Browsing by Author "Tyberg, John V. T."

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    Identifying indices of vulnerability in the aneurysmal abdominal aorta: The interplay between mechanics and morphology
    (2019-09-05) Ismaguilova, Alina; Di Martino, Elena S.; Matyas, John Robert; Tyberg, John V. T.; Edwards, William Brent; Natale, Giovanniantonio
    An abdominal aortic aneurysm is the pathological dilatation of the abdominal aorta that can grow silently and rupture without warning. Over time, the vessel wall becomes weaker as inflammatory processes take over and the microarchitecture is compromised. Understanding the behaviour of the aneurysm wall at the macro and microscopic level can help elucidate the rupture potential of the vessel. This study proposes a novel method in assessing regions-specific differences by which we section the aorta into patches that can be traced back to specific areas on the aneurysm. The present study is thus an exploratory approach at assessing the aneurysms of multiple patients to establish differences between aneurysms, within aneurysms, and compare against healthy tissue. We established these differences, among others, using a variety of methods that assess the tissue microstructure, inflammation, composition, and mechanical response to loading. We also demonstrated the mechanical and structural heterogeneity in case studies exploring region-specific differences within the same patient. Coupling exploration into the pathophysiology of the aneurysm with its mechanical behaviour allowed us to paint a better picture of the disease, with mechanics often explaining biology and vice versa. We conclude that the pathological abdominal aorta exhibits a disruption in its extracellular matrix, profound inflammation, stiffer behaviour, and increased energy loss when compared with non-aneurysmal tissue. Ultimately, rupture risk assessment strategies need to utilize patient-specific parameters, and region-specific considerations need to be made.
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    Mechanisms of Early Filling in the Left Ventricle
    (2021-01-08) Urroz Lopez, Maryell Giofred; Tyberg, John V. T.; Shrive, Nigel Graham; Belenkie, Israel; Sheldon, Robert Stanley; Fewell, James E.
    The mechanism by which ventricular filling occurs has been debated for centuries. Originally, it was believed that ventricular filling was an entirely passive process. However, in the 20th-century, researchers proposed that LV played a role in its own filling sucking blood into itself. At the early phase of diastole, the LV enlarges faster than it is able to fill passively by the Left Atrium (LA); therefore, aspirating blood into itself. Diastolic suction (DS) is the term applied to the phenomenon whereby the left ventricle (our study will be limited to the LV), in part, fills itself. Two approaches to study DS are the volume of filling due to suction, VDS (Katz’ approach), and the energy of the Backward Decompression Wave, BDW (Wang’s approach). Our first aim was to determine if DS exists at LV volumes that exceed the unstressed volume. Our second aim was to compare Katz’ Criterion to the energy of the BDW generated by the LV (Wang’s approach). The study was performed using a porcine model. Hemodynamics and mitral blood flow velocity were measured over a range of filling pressures during baseline, inferior vena cava occlusion (IVCO), and volume loading. Our findings suggests that the LV generates suction when ESV is greater than ESV at baseline (ESVB), which opposes the view that the LV is not capable of generating suction at increasing ESV’s. We found non-zero values of VDS and IW-DS when ESV>ESVB which suggest a non-recoil mechanism responsible for suction. In addition, our data shows that both approaches to DS, measured by VDS and IW-DS, are indeed related as they responded in the same manner to changes in EDV, ESV, and EDP.
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    Novel Approaches to the Assessment of Systemic Circulation and Ventricular Performance
    (2019-02-27) Howell, Sarah; Tyberg, John V. T.; Shrive, Nigel; Phillips, Aaron A.; Sheldon, Robert Stanley; Fedak, Paul
    The purpose of evaluating the systemic circulation using systemic vascular conductance and head-capacity curve to quantify left ventricle performance is to validate these assessment approaches instead of more commonly used cardiovascular indexes (i.e., systemic vascular resistance and ejection fraction) used for physiologic responses. Systemic vascular conductance is reciprocal of systemic vascular resistance and is defined as the flow to the systemic circulation that determines arterial pressure. Systemic vascular conductance is calculated by dividing cardiac output by arteriovenous pressure difference. Left ventricle performance is assessed using head-capacity curve. It states that left ventricle as a pump, works under a head-capacity curve. The aim of this study is to increase our understanding of systemic circulation and ventricular performance by studying how changing loading conditions using drug interventions (phenylephrine, sodium nitroprusside, and isoproterenol), proximal aortic constriction, and volume loading will affect the systemic vascular conductance and left ventricle performance. The ultimate goal of this study is to combine circulatory and ventricular properties quantitatively and define the relationship between output produced by the heart and input of the circulation. Overall, results are consistent with the hypothesized physiological changes and allow the application of both proposed indices to determine the global cardiovascular performance in a simple manner. These results further enhance our understanding that conductance of each intervention in systemic circulation determines the mean arterial pressure. Systemic vascular conductance changes markedly with changing cardiac index and these changes are mediated by left ventricle. The potential benefits of these assessment parameters will help provide insights to optimize circulatory performance in patients with cardiovascular disease.