Date of Award

12-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Mechanical Engineering

Committee Chair/Advisor

Figliola, Ricahrd S

Committee Member

Hsia, Tain-Yen

Committee Member

Beasley, Donald E

Committee Member

Tong, Chenning

Abstract

Hypoplastic left heart syndrome (HLHS) is a congenital heart defect in which the left ventricle is severely underdeveloped. The Norwood procedure is the first stage procedure to make an unrestrictive systemic blood flow and at the same time balance it with the pulmonary flow. This is done by constructing a neo-aorta using the pulmonary artery root and the autologous aorta, and then installing a shunt to the pulmonary artery. Variations of the Norwood surgery include the modified Blalock-Taussig (mBT) shunt, which diverts blood from the innominate artery to the pulmonary artery (PA), and the Right Ventricle Shunt (RVS), which diverts blood from the right ventricle to the PA. Recurrent neo-aortic coarctation (NAO) is a frequent complication of the Norwood procedure. It causes changes in circulation flow rate balances and hypertension in the aortic arch. Conventionally, the value of a coarctation index (CoI) is used in choosing interventions to treat NAO. Aortic arch morphology of Norwood patients is suspected to be a factor of hemodynamic response to NAO. This study aims to develop and validate an in vitro model of the Norwood circulation and to use it to better understand the hemodynamic impact of progressive coarctation severity in the Norwood patients with mBT and RVS shunts. Five patient-specific cases were selected, each case having a different aortic morphology. A multi-scale mock circulatory system (MCS) was developed to simulate patient-specific Norwood circulation. The MCS couples a lumped parameter network (LPN) model of the circulation with the 3D test section of the aorta and superior arteries. The system includes branches for the pulmonary, upper body, lower body and single ventricle. The MCS was set to patient specific conditions based on the clinical measurements. Flow rate and pressure measurements were made around the circulation model. The native arch anatomy of each patient was morphed to simulate coarctation by controlling the amount of narrowing of the aortic isthmus, while keeping the original patient-specific aortic geometry intact. Separate NAO models were created to provide for a range of CoI. Aortic pressure measurements were made to study pressure drop and recovery effects. In a further study, the MCS was modified to simulate the Norwood circulation with RVS. The NAO models were used to study coarctation effects. The MCS was validated against clinical measurements. The experimental measurements demonstrated that the time-based flow rate and pressure developed within the circulation recapitulated clinical measurements (0.72 < R2 < 0.95). The results showed good fidelity in replicating the mean values of the Norwood circulation at the patient-specific level (p > 0.10). The system demonstrated the coarctation effects in the Norwood circulation with mBT. For all patient cases, the single ventricle power (SVP), mean pressure difference, and Qp/Qs increased noticeably when CoI < 0.5 (p<0.05). An increased SVP correlated with abnormal aortic arch morphology (dilated or tubular). Measurements from two of four cases studied showed that substituting the mBT with the RVS can relieve pulmonary overcirculation and improve the pulmonary to systemic flow balance (Qp/Qs). Using the RVS reduced SVP requirements by 74.5 mW on average. A tubular arch morphology was associated with a higher SVP with the RVS than those patients with a dilated arch. The study has shown that the hypothesis, “NAO may not need immediate surgical intervention at an early stage for some patients” was accepted. Aortic arch morphology does affect the hemodynamic response to NAO. Any morphological abnormality causes extra SVP. The RVS can relieve overcirculation and is associated with lower SVP level and SVP changes in some of the patients.

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