An abnormal chronotropic response to exercise is a common finding following surgical repair of tetralogy of Fallot (TOF) which has generally been attributed to a putative sympathetic dysfunction. There exists little information on sympathetic function in patients operated for a congenital heart defect to support such a claim. This study used spectral analysis of heart rate (HRV) and blood pressure (BPV) variability to examine sympathovagal influences on the sinus node in 9 adolescents operated for TOF 13.0 +/- 1.12 years previously and in 8 healthy age and sex-matched control (CTRL) subjects. Continuous ECG and BP recordings were obtained under supine or seated resting positions, with or without controlled respiration at 0.20 Hz (CR); after passive 85° head-up tilt (HUT); during cycling at steady-state heart rates of 100 and 120 bpm (Ex 100, Ex 120), and after 10 and 20 minutes of passive seated recovery. (Abstract shortened by UMI.)

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Single ventricle heart defects are a rare but serious form of congenital heart disease, which affect approximately 2000 children born in the United States each year. Staged surgical palliation, culminating with the “Fontan Procedure,” is typically required to achieve adequate supply of blood to both the pulmonary and systemic circulations while avoiding chronic ventricular volume overload. This surgery reroutes the systemic veins to the pulmonary arteries, forming a total cavopulmonary connection (TCPC), to completely bypass the right side of the heart and restore a series configuration to the cardiovascular circuits. Despite improved survival through this operative course in first weeks and years of life, Fontan patients are subjected gradual attrition and decreased life expectancy through a multitude of chronic complications. It is suspected that the adverse hemodynamics of this surgically altered physiology, including those specific to the surgical TCPC, play a role in determining patient outcome. However, the small and heterogeneous patient population has hindered decisive progress and there is still not a good understanding of the optimal care strategies on a patient-by-patient basis. In recent decades, advances in medical imaging and image-based computational fluid dynamics (CFD) have redefined the realm of possibility for studying complex biomedical phenomena. Combined, these methods provide the means to create and evaluate patient-specific models of a wide range of cardiovascular structures, including the TCPC, with high fidelity. Results from these models can then be used for a wide array of different analyses, such as identifying regions of flow separation or stagnation, calculating hemodynamic power loss, or quantifying local flow distribution patterns. Through significant effort from numerous past investigators, a robust set of validated computational and image processing tools has been assembled, along with the largest library of cardiac magnetic resonance (CMR) data of TCPC anatomy and flow. These tools are leveraged in this thesis to characterize the functional implications of TCPC power loss at an unprecedented scale: we report the largest CFD analysis of patient-specific TCPC hemodynamics to date with particular focus on identifying functional correlates. Combining these data with imaging-based analysis of ventricle function, we directly compare the CFD-derived hemodynamics to the performance of the single ventricle for the first time. Motivated by the physiologic significance of these findings, the same patient-specific CFD framework is used for the translational application of prospective surgery planning for hemodynamic optimization, including the first implementation of a novel TCPC connection design hypothesized to uniquely streamline the energetic performance. We conclude with a first look at the longitudinal evolution of patient functional status to begin understanding how factors such as TCPC hemodynamics contribute to poor long-term performance in these patients.

Single ventricle heart defects, where systemic and pulmonary venous returns mix in the single functional ventricle, represent the most complex form of congenital heart defect, affecting 2 babies per 1000 live births. Surgical repairs, termed "Fontan Repairs," reroute the systemic venous return directly to the pulmonary arteries, thus preventing venous return mixing and restoring normal oxygenation saturation levels. Unfortunately, these repairs are only palliative and Fontan patients are subjected to a multitude of chronic complications. It has long been suspected that hemodynamics play a role in determining patient outcome. However, the number of anatomical and functional variables that come into play and the inability to conduct large scale clinical evaluations, due to too small a patient population, has hindered decisive progress and there is still not a good understanding of the optimal care strategies on a patient-by-patient basis. Over the past decades, image-guided computational fluid dynamics (CFD) has arisen as an attractive option to accurately model such complex biomedical phenomena, providing a high degree of freedom regarding the geometry and flow conditions to be simulated, and carrying the potential to be automated for large sample size studies. Despite these theoretical advantages, few CFD studies have been able to account for the complexity of patient-specific anatomies and in vivo pulsatile flows. In this thesis, we develop an unstructured Cartesian immersed-boundary flow solver allowing for high resolution, time-accurate simulations in arbitrarily complex geometries, at low computational costs. Combining the proposed and validated CFD solver with an interactive virtual-surgery environment, we present an image-based surgical planning framework that: a) allows for in depth analysis of the pre-operative in vivo hemodynamics; b) enables surgeons to determine the optimum surgical scenario prior to the operation. This framework is first applied to retrospectively investigate the in vivo pulsatile hemodynamics of different Fontan repair techniques, and quantitatively compare their efficiency. We then report the prospective surgical planning investigations conducted for six failing Fontan patients with an interrupted inferior vena cava and azygous continuation. In addition to a direct benefit to the patients under consideration, the knowledge derived from these surgical planning studies will also have a larger impact for the clinical management of Fontan patients as they shed light onto the impact of caval offset, vessel flaring and other design parameters upon the Fontan hemodynamics depending on the underlying patient anatomy. These results provide useful surgical guidelines for each anatomical template, which could benefit the global surgical community, including centers that do not have access to patient-specific surgical planning interfaces.