Various congenital heart defects (CHDs) are characterized by the existence of a single functional ventricle, which perfuses both the systemic and pulmonary circulation. A three-stage palliation procedure, including the final Fontan completion, is often adopted by surgeons to treat patients with such CHDs. The completion Fontan involves the creation of a total cavopulmonary connection (TCPC), commonly accomplished with an extracardiac conduit. This TCPC results in nonphysiologic flow conditions that can lead to systemic venous hypertension, reduced cardiac output, and ultimately the need for heart transplantation. A modest pressure rise of 5–6 mm Hg could correct the abnormal flow dynamics in these patients. To achieve this, we propose a novel conceptual design of a dual-propeller pump inside a flared TCPC. The TCPC dual-propeller conjunction was examined for hydraulic performance, blood flow pattern, and potential for hemolysis inside the TCPC using computational fluid dynamics (CFD). The effect of axial distance between the two propellers on the blood flow interference and energy loss was studied to determine the optimal separation distance. Both the inferior vena cava (IVC) and superior vena cava (SVC) propellers provided a pressure rise of 1–20 mm Hg at flow rates ranging from 0.4 to 7 lpm while rotating at speeds of 6,000–12,000 rpm. Larger separation distance provided favorable performance in terms of flow interference, energy loss, and blood damage potential. The ability of a dual-propeller micropump to provide the required pressure rise would help to augment the cavopulmonary flow and mimic flows seen in normal biventricular circulation.
Mechanical circulatory support devices have gained significant importance in recent years as a viable therapeutic option to support paediatric population and children with single functional ventricle. The Fontan operation helps to reroute the deoxygenated blood to the lungs by bypassing the dysfunctional right ventricle. Total Cavopulmonary Connection (TCPC) is usually a method opted by the clinicians to connect the superior vena cava (SVC) and inferior vena cava (IVC) to the left and right pulmonary artery (LPA and RPA). However, the non-physiologic flow patterns created by the Fontan procedure leads to an increase in chances of platelet deposition and pressure loss which calls for heart transplantation to prevent early and late stage pathophysiology. This had led to modification of TCPC geometry to reduce the pressure and energy loss and thereby unload the single functional ventricle to ensure longer survival period. A study on mechanical circulatory device in conjunction with the modified TCPC geometry has seen little exposure and has opened new gates to develop a variety of state-of-art cavopulmonary assist devices. This study is focused on the selection of optimal TCPC to reduce energy loss and the effect of stent inside the modified TCPC on hemodynamics and flow structures. Four TCPC connections, developed for a particular age group of children, were studied for the velocity field, overall pressure and energy loss. In addition, the four TCPC connection geometries were also studied for distribution of hepatic blood from the IVC to both pulmonary arteries, and hence the lungs, to prevent development of any arteriovenous malformations. The entire stent assembly mounted inside the two best performing TCPC connections was examined for the hemodynamic effects using a series of 3D-CFD simulations. The curved-type connection for the TCPC proved to provide minimum pressure and energy loss along with reduced traces of vortex and recirculation. However, it was not efficient in terms of hepatic blood distribution. The flared geometry performed second best in terms of both minimum power loss and even hepatic blood distribution. There was a slight difference in power loss between the flared and the curved TCPC configuration with stent but the flared geometry had better hepatic blood distribution. This study demonstrated that a stent in conjunction with a TCPC leads to development of a helical flow pattern which provides better mixing of blood and even distribution to both the pulmonary arteries. The design of a stent with the best performing flared TCPC configuration can be optimized to reduce the amount of power loss and vortex generation and can be used to design similar scaled models for paediatric population of various age groups.
Hypoplastic Right Heart Syndrome is a type of congenital heart defect where the right ventricle is underdeveloped in an infant to pump blood from the body to the lungs. The three-staged surgical Fontan procedure provides a temporary treatment; however, in most of the cases, a heart transplantation is required due to postoperative complications. Currently, there are no devices commercially available in the market to provide a therapeutic assistance to these patients until a donor heart is available. Thus, a novel dual propeller pump concept is developed to provide cavopulmonary assistance to these patients. The designed blood pump would be percutaneously inserted via the Femoral vein and deployed at the center of the Total Cavopulmonary Connection (TCPC). The two propellers, each placed in the Superior Vena Cava (SVC) and the Inferior Vena Cava (IVC) are connected by a single shaft and rotating at same speed. The device is supported with the help of a self-expanding stent whose outer walls are anchored to the inner walls of the IVC and the SVC. Each of the IVC and the SVC propeller without the stent provides a modest pressure augmentation of 5–6 mm Hg. To expand on this, the current study focusses on studying the effect of the introduction of stent around the propeller on the hemodynamic performance of the pump. Five different stent design parameters, viz. the strut thickness, width, number, the stent length and number of strut columns were selected for a range of values. Each of the design parameters was varied by keeping all others constant and equal to the base stent design. All the stent models were analysed to see their effect on pressure rise, flow pattern and blood damage using 3D CFD analysis. The blood damage potential for different studied designs was predicted using a non-linear mathematical power law model along with Lagrangian particle tracking to predict the blood flow path. The introduction of stent resulted in pressure reduction of around 0.4 and 0.2 mm Hg around the IVC and SVC propeller with an increase in blood damage index (BDI) by almost 2 times for the final dual propeller pump assembly. It was observed that the blood damage potential was directly related to the amount of pressure rise where the stent length, stent column number, strut width, and strut thickness had a converse effect showing a reduction in pressure rise and blood damage with their increment. While the number of struts gave a desirable effect of increasing pressure rise and reducing blood damage with its increment. The study also demonstrated that the introduction of stent around a circulatory pump increases the Wall Shear Stress (WSS) value at the stent-artery wall interface thereby preventing the occurrence of restenosis and thrombosis initiating due to very low WSS (< 0.5 Pa). Thus, this study acts as an initial step to design a protective stent support around a percutaneous assist device by analysing the sensitivity of stent design parameters on the hemodynamic performance of the pump.
