Dissertação
Avaliação numérica do efeito da variação de vazão na hemodinâmica em cateter venoso central para a hemodiálise
Fecha
2020-09-29Autor
Saulo de Freitas Gonçalves
Institución
Resumen
The main limitation of central venous catheters for hemodialysis is associated with the
thrombus formation. Thrombus formation, in turn, depends on physical factors of the
flow field, such as turbulence, recirculation and stagnation. Numerical models, based
on the computational fluid mechanics, open a new perspective for the evaluation of
hemodynamic factors that lead to the thrombus formation and progression. However,
the difficulty of validating these models, as well as the simplifications and sources of
uncertainty intrinsic to numerical modeling, restrict their use, scope and reliability.
Given the present context, this study proposes the development and validation of a
numerical model for the assessment of the hemodynamic environment in central
venous access for hemodialysis. Different turbulence models, as well as different
dialysis flow rate values in the catheters, were analyzed and discussed. The numerical
simulations were performed in transient regime, considering the physiological
conditions of the pulsatile flow in the central veins. The geometric domain of the
simulations was based on computed tomography images of a specific patient. A
methodology for the validation of the numerical model was proposed based on the
development of an experimental protocol, in vitro, for the measurement of pressure
drop in a test bench representative of the central venous access for hemodialysis. The
specimen used in the test bench was manufactured using the silicone casting
technique and 3D printing, which resulted in a body representative of the geometry of
the central veins, compliant and with optical access. The pressure loss values obtained
with the experiments and with the simulations showed a maximum difference of 13%,
making possible to validate the numerical model. From the validated numerical model,
it was possible to evaluate different turbulence models, as well as different dialysis flow
values commonly used in central venous access for hemodialysis. The flow rates of
250, 300 and 350 mL / min resulted in a longer exposure time and greater presence of
recirculation in the flow field than the other evaluated flows (200 and 400 mL / min). In
general, shear stresses increase in a quadratic manner with the increase in dialysis
flow rates in the catheter. However, its variation is percentile less significant than the
variation of maximum exposure times along the flow current lines, which suggests that
the presence of recirculation and the increase in the exposure time of blood particles
are more significant factors for the formation of thrombi in the region. As for the
turbulence models evaluated, the hybrid models (k-ω SST and k-ω SST of Transition)
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presented results consistent with studies involving the histological analysis of
thrombotic tissue in the region close to the venous orifices of the CVC. These last two
models resulted in the same general shear stress distribution and turbulence
intensities, however, the transition model resulted in the characterization of higher
values of these quantities, probably due to its greater capacity to characterize the
transition to turbulence.