Congenital heart valve disease is one of the most common abnormalities in children, with common valve defects being aortic stenosis, mitral stenosis, and valvular regurgitation. Although adult sized mechanical heart valve (MHV) replacements are widely studied and utilized, there are currently no FDA approved prosthetic heart valves available for the pediatric population. This is due to a variety of reasons such as a limited patient pool for clinical trials, limited valve sizes, and complex health histories in children. Much like adult sized mechanical heart valves, potential complications with pediatric heart valve replacements include thrombosis, blood damage due to high shear stresses, and cavitation. Due to pediatric sized MHVs being much smaller in size than adult MHVs, different fluid dynamic conditions and associated complications are expected. In order to accelerate the approval of pediatric sized heart valves for clinical use, it is important to first characterize and assess the fluid dynamics across pediatric sized heart valves. By understanding the hemodynamic performance of the valve, connections can be made concerning potential valve complications such as thrombosis and cavitation. The overall objective of this study is to better characterize and assess the flow field characteristics of a pediatric sized mechanical heart valve using flow visualization techniques in a mock circulatory loop. The mechanical heart valve chosen for this research was a size 17 mm Bjork-Shiley tilting disc valve, as this is a common size valve used for younger patients with smaller cardiovascular anatomy. The mock circulatory loop used in this research was designed to provide realistic pediatric physiological flow conditions, consisting of a Harvard Apparatus Pulsatile blood pump, venous reservoir, and a heart valve testing chamber. In order to expose the valve to realistic pediatric flow conditions, six unique pump operating conditions were tested that involved pre-determined heart rate and stroke volume combinations. In addition, a modified aortic root model was used to hold the mechanical heart valve in place within the loop and to provide more realistic aortic root geometry. This heart valve chamber was made from a transparent acrylic material, allowing for fluid flow visualization. A traditional Particle Image Velocimetry (PIV) experimental set up was used in order to illuminate the particles seeded within the fluid path, and thus allowing for the capture of sequential images using a high speed camera. The data collected throughout this study consisted of flow rate measurements using an ultrasonic flow meter, and the sequential PIV images obtained from the camera in order to analyze general flow characteristics across the pediatric valve. Such information regarding the flow profile across the valve allowed for conclusions to be made regarding the valve performance, such as average flow velocities and regions of regurgitant flow. By gaining a better understanding of the fluid dynamic profile across a pediatric sized heart valve, this may aid in the eventual approval of pediatric sized mechanical heart valves for future clinical use.

A systemic mock circulatory loop plays a pivotal role as the in vitro assessment tool for left heart medical devices. The standard design employed by many research groups dates to the early 1970's, and lacks the acuity needed for the advanced device designs currently being explored. The necessity to update the architecture of this in vitro tool has become apparent as the historical design fails to deliver the performance needed to simulate conditions and events that have been clinically identified as challenges for future device designs. In order to appropriately deliver the testing solution needed, a comprehensive evaluation of the functionality demanded must be understood. The resulting system is a fully automated systemic mock circulatory loop, inclusive of anatomical geometries at critical flow sections, and accompanying software tools to execute precise investigations of cardiac device performance. Delivering this complete testing solution will be achieved through three research aims: (1) Utilization of advanced physical modeling tools to develop a high fidelity computational model of the in vitro system. This model will enable control design of the logic that will govern the in vitro actuators, allow experimental settings to be evaluated prior to execution in the mock circulatory loop, and determination of system settings that replicate clinical patient data. (2) Deployment of a fully automated mock circulatory loop that allows for runtime control of all the settings needed to appropriately construct the conditions of interest. It is essential that the system is able to change set point on the fly; simulation of cardiovascular dynamics and event sequences require this functionality. The robustness of an automated system with incorporated closed loop control logic yields a mock circulatory loop with excellent reproducibility, which is essential for effective device evaluation. (3) Incorporating anatomical geometry at the critical device interfaces; ascending aorta and left atrium. These anatomies represent complex shapes; the flows present in these sections are complex and greatly affect device performance. Increasing the fidelity of the local flow fields at these interfaces delivers a more accurate representation of the device performance in vivo.

This thesis deals with the mechanics of the Intra-Aortic Balloon Pump (IABP), the most widely used temporary cardiac assist device, whose beneficial action is based on the principle of counterpulsation. The investigation is carried out in vitro in increasingly more realistic setups, including a mock circulatory system with physiological distribution of peripheral resistance and compliance in which IABP counterpulsation was simulated. Pressure and flow measurements show the effect of variables such as intra-luminal pressure, angle and aortic compliance on balloon hemodynamics. These data are complemented by results on the duration of balloon inflation and deflation obtained by means of high-speed camera visualisation. Furthermore, wave intensity analysis is carried out and it is identified as a possible alternative method for the assessment of IABP performance. This work includes two prototypes of intra-aortic balloons of novel shape with the balloon chamber tapering both from and toward the balloon tip. In clinical terms, with reference to the semi-recumbent position in which patients assisted with the IABP are nursed in the intensive care unit, the results presented in this thesis indicate that operating the balloon at an angle compromises the benefit of counterpulsation when assessed in vitro.

Heart failure (HF) affects an estimated 5.8 million Americans, accounting for near 250,000 deaths each year. With shortages in available donor hearts, mechanical circulatory support (MCS) has emerged as a life-saving treatment for advanced stage HF. With growth in MCS use, a clinical and developmental need has emerged for a standard characterization and evaluation platform that may be utilized for inter-device comparison and system training. The goal of this research was to harness SynCardia's total artificial heart (TAH) to meet this need. We first sought to characterize the TAH in modern physiological terms - i.e. hemodynamics and pressure-volume loops. We then developed a model of HF using the TAH and mock circulatory system operating in a reduced output mode. We demonstrated that MCS devices could be incorporated and evaluated within the HF model. Finally, we characterized the operational envelope of SynCardia's Freedom (portable), Driver operating against varying loading conditions. Our results describe the hemodynamic envelope of the TAH. Uniquely, the TAH was found not to operate with time-varying elastance, to be insensitive to variations in afterload up to at least 135 mmHg mean aortic pressure, and exhibit Starling-like behavior. After transitioning the setup to mimic heart failure conditions, left atrial pressure and left ventricular pressure were noted to be elevated, aortic flow was reduced, sensitivity to afterload was increased, and Starling-like behavior was blunted, consistent with human heart failure. The system was then configured to allow ready addition of ventricular assist devices, which upon placement in the flow circuit resulted in restoration of hemodynamics to normal. Lastly, we demonstrated that the Freedom Driver is capable of overcoming systolic pressures of 200 mmHg as an upper driving limit. Understanding the physiology and hemodynamics of MCS devices is vital for proper use, future device development, and operator training. Characterization of the TAH affords insight into the functional parameters that govern artificial heart behavior providing perspective on differences compared to the human heart. The use of the system as a heart failure model has the potential to serve as a valuable research and teaching tool to foster safe MCS device use.

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