Biomedical Engineering Reference
In-Depth Information
et al.,
2009
). An indispensable tool in the treatment for ARDS is mechanical ven-
tilation. However, heterogeneity of the ARDS lung predisposes patients towards
a number of complications which are collectively termed ventilator associated lung
injuries (VALI) and deemed one of the most important factors in the pathogenesis of
ARDS (Ranieri et al.,
1999
). VALI mainly occurs in the walls of the small lung com-
partments constituting the blood-gas barrier. In these so-called alveoli, both primary
mechanical and secondary inflammatory injuries occur (DiRocco et al.,
2005
). Pri-
mary injuries are consequences of alveolar overexpansion or frequent recruitment
and derecruitment inducing high shear stresses. Since mechanical stimulation of
cells can result in the release of proinflammatory mediators—a phenomenon com-
monly called mechanotransduction—secondary inflammatory injuries often directly
follow, possibly starting a cascade of events leading to sepsis and multi-organ fail-
ure. Understanding the reason why the lungs still become damaged despite recent
developments towards more 'protective' ventilation protocols (Amato et al.,
1998
)
is a key question sought by the medical community.
Computational models of the respiratory system can provide essential insights
into involved phenomena and open up new vistas towards improved patient-specific
ventilation protocols. In particular, computational models offer the possibility to
predict data that cannot be measured in vivo such as local alveolar strains and
stresses which are relevant for the development and progress of VALI. However,
establishing reasonable models is difficult since the lung comprises more than 20
generations of bifurcating airways ending in approximately 500 million alveoli. This
complexity inhibits a direct numerical simulation resolving all levels of the respira-
tory system from the onset. Therefore, we currently pursue two distinct modeling
strategies, which will be reviewed in the following. The first approach is based on
three-dimensional (3D) continuum models of both the airways and the tissue. As
only parts of the lung can be resolved in detail in the model, advanced multi-scale
techniques are utilized to adequately consider the unresolved parts. After having
discussed the 3D approach in Sect.
28.2
, an alternative reduced-dimensional (0D)
lung model will be presented in Sect.
28.3
. This approach allows to effectively study
pressure and flow characteristics in the entire conducting region of the lung, albeit at
the cost of detailed information on local alveolar stresses and strains. In Sect.
28.4
,
the presented models will be briefly summarized and an outlook to future work will
be provided.
28.2 3D Lung Model
To enable the quantification of local stresses and strains during ventilation, we have
established a comprehensive 3D continuum model of the respiratory system. In the
following, the basic building blocks and their combination to one overall lung model
will be surveyed.
