Biomedical Engineering Reference
In-Depth Information
Fig. 7.15
Boundary condi-
tions determination based
on flow rates when the pres-
sures at inlets and outlet are
unknown
7.2.7
Downstream peripheral vascular impedance modelling
During past decades, various haemodynamic studies of carotid bifurcations have
been carried out from the aspects of experimental and numerical modelling (Klein-
streuer et al. 1991; Ku et al. 1985b; Ojha 1993; Steinman and Rutt 1998; Yuan C et
al. 1995). For most of these models, the inlet and outlet flow velocity waveforms
from experimental measurements are able to establish accurate boundary conditions
for a physiologically realistic model. For example, based on configuration of flow
rates as boundary conditions, waveforms were imposed at the inlet artery of the
stenosed model, and also with the fully developed velocity profiles applied at the
CCA inlet and opening boundary conditions at the ICA and ECA outlets. However,
such flow profiles are not readily available. The artery outlet branches are often set
at specific pressures as outflow boundary conditions whenever the outlet volume
flow profiles are not available (Steinman et al. 2000; Zhao et al. 1999). As such,
velocity profiles may be poorly defined, not patient-specific, or lack clinical valida-
tion. The results generated by these configurations may not be accurate enough to
reflect physiological realism, and further assumptions are required to approximate
the modelling as close as possible.
The lack of measured inlet and outlet boundary conditions will jeopardise the ac-
curacy of the modelling significantly, and it is important that sufficient assumptions
such as accurate flow division are established to maintain the integrity of the model-
ling. Configurations of existing studies that utilized specific or measured flow divi-
sion ratios are examined. The carotid bifurcation study by Steinman et al. (2000)
utilised assumptions that involved a flow division of 56:44 between the ICA and
ECA branches. It may be worthwhile noting that according to the statistics provided
by such clinical studies, this ratio is not a good representative of the physiological
condition, and does not come with experimental support all the time. For example,
in a study by Zhao et al. (1999), fully developed flow profiles were set at both the
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