Far-field recording of compound muscle action potentials produced during brachial plexus stimulation

In 15 normal subjects, compound muscle action potentials (CMAPs) produced during electrical stimulation of the brachial plexus were obtained at a distance (far field) from their sources (upper-limb muscles). These far-field CMAPs were recorded at the forehead, neck, and tip of the middle finger and were referred to a relatively inactive region (contralateral forearm or knee). At the proximal recording site (neck or forehead), the far-field CMAPs were generally triphasic in appearance when all of the muscles of the upper limb were activated during brachial plexus stimulation. The initial component was a negative wave; this was followed by a positive component which terminated in a negative potential. At the distal recording site (tip of middle finger), the far-field CMAPs usually had a biphasic appearance, with an initial positivity followed by a period of negativity. The contours of the major components of the far-field CMAPs were frequently interrupted by a series of small amplitude negative and positive peaks or inflections.