Complex flow in the proximal internal carotid artery challenges Doppler flow assumptions

Conventional duplex ultrasound is commonly used to measure blood velocity in the internal carotid artery (ICA) for disease screening and cerebral or extracranial flow estimation. Pouseillian-based flow calculations assume laminar flow, which is violated by turbulence and recirculation zones generated by the complex ICA geometry downstream of the carotid bifurcation. This study used vector flow imaging (VFI) to determine how far downstream of the carotid sinus complex flow persists and evaluate guidelines recommending flow measurement 2 cm distal to the bifurcation. Sixteen healthy adults (8 females, 24 ± 3 yrs) completed VFI scans at rest and during dynamic handgrip exercise (DHG) at 20% and 40% maximal voluntary contraction (MVC). Multidirectional flow length (MDFL) was assessed by visual analysis of where flow returned to a laminar pattern. At rest, MDFL was 2.45 ± 0.36 cm distal to the carotid sinus. MDFL did not change from baseline to 20% MVC (p = 0.23) but decreased from 20% to 40% MVC (2.24 ± 0.43 vs. 1.98 ± 0.47 cm; p < 0.01). During 20% MVC, flow complexity increased at all scanning locations, indicating decreased flow coherence with moderate DHG. Conventional Doppler-derived ICA blood flow remained unchanged across conditions, highlighting discrepancies between imaging modalities. These results underscore the need for caution when interpreting Doppler flow measurements in the ICA. We recommend landmarking blood flow locations as distal along the ICA as possible to minimize the effects of non-laminar flow patterns on ICA blood flow estimation.