Impact of side hole design of drainage cannulas on flow velocity and predilection sites for low-flow areas in veno-venous ECMO circuits.

Abstract

Background: Veno-Venous Extracorporeal Membrane Oxygenation (VV-ECMO) (VV-ECMO) and Veno-Artial Extracorporeal Membrane Oxygenation (VA-ECMO) have become evidence-based therapy options for patients with most severe heart and/or lung failure. Cannulas continue to play a central role as the direct interface between patients and extracorporeal circulation. However, severe side-effects, such as coagulation and haemolysis, may result not only from a blood pump but also from inappropriate cannula design.

Methods: Based on the multi-stage lighthouse design of a draining ECMO cannula, the area of the side holes was varied in both shape and size in the distal and proximal directions. Using a Computer-Aided Design (CAD)- Model of cannulas (19,21,23 French (Fr)) appropriate simulation models were created in the non-reactive plenum. The resultant numerical 3D simulations are based on physiological boundary conditions and on the flows covering different ECMO-flow rates.

Results: The current cannula design reveals no-flow zones, which induce thrombus formation, especially when using blood-flow rates below 2 L/min. New geometric cannula design variants revealed an improvement in pressure loss, but at the expense of pronounced no-flow zones in the distal region. During drainage, the largest proportion of the total volume flow drains through the proximal row of side holes. Here, the pressure and thus the flow for both, immediately drops to the level of the surrounding plenum. This means that risk areas for haemostasis develop more distally. The direct influence of the design is evident in the proximally exponentially reduced side holes. They significantly reduce the extent of the no-flow zones. Simulations of the reference cannula showed that suction to the vessel wall and thus closure of some side holes contributes to an overall improvement in flow within the cannula.

Conclusions: When reducing the blood-flow rate in ECMO therapy, stagnation areas may develop within the drainage cannula. The geometry of the side holes has an important impact. A comparison of the different designs shows that distal growth of the side hole area results in improved flow behaviour.