Confined jets in co-flow: effect of the flow rate ratio and lateral position of a return cannula on the flow dynamics

Effect of flow rate ratio and positioning on a lighthouse tip ECMO return cannula

Extracorporeal membrane oxygenation is a life-saving support therapy in the case of cardiopulmonary refractory failure. Its use is associated to complications due to the presence of artificial surfaces and supraphysiological stress conditions. Thus, knowledge of the fluid structures associated to each component can give insight into sources of blood damage. In this study, an experimentally validated numerical study of a conventional lighthouse tip cannula in return configuration was carried out to characterize the flow structures using water or a Newtonian blood analog with different flow rate ratios and cannula positioning and their influence on hemolysis. The results showed that strong shear layers developed where the jets from the side holes met the co-flow. Stationary backflow regions at the vessel wall were also present downstream of the cannula. In the tilted case, the recirculation was much more pronounced on the wide side and almost absent on the narrow side. Small vortical backflow structures developed at the side holes which behaved like obstacles to the co-flow, creating pairs of counter-rotating vortices, which induced locally higher risk of hemolysis. However, global hemolysis index did not show significant deviations. Across the examined flow rate ratios, the holes on the narrow side consistently reinfused a larger fraction of fluid. A radial force developed in the tilted case in a direction so as to recenter the cannula in the vessel.

Extracorporeal Membrane Oxygenation Blood Flow and Blood Recirculation Compromise Thermodilution-Based Measurements of Cardiac Output

The contribution of veno-venous (VV) extracorporeal membrane oxygenation (ECMO) to systemic oxygen delivery is determined by the ratio of total extracorporeal blood flow () to cardiac output (). Thermodilution-based measurements of may be compromised by blood recirculating through the ECMO (recirculation fraction; Rf). We measured the effects of and Rf on classic thermodilution-based measurements of in six anesthetized pigs. An ultrasound flow probe measured total aortic blood flow () at the aortic root. Rf was quantified with the ultrasound dilution technique. was set to 0-125% of and was measured using a pulmonary artery catheter (PAC) in healthy and lung injured animals. PAC overestimated () at all settings compared to . The mean bias between both methods was 2.1 L/min in healthy animals and 2.7 L/min after lung injury. The difference between and increased with an of 75-125%/ compared to QEC <50%/. Overestimation of was highest when resulted in a high Rf. Thus, thermodilution-based measurements can overestimate cardiac output during VV ECMO. The degree of overestimation of depends on the EC/ ratio and the recirculation fraction.

The Hemodynamics of Small Arterial Return Cannulae for Venoarterial Extracorporeal Membrane Oxygenation

BACKGROUND

Venoarterial extracorporeal membrane oxygenation (ECMO) provides mechanical support for critically ill patients with cardiogenic shock. Typically, the size of the arterial return cannula is chosen to maximize flow. However, smaller arterial cannulae may reduce cannula-related complications and be easier to insert. This in vitro study quantified the hemodynamic effect of different arterial return cannula sizes in a simulated acute heart failure patient.

METHODS

Baseline support levels were simulated with a 17 Fr arterial cannula in an ECMO circuit attached to a cardiovascular simulator with targeted partial (2.0 L/min ECMO flow, 60-65 mmHg mean aortic pressure - MAP) and targeted full ECMO support (3.5 L/min ECMO flow and 70-75 mmHg MAP). Return cannula size was varied (13-21 Fr), and hemodynamics were recorded while keeping ECMO pump speed constant and adjusting pump speed to restore desired support levels.

RESULTS

Minimal differences in hemodynamics were found between cannula sizes in partial support mode. A maximum pump speed change of +600 rpm was required to reach the support target and arterial cannula inlet pressure varied from 79 (21 Fr) to 224 mmHg (13 Fr). The 15 Fr arterial cannula could provide the target full ECMO support at the targeted hemodynamics; however, the 13 Fr cannula could not due to the high resistance associated with the small diameter.

CONCLUSIONS

A 15 Fr arterial return cannula provided targeted partial and full ECMO support to a simulated acute heart failure patient. Balancing reduced cannula size and ECMO support level may improve patient outcomes by reducing cannula-related adverse events.

Flow Dynamics and Mixing in Extracorporeal Support: A Study of the Return Cannula

Cannulation strategies in medical treatment such as in extracorporeal life support along with the associated cannula position, orientation and design, affects the mixing and the mechanical shear stress appearing in the flow field. This in turn influences platelet activation state and blood cell destruction. In this study, a co-flowing confined jet similar to a return cannula flow configuration found in extracorporeal membrane oxygenation was investigated experimentally. Cannula diameters, flow rate ratios between the jet and the co-flow and cannula position were studied using Particle Image Velocimetry and Planar Laser Induced Fluorescence. The jet was turbulent for all but two cases, in which a transitional regime was observed. The mixing, governed by flow entrainment, shear layer induced vortices and a backflow along the vessel wall, was found to require 9–12 cannula diameters to reach a fully homogeneous mixture. This can be compared to the 22–30 cannula diameters needed to obtain a fully developed flow. Although not significantly affecting mixing characteristics, cannula position altered the development of the flow structures, and hence the shear stress characteristics.