The physiological and clinical considerations of centrifugal and axial pumps as ven- tricular assist devices (VADs) demands limitations on the power, size and geometry of the impellers. A typical pump design method is to rely on the characteristics of previously designed pumps with known performance using empirical equations and nondimensional parameters based on uid dynamics similarity law. Such data are widely available for industrial pumps operating in Reynolds number region of 108. VADs operate in Re < 106 and therefore the similarity concept does not apply between the industrial diagrams and the medical application of small pumps. The present dissertation employs a parametric approached analytical model to in- vestigate more than 150 axial and centrifugal pumps. The design parameters are optimised using the response surface methodology. The effect of different design parameters on the performance, force analysis and hemocompatibility of the pumps is thoroughly investigated by modelling the haemolysis through a power-law equation. The results show an explicit and consistent relationship between the number of blades, outlet width, outlet angle and the hemocompatibility of the device. Centrifu- gal pumps showed signi cantly lower probability of blood complications compared to axial pumps. The evaluation of the design characteristics helps pump designers to select their parameters accordingly for a low probability of blood complications. Furthermore, experimental techniques are employed to test more than 70 pumps in different conditions of flow, pressure and rotational speed. The experimental results validate the numerical simulations and create a database of empirical equations and data points for small axial and centrifugal pumps. The specifi c speed and speci fic diameters of the pumps are plotted on an ns − ds diagram to enable preliminary design of small pumps for VADs suitable for different stages of congestive heart failure (CHF).

<p>Ventricular Assist Devices (VADs) are mechanical pumps used to off-load a deceased heart, primarily in late stage congestive heart failure patients. VAD employment may facilitate cardiac recovery, but most often provides time before a suitable heart transplant can be found. Lately, long term use VAD systems have been introduced as an alternative to a heart transplant.</p><p>Traditionally, design of VADs has employed either displacement based pump technologies or radial-flow pumps, also known as rotodynamic pumps. A displacement pump induces a mechanical force on a fluid contained within a defined space, hence giving it motion. Radial-flow pumps impart momentum to a fluid, most often by placing a rotating device in the fluid.</p><p>This thesis introduces a novel pumping concept, combining features from both displacement and radial-flow pumps. A first prototype, the Vivicor^TM pump, has been designed, fabricated and evaluated In Vitro, the results reported in this thesis.</p><p>The In Vitro evaluation of the Vivicor^TM pump provides evidence of a pump with mechanical self-regulation based on pump pre-load level, much like a displacement pump. The Vivicor^TM pump also displays pulsating outflow in combination with an inflow both during pump systole and diastole. The latter provides potential advantages over traditional displacement pumps as smaller cannulae or catheters can be used, facilitating miniaturization. Continuos filling throughout the pumping cycle also require less pressure to be exerted on the fluid, compared to displacement pumps, limiting the risk of mechanical damage to the pumped fluid. The In Vitro evaluation has also provided further insights on necessary design modifications in the second-generation Vivicor^TM prototype, currently planned. The Vivicor^TMpumping technology is highly interesting for further development and evaluation for use in ventricular assist applications.</p>

A computational model has been developed to study the effects of left ventricular assist devices (LVADs) on the cardiovascular system during a ventricular collapse. The model consists of a toroidal pulsatile blood pump and a closed loop circulatory system. Together, they predict the pump's motor current traces that reflect ventricular suck-down and provide insights into torque magnitudes that the pump experiences. In addition, the model investigates likeliness of a suction event and predicts reasonable outcomes for a few test cases. Ventricular collapse was modeled with the help of a mock circulatory loop consisting of a artificial left ventricle and centrifugal continuous flow pump. This study also investigates different suction detection schemes and proposes the most suitable suction detection algorithm for the TORVAD pump, toroidal left ventricular assist device. Model predictions were further compared against the data sampled during in vivo animal trials with the TORVAD system. The two sets of results are in good accordance. / text

