the sternness of the coronary stenosis disease. The computational fluid dynamics (CFD) model is being used by the doctors for showing the pattern of blood flow in various anatomical geometries. CFD simulation of the hemodynamic is considered to be very much in the research of the flows of biological fluid.

Design of a bypass graft

In the clinical applications, imagining techniques are used for providing only the anatomic Coronary artery disease. The pressure data and flow rate information of coronary artery disease can be identified by MRI and angiography method. CFD analysis can be used to determine hemodynamic information and data of coronary heart disease. The application of computational fluid dynamics method assists to examine the hemodynamic strictures of the coronary heart disease (Dur et al., 2010). However, it is found that Coronary Artery Bypass Graft fails after a short time period because within the junction of the bypass graft a plaque is developed which is referred to as intimal hyperplasia. The intimal hyperplasia development within the bypass graft leads to compliance mismatch and abnormal hemodynamic. Thus, it is necessary to understand and analyse the hemodynamic forces into the coronary artery bypass and its geometrical characteristics and mechanical structure.

 In vivo, it difficult to obtain the information and in vitro numerical and experimental investigations of realistic and idealistic CABG surgery models have been used extensively for obtaining the mechanical and hemodynamic forces in the bypass. The result is being used for designing and optimising the configurations of bypass graft because the development of artery diseases decreases and increase in the patency of bypass.  Computational fluid dynamics is being used extensively as a numerical tool for investigating geometrical and physical factors that influences the hemodynamic of different CABG configurations. The flow characteristic and pressure and wall shear stress for cyclic and steady flows with CFD can be effectively and accurately examined (Ellis and Holmes, 2015).

The use of CFD in hemodynamic and biomechanics is accepted widely as an appropriate alternative in vivo and vitro measurements that can be very time to consume and expensive. There is always a requirement for obtaining effective quantitative experimental data for validation and verification of the numerical predictions. The combination of CFD with imaging and experimental methods has been widely used for examining the hemodynamic of different graft and bypass junctions. The use of the two complementary methods allows determining different factors such as gradients, wall shear stress, blood flow fields, deformation of graft and artery junction and compliance mismatch degree for the examination of the cardiovascular diseases (Fan, 2011).

CFD can be used for assessing the hemodynamic of bypass graft using data of a patient obtained from the computed tomography angiography. The 3D model is considered to be a rigid wall for stimulating the flow of blood in the venous narrowed bypass graft in order to recognize the detailed transition of flow physics to the downstream turbulence of the graft curvature. The study focuses on the implementation of new techniques that would enable the development of the postoperative vascular models with the use of simulation-based planning system. The computer-aided design with computational fluid dynamics method is used for optimizing hemodynamic of the coronary artery bypass graft. The configuration of CABG is relied on the surgical planning hypothesis and used for evaluating the acute hemodynamic and local hemodynamic for readjusting the coronary bypass surgery (Ghista and Kabinejadian, 2013). The proposed procedure is found to be used to help the decision-making processes in surgery. However, it is necessary to minimize the circulation zone, local flow turbulence and maximizing the conduit energy efficiencies of the given configurations. The hemodynamic improvements can be achieved with the optimization of the anastomosis geometry, transitional curvature and graft vessel of the vessel.      

The Coronary Artery Bypass Graft is the surgical procedure used to graft veins or arteries from the body of the patient or imitation conduits to coronary occluded arteries for bypassing the atherosclerotic narrowing and improving the supply of blood for nurturing the myocardium to the coronary circulations. The below figure shows both venous and arterial grafts bypassing each the coronary blockage created by the cholesterol build-ups (He and Loop, 2006).

Figure: Coronary arteries bypass grafting

Source: (Ghista and Kabinejadian, 2013)

The above figure shows the coronary artery bypass grafting. The saphenous vein graft is proximally anastomosed to aorta and stenosis downstream of the right coronary artery. Internal mammary artery created from the aorta is being anastomosed to LAD (left anterior descending) coronary artery. Coronary artery bypass grafting is considered to be extremely effective for prognostic improvement and symptomatic aid in computational fluid dynamics and it is used for patients with high risk. It provides long-term benefits associated with the continuing conduit patency. It is seen that after operation during the first year approximately 10-15 per cent of the vein grafts stops. For five to ten year, about half of vein graft is seen to be effective. After 10 years, about 60 percent of the vein grafts are obvious and 50 percent remain free of important stenosis (Dur et al., 2010).

