far-field potentials

INVITED REVIEW Far-field potentials are produced by neural generators located at a distance from the recording electrodes. These potentials were initially characterized incorrectly as being of positive polarity, widespread distribution, and constant latency; however, recent advances have clearly demonstrated that far-field potentials may be either positive or negative depending upon the location of the electrodes with respect to the orientation of the dipole generator. Additionally, peak latencies in the far-field can vary with alterations in body position and the spatial distribution of far-field potentials, while widespread, is not uniform. Recent studies of far-field potentials suggest how such waveforms are produced when the symmetry of an action potential, as recorded by distant electrodes, is broken by such factors as differing conductivities of volume conductor compartments, direction of action potential propagation, size differentials in adjoining body segments, or the termination of action potential propagation in excitable tissue. Human, animal, and computer experiments support the preceding generalizations. These new explanations are directly applicable to such far-field potentials as the short latency somatosensory-evoked potential. Furthermore, since far- field potentials can also occur in muscle tissue, one should expect that these generalizations will hold with respect to electromyographic potentials. 0 1993 John Wiley & Sons, Inc. Key words: far-field potentials action potentials stationary potentials volume conduction

MUSCLE 81 NERVE 16~237-254 1993

FAR-FIELD POTENTIALS

DANIEL DUMITRU, MD, and DON L. JEWETT, MD, DPhil

Peripheral nerve activation combined with analysis of the ensuing cortical and subcortical responses is an important technique to assess the functional status of both the peripheral and central nervous systems. Depending upon the recording electrode montage, it is possible to detect waveforms originating from subcortical generator sites that display a fixed latency, irrespective of the recording electrode location. These waveforms have been referred to as far-field potentials. As knowledge regarding far-field potentials increases, it is becoming clear that far-field potentials can be generated when there are transient potential im- balances at far-field recording electrodes from the two di olar components of an action potential.”‘-’ The new insights regarding far-field the-

From the University of Texas, Health Science Center at San Antonio, San Antonio, Texas (Dr Dumitru), and University of California at San Fran- cisco, and Abratech Corporation, Mill Valey, California (Dr. Jewett).

Address reprint requests to Daniel Dumitru, MD, University of Texas, Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Anto- nio, TX 78284-7798

Acknowledgments Supported in part by Grant DC000328 from the Na- tional Institute for Deafness and Communicative Disorders

Accepted for publication July 15, 1992

0 1993 John Wiley & Sons, Inc CCC 0148-639X/93/030237-18

ory suggests innovative interpretations of both peripheral and cortical-evoked far-field waveforms.

TERMINOLOGY AND ELECTROPHYSIOLOGIC RECORDING PRINCIPLES

We may define a uolume conductor as any medium with the capability of sustaining passive current flows on divergent and convergent paths throughout a volume, as distinguished from parallel current lines in a wire.”2,58,62 A current dipole consists of a current source (positive pole when current is considered as positive) and a current sink (negative pole) between which there is current flow in the volume conductor.35 The dipole moment for a dipolar current source is computed as the product of the dipole separation and the dipole current. Current density (number of current lines per unit area within the volume) is greater near the generator’s source and sink than further away. A linear quadrupole (action potential on a straight axon) may be thought of as two dipoles of equal dipole moment back-to-front, i.e., source- sink, sink- source (+ -, - +).Y1,54 The two sinks may, under some circumstances, be considered as a single centrally located sink; this resulting source- sink- source configuration has been called a tripole. l2

Physically constructed dipoles and tripoles have

Far-Field Potentials MUSCLE & NERVE March 1993 237

been used to generate current flows in volume conductors, thus mimicking waveforms generated by action otentials from isolated nerves in similar volumes.

'The designations near-field and fur-jield were first introduced to distinguish between two aspects of an action potential's current distribution in a volume conduction.""."' A spatial pudient of potential associated with a current generator is the rate of change o f potential as a function of distance. This gradient is high when differences in potential are large between adjacent points and low when the differences in potential are small. Neur- field potentids are the waveforms recorded close to a current generator source where the spatial gradient associated with an action potential's current flow is high. Far-fidd polentials, in contrast, are waveforms detected when both electrodes are at such a distance from a current generator that the spatial gradient is low.

