Computational fluid dynamic analysis of the flow field in the newly developed inflow cannula for a bridge-to-decision mechanical circulatory support

Numerical investigation of the effects of cannula geometry on hydraulic blood flow to prevent the risk of thrombosis

Despite significant advances in left ventricular assist devices and the cannula, unfavorable events leading to the death of patients, including bleeding, infection, neurological disorders, hemolysis, and thrombosis, are still being reported. Local parameters of blood flow, including static flow, vorticity and critical values of shear stress on the wall of ventricle and cannula, increase the risk of thrombosis. Therefore, the analysis of blood flow domains inside the ventricle and cannula is necessary to investigate the probability of forming thrombosis in the cannula of left ventricular assist devices. In this study, blood flow is investigated in a Medtronic DLP 16F clinical cannula by using computational fluid dynamics through three-dimensional modeling of the left ventricle and cannula based on real geometry. Apart from the fact that blood is considered non-Newtonian fluid, the effect of heart movement in the left ventricle is also applied. In this research, blood flow in the cannula has been examined and some problems resulting from the use of the cannulas have been investigated. The results indicate that changing the geometry of input holes, such as their number and size, on the tip of the cannula, alter the probability of forming thrombosis and the standard mode shows a better performance.

Ventricular flow dynamics with varying LVAD inflow cannula lengths: In-silico evaluation in a multiscale model

Left ventricular assist devices are associated with thromboembolic events, which are potentially caused by altered intraventricular flow. Due to patient variability, differences in apical wall thickness affects cannula insertion lengths, potentially promoting unfavourable intraventricular flow patterns which are thought to be correlated to the risk of thrombosis. This study aimed to present a 3D multiscale computational fluid dynamic model of the left ventricle (LV) developed using a commercial software, Ansys, and evaluate the risk of thrombosis with varying inflow cannula insertion lengths in a severely dilated LV. Based on a HeartWare HVAD inflow cannula, insertion lengths of 5, 19, 24 and 50 mm represented cases of apical hypertrophy, typical ranges of apical thicknesses and an experimental length, respectively. The risk of thrombosis was evaluated based on blood washout, residence time, instantaneous blood stagnation and a pulsatility index. By introducing fresh blood to displace pre-existing blood in the LV, after 5 cardiac cycles, 46.7%, 45.7%, 45.1% and 41.8% of pre-existing blood remained for insertion lengths of 5, 19, 24 and 50 mm, respectively. Compared to the 50 mm insertion, blood residence time was at least 9%, 7% and 6% higher with the 5, 19 and 24 mm insertion lengths, respectively. No instantaneous stagnation at the apex was observed directly after the E-wave. Pulsatility indices adjacent to the cannula increased with shorter insertion lengths. For the specific scenario studied, a longer insertion length, relative to LV size, may be advantageous to minimise thrombosis by increasing LV washout and reducing blood residence time.

Numerical prediction of thrombus risk in an anatomically dilated left ventricle: the effect of inflow cannula designs

BACKGROUND

Implantation of a rotary blood pump (RBP) can cause non-physiological flow fields in the left ventricle (LV) which may trigger thrombosis. Different inflow cannula geometry can affect LV flow fields. The aim of this study was to determine the effect of inflow cannula geometry on intraventricular flow under full LV support in a patient specific model.

METHODS

Computed tomography angiography imaging of the LV was performed on a RBP candidate to develop a patient-specific model. Five inflow cannulae were evaluated, which were modelled on those used clinically or under development. The inflow cannulae are described as a crown like tip, thin walled tubular tip, large filleted tip, trumpet like tip and an inferiorly flared cannula. Placement of the inflow cannula was at the LV apex with the central axis intersecting the centre of the mitral valve. Full support was simulated by prescribing 5 l/min across the mitral valve. Thrombus risk was evaluated by identifying regions of stagnation. Rate of LV washout was assessed using a volume of fluid model. Relative haemolysis index and blood residence time was calculated using an Eulerian approach.

RESULTS

The inferiorly flared inflow cannula had the lowest thrombus risk due to low stagnation volumes. All cannulae had similar rates of LV washout and blood residence time. The crown like tip and thin walled tubular tip resulted in relatively higher blood damage indices within the LV.

CONCLUSION

Changes in intraventricular flow due to variances in cannula geometry resulted in different stagnation volumes. Cannula geometry does not appreciably affect LV washout rates and blood residence time. The patient specific, full support computational fluid dynamic model provided a repeatable platform to investigate the effects of inflow cannula geometry on intraventricular flow.

Numerical Simulation of LVAD Inflow Cannulas with Different Tip

The tip structure of LVAD inflow cannula is one of major factors to lead adverse events such as thrombosis and suction leading to obstruction. In this research, four kinds of tips that had been used in inflow cannulas were selected and designed. The flow field of the four inflow cannulas inserted into the apex of left ventricle (LV) was numerically computed by computational fluid dynamics. The flow behavior was analyzed in order to compare the blood compatibility and suction in left ventricle and cannulas after the inflow cannulas with different tips were inserted to the apex of LV. The results showed that the cannula tip structure affected the LVAD performance. Among these four cannulas, the trumpet-tipped inflow cannula owned the best performance in smooth flow velocity distribution without backflow or low-velocity flow so that it was the best in blood compatibility. Nevertheless, the caged tipped cannula was the worst in blood compatibility. And the blunt-tipped and beveled tipped inflow cannulas may obstruct more easily than trumpet and caged tipped inflow cannulas because of their shape. The study indicated that the trumpet tip was the most preferable for the inflow cannula of long-term LVAD.