Early failure of bypass grafts is due to the technical errors during the surgical process which results in thrombosis. Graft failures at a later stage are caused by the progression of intimal hyperplasia and atherosclerosis. The major cause of the occurrence of graft stenosis is found to be IH. IH is the continued, abnormal, overgrowth and proliferation of the smooth muscle cells in response to dysfunction or endothelial injury. There are both biomechanical and biological factors which lead to IH.  Hemodynamic parameters are important information of IH and genesis. IH predominantly develops at the heel and toe of the artery and anastomosis bed junction where the hemodynamic factors and patterns of distributed flow are observed. The distribution patterns of flow, hemodynamic factors and intimal thinking distribution are correlated with the progression and onset of anastomic intimal hyperplasia and atherosclerosis (Masahiko Kuinose, 2014).

Many studies were carried out for various anastomotic devices and geometrics for the improvement of HPs distribution and flow fields at ETS anastomosis for enhancing the graft patency. The efforts have been made for achieving the enhancement of the optimal patency coronary artery bypass graft anastomotic configuration. The enhancement of the patency rate and longevity of CABGs will improve the ejection fraction and left contractility ventricular index of patients with the coronary artery disease, elimination of the necessity of re-operation, lower morbidity and decreased medical costs (Menees and Bates, 2010).           

Coronary artery disease is the blockage or narrowing of coronary arteries which is generally caused by atherosclerosis. The build-up of fatty and cholesterol deposits on the arteries' inner walls are called as atherosclerosis. The plaque can obstruct the flow of blood to the muscle of heart by clogging physically the arteries or by causing abnormal function and tone of the artery. The heart suffers due to the lack of oxygen and significant nutrients which are needed to work appropriately because there is an inadequate supply of blood (Pak, 2017). It can cause chest pain which is referred to as angina. A heart attack can occur if the heart’s energy demands become higher than the supply of blood or if the supply of blood is cut off to the heart muscle.  Thus, a complete blockage results into a heart attack. The coronary artery disease develops generally over decades but the problem can be noticed only when the individual has a heart attack or blockage. In the coronary artery, the increase in the size of blockage enables the narrowed coronary artery to form collateral circulations. It is the formation of the new blood vessels that redirect the flow of blood around the blockages. The new arteries during the increase in stress or exertion may not be able for supplying adequate oxygen-rich blood to the muscle of blood (Leopold, 2015).      

Narrow coronary arteries cannot supply provide enough blood or oxygen to the heart. It generally happens during exercise or while doing any physical activity, because heart beats hard during this time. Coronary artery bypass surgery has been saving thousands of people each year. At the same time, it has also been blamed for having many other problems just after the surgery (LeWinter, 2005).

Reduce Kidney function: There is a high risk of kidney failure just after the surgery.  It may stop working properly. This has been noticed in 5 to 20 out of 100 patients who have Coronary artery bypass surgery. Sometimes it takes to dialysis for getting the kidney starts working again. The kidney is being attached to a machine which works for the kidney and also works as a filter that separates unnecessary stuff from the blood, it is called dialysis. Some of the patients face reduced kidney function just after the surgery. But it remains for a short period of time, in most of the cases, it starts working again just after a few days or weeks (Song et al., 2017).  

Blood clots: This is a common problem which may happen with any surgery. But the risk is associated with it is very high for those patients who smoke, overweight or not able to move much after the surgery. This starts from the legs and it is called deep vein thrombosis or DVT. It can be a threat to life because it can move to the lungs and may occur some serious problems. It generally starts from the legs and can move to the lungs, swelling and pain can be noticed in the leg and in the calf. There is also a chance of having pain in chest and breathing problem (Ctsnet, 2018).

Doctors generally have a discussion with the patients and take them through their medical history and let them know if they are at high risk of clots or not. So that they can have preventative treatment before they go for the coronary bypass surgery (LeWinter, 2005).

Compression stocking: Doctors might advise patients to use compression stocking after having the surgery. It helps in reducing the beating rate of the heart and keeps it to a stable rhythm which might increase during the medication. If the patient is having an issue of atrial fibrillation continually for more than two days then he/she might require a procedure to make it correct (Cleveland, 2018). It does not take more than 5 minutes and general anaesthesia is being used to carry out with the procedure. A very short electric shock use to give to make the heart beat normal. This procedure is known as cardioversion.

