Hemodynamics of the aorta

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Anatomical location of the thoracic aorta Aorta segments.svg
Anatomical location of the thoracic aorta

The hemodynamics of the aorta is an ongoing field of research in which the goal is to identify what flow patterns and subsequent forces occur within the thoracic aorta. These patterns and forces are used to identify the presence and severity of cardiovascular diseases such as aortic aneurysm and atherosclerosis. [1] Some of the methods used to study the hemodynamics of aortic flow are patient scans, computational fluid dynamics models, and particle tracking velocimetry (PTV). The information gathered through these studies can be used for surgery planning and the development of implants. [2] Greater understanding of this topic reduces mortality rates associated with cardiovascular disease.

Contents

General flow patterns

The mean velocity in the aorta varies over the cardiac cycle. During systole the mean velocity rises to a peak, then it falls during diastole. This pattern is repeated with each squeezing pulse of the heart. The highest velocities are found at the exit of the valve during systole. At this stage the majority of the flow can be described with velocity vectors normal to the entrance, but in plane velocities tangent to the flow are present. [3] As the path starts to curve in the ascending aorta, the blood towards the outside of the arch tends to rotate towards the inner wall, causing a helical pattern that is observed in most individuals. As the blood moves into the aortic arch, the area with the highest velocity tends to be on the inner wall. Helical flow within the ascending aorta and aortic arch help to reduce flow stagnation and increase oxygen transport. [4] As the blood moves into the descending aorta, rotations in the flow are less present. Physiological abnormalities due to plague formation or aneurysm lead to helical flows and high velocity flows in locations where they would not normally be present or as prominent. The abnormal high velocity areas generate a higher amount of wall shear stress than normal and contribute to stenosis and further plaque formation. [5] Abnormal helical structures expose tissue to low wall shear stresses that it would not normally experience. Simulations of these flow patterns seek to identify what normal wall shear stress conditions and helical flows are present at specific location within the aorta.[ citation needed ]

Effect of age and sex

When evaluating the significance of the hemodynamics of a patient their age and gender play a role. [6] Each individual will have specific aortic geometries but trends can be identified when observed as a group. As age increases the aortic diameter tends to increase and the peak velocity of systolic flow tends to decrease until patients reach an age greater than 60 years old. [6] Patients over the age of 60 tend to have an increase of peak systolic velocity. [6] While both sexes experience the same pattern of velocity change with age, men tend to experience a wider range and higher peak velocity with age.[ citation needed ]

Effect of diabetes

Diabetes mellitus (diabetes) is a significant risk factor for cardiovascular diseases. [7] The presence of diabetes affects the dynamic viscosity of blood and the compliance of the aortic walls. [8] The dynamic viscosity of blood where diabetes is present is higher than that of healthy blood making it slightly less resistive to flow. The Young's modulus of the aortic walls where diabetes is present is higher than that of a healthy patient making it more stiff. When comparing CFD models of normal blood and wall properties with CFD models where the blood and wall properties to replicate that of an individual with diabetes, it is found that the models with diabetes have a lower mean velocity. [8] It is also observed that the outlet velocity of the descending aorta is lower in the diabetes model. [8] The blood pressure in the diabetes model is lower than that of the control model, but the mean pressures of the entire aorta are similar between both models. [8]

Modeling of aortic flow

CFD modeling of the aorta

CFD models allow for researchers to recreate flows happening within the aorta and evaluate factors that cannot be obtained through normal patient scans. These factors include wall shear stress and helicity. These factors are then used to evaluate the progression and severity of cardiovascular diseases.[ citation needed ]

Patient specific information

In order to replicate patient-specific geometries a CT scan or an MRI is taken. [2] From this scan the inlet, various outlets, and walls can be digitally reconstructed to create the control volume. A common software used to construct the geometry and discretize the mesh is ANSYS. The inlet is identified as the cross section directly above the aortic valve. The outlets are identified as the brachiocephalic artery, left and right common carotid artery, subclavian artery, and the descending aorta.

