Margin of safety for needle puncture of a radial artery.

The radial artery is the preferred site for arterial cannulation because of consistent anatomic accessibility, ease of cannulation, and a low incidence of complications. One possible risk associated with cannulation is an injury to the superficial branch of the radial nerve. The radial nerve divides to the superficial branch and the deep branch at the top of the forearm, and the superficial branch lies behind the brachioradialis muscle close to the lateral side of the radial artery and terminates the lateral side of the radial artery about 7 cm proximal to the wrist. The segment where the superficial branch of the radial nerve runs side-by-side the radial artery ("run side-by-side segment) is an increased risk area of nerve injury. We studied 100 volunteers to measure a "safe" segment (the length between the styloid process and the distal edge of the "run side-by-side segment), using an ultrasonograph. The 95%reference interval (which indicates the central 95% of a reference population) of the "safe" segment was 6.8-11.6 cm [with the mean 9.4 (standard variation: 1.7)] in men, and 5.4-11.0 [8.2 (1.4)] cm in women. In conclusion, we have shown that the margin of safety for needle puncture of a radial artery is about 6.8 cm from the wrist for in men, and 5.4 cm from the wrist for in women.