The quintessential characteristic of the forearm that makes transradial catheterization so safe and forgiving is the presence of abundant and redundant vascular supply. The robust vascular supply of the hand has impressed generations of physicians and has provided a readily available example of collateral circulation to demonstrate to healthcare and physiology students. When modern transradial catheterization was first developed, evidence of these collaterals was used to triage access decisions. It is now evident that collateral testing does not predict ischemic outcomes, but nevertheless understanding the collateral dynamics can facilitate management of the access site after the procedure and provide evidence for radial artery occlusion.
The circulation to the hand is often displayed schematically as a dual- arched palmar system fed by dual blood sources from the radial and ulnar arteries (Figure 1). While this simplistic design is adequate to explain basic physiology, it is not reflective of the actual anatomy and dynamic changes that can occur under ischemic stress. Absent in many diagrams are the interosseous arteries (anterior and posterior) and/or median artery that exists in many arms as a source of blood to the hand in addition to a multitude of variable and unnamed collateral connections that can be seen in high-resolution angiograms or post-mortem casts (Figure 2). In addition, intra-arterial channels are recruitable under ischemic stress and simple tests of collateral flow may not account for these. This robust protection from ischemic injury to the hand is a highly conserved characteristic of mammalian evolution that protects from loss of function to this most valuable part of the upper limb, just as it protects man from ischemic injury from radial catheterization.