Date of Award
5-2016
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Legacy Department
Mechanical Engineering
Committee Member
Dr. Richard Figliola, Committee Chair
Committee Member
Dr. Donald Beasley
Committee Member
Dr. Chenning Tong
Committee Member
Dr. Ethan Kung
Abstract
Single ventricle physiology can result from various congenital heart defects in which the patient has only one functional ventricle. Hypoplastic left heart syndrome refers to patients born with an underdeveloped left ventricle. A three stage palliation strategy is applied over the first several years of life to establish a viable circulation path using the one functioning ventricle. Results of the first stage Norwood procedure on neonates with hypoplastic left heart syndrome are unsatisfactory with high morbidities and mortalities primarily due to high ventricle load and other complications. An early second stage Bidirectional Glenn (BDG) procedure is not a suitable option for neonates due to their high pulmonary vascular resistance (PVR), which limits pulmonary blood flow. Realistic experimental models of these circulations are not well established and would be useful for studying the physiological response to surgical decisions on the distribution of flows to the various territories, so as to predict clinical hemodynamics and guide clinical planning. These would serve well to study novel intervention strategies and the effects of known complications at the local and systems-level. This study proved the hypothesis that it is possible to model accurately the first and second stage palliation circulations using multi-scale in vitro circulation models and to use these models to test novel surgical strategies while including the effects of possible complications. A multi-scale mock circulatory system (MCS), which couples a lumped parameter network model (LPN) of the neonatal circulation with an anatomically accurate three-dimensional model of the surgical anastomosis site, was built to simulate the hemodynamic performance of both the Stage 1 and Stage 2 circulations. A pediatric ventricular assist device was used as the single ventricle and a respiration model was applied to the Stage 2 circulation system. Resulting parameters measured were pressure and flow rates within the various territories, and systemic oxygen delivery (OD) were calculated. The Stage 1 and Stage 2 systems were validated by direct comparisons of time-based and mean pressures and flow rates between the experimental measurements, available clinical recordings and/or CFD simulations. Regression and correlation analyses and unpaired t-tests showed that there was excellent agreement between the clinical and experimental time-based results as measured throughout the circulations (0.60 < R^2 < 0.99; p > 0.05, r.m.s error< 5%). A novel, potentially alternative surgical strategy for the initial palliation, was proposed and was tested, called the assisted bidirectional Glenn (ABG) procedure. The approach taps the higher potential energy of the systemic circulation through a systemic to caval shunt with nozzle to increase pulmonary blood flow and oxygen delivery within a superior cavopulmonary connection. Experimental model was validated against a numerical model (0.65 < sigma < 0.97; p > 0.05). The tested results demonstrated the ABG had two main advantages over the Norwood circulation. First, the flow through the ABG shunt is a fraction of the pulmonary flow, reducing the volume overload on the single ventricle and improving systemic and coronary perfusion. Second, the ABG should provide a more stable source of pulmonary flow, which should reduce thrombotic risk or intimal thickening over an mBT shunt. A study to examine the ejector pump effect was conducted. Two parameters were investigated: (1) the superior vena cava (SVC) and pulmonary artery (PA) pressure difference; and (2) the SVC and PA pressure difference relative to PA flow rate. Results validated the hypothesis that an ejector pump advantage can be adopted in a superior cavo-pulmonary circulation, where the low-energy pulmonary blood flow can be assisted by an additional source of high energy flow from the systemic circulation. But the ejector pump effect produced by the current nozzle designs was not strong. Parametric study includes nozzle size, placement, and nozzle shape was conducted. Results shown that nozzle to shunt diameter ratio had the most important effects on the ABG performance. As β increased, pulmonary artery flow rate and systemic oxygen delivery increased. A suggested β value falls between 0.48 and 0.72. The study showed that a bigger β produced a smaller resistance value. The shape of the nozzle did not change the resistance value. The effects of shunt angle, nozzle placement and nozzle shape on the ABG circulation were not statistical significant. The aortic coarctation study showed that the aortic coarctation could have an effect on the ABG circulation. The coarctation index (CoI) around 0.5 was found to be the transition point between no effects (CoI > 0.5) and discernible effects on the ABG circulation. These effects include changes in pulmonary to systemic flow distribution. In summary, this research verified and validated an in vitro mock circulatory system (MCS) for Stage 1 and Stage 2 circulations. The system was used to assess a novel conceptual surgery option named the ABG. Parametric studies were conducted to give guidance on designing the important element for the ABG: the shunt (nozzle) connecting the SVC and systemic circulation. The performance of the ABG under one unhealthy condition, namely, aortic coarctation was assessed.
Recommended Citation
Zhou, Jian, "In Vitro Multi Scale Models to Study the Early Stage Circulations for Single Ventricle Heart Diseases Palliations" (2016). All Dissertations. 1632.
https://open.clemson.edu/all_dissertations/1632