A method to facilitate regional anaesthesia by detection of mixed nerve action potentials

We have studied conduction anaesthesia of the median and ulnar nerves at the axilla using mixed nerve action potentials (MNAP) as a guide to needle position in three volunteers. The median and ulnar nerves were stimulated distally at the wrist using surface electrodes and MNAP were detected proximally in the axilla using an insulated needle electrode. The increase in amplitude of MNAP to maximum as the insulated needle electrode approached the nerve trunk was taken as an indication that the tip of the needle was in close proximity to the nerve trunk. Instillation of 1% lignocaine plain 4 ml decreased the amplitude of the MNAP with the onset of nerve block. We conclude that this method may be of potential use as an aid to peripheral regional anaesthesia by allowing accurate localization of needle position close to specific nerve trunks.