In order to replicate the flow velocities that occur in individual patients a PC-MRI is taken. The PC-MRI can be taken be 1D, 3D, or 4D. 1D PC-MRIs only capture the velocity in one direction, typically axially with the inlet. A 4D PC-MRI can capture the axial through plane velocity, as well as orthogonal in plane velocities. Although 4D PC-MRIs provide more accurate and useful information on the flow, 1D PC-MRIs are more commonly taken and used in CFD modeling of the aorta. [9] Wall shear stress and helicity of the flow tend to be influenced by which type of velocity information os used in the model. [9]

Boundary conditions

There are a variety of flows that have been modeled and studied as the inlet boundary condition. Some simplified flows include plug flow, parabolic flow, linear shear flow, and skewed cubic flow. [2] 1D and 3D flows generated from patient scans can be used as more accurate inlet conditions. [9] 1D flows include the patient-specific variation of velocity normal to inlet. 3D flows include patient-specific velocities in the inlet plane in addition to the velocities normal to the inlet. The more accurate inlet conditions are oftentimes not used because the high acquisition time and low spatial resolution of the PC-MRIs needed. [1]

In each patient-specific model there are multiple outlets. The most common outlet boundary conditions used are the two and three-element Windkessel models. [2] The two-element Windkessel model replicates viscous resistance immediately downstream from the outlet and the three-element Windkessel model accounts for the resistance of capillaries and venous circulation. [2] Comparing the results of the two outlet conditions there is no significant difference in wall shear stress. [2] It has been found that the outlet boundary condition has an effect on less of the total flow than the inlet boundary condition. [2] Because of this, the inlet boundary condition has been of higher focus in most CFD studies.

Limitations in modeling

Because there is no standard for setting inlet boundary condition in CFD models, they have to be verified with experimental results. Those results can be obtained either through in vivo measurements using 4D PC-MRIs. 4D PC-MRI's are also limited because the acquisition time is high, the spatial and temporal resolution is low, and the signal to noise ratio is also low. [10]

Particle tracking velocimetry

Particle tracking velocimetry (PTV) allows for researchers to create an experimental setup to evaluate aortic flow patterns.

Methods

A CT scan or MRI is taken of a patient to obtain the geometry of the aorta. The information from that scan is then used to create a physical model made from a transparent silicone material. [11] The material used can either be compliant, in order to replicate the dilation of the valve, or rigid. [10] [1] The working fluid within the model should have a refractive index to match that of the material used to create the model. Fluorescent tracers in the working fluid are illuminated by a laser in the volume of interest. A single high speed camera can be used to capture four separate images of the same illuminated volume using and image splitter and four mirrors. [12]

The pulsating flow of the aorta is replicated by a ventricular assist device (VAD). The VAD is driven by a pump with a waveform to replicate the systole and diastole of the flow. When the pump is running, the high speed camera collects images of the tracers within the volume under investigation. A 3D velocity profile of the volume under investigation can be created from the particle movement from frame to frame. Focusing on different control volumes within the model allow for the creation of velocity profiles at different locations within the aorta.

Application of results

PTV velocity information can be used in place of 4D PC-MRI information in a CFD model. [10] The 3D velocity information from the inlet of the PTV model can be applied as the inlet boundary condition in a CFD model. That CFD model can then solve for wall shear stresses. The velocity information from PTV can also be used to create an MRI simulation. [1] The MRI simulation can then be used to assess progression of cardiovascular diseases.[ citation needed ]

Related Research Articles

<span class="mw-page-title-main">Aortic valve</span> Valve in the human heart between the left ventricle and the aorta

The aortic valve is a valve in the heart of humans and most other animals, located between the left ventricle and the aorta. It is one of the four valves of the heart and one of the two semilunar valves, the other being the pulmonary valve. The aortic valve normally has three cusps or leaflets, although in 1–2% of the population it is found to congenitally have two leaflets. The aortic valve is the last structure in the heart the blood travels through before stopping the flow through the systemic circulation.

