Far-field potentials

Phrenic nerve conduction study in normal subjects

Phrenic nerve conduction studies were performed in 50 phrenic nerves from 25 normal subjects using a technique modified from previously described methods. The normal ranges for latency, amplitude, negative peak area, and duration were established. The latency correlates with age and the amplitude increases with chest circumference. With our method, the amplitude increases and the duration decreases with lung volume. We found good right-left agreement and reproducibility. Therefore, the unaffected side can be used as a reference in unilateral phrenic nerve lesions and previous studies can be used for comparison in serial studies. We recommend that phrenic nerve conduction studies be used routinely to diagnose and monitor patients with respiratory involvement from neuromuscular diseases.

Far-field potential production by quadrupole generators in cylindrical volume conductors

Far-field potentials have been observed clinically and recognized as such for approximately 30 years. Unfortunately a complete understanding of far-field potential generation is not yet at hand. An attractive model is the representation of an action potential by a quadrupole consisting of a leading and trailing dipole with respect to the direction of propagation. This investigation physically models an action potential by using a quadrupole constant current source and substantiates the concept that an action potential as modeled by two dipoles back-to-back is capable of producing far-field potentials in cylindrical volume conductors. The 4 postulated mechanisms of generating far-field potentials are validated, i.e., an action potential encountering (1) different size volume conductors, (2) the termination of excitable tissue, (3) a change in conducting medium conductivity, and (4) a bend in the nerve. A fifth postulated but previously not demonstrated method of far-field production, neural branching, is shown by the quadrupole model to also be capable of yielding far-field potentials. The termination of a volume conductor is also shown to be capable of generating a voltage difference across the quadrupole. Any of the above 6 conditions create an alteration in the symmetry of the leading and trailing dipole moments resulting in a transient potential difference across the quadrupole as recorded with a far-field recording montage. The potential difference produced by the asymmetric electric field between the leading and trailing dipoles recorded distantly in areas of low potential gradient is the so-called far-field potential. This investigation substantiates the utility of the leading/trailing dipole model of far-field production and offers a simple model of passive voltage distributions secondary to dipolar moment imbalances to better understand the generation of far-field potentials in cylindrical volume conductors.