An ultrasound imaging and computational fluid dynamics protocol to assess hemodynamics in iliac vein compression syndrome

OBJECTIVE: Elevated shear rates are known to play a role in arterial thrombosis; however, shear rates have not been thoroughly investigated in patients with iliac vein compression syndrome (IVCS) owing to imaging limitations and assumptions on the low shear nature of venous flows. This study was undertaken to develop a standardized protocol that quantifies IVCS shear rates and can aid in the diagnosis and treatment of patients with moderate yet symptomatic compression. METHODS: Study patients with and without IVCS had their iliac vein hemodynamics measured via duplex ultrasound (US) at two of the following three vessel locations: infrarenal inferior vena cava (IVC), right common iliac vein, and left common iliac vein, in addition to acquiring data at the right and left external iliac veins. US velocity spectra were multiplied by a weighted cross-sectional area calculated from US and computed tomography (CT) data to create flow waveforms. Flow waveforms were then scaled to enforce conservation of flow across the IVC and common iliac veins. A three-dimensional (3D), patient-specific model of the iliac vein anatomy was constructed from CT and US examination. Flow waveforms and the 3D model were used as a basis to run a computational fluid dynamics (CFD) simulation. Owing to collateral vessel flow and discrepancies between CT and US area measurements, flows in internal iliac veins and cross-sectional areas of the common iliac veins were calibrated iteratively against target common iliac flow. Simulation results on mean velocity were validated against US data at measurement locations. Simulation results were postprocessed to derive spatial and temporal values of quantities such as velocity and shear rate. RESULTS: Using our modeling protocol, we were able to build CFD models of the iliac veins that matched common iliac flow splits within 2% and measured US velocities within 10%. Proof-of-concept analyses (1 subject, 1 control) have revealed that patients with IVCS may experience elevated shear rates in the compressed left common iliac vein, more typical of the arterial rather than the venous circulation. These results encourage us to extend this protocol to a larger group of patients with IVCS and controls. CONCLUSIONS: We developed a protocol that obtains hemodynamic measurements of the IVC and iliac veins from US, creates patient-specific 3D reconstructions of the venous anatomy using CT and US examinations, and computes shear rates using calibrated CFD methods. Proof-of-concept results have indicated that patients with IVCS may experience elevated shear rates in the compressed left common iliac vein. Larger cohorts are needed to assess the relationship between venous compression and shear rates in patients with IVCS as compared with controls with noncompressed iliac veins. Further studies using this protocol may also give promising insights into whether or not to treat patients with moderate, yet symptomatic compression.