<span class="mw-page-title-main">Cardiac output</span> Measurement of blood pumped by the heart

In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols , , or , is the volumetric flow rate of the heart's pumping output: that is, the volume of blood being pumped by a single ventricle of the heart, per unit time. Cardiac output (CO) is the product of the heart rate (HR), i.e. the number of heartbeats per minute (bpm), and the stroke volume (SV), which is the volume of blood pumped from the left ventricle per beat; thus giving the formula:

<span class="mw-page-title-main">Aortic dissection</span> Injury to the innermost layer of the aorta

Aortic dissection (AD) occurs when an injury to the innermost layer of the aorta allows blood to flow between the layers of the aortic wall, forcing the layers apart. In most cases, this is associated with a sudden onset of severe chest or back pain, often described as "tearing" in character. Vomiting, sweating, and lightheadedness may also occur. Damage to other organs may result from the decreased blood supply, such as stroke, lower extremity ischemia, or mesenteric ischemia. Aortic dissection can quickly lead to death from insufficient blood flow to the heart or complete rupture of the aorta.

Hemodynamics or haemodynamics are the dynamics of blood flow. The circulatory system is controlled by homeostatic mechanisms of autoregulation, just as hydraulic circuits are controlled by control systems. The hemodynamic response continuously monitors and adjusts to conditions in the body and its environment. Hemodynamics explains the physical laws that govern the flow of blood in the blood vessels.

<span class="mw-page-title-main">Aneurysm</span> Bulge in the wall of a blood vessel

An aneurysm is an outward bulging, likened to a bubble or balloon, caused by a localized, abnormal, weak spot on a blood vessel wall. Aneurysms may be a result of a hereditary condition or an acquired disease. Aneurysms can also be a nidus for clot formation (thrombosis) and embolization. As an aneurysm increases in size, the risk of rupture, which leads to uncontrolled bleeding, increases. Although they may occur in any blood vessel, particularly lethal examples include aneurysms of the Circle of Willis in the brain, aortic aneurysms affecting the thoracic aorta, and abdominal aortic aneurysms. Aneurysms can arise in the heart itself following a heart attack, including both ventricular and atrial septal aneurysms. There are congenital atrial septal aneurysms, a rare heart defect.

<span class="mw-page-title-main">Afterload</span> Pressure in the wall of the left ventricle during ejection

Afterload is the pressure that the heart must work against to eject blood during systole. Afterload is proportional to the average arterial pressure. As aortic and pulmonary pressures increase, the afterload increases on the left and right ventricles respectively. Afterload changes to adapt to the continually changing demands on an animal's cardiovascular system. Afterload is proportional to mean systolic blood pressure and is measured in millimeters of mercury.

<span class="mw-page-title-main">Aortic regurgitation</span> Medical condition

Aortic regurgitation (AR), also known as aortic insufficiency (AI), is the leaking of the aortic valve of the heart that causes blood to flow in the reverse direction during ventricular diastole, from the aorta into the left ventricle. As a consequence, the cardiac muscle is forced to work harder than normal.

<span class="mw-page-title-main">Bicuspid aortic valve</span> Medical condition

Bicuspid aortic valve (BAV) is a form of heart disease in which two of the leaflets of the aortic valve fuse during development in the womb resulting in a two-leaflet (bicuspid) valve instead of the normal three-leaflet (tricuspid) valve. BAV is the most common cause of heart disease present at birth and affects approximately 1.3% of adults. Normally, the mitral valve is the only bicuspid valve and this is situated between the heart's left atrium and left ventricle. Heart valves play a crucial role in ensuring the unidirectional flow of blood from the atrium to the ventricles, or from the ventricle to the aorta or pulmonary trunk. BAV is normally inherited.

<span class="mw-page-title-main">Aortic aneurysm</span> Excessive enlargement of the human aorta

An aortic aneurysm is an enlargement (dilatation) of the aorta to greater than 1.5 times normal size. They usually cause no symptoms except when ruptured. Occasionally, there may be abdominal, back, or leg pain. The prevalence of abdominal aortic aneurysm ("AAA") has been reported to range from 2 to 12% and is found in about 8% of men more than 65 years of age. The mortality rate attributable to AAA is about 15,000 per year in the United States and 6,000 to 8,000 per year in the United Kingdom and Ireland. Between 2001 and 2006, there were approximately 230,000 AAA surgical repairs performed on Medicare patients in the United States.

