Aortic stiffening after thoracic aortic stent grafting: A multi-patient specific computational study

Treatment of Descending Thoracic Aortic Aneurysms (DTAA) using open surgical or endovascular methods (Thoracic EndoVascular Aortic Repair) is widely accepted in medicine. However, these procedures modify the aorta's anatomy and biomechanics, triggering anomalous wave reflections and cardiac remodeling. The complex interplay among these factors is largely unexplored, hampering procedural efficacy and long-term predictability. Computational fluid structure interaction (FSI) is a powerful tool for exploring these dynamics, but the computational complexity of simulating 3D scenarios often poses practical challenges, in terms of efficiency and reliability. To address this, we employ cost-effective geometrical multiscale 0D-1D FSI models.

We integrate a simplified lumped parameter model of the left heart and an extended 1D systemic circulation model (covering the largest 55 arteries), implemented in the Multiscale module in the C++ finite element library LifeV. Patient-specific preoperative and postoperative 1D-FSI models were derived from CT angiography data of 11 patients (6 open surgery, 5 TEVAR), with implant stiffness adhering to established literature values (1.2 MPa Dacron grafts, 51.7 MPa metallic stents). Physiological inflow conditions are imposed in the ascending aorta, while three-element Windkessel models account for peripheral circulation. We simulated a total of 22 cases (baseline and postoperative for each case, spanning five heartbeats each) and compared the peak systolic pressure, pressure-volume loops (PV) and pulse-wave velocity (PWV) between vessels.

Our findings demonstrate that the presence of implants can increase ascending aorta pressure due to the backpropagation of pressure waves. The cases of TEVAR exhibit significantly higher PWV. Complex geometric cases exhibit smoother pressure-flow profiles after surgery. In general, the combination of altered arterial geometry and stiffness in postoperative conditions can significantly alter cardiac PV loops. We quantitatively demonstrate that descending aortic elongation independently compensates for material stiffness effects, with a strong negative correlation (r=-0.762,p=0.0065) between length change and post-operative pressure elevation. Cases achieving ≥15% descending elongation uniformly showed pressure reduction or minimal elevation.

Extending these insights to larger cohorts of patients has the potential to reveal mechanisms shaping the long-term effects of DTAA repair. Our results highlight the crucial need for a combined analysis of both stiffness and geometrical changes following surgery, as a more significant local stiffening (such as that caused by a stent) does not necessarily lead to cardiac overload if simultaneous changes in aortic geometry can compensate for the stent's impact.