Muffled hearing: patients can feel their heartbeat because the sac around the heart is used to open by the surgeon during the surgery. The sac is known as pericardium. However, this is nothing to worry about as opening the sac is a part of the operation. This automatically goes away in a few days.

Delirium: This is a common problem and it may be found in one out of four people who have coronary bypass surgery. And this happens because of anaesthesia. It makes feel like confused and emotional and it happens after a few days of the surgery. It makes patients angry, tearful or irritable. Sleep may also be affected by the cause of it. This may affect patients' memory as well; it may cause memory loss as patients may not remember his/her name or the name of the family members. This generally happens due to infection, dehydration and low oxygen level (Hu and Gao, 2014).

Sleeping problem: Sleeping disorder is the other issue with the surgery and it takes a couple of weeks to become normal. Patients might find hard to go to the sleep or wake up early hours. They might find themselves very sweaty during the sleep and it goes away as the body recover.

Pain: This is a common problem and discomfort for the patients after having heart surgery. The patients usually suffer from stiffness in their chest, neck and arms. Like all other problems it also goes away gradually within 3-4 weeks. But chest pain and discomfort usually take six months to away (Iwasaki and Matsumoto, 2011).   

Poor appetite: Sometimes patients do not feel hungry. Even they do find taste in the food. And it also gets better with the time. Patients should have food more often to stay hydrated.

The coronary artery bypass grafting is being used by doctors for improving the flow of blood to the heart. It is the type of surgery used for the patient suffering from coronary artery disease.  The coronary artery bypass graft is considered to be an effective treatment for coronary heart disease. The doctors connect or graft the healthy vein or artery from the body to the blocked or narrowed coronary artery. The linked vein or artery bypasses the portion which is blocked of the coronary artery. It assists in creating a new route allowing the oxygen-rich blood to reach the heart muscle (Poon et al., 2015). Computational fluid dynamics is being used by the doctors for determining the optimal design which is based on the error and trial among the small number of intuitive design and geometrical variations alternatives. It has been found that the simulation of a computer with numerical optimization shape has many benefits as it provides cost-effective techniques for the structure of the medical devices (Tan et al., 2005).  

Coronary bypass surgery is the only an effective way to prevent the risk of heart attack or to get relief from serious chest pain caused by a blockage in a blood vessel that provides blood and oxygen to the heart. A blood vessel needs to be attached to the affected part of the heart artery, which use to take from the other part of the body by the surgeon.  

Coronary bypass surgery contains multiple benefits, especially for those patients who are suffering from serious cardiovascular disease. The surgery may help in saving the patient’s life if he/she has already faced a heart attack or at risk of having one. Coronary bypass surgery is highly effective for those patients, who are suffering from breathing problems or angina due to heart arteries disease, it can reduce the discomfort or eliminate permanently (Dewantoro et al., 2018). Many of the patients have improvement from their surgery or permanent relief for several years from the symptoms that they had been facing. The surgery can help in getting relief more quickly than going “on pump” that may make suffer from post-operative difficulties.

There are a few more benefits which can be achieved from Coronary bypass surgery, such as:

• Less need for transfusion
• Shorter hospital stay
• Fewer issues with thinking skills and memory loss
• A lower risk of stroke
• Less need for transfusion
• Lower death rate- especially among woman and “high risk” patients
• Decrease injury to the heart
• Relief from the chest pain
• Having more energy for physical activity  

• Bleeding during or after the surgery: It is noticed that almost 30% of the patients generally need blood transfusions just after the operation (Heart and Vascular Institute, 2015).
• The problem in the rhythm of the heart: A conation may arise where the upper chamber of the heart does not beat properly instead starts quivering and it's known as atrial fibrillation. This is a common scenario which may arise during the surgery and can cause to blood clots starts from the heart to the other parts of the body.
• Blood clots: Formation of blood clots can lead to the heart attack, lungs problems and stoke.
• There is a risk of getting the infection to that particular area of the chest which was opened during the surgery. However, the risk of getting infected is very rare (Medtronic, 2018).
• Post-pericardiotomy syndrome:   This syndrome is being faced by around 30% of the patients. Patients suffer from fever and chest pain which may start just after the surgery or after six months.
• The surgery may affect the kidney; there is a high probability that kidney stop working after the coronary bypass surgery.
• Loss of memory: Thinking difficulty just after coronary bypass surgery has been reported by many patients. It takes six months to one year in getting improvement (National Institutes of Health, 2018).
• It is being noticed that the patents face any type of reactions with anaesthesia such as allergic problem and difficult breathing.
• Death: After coronary bypass surgery in-hospital death is rare. It generally happens by stork or heart attack.
• Injuries in nerves, arms, chest or leg may occur during the surgery (Secondscount, 2015).