<span class="mw-page-title-main">Abdominal aortic aneurysm</span> Medical condition

Abdominal aortic aneurysm (AAA) is a localized enlargement of the abdominal aorta such that the diameter is greater than 3 cm or more than 50% larger than normal. An AAA usually causes no symptoms, except during rupture. Occasionally, abdominal, back, or leg pain may occur. Large aneurysms can sometimes be felt by pushing on the abdomen. Rupture may result in pain in the abdomen or back, low blood pressure, or loss of consciousness, and often results in death.

<span class="mw-page-title-main">Magnetic resonance angiography</span> Group of techniques based on magnetic resonance imaging (MRI) to image blood vessels.

Magnetic resonance angiography (MRA) is a group of techniques based on magnetic resonance imaging (MRI) to image blood vessels. Magnetic resonance angiography is used to generate images of arteries in order to evaluate them for stenosis, occlusions, aneurysms or other abnormalities. MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs.

Impedance cardiography (ICG) is a non-invasive technology measuring total electrical conductivity of the thorax and its changes in time to process continuously a number of cardiodynamic parameters, such as stroke volume (SV), heart rate (HR), cardiac output (CO), ventricular ejection time (VET), pre-ejection period and used to detect the impedance changes caused by a high-frequency, low magnitude current flowing through the thorax between additional two pairs of electrodes located outside of the measured segment. The sensing electrodes also detect the ECG signal, which is used as a timing clock of the system.

Arterial stiffness occurs as a consequence of biological aging and arteriosclerosis. Inflammation plays a major role in arteriosclerosis development, and consequently it is a major contributor in large arteries stiffening. Increased arterial stiffness is associated with an increased risk of cardiovascular events such as myocardial infarction, hypertension, heart failure and stroke, the two leading causes of death in the developed world. The World Health Organization predicts that in 2010, cardiovascular disease will also be the leading killer in the developing world and represents a major global health problem.

<span class="mw-page-title-main">Traumatic aortic rupture</span> Medical condition

Traumatic aortic rupture, also called traumatic aortic disruption or transection, is a condition in which the aorta, the largest artery in the body, is torn or ruptured as a result of trauma to the body. The condition is frequently fatal due to the profuse bleeding that results from the rupture. Since the aorta branches directly from the heart to supply blood to the rest of the body, the pressure within it is very great, and blood may be pumped out of a tear in the blood vessel very rapidly. This can quickly result in shock and death. Thus traumatic aortic rupture is a common killer in automotive accidents and other traumas, with up to 18% of deaths that occur in automobile collisions being related to the injury. In fact, aortic disruption due to blunt chest trauma is the second leading cause of injury death behind traumatic brain injury.

<span class="mw-page-title-main">Endovascular aneurysm repair</span> Surgery used to treat abdominal aortic aneurysm

Endovascular aneurysm repair (EVAR) is a type of minimally-invasive endovascular surgery used to treat pathology of the aorta, most commonly an abdominal aortic aneurysm (AAA). When used to treat thoracic aortic disease, the procedure is then specifically termed TEVAR for "thoracic endovascular aortic/aneurysm repair." EVAR involves the placement of an expandable stent graft within the aorta to treat aortic disease without operating directly on the aorta. In 2003, EVAR surpassed open aortic surgery as the most common technique for repair of AAA, and in 2010, EVAR accounted for 78% of all intact AAA repair in the United States.

<span class="mw-page-title-main">Cardiac magnetic resonance imaging</span>

Cardiac magnetic resonance imaging, also known as cardiovascular MRI, is a magnetic resonance imaging (MRI) technology used for non-invasive assessment of the function and structure of the cardiovascular system. Conditions in which it is performed include congenital heart disease, cardiomyopathies and valvular heart disease, diseases of the aorta such as dissection, aneurysm and coarctation, coronary heart disease. It can also be used to look at pulmonary veins. Patient information may be found here.

