In Vitro Aortic Arch Flow Model For Vector Flow Imaging Testing

Pediatric stenosis is the narrowing of the aorta at the aortic valve, above the aortic valve, or below the aortic valve. Typically, this disease’s severity is diagnosed by conventional Doppler ultrasound methods, or echocardiography. Conventional Doppler can sometimes overestimate the pressure gradient over the area of stenosis, diagnosing some cases of PAS to be more severe than they actually are. This causes earlier intervention than is desired in children. A new US modality, Vector Flow Imaging (VFI), is an angle-independent US imaging method that can potentially more accurately quantify peak blood flow velocities and pressure gradients across stenotic sections of vasculature. This project’s goal was to create an in vitro cardiac flow loop that could validate the accuracy with which VFI could obtain these measurements. An in vitro healthy aortic arch was created so that pressure gradients could be measured directly by pressure transducers located at the inlet and outlet of the stenosis and compared to the pressure gradients obtained across the same region by use of VFI. A LabView VI was designed to display pressure vs. time data in Excel to obtain these pressure measurements. A mechanical pump was also designed to create pulsatile flow throughout the loop. Introduction Pediatric aortic stenosis (PAS) is the narrowing of the aorta in children, obstructing the flow of blood pumped from the left ventricle. This congenital heart defect can exist as valvar, subvalvar, or supravalvar AS [1]. AS is incident in 5% of children diagnosed with heart disease [1]. Valvar aortic stenosis, or VAS, is the most common obstruction to left ventricular flow [1]. In adults, VAS manifests as a sclerotic aortic valve with thickened leaflets that eventually becomes inflamed or calcified. [2] A reduction in the area of the orifice will occur in the beginning of the onset of the stenosis when the valve is sclerotic and will continue to narrow until the valve becomes calcified. [2] Aortic valves normally exist as tricuspid tissue valves, valves with three leaflets or cusps. Being born with an abnormal bicuspid valve predisposes a person to a higher likelihood of developing aortic stenosis because of a differing functionality of the valve. [2] If aortic stenosis is present in a young infant, it is called neonatal critical aortic stenosis, and is nearly always fatal if there is no surgical intervention [1]. This case of AS in children typically occurs when there is a bicuspid or sclerotic valve with a reduced crosssectional area. [1] In both children and in adults, the reduction in cross-sectional area at the valve results in an elevated pressure gradient across the stenosis that puts significant strain on the left ventricle. [2] This can result in concomitant anomalies such as left ventricular hypertrophy, where the cardiac myocytes of the left ventricle enlarge to try and compensate for the elevated load requirement placed on the heart. [2] Supravalvar aortic stenosis (SVAS) is the most common cardiovascular anomaly in William’s Syndrome. [3] William’s Syndrome is a “congenital, multisystem disorder involving the cardiovascular, connective tissue, and central nervous systems. ” [4] It is characterized and diagnosed on the basis of many phenotypic characteristics such as stellate eyes, a broad forehead, and full lips. [3] In this population of children, SVAS is manifested in one of two ways. It either exists as an hourglass-shaped narrowing of the aorta near the sinotubular junction or a diffuse stenosis of the ascending aorta. [3] Both instances result in a narrowing of a section of the aorta that introduce abnormal hemodynamics in that section. These complications in conjunction with common concomitant problems of SVAS such as coronary perfusion [5] make correctional surgery a requirement for SVAS. SVAS correctional surgery is usually done by patch aortoplasty (using a synthetic patch to widen a section of the aorta) and sometimes by multi-sinus repair [1]. Subvalvar aortic stenosis (SAS) is one of the more rare forms of PAS, making up only 8 to 10% of all congenital aortic stenosis [6]. It is not always characterized by discrete symptoms or pathologies, but more-so by a spectrum. [1] The most common form is a “discrete ridge of fibrous tissue that projects into the left ventricle completely circling the outflow tract. ” [7] Some cases of this type of SAS include bulging and hypertrophy of the interventricular septum underneath. [7] Sometimes, if there is hypertrophy of just the septum and left ventricle, the thickened muscle will come together during left ventricular contraction and occlude the path to the systemic circulation. [7] Angina, dyspnea, and palpitations are some of the common symptoms of SAS. [7] Correctional operations for SAS are usually very low risk. [6] All three levels of pediatric aortic stenosis share a few commonalities. First, every type’s underlying problem is an obstruction to the systemic circulation. This obstruction or narrowing of a particular point in the aorta will substantially raise the pressure and maximum velocity of the blood flowing over that stenotic region. Second, symptomatic children have a poor prognosis and require intervention [2]. Third, and finally, careful consideration must be taken into each type of surgical intervention, because children are the recipient of such intervention. Whether the intervention be an aortic valve replacement or an aortoplasty, because it is being done in a child, it will almost certainly not be the last correctional procedure required to fix the disease. Children’s bodies, both internally and externally, continue to grow. Because most replacement aortic valves or grafts will not continue to grow with the child, they will soon be too small to support healthy systemic circulation, and subsequent operations will have to be performed. Thus, to avoid continual and potentially risky operations, it is best to surgically intervene at the latest possible moment. This means that diagnosing the severity of the stenosis is almost more important than diagnosing the stenosis itself. In children, the surgeon should only operate on the child when there is no other choice when the stenosis is most severe. Transthoracic echocardiography or Doppler echocardiography is the standard imaging modality for diagnosing PAS and determining its severity. [2] This ultrasound modality works by measuring the jet velocity of blood across the stenosis. [2] Multiple views or angles are required to correctly identify the maximum stenotic jet velocity. [2] Common transducer angles that yield the highest velocity are the apical, suprasternal, and right parasternal views [2]. After the peak velocity across the stenosis is correctly identified, the simplified Bernoulli equation can be used to obtain the mean and maximum pressure gradients across the area of stenosis. [2] These are termed, ∆P#$%& and , ∆P#%', respectively. [2] The simplified Bernoulli equation is given below.