Background and Objectives Unintentional extrasheath injection causes failed axillary brachial plexus block. We wanted to find out if extrasheath injections produce higher plasma concentrations of local anesthetics compared to intrasheath injections. We also studied the incidence of extrasheath injection with radiographs.

Methods Axillary brachial plexus blocks were established using a catheter technique, Fifty milliliters of 1.5% mepivacaine without epinephrine mixed with contrast medium was injected through the catheter. An anteroposterior radiograph was used to determine the distribution of contrast medium. Mepivacaine concentrations in arterial plasma were compared when local anesthetic solution was injected unintentionally outside of the axillary neurovascular sheath (n = 6) and when it was injected correctly into the sheath (n = 6). The incidence of extrasheath injection was studied in a different series of 109 patients.

Results Arterial plasma mepivacaine concentrations were higher after extrasheath injection [8.0 (6.3-9.7) vs 5.8 (4.5-7.0), μg/mL, means (95% confidence intervals), P < .05]. Pharmacokinetic parameters such as mean residence time and total clearance did not differ between intra- and extrasheath injections. Extrasheath injection was observed in 3.7% (4/109) of cases.

Conclusion Failed extrasheath injection of 50 mL 1.5% plain mepivacaine produces higher arterial plasma concentration in axillary brachial plexus block.