Electrical cardiometry is a method based on the model of Electrical Velocimetry, and non-invasively measures stroke volume (SV), cardiac output (CO), and other hemodynamic parameters through the use of 4 surface ECG electrodes. Electrical cardiometry is a method trademarked by Cardiotronic, Inc., and is U.S. FDA approved for use on adults, children, and neonates.

<span class="mw-page-title-main">Familial aortic dissection</span> Medical condition

Familial aortic dissection or FAD refers to the splitting of the wall of the aorta in either the arch, ascending or descending portions. FAD is thought to be passed down as an autosomal dominant disease and once inherited will result in dissection of the aorta, and dissecting aneurysm of the aorta, or rarely aortic or arterial dilation at a young age. Dissection refers to the actual tearing open of the aorta. However, the exact gene(s) involved has not yet been identified. It can occur in the absence of clinical features of Marfan syndrome and of systemic hypertension. Over time this weakness, along with systolic pressure, results in a tear in the aortic intima layer thus allowing blood to enter between the layers of tissue and cause further tearing. Eventually complete rupture of the aorta occurs and the pleural cavity fills with blood. Warning signs include chest pain, ischemia, and hemorrhaging in the chest cavity. This condition, unless found and treated early, usually results in death. Immediate surgery is the best treatment in most cases. FAD is not to be confused with PAU and IMH, both of which present in ways similar to that of familial aortic dissection.

<span class="mw-page-title-main">Apicoaortic conduit</span> Cardiothoracic surgical process

Apicoaortic Conduit (AAC), also known as Aortic Valve Bypass (AVB), is a cardiothoracic surgical procedure that alleviates symptoms caused by blood flow obstruction from the left ventricle of the heart. Left ventricular outflow tract obstruction (LVOTO) is caused by narrowing of the aortic valve (aortic stenosis) and other valve disorders. AAC, or AVB, relieves the obstruction to blood flow by adding a bioprosthetic valve to the circulatory system to decrease the load on the aortic valve. When an apicoaortic conduit is implanted, blood continues to flow from the heart through the aortic valve. In addition, blood flow bypasses the native valve and exits the heart through the implanted valved conduit. The procedure is effective at relieving excessive pressure gradient across the natural valve. High pressure gradient across the aortic valve can be congenital or acquired. A reduction in pressure gradient results in relief of symptoms.

<span class="mw-page-title-main">Phase contrast magnetic resonance imaging</span>

Phase contrast magnetic resonance imaging (PC-MRI) is a specific type of magnetic resonance imaging used primarily to determine flow velocities. PC-MRI can be considered a method of Magnetic Resonance Velocimetry. It also provides a method of magnetic resonance angiography. Since modern PC-MRI is typically time-resolved, it provides a means of 4D imaging.

