The anatomy and morphology of the radial artery
Arteries can be classified into three types: a) type 1: ‘somatic’ b)
type 2: ‘splanchnic’ c) type 3: ‘limb arteries’5, 6.
The RA is an example of type 3 artery. As the other arteries, the RA has
three layer of tunica. The tunica intima is a thin layer with a
prominent internal elastic membrane. The tunica media contains myocytes,
connective tissue and elastic fibers. The tunica adventitia is very
prominent and mainly consists of collagen and elastic fibers, fibroblast
and clusters of smooth muscle cells. Notably, there are also adventitial
sympathetic and parasympathetic nerves that might be involved in
arterial spasm.
The RA is also defined as ‘muscular artery’ given the abundant myocytes
in the media layer 7. The more muscular media compared
to the left internal mammary artery (LIMA) is the theoretical background
for the described concerns of RA spasm8. Chester et
al. demonstrated that there are more muscle cells in the proximal RA
vessel compared to the distal RA, as well as variation in the profile of
adrenoceptors along the RA vessel9.
Notably, the RA vasa vasorum do not penetrate into the media and
nutrients and oxygen are provided by luminal diffusion; this may suggest
that its use as free graft should not be subjected to ischemic events
over the long term. However that was challenged by other studies. van
Son and colleagues measured a mean width in the media of the RA
approximately 500 μm, as opposed to 330 μm for that of the LIMA, 280 μm
for that of the gastroepiploic artery and 240 μm for the inferior
epigastric artery10. These authors suggested that the
RA thick media layer can be prone to fibrosis by the time, given the low
oxygen penetration at this level10.
Local factors and systemic hormones can modulate RA vasoconstriction
acting at the level of the receptors in the vascular smooth cells. The
RA endothelium is pivotal in regulating smooth muscle
activity5. Vasodilation in fact can be achieved by the
release of endothelium derived nitric oxide, prostaglandin and
prostacyclin. Endothelin -1, thromboxane A2 and prostaglandin 2a are
among the most potent vasoconstrictors. In terms of systemic hormones,
angiotensin II and arginine vasopressin are both potent vasoconstrictor
even at low concentration acting by opening calcium channels and via the
α1-adrenergic receptor5.
Remarkably, there is also evidence of cross-talk between signaling
pathway that mediate vascular contraction and those that are involved
with muscle cell growth; RA vessel contraction and spasm can be an
important step in the activation of growth-promoting
pathways11.
There are some concerns with regard to pre-existing disease in the
radial artery; analysis from histological specimens from 177 radial
arteries showed increased prevalence of intimal thickening, medial
sclerosis and calcification in the radial artery compared to other
conduits12.
In summary, the functional and morphological anatomy of the RA is very
complex and the way it reacts to internal or external stimuli remains,
also, not fully understood. There are a number of mechanisms that can be
responsible for vasodilation and vasoconstriction; thereby, it is
unlikely that a single agent could completely eliminate RA spasm.