The most common surgical procedure used to treat right ventricular heart failure is the Fontan procedure, which connects the superior vena cava and the inferior vena cava directly to the left and right pulmonary arteries bypassing the right atrium. Many studies have been performed to improve the Fontan procedure. Research has been done on a four-way connector that can both passively and actively improve flow characteristics of the junction between the Superior Vena Cava (SVC), Inferior Vena Cava (IVC), Left Pulmonary Artery (LPA) and Right Pulmonary Artery (RPA), using an optimized connector and dual propeller system. However, the configuration of these devices do not specify propeller motor placement and has a stagnation point in the center of the connector. This study focuses on creating a housing for the motor in the center of the connector to reduce the stagnation area and further stabilize the propellers. To do this, we created a program in ANSYS that utilizes the design-of-experiment (DOE) function to minimize power-loss and stagnation points in the connector for a given geometry. First, a CFD model is created to simulate the blood flow inside the connector with different housing geometries. The shape and size of the housing are used as parameters for the DOE process. In this study, an enhanced central composite design technique is used to discretize the design space. The objective functions in the DOE are red blood cell residence time and power loss. It was confirmed that the addition of the housing did decrease the size of the stagnation point. In fact, the housing added in stabilizing the flow through the connector by creating a more defined flow path. Because the flowrates from the IVC and SVC are not the same, the best configuration for the housing was found to be asymmetric along the axis of the pulmonary artery. While this is a continuation of previous studies, the creation of an optimized housing for the motors for the propellers makes implementation of the propeller idea more viable in a real life situation. The added stability of the propellers provided by the housing can also decrease the risk of propeller failure due to rotordynamic instability.
A single ventricular physiology of the human heart caused by a dysfunctional right ventricle is usually treated with the three-stage Fontan operation. The outcome of this operation is an extra-cardiac total cavopulmonary connection (TCPC) which supplies the deoxygenated blood from the body to the lungs by directly connecting the inferior and superior vena cava (IVC and SVC) to the left and right pulmonary arteries (LPA and RPA). However, the situation is worsened due to non-physiologic flow conditions and pressure loss inside the cavopulmonary track, which ultimately calls for a heart transplantation. A modest pressure rise of 5–6 mm Hg will help to regain the normal physiology of the patient. In order to achieve this, a conceptual design of a dual propeller pump inside a flared TCPC is developed and studied. In order to provide a modest pressure rise, a blood pumping device was inserted inside the flared TCPC connection which consisted of two propellers, each placed in the SVC and the IVC and connected by a single shaft. The IVC and the SVC propellers were designed to rotate at the same rotational speed, having the same pressure rise but different blood inflow rate. The equal pressure rise across both the propellers was necessary at the design speed and flow rate to prevent any blood flow into the opposite vena cava. The TCPC-dual propeller conjunction was examined for the hydraulic performance and the flow pattern inside the TCPC using the 3D-CFD simulations on Ansys-CFX. The effect of axial distance between the two propellers on the blood flow interference and energy loss was also studied to select an optimal separation distance between them. The introduction of dual propeller pump inside the flared TCPC led to a pressure rise of 2–15 mm Hg at a total flow rate of 4.5 lpm (63% from IVC and 37% from SVC) with the rotational speed ranging from 6000–12000 rpm. It was seen that an axial separation of 70 mm between the two propellers provided the best performance in terms of flow interference and energy loss. A dual propeller pump assembled with an optimized TCPC could provide the required pressure rise for a particular age group of patients with univentricular Fontan physiology. The ability of dual micro-propeller pump to provide the required pressure rise will help to augment the cavopulmonary flow and hence help to regain the normal flow physiology as that witnessed by a human with biventricular circulation.
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