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.