Ventricular Assist Devices (VADs) are mechanical pumps used to off-load a deceased heart, primarily in late stage congestive heart failure patients. VAD employment may facilitate cardiac recovery, but most often provides time before a suitable heart transplant can be found. Lately, long term use VAD systems have been introduced as an alternative to a heart transplant. Traditionally, design of VADs has employed either displacement based pump technologies or radial-flow pumps, also known as rotodynamic pumps. A displacement pump induces a mechanical force on a fluid contained within a defined space, hence giving it motion. Radial-flow pumps impart momentum to a fluid, most often by placing a rotating device in the fluid. This thesis introduces a novel pumping concept, combining features from both displacement and radial-flow pumps. A first prototype, the VivicorTM pump, has been designed, fabricated and evaluated In Vitro, the results reported in this thesis. The In Vitro evaluation of the VivicorTM pump provides evidence of a pump with mechanical self-regulation based on pump pre-load level, much like a displacement pump. The VivicorTM pump also displays pulsating outflow in combination with an inflow both during pump systole and diastole. The latter provides potential advantages over traditional displacement pumps as smaller cannulae or catheters can be used, facilitating miniaturization. Continuos filling throughout the pumping cycle also require less pressure to be exerted on the fluid, compared to displacement pumps, limiting the risk of mechanical damage to the pumped fluid. The In Vitro evaluation has also provided further insights on necessary design modifications in the second-generation VivicorTM prototype, currently planned. The VivicorTM pumping technology is highly interesting for further development and evaluation for use in ventricular assist applications. / QC 20101129

Physical bench top experiments are performed to validate and complement ongoing computational fluid dynamics (CFD) analyses of ventricular assist device (VAD) circulation. VADs are used in patients whose hearts do not function to their maximum potential due advanced stages of heart disease and, consequently, are unable to adequately supply blood to the systemic circulation. VADs are commonly utilized as a bridge-to-transplantation, meaning that they are implanted in patients while waiting for a heart transplant. In such cases of long term utilization of VADs, it has been reported in the literatures that thrombo-embolic cerebral events occur in 14-47% of patients over the period of 6 to 12 months. This is a result of thrombus forming despite the use of anticoagulants and advances in VAD design. Accepting current rates of thrombo-embolisms, the main objective of the project is to identify and propose an optimal surgical cannula implantation orientation aimed at reducing the rate of thrombi reaching the carotid and vertebral arteries and thus reduce the morbidity and mortality rate associated with the long term use of VADs to patients suffering from advanced heart failure. The main focus of the experiment is on the physical aspect using a synthetic anatomically correct model constructed by rapid prototyping of the human aortic arch and surrounding vessels. Three VAD cannula implantation configurations are studied with and without bypass to the left carotid artery or to the Innominate artery with ligation of the branch vessel at its root. A mixture of water and glycerin serves to match blood viscosity measured with a rotating cone-plate viscometer. The Reynolds number in the ascending aorta is matched in the flow model. A closed loop mock circulatory system is then realized. In order to match the Reynolds number in the ascending aorta and LVAD cannula with that of the CFD model, a volumetric flow rate of 2.7 liters per minute is supplied through the synthetic VAD cannula and 0.9 liter per minute is supplied to the ascending aorta. Flow rates are measured using rotary flow meters and a pressure sensor is used to ensure a mean operating pressure of 100 mmHg is maintained. Synthetic acrylic blood iv clots are injected at the inlet of the VAD cannula and they are captured and counted at the vertebral and carotid arteries. The sizes of the thrombi simulated are 2, 3.5 and 5 mm which are typical of the range of diameters encountered in practice. Nearly 300 particles are released over 5 separate runs for each diameter, and overall embolization rates as well as individual embolization rates are evaluated along with associated confidence levels. The experimental results show consistency between CFD and experiment. Means comparison of thromboembolization rates predicted by CFD and bench-top results using a Z-score statistic with a 95% confidence level results in 22 of 24 cases being statistically equal. This study provides confidence in the predictive capabilities of the bench-top model as a methodology that can be utilized in upcoming studies utilizing patient-specific aortic bed model.

