Numerical and experimental investigation of a lighthouse tip drainage cannula used in extracorporeal membrane oxygenation

Abstract

Background: Extracorporeal membrane oxygenation is a life-saving therapy used in case of acute respiratory/circulatory failure. Exposure of blood to non-physiological surfaces and high shear stresses is related to hemolytic damage and platelet activation. A detailed knowledge of the fluid dynamics of the components under different scenarios is thus paramount to assess the thrombogenicity of the circuit.

Methods: An investigation of the flow structures developing in a conventional lighthouse tip (single-staged) drainage cannula was performed with cross-validated computational fluid dynamics and particle image velocimetry. The aim was to quantify the variation in drainage performance and stress levels induced by different fluid models, hematocrit and vessel-to-cannula flow rate ratios.

Results: The results showed that the 90° bends of the flow through the side holes created a recirculation zone inside the cannula which increased residence time. Flow structures resembling a jet in a crossflow were also observed. The use of different hematocrits did not significantly affect drainage performances. The most proximal set of holes drained the largest fraction of fluid. However, different flow rate ratios altered the flow rate drained through the tip. The use of 2D data led to a 50% underestimation of shear rate levels. In the drainage zone the non-Newtonian behavior of blood was less relevant.

Conclusions: The most proximal holes drained the largest amount of fluid. The flow features and distribution of flow rates among the holes showed little dependence on the hematocrit. The non-Newtonian behavior of blood had a small influence on the dynamics of the flow.

Keywords: CFD; ECMO; PIV; drainage; flow structures; jet in crossflow; non-Newtonian.

FIGURE 1

(A) A sketch of the domain with a focus on the drainage area. The red arrows show the direction of the vessel flow, whereas the blue arrow indicates the drainage direction. The origin of the coordinate system is placed on the centerline of the cannula at its tip. According to the coordinate system, z and w represent the position and velocity components in the streamwise direction. The inner cannula diameter is D = 6 mm and each side hole has a diameter d = 3 mm. (B) The cannula geometry used in the experiments and simulations. The labels A′, B′, C′ refer to the 90° shifted holes visible in the picture. The remaining holes on the opposite sides will be indicated as D′, E', F′. (C) Representation of the grid used for the simulations. A to F indicate six of the twelve side holes of the cannula, with A–F being the most proximal row.

FIGURE 2

Comparison between numerical and experimental results. For the profiles taken at the centerline of the holes AF (z = −12 mm), BE (z = −22 mm), CD (z = −32 mm), (A) shows the streamwise (z) velocity, (B) shows the normal (y) velocity, (C) the shear rate norm. (D) shows the y velocity profiles taken across the side holes diameters (in the z direction). In panels B and C, the black dashed lines indicate the position of the cannula walls.

FIGURE 3

Averaged velocity field of Case 6 in the mid zy plane. The gray dashed lines show the location of the cross planes. The red circles highlight the jets in tandem, with the crossflow being the flow coming from the right side of the domain. The black ovals show the locations of the recirculation bubbles developing behind the 90° bend of the side holes.

FIGURE 4

The figure shows the axial vorticity in the zy plane. The counter‐rotating vortex pairs are visible in the cross‐planes. The zoom‐in depict regions of high magnitude of helicity in the vicinity of the most proximal holes, with the red and blue clusters representing high positive and negative helicity respectively.

FIGURE 5

Pressure gradient along the centerline of the cannula for Case 2. The black dashed lines show the location of the side holes and the cannula tip. The zoom‐in of the drainage area is colored by velocity magnitude. The arrows in the panel show the bulk velocities of the drainage flow and vessel flow computed by diving the flow rates by the respective cross‐sectional areas.

FIGURE 6

Viscosity field for Case 6. The gray dashed lines mark the location of the cross planes. Dark blue areas depict the locations where viscosity reached its asymptotic value, while dark red areas are more influenced by non‐Newtonian effects.

FIGURE 7

Normal (y) velocity profiles along each side hole diameter (z direction) for Cases 1 and 6, scaled using the bulk velocity of the drainage flow.