Introduction: Intracranial arterial stenosis accounts for about 10% of all ischemic strokes and has a high recurrence rate. The EC-IC Bypass Trial failed to demonstrate benefit over medical management, particularly for patients with severe stenosis rather than occlusion, potentially explained by retrograde flow from the bypass competing with flow through the stenosis, creating conditions at risk for thrombosis, such as stagnant or turbulent flow. This theoretical concern has not been thoroughly evaluated. To better understand the hemodynamics caused by introduction of a bypass, we employed a 3D computational model.
Methods: The model was initially constructed from biplane angiograms of a patient following STA-to-MCA bypass using 2D-to-3D reconstruction of semi-automatically delineated vessel branches. Additional details, such as the anterior temporal branch and lenticulostriates, were added. The vessel model was then modified with varying degrees of stenosis. The ANSYS: Simulation Technology package simulated blood flow through the model to evaluate hemodynamic parameters at 70% and 90% stenosis.
Results: The 70% and 90% models (Figure 1 A&B) showed comparable pressure drops (5861Pa, 6335Pa), local flow velocity (FV; 4.5m/s, 3.4m/s), and wall shear stress (WSS) distal to the stenosis (2.97Pa, 3.42Pa). The distal M1 FV was greater in the 70% model (0.516m/s, 0.030m/s). Conversely, retrograde FV through lateral M3 branches was lower in the 70% model (0.099m/s, 0.558m/s). The medial M3 was supplied by the ICA at 70% and by the bypass at 90% (Figure 1 C). Lastly, the maximal turbulent kinetic energy (TKE) was lower at 70% stenosis (0.00586J/kg, 0.338J/kg).
Conclusions: These results demonstrate that introduction of a bypass causes dramatic hemodynamic changes in the form of competitive flow within the M1 and M3 segments. These changes are evidenced by near complete stagnation of flow in regions of the MCA with high WSS and TKE, a set of conditions at high risk for intra-arterial thrombosis.
Patient Care: This research will improve our understanding of the hemodynamics underlying clinical observations of patients who have received an extracranial-intracranial bypass for the treatment of intracranial arterial stenosis
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the effects that hemodynamic changes can create in a bypass, 2) Discuss how the information obtained from computational models can aid in research and practice