Avaliação numérica do efeito da variação de vazão na hemodinâmica em cateter venoso central para a hemodiálise

Abstract

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) 9 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.