References

  1. 1 2 3 4 Gülan, Utku; Calen, Christelle; Duru, Firat; Holzner, Markus (July 2018). "Blood flow patterns and pressure loss in the ascending aorta: A comparative study on physiological and aneurysmal conditions". Journal of Biomechanics. 76: 152–159. doi: 10.1016/j.jbiomech.2018.05.033 . ISSN   0021-9290. PMID   29907330.
  2. 1 2 3 4 5 6 7 Madhavan, Sudharsan; Kemmerling, Erica M. Cherry (2018-05-30). "The effect of inlet and outlet boundary conditions in image-based CFD modeling of aortic flow". BioMedical Engineering OnLine. 17 (1): 66. doi: 10.1186/s12938-018-0497-1 . ISSN   1475-925X. PMC   5975715 . PMID   29843730.
  3. ↑ Pirola, S.; Jarral, O. A.; O'Regan, D. P.; Asimakopoulos, G.; Anderson, J. R.; Pepper, J. R.; Athanasiou, T.; Xu, X. Y. (June 2018). "Computational study of aortic hemodynamics for patients with an abnormal aortic valve: The importance of secondary flow at the ascending aorta inlet". APL Bioengineering. 2 (2): 026101. doi:10.1063/1.5011960. hdl:10044/1/57522. ISSN   2473-2877. PMC   6481743 . PMID   31069298.
  4. ↑ Liu, Xiao; Fan, Yubo; Deng, Xiaoyan (2009-12-24). "Effect of Spiral Flow on the Transport of Oxygen in the Aorta: A Numerical Study". Annals of Biomedical Engineering. 38 (3): 917–926. doi:10.1007/s10439-009-9878-8. ISSN   0090-6964. PMID   20033776. S2CID   11591750.
  5. ↑ Cecchi, Emanuele; Giglioli, Cristina; Valente, Serafina; Lazzeri, Chiara; Gensini, Gian Franco; Abbate, Rosanna; Mannini, Lucia (February 2011). "Role of hemodynamic shear stress in cardiovascular disease". Atherosclerosis. 214 (2): 249–256. doi:10.1016/j.atherosclerosis.2010.09.008. ISSN   0021-9150. PMID   20970139.
  6. 1 2 3 Garcia, Julio; van der Palen, Roel L.F.; Bollache, Emilie; Jarvis, Kelly; Rose, Michael J.; Barker, Alex J.; Collins, Jeremy D.; Carr, James C.; Robinson, Joshua (2017-05-26). "Distribution of blood flow velocity in the normal aorta: Effect of age and gender". Journal of Magnetic Resonance Imaging. 47 (2): 487–498. doi:10.1002/jmri.25773. ISSN   1053-1807. PMC   5702593 . PMID   28556277.
  7. ↑ Uwe Janka, Hans (January 1996). "Increased cardiovascular morbidity and mortality in diabetes mellitus: identification of the high risk patient". Diabetes Research and Clinical Practice. 30: S85–S88. doi:10.1016/s0168-8227(96)80043-x. ISSN   0168-8227. PMID   8964198.
  8. 1 2 3 4 Shin, Eunji; Kim, Jung Joo; Lee, Seonjoong; Ko, Kyung Soo; Rhee, Byoung Doo; Han, Jin; Kim, Nari (2018-08-23). "Hemodynamics in diabetic human aorta using computational fluid dynamics". PLOS ONE. 13 (8): e0202671. Bibcode:2018PLoSO..1302671S. doi: 10.1371/journal.pone.0202671 . ISSN   1932-6203. PMC   6107202 . PMID   30138473.
  9. 1 2 3 Morbiducci, Umberto; Ponzini, Raffaele; Gallo, Diego; Bignardi, Cristina; Rizzo, Giovanna (January 2013). "Inflow boundary conditions for image-based computational hemodynamics: Impact of idealized versus measured velocity profiles in the human aorta". Journal of Biomechanics. 46 (1): 102–109. doi:10.1016/j.jbiomech.2012.10.012. ISSN   0021-9290. PMID   23159094.
  10. 1 2 3 Gallo, Diego; Gülan, Utku; Di Stefano, Antonietta; Ponzini, Raffaele; Lüthi, Beat; Holzner, Markus; Morbiducci, Umberto (September 2014). "Analysis of thoracic aorta hemodynamics using 3D particle tracking velocimetry and computational fluid dynamics". Journal of Biomechanics. 47 (12): 3149–3155. doi:10.1016/j.jbiomech.2014.06.017. ISSN   0021-9290. PMID   25017300.
  11. ↑ Doyle, B. J.; Morris, L. G.; Callanan, A.; Kelly, P.; Vorp, D. A.; McGloughlin, T. M. (2008). "3D Reconstruction and Manufacture of Real Abdominal Aortic Aneurysms: From CT Scan to Silicone Model". Journal of Biomechanical Engineering. 130 (3): 034501. doi:10.1115/1.2907765. ISSN   0148-0731. PMID   18532870.
  12. ↑ Gülan, Utku; Lüthi, Beat; Holzner, Markus; Liberzon, Alex; Tsinober, Arkady; Kinzelbach, Wolfgang (2012-09-02). "Experimental study of aortic flow in the ascending aorta via Particle Tracking Velocimetry" (PDF). Experiments in Fluids. 53 (5): 1469–1485. Bibcode:2012ExFl...53.1469G. doi:10.1007/s00348-012-1371-8. hdl: 20.500.11850/58740 . ISSN   0723-4864. S2CID   53508056.


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