How to Measure Mean Flow Velocity Index Post-Thrombectomy
Mean flow velocity index is measured by transcranial color-coded duplex sonography (TCCD) by calculating the ratio of mean blood flow velocity in the recanalized middle cerebral artery (MCA) divided by the mean blood flow velocity in the contralateral MCA. 1
Technical Measurement Protocol
Equipment and Approach
- Use transcranial color-coded duplex sonography (TCCD) to obtain Doppler spectrum measurements of the MCA 2
- Perform measurements at the same insonation depth and angle correction used at baseline to ensure consistency 2
- Obtain mean blood flow (MBF) velocities from both the recanalized MCA and the contralateral MCA 1
Calculation Method
- Calculate the MCA MBF velocity index as: MBF velocity of recanalized MCA ÷ MBF velocity of contralateral MCA 1
- Normal values typically range around 1.0, indicating symmetric flow between hemispheres 1
- Values >1.32 indicate abnormally increased flow velocity and elevated risk for complications 1
Timing of Measurements
Post-Thrombectomy Assessment Window
- Perform TCCD examination within 72 hours after mechanical thrombectomy 3
- Earlier measurements may detect evolving hemodynamic changes before clinical deterioration 4
- Serial measurements are recommended to track changes over time, as the hemodynamic situation can fluctuate during the acute phase and recanalization progress 2
Critical Interpretation Guidelines
Normal vs. Abnormal Flow Patterns
- An MCA MBF velocity index of 1.02 ± 0.32 represents normal post-thrombectomy hemodynamics 1
- An elevated index of 1.32 ± 0.39 or higher indicates hyperperfusion and significantly increased risk for intracranial hemorrhage 1
- Abnormal sonographic flow (Thrombolysis in Brain Ischemia grade 0-4) occurs in approximately 36% of patients despite successful angiographic recanalization 3
Prognostic Implications
- Higher MCA MBF velocity index independently predicts postinterventional intracranial hemorrhage in multivariate analysis 1
- Elevated velocity index is associated with poor 90-day functional outcome 1
- Abnormal middle cerebral artery hemodynamics on TCCD predict poor short-term outcome even when angiographic recanalization appears successful 3
Important Technical Considerations
Prerequisites for Accurate Measurement
- Rule out upstream obstruction of the internal carotid artery before attributing flow changes to intracranial pathology 2
- Ensure the contralateral MCA and supplying carotid arteries can be assessed without relevant stenosis or occlusion before calculating asymmetry indices 2
- Use standardized insonation depths and angle corrections to maintain measurement consistency 2
Common Pitfalls to Avoid
- Do not calculate velocity indices if there is significant contralateral carotid or MCA disease, as this will falsely elevate the index 2
- Avoid comparing measurements obtained at different insonation depths or angles, as this introduces measurement error 2
- Recognize that reactive hyperperfusion after recanalization may cause transiently elevated velocities that do not necessarily indicate stenosis 2
Clinical Utility for Risk Stratification
Identifying High-Risk Patients
- Rising blood flow velocities on serial TCCD examinations may precede reocclusion of the recanalized vessel 4
- TCCD serves as a valuable bedside tool for detecting patients at risk for secondary vascular events after mechanical thrombectomy 4
- Approximately 70% of patients with acute ischemic stroke demonstrate MCA occlusion or stenosis on initial TCD, with 86% showing recanalization by 2 weeks 5