Continuous-flow left ventricular assist devices provide mechanical circulatory assistance for patients suffering from end-stage heart failure that are awaiting or ineligible for heart transplantation. Although actuarial survival and quality of life with these devices is comparable to allograft transplant, they are associated with severe adverse events, including cerebrovascular accidents. Recent advances in continuous-flow technology aim to mitigate the risk of stroke by including design features that minimize flow stasis, turbulence and endothelial dysfunction, as well as promote near-normal pulse pressures. The proposed study is a multicenter, prospective, randomized clinical trial that aims to compare the stroke-free survival and associated incidence and risk of cerebrovascular accidents between three continuous-flow left ventricular assist devices in patients with refractory, end-stage heart failure planning to undergo bridge-to-transplant or destination therapy. Patients will be randomized to receive one of three devices (HeartMate II, Thoratec Corporation, Pleasanton, CA; HeartWare HVAD, HeartWare International Inc., Framingham, MA; HeartMate III, Thoratec Corporation, Pleasanton, CA). Patients will be monitored for stroke-free survival and incidence of cerebrovascular accident for 24 months post-implantation. Investigators will compare stroke-free survival with Kaplan-Meier survival curves and log-rank testing; in addition, investigators will examine each device’s level of risk for causing a cerebrovascular accident with chi square and odds ratio analysis. The data from this study will be used to guide treatment paradigms, device assignment and future development of technologies that mitigate stroke risk in this high-risk population.

Purpose: Patients living with advanced heart failure experience dyspnea, fatigue, poor quality of life, depression and cognitive impairment which may threaten their ability to provide informed consent to undergo heart transplant (HTx) or mechanical support (LVAD). Using qualitative and quantitative methods, we asked how patients with advanced heart failure make decisions regarding HTx and LVAD. The variables chosen to reflect the elements of consent included quality of life and symptom severity (voluntariness), depression and cognitive impairment (capacity) and treatment preferences (decision-making). Methods: 76 patients enrolled in the quantitative arm completed the Minnesota Living with Heart Failure Questionnaire; Visual Analog scales for dyspnea, fatigue and overall health; Beck Depression Inventory; Montreal Cognitive Assessment; Standard Gamble and Time Tradeoff. Qualitative methods were used to discover concepts, relationships and decision-making processes described by 17 of the 76 patients considering HTx and LVAD. Results: Patients reported poor quality of life and high symptom severity scores which compelled them to consider surgery as a way to relieve unpleasant symptoms and improve quality of life. Although 30% of patients had evidence of depression and/or cognitive impairment, no patient was deemed incapable of decision-making. Patients were willing to take considerable risk (35%) and trade considerable time (4months) to improve their health. While heart failure-related concepts were important to the decision, entrustment emerged as the meaningful process for decision-making. Conclusions: Patients who participated in this study were capable of decision-making and understood the risks associated with the surgery. Voluntariness was diminished by disease but not absent, and decisions were free of coercion. These results suggest the entrustment model of decision-making is the dominant process for patients considering high-risk surgical procedures and meets criteria for informed consent. Understanding the process of decision-making will help clinicians support and enable treatment decisions made by patients living with advanced heart failure.

