Mean arterial pressure required for maintaining patency of extracranial-to-intracranial bypass grafts : an investigation with computational hemodynamic models-case series

BACKGROUND: Maintaining flow in a newly established high-flow bypass into the intracranial circulation may be threatened by low blood pressure. OBJECTIVE: To identify mean arterial blood pressure below which early graft failure may ensue. METHODS: Computational fluid dynamic blood flow simulation and Doppler ultrasound-derived velocities were combined to study 12 patients with common carotid-to-intracranial (internal carotid artery in 9 and middle cerebral artery in 3) arterial brain bypass with interposition of the saphenous vein. Patients underwent carotid duplex and high-resolution computed tomography angiography to obtain the necessary data. A mean time-averaged pressure gradient across both anastomoses of the graft was then calculated. RESULTS: The bypass graft mean blood flow +/- SD was 180.3 +/- 76.2 mL/min (95% confidence interval: 132-229). The mean time-averaged pressure gradient +/- SD across the bypass graft was 10.2 +/- 8.7 mm Hg (95% confidence interval: 4.6-15.7). This compared with a mean pressure gradient +/- SD on the contralateral carotid of 21.7 +/- 13.8 mm Hg. From these data, the minimum mean +/- SD systemic pressure necessary to maintain graft flow of at least 40 mL/min was 61.6 +/- 2.31 mm Hg, and the mean peak wall shear stress +/- SD at the proximal anastomosis was 0.8 +/- 0.7 Pa (95% confidence interval: 0.3-1.2). CONCLUSION: Early postoperative mean arterial pressure less than approximately 60 mm Hg may induce blood flow in the bypass to decrease to less than 40 mL/min, a flow below which low shear stress may lead to early graft occlusion. ABBREVIATIONS: CCA, common carotid artery CFD, computational fluid dynamics CI, confidence interval ECA, external carotid artery EC-IC, extracranial-to-intracranial ICA, internal carotid artery MAP, mean arterial pressure TAV, time-averaged flow velocity WSS, wall shear stress.