A Multiscale Model of the Neonatal Circulatory System Following Hybrid Norwood Palliation

Hypoplastic left heart syndrome (HLHS) is a complex cardiac malformation in neonates suffering from congenital heart disease and occurs in 1 per 5000 births [1]. HLHS is uniformly fatal within the first hours or days after birth as the severely malformed anatomies of the left ventricle, mitral and aortic valves, and ascending aorta are not compatible with life. The regularly implemented treatment, the Norwood operation, is a complex open heart procedure that attempts to establish univentricular circulation by removing the atrial septum (communicating the right and left ventricle), reconstructing the malformed aortic arch, and connecting the main pulmonary artery into the reconstructed arch to allow direct perfusion from the right ventricle into the systemic circulation. A relatively new treatment being utilized, the Hybrid Norwood procedure, involves a less invasive strategy to establish univentricular circulation that avoids a cardiopulmonary bypass (heart-lung machine), deliberate cardiac arrest, and circulatory arrest of the patient during the procedure. The resulting systemic-pulmonary circulation is unconventional; blood is pumped simultaneously and in parallel to the systemic and pulmonary arteries after the procedure. Cardiac surgeons are deeply interested in understanding the global and local hemodynamics of this anatomical configuration. To this end, a multiscale model of the entire circulatory system was developed utilizing an electrical circuit model analogy for the peripheral or distal circulation coupled with a 3D Computational Fluid Dynamics (CFD) model to understand the local hemodynamics. The electrical circuit model is mainly a closed loop circuit comprised of RLC compartments that model the viscous drag, flow inertia, and compliance of the different arterial and venous beds in the body. A system of 32 first-order differential equations is formulated and solved for the electrical circuit model using a fourth-order adaptive Runge-Kutta solver. The output pressure and flow waveforms obtained from the electric circuit model are imposed as boundary conditions on the 3D CFD model. The CFD model domain is a representative HLHS anatomy of an infant after undergoing the Hybrid Norwood procedure and is comprised of the neo-aorta, pulmonary roots, aortic arch with branching arteries, and pulmonary arteries. The flow field is solved over several cardiac cycles using an implicit-unsteady RANS equation solver with the K-Epsilon turbulence model. Congenital heart disease is the leading cause of death for infants born with birth defects, with more than a 30% mortality attributed to this disease. One of the most complex forms of congenital heart disease is in 4 to 8 percent of infants born with cardiac malformations malformed anatomies of the left ventricl currently two approaches to therapy for children with HLSH: heart transplantation or surgical intervention to establish univentricular circulation. Unfortunately heart transplantation i number of donors at this early age [3] common method to establish an early blood circulation that is compatible with life. of removal of the atrial septum, reconstruction the innominate artery to the right pulmonary artery (RPA) to allow for pulmonary perfusion (Figure Figure 1: HLHS Anatomy (left)[4], Standard Norwood Anatomy (center) recovery of the patient from the Norwood operation is often followed by subsequent surgical procedures to establish a more normal circulatory system where pulmonary perfusion is achieved with deoxygenated blood. A relatively new alternative to the first-stage Norwood procedure is the Hybrid Norwood procedure, which consists of removal of the atrial septum, implantation of a stent in the patent ductus arteriosis, and banding of the pulmonary arteries The PDA, a small vessel that connects the pulmonary arteries with the aortic arch and normally closes shortly after birth, is enlarged with the use of the stent to allow through the aortic arch. The pulmonary banding is put in place to prevent excessive pulmonary circulation, with an ideal adjustment providing a pulmonary to systemic flow ratio of 1 Hybrid Norwood is a less invasive procedure that avoids the use of a heart machine as well as deliberate cardiac and circulatory arrest. Though there are numerous surgical advantages of the Hybrid versus the conventional method, there are also diffe important to surgeons participating in this study is the obstruction of flow due to stenosis of the aortic arch proximal to the stent. This condition is fatal if coronary perfusion connecting the pulmonary root to the innominate artery resulting hemodynamics are very complex since the flow r innominate circulation flows downstream through the innominate and part upstream to feed the coronary and remaining aortic branches. Furthermore, secondary motions h circulatory Figure 2: Hybrid Norwood Anatomy are therefore deeply interested in understanding the loc with RBTS in place Hybrid Norwood anatomy especially when severe stenosis is present. The main objectives of this study are therefore loop, multiscale model of the cardiovascular system able to describe detailed local hemodynamics and its effects on Figure 2: Hybrid Norwood Anatomy with RBTS in place Introduction [2]. The main complications of HLHS are severely e, mitral and aortic valves, and ascending aorta (Figure 1). s often not an alternative . Surgical techniques such as the Norwood operation are The Norwood operation malformed aorta, and connection of a bypass (BT [5], Hybrid Norwood Anatomy (right) circulation from the right ventricle into the systemic circulation rent complications that may arise with this method. The most and/or carotid flow is critically low. With the expectation to these vessels, a bypass called Reverse BT-shunt (RBTS) is placed artery is both antegrade and retrograde, meaning part of the ave been detected throughout the cardiac cycle configuration far from intuitive al hemodynamics of and the potential benefits of the reverse BT shunt, : 1) To develop a closed HLSH and occurs