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Research Papers

# Carotid Bifurcation Hemodynamics in Older Adults: Effect of Measured Versus Assumed Flow Waveform

[+] Author and Article Information
Yiemeng Hoi

Department of Mechanical and Industrial Engineering, Biomedical Simulation Laboratory, University queryof Toronto, Toronto, ON, Canada M5S 3G8

Bruce A. Wasserman

Department of Radiology, Johns Hopkins University, Baltimore, MD 21210

Edward G. Lakatta

Laboratory of Cardiovascular Science, Intramural Research Program, National Institute on Aging, NIH, Baltimore, MD 21224

David A. Steinman1

Department of Mechanical and Industrial Engineering, Biomedical Simulation Laboratory, University of Toronto, Toronto, ON, Canada M5S 3G8steinman@mie.utoronto.ca

1

Corresponding author.

J Biomech Eng 132(7), 071006 (May 26, 2010) (6 pages) doi:10.1115/1.4001265 History: Received October 26, 2009; Revised February 05, 2010; Posted February 15, 2010; Published May 26, 2010; Online May 26, 2010

## Abstract

Recent work has illuminated differences in carotid artery blood flow rate dynamics of older versus young adults. To what degree flow waveform shape, and indeed the use of measured versus assumed flow rates, affects the simulated hemodynamics of older adult carotid bifurcations has not been elucidated. Image-based computational fluid dynamics models of $N=9$ normal, older adult carotid bifurcations were reconstructed from magnetic resonance angiography. Subject-specific hemodynamics were computed by imposing each individual’s inlet and outlet flow rates measured by cine phase-contrast magnetic resonance imaging or by imposing characteristic young and older adult flow waveform shapes adjusted to cycle-averaged flow rates measured or allometrically scaled to the inlet and outlet areas. Despite appreciable differences in the measured versus assumed flow conditions, the locations and extents of low wall shear stress and elevated relative residence time were broadly consistent; however, the extent of elevated oscillatory shear index was substantially underestimated, more by the use of assumed cycle-averaged flow rates than the assumed flow waveform shape. For studies of individual vessels, use of a characteristic flow waveform shape is likely sufficient, with some benefit offered by scaling to measured cycle-averaged flow rates. For larger-scale studies of many vessels, ranking of cases according to presumed hemodynamic or geometric risk is robust to the assumed flow conditions.

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## Figures

Figure 1

Measured flow waveforms at the CCA for all nine subjects (dotted lines) compared with characteristic young adult (green/light solid line) and older adult (blue/dark solid line) waveforms. One potential outlier (subject 4), exhibiting a highly damped waveform, is shown as a magenta/dashed line.

Figure 2

Color-coded distributions of OSI, for all nine subjects, for the various waveform boundary conditions indicated at the top row. Black contour lines on each surface identify the area exposed to OSI above its 80th percentile value, as described in Sec. 2. Geometries are clipped at the CCA5, ICA10, and ECA5 locations used to scale the flow rates. CCA3, ICA5, and ECA2 locations, used to bind the surface for disturbed flow calculations, are shown in dark gray.

Figure 3

SA exposed to various disturbed flow indicators (WSS, RRT, and OSI), derived from assumed-waveform models and compared against the corresponding SA from the subject-specific-waveform models. Colored lines are linear regressions through the respectively colored data points. The black dotted line represents the line of unity.

Figure 4

SA exposed to various disturbed flow indicators (WSS, RRT, and OSI), as a function of the waveform boundary condition used. The dashed lines separate SA (vertical axis) into lower, middle, and upper tertiles according to the distributions of exposure levels found by Lee (8). Symbols denote the subject number, and green, yellow, and red are used to highlight Truth cases falling into the lower, middle, and upper tertiles, respectively.

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