Volume conduction, waveform analysis, and near- and far-field potentials

Waveform analysis plays an important role in the assessment of nerve and muscle action potentials. A sequence of potential changes arises as two sufficiently close wave fronts, leading and trailing dipoles, travel in the volume conductor from left to right. This results in a positive-negative-positive triphasic wave as depolarization and repolarization approach, reach, and finally pass beyond the point of the recording electrode. Physiologic temporal dispersion can reduce the area of a short-duration sensory potential by phase cancellation. Pathologic temporal dispersion, which can reduce the size of muscle action potential, may conversely increase the size of a sensory response by countering the physiologic phase cancellation. The near-field potential relates to the propagating signal recorded when the impulse passes under the pickup electrodes, whereas the far-field potential implies either a distant nonpropagating signal or a stationary peak generated by a propagating signal when it crosses a volume conductor junction located far from the recording site. The second type of far-field signal, or junctional potential, helps detect a voltage source generated at a distance before the signal reaches the pickup electrodes. A consensus has emerged that the volume entered becomes initially positive compared with the volume departed when the generator approaches the boundary of a volume conductor followed by a negative rebound.