Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 303

were deemed unsuitable. Bypass surgery, employing small diameter, i.e.
less than 6 mm, bypass conduits, is an important surgical technique for
managing cardiovascular disease, including peripheral vascular disease and
myocardial ischaemia. Bypass grafts with the most consistent medium to
long-term patency rates are autologous arteries and veins (Shah et al. ,
2005). In the absence of suitable autogenous bypass grafts, synthetic con-
duits are considered.
Arterial conduits, such as the internal mammary artery or radial artery
are commonly used for coronary artery bypass grafting (CABG) (Desai
and Fremes, 2007; Jorapur et al. , 2009). These arterial conduits are pre-
ferred because they reliably and repeatedly demonstrate satisfactory
medium and long-term patency when compared with venous conduits
which in turn are preferred to synthetic conduits (Shah et al. , 2005). A
drawback to utilising arteries as bypass grafts is the morbidity involved in
their harvest, their limited dimensions, especially in respect to their length,
and their limited availability.
Vein grafts are mainly harvested from the long saphenous vein. These
grafts have a less favourable long-term patency rate than arterial conduits
in CABG procedures but are still preferred over synthetic grafts. Veins are
often used as small diameter bypass grafts in the limbs as they are longer
than any available arterial conduit and have a marginally better medium
and long-term patency rate when compared with synthetic grafts such as
expanded polytetrafl uoroethylene (ePTFE) and Dacron (Klinkert et al. ,
2003; Mamode and Scott, 2000; Sala et al. , 2003). Vein grafts are relatively
straightforward to harvest with less associated potential morbidity com-
pared with the harvesting of arterial conduits. Vein grafts are also less
prone to infection when compared with synthetic conduits (Vinard et al. ,
1991). Vein grafts are, however, delicate structures which may be injured
when they are incorporated into the arterial circulation as bypass conduits
(Davies and Hagen, 1994). Synthetic grafts are still considered a viable
alternative (Klinkert et al. , 2003).
When a bypass procedure is complete, the re-establishment of arterial
circulation abruptly subjects the venous graft to extraordinary magnitudes
of mechanical stretch and shear stress. Venous conduits, usually exposed
to a blood pressure ranging from 3 to 7 mm Hg, are now subjected to a
systolic arterial pressure ranging from 100 to 140 mm Hg after incorpora-
tion into the arterial circulation. Vein grafts are therefore required to
permanently maintain their integrity within the arterial circulation despite
not having been designed to do so. Subjecting a vein graft to magnitudes
of stretch normally reserved for the arterial circulation, induces vessel wall
modifi cation (Fuchs et al. , 1978). Vascular smooth muscle cells (VSMCs),
located within the vein's tunica media, adopt features of a synthetic
phenotype. VSMCs are inherently plastic, apparently never attaining a

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