Mean Arterial Pressure Required for Maintaining Patency of Extracranial-to-Intracranial Bypass Grafts

Abstract

Maintaining flow in a newly established high-flow bypass into the intracranial circulation may be threatened by low blood pressure. To identify mean arterial blood pressure below which early graft failure may ensue. 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. 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). 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.