Impact of altered vena cava flow rates on right atrium flow characteristics

Abstract

The right atrium (RA) combines the superior (SVC) and inferior vena cava (IVC) flows. Treatments like extracorporeal membrane oxygenation (ECMO) and hemodialysis by catheter alter IVC/SVC flows. Here we assess how altered IVC/SVC flow contributions impact RA flow. Four healthy volunteers were imaged with CT, reconstructed and combined into a patient-averaged model. Large Eddy Simulations (LES) were performed for a range of IVC/SVC flow contributions (30-70% each, increments of 5%) and common flow metrics were recorded. Model sensitivity to reconstruction domain extent, constant/pulsatile inlets and hematocrit was also assessed. Consistent with literature, a single vortex occupied the central RA across all flowrates with a smaller counter-rotating vortex, not previously reported, in the auricle. Vena cava flow was highly helical. RA turbulent kinetic energy (TKE) (P=0.027) and time-averaged wall shear stress (WSS) (P<0.001) increased with SVC flow. WSS was lower in the auricle (2 Pa, P<0.001). WSS in the vena cava were equal at IVC/SVC =65/35%. The model was highly sensitive to the reconstruction domain with cropped geometries lacking helicity in the vena cavae, altering RA flow. RA flow was not significantly affected by constant inlets or hematocrit. The rotational flow conventionally described in the RA is confirmed however a new, smaller vortex was also recorded in the auricle. When IVC flow dominates, as is normal, TKE in the RA is reduced and WSS in the vena cavae equalize. Significant helicity exists in the vena cava, a result of distal geometry and this geometry appears crucial to accurately simulating RA flow.

Hemodynamic and recirculation performance of dual lumen cannulas for venovenous extracorporeal membrane oxygenation

“…Thirdly, inflows to the model were assumed to be constant, this may reflect more closely the deeply sedated rather than awake ECMO patient. Constant inflows have been assessed previously to have little effect on time-averaged velocities in the RA 8 . Realistic transient inlet waveforms for an ECMO patient were not available.…”

“…All methods were performed in accordance with the relevant guidelines and regulations and ethical approval was obtained from the Swedish Ethical Review Authority. A rigid-wall patient-averaged model of the adult RA and venae cavae reported previously was used to model venous flow 8 . This model was based on four healthy volunteers (3 female, 1 male) with mean age = 58.3 ± 3.5yrs, weight = 77.0 ± 11.9 kg and height = 173.0 ± 2.8 cm.…”

“…Averaging of the geometry was achieved by quantifying minimum/maximum diameters and length for each venous branch as well as the dimensions of the right atrium. Using these, individual vein segments were selected, scaled and merged to create a model that best reflected the group mean of these measurements, as previously described 8 . A constant total venous return of 6 L/min was applied with 35% entering the RA from the superior vena cava (SVC) and 65% from the inferior vena cava (IVC) 11 , 12 .…”

“…The CFD model follows the same setup used previously 8 – 10 . Briefly, it is a large Eddy simulation (LES) with wall-adapting local-eddy viscosity (WALE) model and an all y+ wall function 13 .…”

“…We have developed a computational fluid dynamics (CFD) model of the right atrium (RA) and venae cavae 8 , tested its sensitivities 9 , and validated it against clinical data 10 . The model serves as a platform to investigate the impact of VV ECMO cannulation variables on performance.…”