The process of the computational fluid dynamics relies on three fundamental principles that are conservation of mass, conservation of energy and Newton's second law. The fundamental principles can be expressed in mathematical equations terms that can be calculated and depicted as numerical results. The numerical results in CFD methods are directly related to the adoption of the mesh quality. There are many meshing methods that can optimize the accuracy, time and number of elements for the optimization. There are many shapes of elements which can be distributed automatically by the meshing algorithms.

The haemodynamics plays a significant role in the graft anastomosis's patency rate and there is no guarantee about the optimal pattern of flow that enhances the effectiveness of grafting. The Wall Shear Stress benefits assist to avoid the creation of plaque, the proliferation of intermediate lesion and fibroatheroma and increase in the intimal-medial thickness. Wall Shear Stress high values may lead to endothelial lesions. In vivo data, the paucity for supporting the existing hypotheses to develop atherosclerosis and intimal hyperplasia in various bypass configurations is a major challenge (Ruiz-Soler, Kabinejadian, Slevin, Keshmiri and Bartolo, 2017). The swirling flow is a useful physiological mechanism for decreasing the abnormal flow conditions for preventing intimal hyperplasia, atherosclerotic lesions and thrombosis. The optimisation criteria are based on the assumptions of high secondary velocity, high wall shear stress and reduction of recirculation and separation zones.     

Figure: Contours of cross-flow streamlines and secondary velocity for various ridge designs at the monitoring plane

Source: (Ruiz-Soler, Kabinejadian, Slevin, Keshmiri and Bartolo, 2017)   

Figure: Distribution of Wall Shear Stress on the developed surface of the host artery for various ridge designs

Source: (Ruiz-Soler, Kabinejadian, Slevin, Keshmiri and Bartolo, 2017)   

Source: Areas of retrograde flow of various designs at the monitoring planes one and two

Figure: (Ruiz-Soler, Kabinejadian, Slevin, Keshmiri and Bartolo, 2017)   

The supply of blood to tissues and organs is monitored by the flow-resistant that leads to pressure drop. The vessels geometry is the main parameters constituted in the adjustment of blood flow by means of vasodilator and vasoconstrictor mechanisms. The swirling motion is being characterised through cross-flow streamlines and secondary velocity contours at the monitoring plane. The increase in the secondary velocity leads to a decrease in the pitch. The Wall Shear Stress is the tangential fluid forces which act on the vessel wall. The restenosis and intimal thickening are due to the intimal hyperplasia which is characterised by low Wall Shear Stress. Recirculation and abnormal flow conditions are associated with the areas of low Wall Shear Stress that leads to atheroma thrombus growth, IH development and cholesterol deposition.      

The recognition of the spiral shape of the blood flow in the arterial system leads to the novel shape of the spiral suggesting prosthetic graft consisting of Spiral Laminar Flow peripheral vascular graft. The use of internal ridge for inducing the spiral flow blood enabled SLF graft to decrease Oscillatory Shear Index and WSS gradient, decrease near wall turbulence and directed forces, suppress thrombus acute formation with the platelet activation, decreasing the low-density luminal surface lipoproteins concentration and enhancing oxygen fluxes to the arterial wall.

The findings need further investigations on the improvement of the performance of the grafts with appropriate spiral design. The focus should be done on the role played by the haemodynamic parameters and sensitivity recognized of the geometry flow pattern. The improvement of the performance and design of the spiral inducing grafts need to be done for finding optimum configuration. A further research is needed for constructing an appropriate cost function for shaping the optimization. Thus, a wide range of design parameters should be introduced for completing the shape optimisation.  

Conclusion 

The knowledge of appropriate hemodynamic patterns of flow, structure deformation in coronary artery bypass graft and complex interactions between the fluid forces are considered to be important for the study of the occurrence of cardiovascular diseases. In vivo, the detection and measurement are difficult. In numerical investigations and vitro experiments under complete simulation and correct physiological is being adopted for determining mechanical and hemodynamic forces for designing and optimising the configurations of the bypass graft. The computational fluid dynamics is being used by the doctors for examining the geometrical and physical factors that influence the hemodynamic of different CABG configurations. The measurement of pressure, flow and wall shear stress can be effectively and accurately examined with the use of the computational fluid dynamics.     

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