Computational modelling of blood flow in medical assist devices

QC 230503

Abstract

Extracorporeal membrane oxygenation (ECMO) is a life-saving support treat-ment in case of pulmonary and/or cardiac failure. An artificial extracorporealcircuit is used to offload the function of lungs and/or heart. Patient blood is drained through a drainage cannula, pumped with a centrifugal pump, oxygenated in a membrane lung and returned to the body through a reinfusion cannula. Tubing and connectors complete the circuit. However, its use canlead to thromboembolic and haemolytic complications, which are related to mechanical stresses arising in the flow of blood through its components. Numerical simulations of some of the pumps and cannulae used in the circuit were performed to investigate the flow structures developing in these components and their relation to measures of blood damage in the form of platelet activation state (PAS) and haemolysis index (HI). Simulations of two magnetically levitated centrifugal ECMO pumps were performed both in on- and off-label conditions with flow rates compatible with adult and neonatal use. The results showed that off-label low flow rate can be damaging due to an increase of residence time of the particles, which exposed them for longer to non-physiological stress. This held true for both passive tracers and inertial particles subjected to lift and drag. The neonatal pump showed a backflow structure with flow swirling back to the inlet tubing over its whole labelled range. Simulations of a lighthouse drainage cannula were undertaken to assess drainage characteristics at different haematocrits and flow rate ratios. The results indicated that the flow field was dominated by a jet in crossflow type of structure, with the most proximal holes draining the largest amount of fluid in all the studied cases and for all the considered haematocrits. The effects due to non-Newtonian behaviour of blood were less relevant in the drainage area, allowing to use a Reynolds number analogy to bridge between water and blood results.A lighthouse cannula in return configuration was also considered in both a centred and a tilted position. A characteristic confined jet configuration was found, with a backflow developing at the vessel wall, increasing residence time. In the tilted case, a group of small vortical structures developed at the holes close to the wall, which behaved as an obstacle to the vessel flow and increased both residence time and stress. This led to locally increased haemolysis which, however, did not impact haemolysis at large due to the low flow exposed to this area. The use of different viscosity models in this case led to small variations in the results, which were minor compared to the uncertainty introduced by the use of different model coefficients in the computation of the haemolysis index.

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