Purpose: Patients living with advanced heart failure experience dyspnea, fatigue, poor quality of life, depression and cognitive impairment which may threaten their ability to provide informed consent to undergo heart transplant (HTx) or mechanical support (LVAD). Using qualitative and quantitative methods, we asked how patients with advanced heart failure make decisions regarding HTx and LVAD. The variables chosen to reflect the elements of consent included quality of life and symptom severity (voluntariness), depression and cognitive impairment (capacity) and treatment preferences (decision-making). Methods: 76 patients enrolled in the quantitative arm completed the Minnesota Living with Heart Failure Questionnaire; Visual Analog scales for dyspnea, fatigue and overall health; Beck Depression Inventory; Montreal Cognitive Assessment; Standard Gamble and Time Tradeoff. Qualitative methods were used to discover concepts, relationships and decision-making processes described by 17 of the 76 patients considering HTx and LVAD. Results: Patients reported poor quality of life and high symptom severity scores which compelled them to consider surgery as a way to relieve unpleasant symptoms and improve quality of life. Although 30% of patients had evidence of depression and/or cognitive impairment, no patient was deemed incapable of decision-making. Patients were willing to take considerable risk (35%) and trade considerable time (4months) to improve their health. While heart failure-related concepts were important to the decision, entrustment emerged as the meaningful process for decision-making. Conclusions: Patients who participated in this study were capable of decision-making and understood the risks associated with the surgery. Voluntariness was diminished by disease but not absent, and decisions were free of coercion. These results suggest the entrustment model of decision-making is the dominant process for patients considering high-risk surgical procedures and meets criteria for informed consent. Understanding the process of decision-making will help clinicians support and enable treatment decisions made by patients living with advanced heart failure.

L'insuffisance cardiaque est une maladie à grande incidence dont le traitement définitif est difficile. Les pompes d'assistance ventriculaire ont été proposées comme thérapie alternative à long terme, mais la technologie est relativement jeune et selon son design, axial ou centrifuge, le dispositif favorise soit l'hémolyse, soit la stagnation de l'écoulement sanguin. Les pompes à écoulement mixte, combinant certaines propriétés des deux types, ont été proposées comme solution intermédiaire. Pour évaluer leurs performances, nous avons effectué des comparaisons numériques entre huit pompes, deux axiales, deux centrifuges, et quatre mixtes, en employant un modèle Windkessel du système cardiovasculaire avec paramètres optimisés pour l'insuffisance cardiaque résolu avec une méthode Radau IIA3, une méthode de résolution de système d'équations différentielles ordinaires L-stable appartenant à la famille des méthodes Runge-Kutta implicites. Nos résultats semblent suggérer que les pompes d'assistance mixtes ne démontrent qu'un léger avantage comparativement aux autres types en terme de performance optimale dans le cas de l'insuffisance cardiaque, mais il faudrait effectuer plus d'essais numériques avec un modèle plus complet, entre autres avec contrôles nerveux implémentés. / Heart failure is a disease with a high incidence rate that is difficult to treat definitively. Ventricular assist devices have been proposed as alternative long-term therapeutic options, but the technology is fairly recent and depending on the design, axial or centrifugal, the device will favor hemolysis or blood flow stagnation. Mixed flow devices, which combine properties of the two previous types, have therefore been proposed as intermediate solutions. To evaluate their performance, we did a numerical comparison between eight pumps (two axial, two centrifugal, and four mixed) using a Windkessel model of the cardiovascular system with parameters optimised for heart failure and solved using a Radau IIA3 method, an L-stable method used to numerically solve systems of ordinary differential equations that belongs to the family of implicit Runge-Kutta methods. Our results suggest that in terms of optimal performance, mixed flow pumps are modest improvements over the other two types, and more tests would have to be performed using a more complete model with, among other modifications, neural control systems implemented.

