Computational Prediction of Thrombosis in Food and Drug Administration's Benchmark Nozzle

Enhancing thrombosis prevention in medical devices: The role of turbulence in washout performance using FDA benchmark nozzle model

BACKGROUND AND OBJECTIVES

Thrombosis presents a significant and potentially lethal risk in medical devices. Turbulence has been associated with increased thrombosis risk, primarily due to heightened shear stress and resultant blood damage. However, it can be inferred that turbulence might also enhance washout performance through efficient transport and mixing, thereby mitigating thrombosis. This study explores the underappreciated role of turbulence.

METHODS

The FDA benchmark nozzle model was used as a representative framework for medical devices. To elucidate the isolated role of turbulence on washout performance, comparative simulations were conducted at Reynolds numbers of 500 and 6500 using Large Eddy Simulation (LES) and Menter's Shear Stress Transport (SST) k-ω turbulence models. Washout performance, a critical indicator in thrombosis, is evaluated by a passive scalar transport model.

RESULTS

The validation results align well with published data, confirming the reliability of the simulations. Reynolds numbers and turbulence models play a crucial role in the washout performance. Turbulence improves volume washout by disrupting flow recirculation zones and enhancing the mixing of old and new blood. Furthermore, turbulence aids in surface washout by altering flow patterns in the near-wall region and increasing wall shear stress.

CONCLUSION AND SIGNIFICANCE

The improved washout and dynamic environment facilitated by turbulence potentially minimize platelet adhesion and aggregation, which ultimately benefits the anti-thrombotic properties of medical devices. This research offers a novel perspective on the role of turbulence in thrombosis, extending beyond its traditionally recognized detrimental effects, and provides valuable insights into the design of specific flow patterns in achieving optimal washout performance in medical device applications. Further research is warranted to explore how to effectively leverage the washout-enhancing effects of turbulence while minimizing its potential adverse impacts.

A fibrin enhanced thrombosis model for medical devices operating at low shear regimes or large surface areas

Over the past decade, much of the development of computational models of device-related thrombosis has focused on platelet activity. While those models have been successful in predicting thrombus formation in medical devices operating at high shear rates (> 5000 s⁻¹), they cannot be directly applied to low-shear devices, such as blood oxygenators and catheters, where emerging information suggest that fibrin formation is the predominant mechanism of clotting and platelet activity plays a secondary role. In the current work, we augment an existing platelet-based model of thrombosis with a partial model of the coagulation cascade that includes contact activation of factor XII and fibrin production. To calibrate the model, we simulate a backward-facing-step flow channel that has been extensively characterized in-vitro. Next, we perform blood perfusion experiments through a microfluidic chamber mimicking a hollow fiber membrane oxygenator and validate the model against these observations. The simulation results closely match the time evolution of the thrombus height and length in the backward-facing-step experiment. Application of the model to the microfluidic hollow fiber bundle chamber capture both gross features such as the increasing clotting trend towards the outlet of the chamber, as well as finer local features such as the structure of fibrin around individual hollow fibers. Our results are in line with recent findings that suggest fibrin production, through contact activation of factor XII, drives the thrombus formation in medical devices operating at low shear rates with large surface area to volume ratios.

Patients treated with blood-contacting medical devices suffer from clotting complications. Over the past decades, a great effort has been made to develop computational tools to predict and prevent clot formation in these devices. However, most models have focused on platelet activity and neglected other important parts of the problem such as the coagulation cascade reactions that lead to fibrin formation. In the current work, we incorporate this missing element into a well-established and validated model for platelet activity. We then use this novel approach to predict thrombus formation in two experimental configurations. Our results confirm that to accurately predict the clotting process in devices where surface area to volume ratios are large and flow velocity and shear stresses remain low, coagulation reactions and subsequent fibrin formation must be considered. This new model could have great implications for the design and optimization of medical devices such as blood oxygenators. In the long term, the model could evolve into a functional tool to inform anticoagulation therapies for these patients.