The Efficacy of Monitoring the Middle Cerebral Artery Pressure during the Extracranial-intracranial Bypass

Introduction: Monitoring for intra-operative patency of the extracranial (EC)-M2 bypass is important to avoid ischemic complications. Tools such as ultrasound micro-flow probe and indocyanine green videoangiography are commonly used for EC-M2 monitoring. Middle cerebral artery pressure (MCAP) monitoring is another tool to assess the patency of the EC-intracranial (IC) bypass. We retrospectively analyze and discuss a series of cases of EC-M2 bypass with MCAP monitoring.Methods: We performed EC-M2 bypass with MCAP monitoring in 84 patients at our institution. Before EC-M2 bypass, we performed superficial temporal artery (STA)-MCA bypass, and MCAP was measured through another branch of the STA.Initial MCAP (iMCAP), MCAP after clamping of the ICA (cMCAP), and MCAP after releasing the graft (actual gMCAP) were intraoperatively monitored. The MCAP ratio was defined as gMCAP/iMCAP. On the basis of the Hagen-Poiseuille's law, the expected MCAP ratio (expected gMCAP/iMCAP) was hypothesized as: (1－cMCAP/iMCAP)(graft radius/ICA radius)2 + cMCAP/iMCAP. Graft malfunction was defined as a discrepancy between the expected gMCAP and actual gMCAP.Results: Eight of 84 cases were judged to have graft malfunction by MCAP monitoring and had to be re-anastomosed.The cause of graft malfunction was twisting of the graft in one case, insufficient graft length in 2 cases, severe atherosclerosis of the external carotid artery in 3 cases, M2 proximal twisting owing to pressure of the EC-M2 bypass in one case, and kinking due to an elongated styloid process in one case. After re-anastomosis, the MCAP in 7 cases improved, whereas in 1 case, acute graft occlusion after operation was seen.Conclusions: MCA pressure monitoring, combined with other monitoring tools, is effective for assessing graft malfunction during surgery and can help achieve favorable patency of the EC-M2 bypass.