Biomedical Engineering Reference
In-Depth Information
8.4.3 Haemodynamic factors
The endothelium has the capacity to transduce the physical stimuli of
fl owing blood. The mechanical forces experienced by the vessel wall may
be classifi ed into fl uid and solid dynamic forces, represented by wall shear
and tensile stress respectively. Tensile stress forces are a function of blood
pressure and arterial wall radius. WSS has been previously defi ned as the
product of velocity gradient and blood viscosity. The biological response
within the arterial wall attempts to restore the physiological equilibrium
within the vessel, and it is this adaptive property that is believed to play an
important role in the pathophysiology of IH. Endothelium remains quies-
cent when exposed to WSS within physiological parameters (1-2 N/m
2
in
aorta, 2-4 N/m
2
in popliteal artery, 0.15-0.6 N/m
2
in vein) (Ando
et al.
,
1991). Abnormally low or high stress, however, imposes a hostile environ-
ment upon the endothelium, resulting in IH. The resultant reduction in
lumen diameter will serve to increase low WSS, while the deposition of IH
in pathologically high WSS is likely to be a result of the healing process due
to mechanical trauma.
There is substantial evidence for the inverse relationship between WSS
and the development of IH. One of the fi rst studies demonstrating an
inverse relationship was reported by Rittgers
et al.
(1978) who found focal
deposition of IH in ilio-iliac vein grafts in dogs corresponding to areas of
fl ow separation and therefore reduced shear rate. Kraiss
et al.
(1991)
implanted tandem aorto-aortic and bilateral aorto-iliac ePTFE grafts into
baboons and confi rmed the luminal WSS in the proximal graft to be double
that in the distal grafts. They reported that the corresponding intimal thick-
ness after 12 weeks was signifi cantly greater in the distal grafts compared
with the proximal one, affi rming an inverse relationship between WSS and
intimal thickness. Further studies have added veracity to this conclusion
which is now widely accepted and has infl uenced subsequent research
(Kohler
et al.
,
1991; Sho
et al.
,
2002). Intimal thickening is also a feature of
early atherosclerotic lesions and its development was studied by Pedersen
et al.
(1999) using high-resolution magnetic resonance
in vivo
. They also
found that intimal thickness showed a linear decrease with mean and
maximum WSS, but interestingly a linear increase with oscillating shear
index. The oscillating shear index (OSI) is defi ned as:
A
AA
neg
OSI
=
+
neg
pos
where |
A
neg
| and |
A
pos
| are the absolute numerical areas under the WSS
versus time curve. By convention WSS is considered negative when fl ow is
directed towards the heart. The OSI therefore describes the degree of
deviation of WSS from the antegrade fl ow direction. These fi ndings are
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