Os dispositivos de assistência ventricular (DAVs) podem ser utilizados para a estabilização hemodinâmica de pacientes à espera do transplante cardíaco. Os avanços nas tecnologias e a utilização de materiais biocompatíveis vem contribuindo para o desenvolvimento de dispositivos com dimensões reduzidas e menor trauma ao sangue. A avaliação do desempenho desses dispositivos demanda a utilização de simuladores hidráulicos do sistema circulatório que reproduzam as pressões e fluxos existentes nas condições fisiológicas de interesse. Este trabalho tem como objetivo o desenvolvimento de um simulador da circulação pediátrica com ajustes automatizados de pressões. O simulador é composto por um circuito hidráulico modelando os laços sistêmico e pulmonar e um sistema microcontrolado com uma interface de usuário para medição e visualização dos fluxos e pressões ventriculares e automatização dos ajustes das pressões arteriais aórtica e pulmonar (PAo, PAP) e das pressões atriais esquerda e direita (PAE, PAD). Duas bombas pulsáteis com 15 ml de volume de ejeção são utilizadas para modelar os comportamentos mecânicos dos ventrículos esquerdo e direito. As complacências da aorta e da artéria pulmonar e as pré-cargas dos ventrículos são simuladas por câmaras com volumes ajustáveis de ar e líquido (análogo sanguíneo) utilizando uma bomba de ar. As resistências hidráulicas dos laços são ajustadas por oclusores motorizados. Os sinais instantâneos dos fluxos de entrada e saída dos DAVs e das pressões arteriais, atriais e ventriculares são obtidos por transdutores e digitalizados em um microcontrolador que comanda os oclusores e a bomba de ar. Foram desenvolvidos algoritmos para ajustes das resistências, complacências e pré-cargas. Uma interface gráfica de usuário apresenta os sinais em tempo real (ou gravados) permitindo a escolha dos parâmetros e condições de simulação. O desempenho do sistema de automatização foi avaliado nas simulações de: 1) condições definidas pelo aplicativo da interface e 2) condições fisiológicas (normal e redução na contratilidade do miocárdio). No modelo hidráulico sistêmico as pressões foram ajustadas em ambas as situações com erro máximo de 0,5% para a PAo e 5% para a PAE em aproximadamente 80 segundos. No modelo completo da circulação o erro máximo para as simulações de condições fisiológicas foi de 4% para as pressões arteriais e 5% para as atriais. Os resultados obtidos demonstram que o simulador desenvolvido permite reproduzir adequadamente as características da circulação pediátrica essenciais para a avaliação do desempenho de dispositivos de assistência mecânica. O simulador é portátil, de fácil utilização e pode ser utilizado como ferramenta didática ou para o treinamento de profissionais da saúde envolvidos em assistência a pacientes com suporte circulatório. / Ventricular assist devices (VADs) can be used for the hemodynamic stabilization of patients waiting for heart transplantation. Advances in the technologies and the use of biocompatible materials have contributed to the development of devices with reduced dimensions and blood trauma. Evaluation of the performance of these devices demands the use of hydraulic simulators of the circulatory system that reproduce pressures and flows existing in physiological conditions of interest. This work aims to develop a simulator of the pediatric circulation with automated adjustments of pressures. The simulator consists of a hydraulic circuit modeling the systemic and pulmonary branches and a microcontrolled system with a user interface for monitoring flows and ventricular pressures, and automating adjustments of aortic and pulmonary arterial pressures (AoP, PAP) and left and right atrial pressures (LAP, RAP). Two pulsatile pumps with 15 ml ejection volume are used to model the mechanical behavior of the left and right ventricles. Aortic and pulmonary arterial compliances and preloads of the ventricles are simulated by chambers with adjustable volumes of air and fluid (blood analog) using an air pump. Motorized clamps adjust the hydraulic resistances of the loops. Instantaneous signals of VAD input and output flows and of arterial, atrial and ventricular pressures are obtained by transducers and digitalized in a microcontroller that commands the clamps and the air pump. Algorithms were developed to adjust the resistances, compliances and preloads. A graphical user interface displays signals in real time (or recorded) and allows selection of simulation parameters. The performance of the automation system was tested setting pressures in two situations: 1) simulations of random conditions defined by the interface software and 2) simulations of physiological conditions (normal and low myocardial contractility). In the systemic model, the pressures were adjusted in both cases (maximum error of 0.5% for AoP and 5% for LAP) in approximately 80 seconds. In the complete model, the maximum error for simulations of physiological conditions was 4% for arterial pressures and 5% for atrial pressures. The results obtained demonstrate that the simulator developed allows mimicking the relevant features of the systemic and pulmonary branches of the circulation needed to assess the performance of mechanical circulatory assist devices. The simulator is portable, easy to operate and can be applied in teaching and training of health professionals working with mechanical circulatory support of patients.