Flexion at the elbow will kink the catheter. It is OK to use the AC when you need a big bore catheter for trauma. The AC is also a good choice for patients in supraventricular tachycardia. The t of adenosine is so short, that the distance between the hand and the elbow can actually make a difference. Try to avoid any site near a valve.
Valves appear as knot-like bulges in the vein in some patients.
It may be difficult to advance the catheter through a valve. If the tip of the catheter abuts a valve, it may not flow. Another site is the great saphenous vein, anterior and superior to the medial malleolus. This site is usually avoided because it makes it difficult for the patient to ambulate. It is usually used for infants and other patients with no other suitable veins.