'1'0 illustrate these concepts, Figure 1 shows the current lines (lines connecting the positive and negative poles) that occur around a dipole located in a volume conductor where the current flows from the positive to the negative pole. The current tlensitv is greatest near the poles of the dipole, and &minishes with distance fkom the dipole. 'Ihe iwpotmtial lines are at right angles to the current lines and define lines of a constant potential extending away from the current source. When two recording electrodes are on different isopotential lines, a potential difference will be re-

,L!4

+ - FIGURE 1. The distribution of current lines about a dipole directed from the positive to negative poles. Isopotential lines ex- tend perpendicularly away from the current lines. Five recording electrodes are located at various locations within the current distribution of the dipole. Electrodes A and B are in a region where the current's spatial gradient is high. Electrodes C and D are in a lower spatial gradient compared with A and B, but in a higher spatial gradient than electrode E.

corded. Additionally, any two points at different locations on the same current path must be at a different potential. Note that the "potential" at a single point is a mathematical abstraction which is useful for didactic purposes. However, in reality, only potential diffprences can be measured. Thus, in a measurement, a value can be ascribed to only one of- the electrode locations if and only if the other electrode is located such that the potential at that point is vanishingly small (the so-called "reference" point). In practice, for far-field potentials, there is no such point, i.e., no location has an in- significant potential.

In Figure 1, two electrodes, C arid D, are adjacent to each other, in a bipolar rpcording configuration'7.'Y.42.4." and positioned relatively far away from the dipole. Bipolar montages do not typically record far-field potentials because the differential amplifier records the difference between the two inputs. If the electrodes are far from the dipole, then the detected signal (the difference between the two electrodes) will be small. That is, where the spatial gradient of the dipole current distribution is low, there is little difference in potential to detect with a small interelectrode separation. For the purposes of discussion, let us assume that the small potential difference of C minus D is lost in the noise of the recording and an averager is required to improve the signal-to-noise ratio in order to observe this potential difference.16 However, it is important to note that a small potential does not guarantee that the electrodes are located at a distance from the dipole, because if the electrodes are close to the dipole but located on the same isopotential line, then the recorded potential will be zero.

At any given instant, electrodes C and D mea- sure slightly different potentials (relative to a third point, e.g., E) at some distance from the dipole because they are on different isopotential lines. The result is that the differential recording between C and D (C - D) detects a minimal signal with aver-

However, a bipolar electrode montage with the same interelectrode separation as that between C and D placed closer to the dipole (A and B in Fig. l ) , can readily record a significant potential. 'The potential difference of A - B is greater than that between C - D, because the spatial gradient of the dipole current is higher near the dipole. As a result, the potential changes significantly over relatively small distances and a comparatively larger difference in potential exists between these two electrodes (provided they are not located on the same isopotential line). Bipolar

aging. 13,15,28,34,35

238 Far-Field Potentials MUSCLE & NERVE March 1993

recordings are, therefore, useful to document the presence of neural or muscular activity in the region of the generator (the near-field).

If one of the near-field electrodes (A or B, Fig. 1) is used with electrode E, a referential record- ing42343 results, assuming that E is located in a region in which the potential is of a very small magnitude (for the purposes of recording). In this instance, a near-field recording of the dipole is obtained, but A or B is referenced to an electrode in a region where the potential from the dipole is low. A larger potential difference exists between A or B referenced to E than when the bipolar recording is made between either A and B, or C and D, because a less common mode signal is present.

One may then consider a referential montage with respect to far-field recordings. For example, electrode C or D can be utilized with electrode E in a far-field recording montage (Fig. 1). If the two recording electrodes, e.g., C and E, are not on exactly the same isopotential line and an averaging computer is used to improve the signal-to- noise ratio and detect the small potential difference in the far-field between these electrode pairs, a far-field waveform may be observed. Note, however, that in contrast to the near-field referential montage, in the far-field, both electrodes record some small signal relative to each other which can contribute to the differentially recorded potential. In other words, there is no reference electrode (as the term is commonly utilized) in far-field recordings. Because pure referential recordings with respect to far-field potentials are not possible, the terms active and reference are inadequate to cor- rectly describe the recording configurations for far-field waveforms. Specifically, the error is to consider that the potentials recorded occur only under the active electrode; in far-field recordings it is not possible to tell from a single channel recording where the dipole is located. For the purposes of this discussion, the terms E-l and E-2 will be utilized to describe the recording electrodes, where the differential amplifier output is proportional to E-1 minus E-2 (in near-field recording conditions called the "active" minus the "reference," re~pectively).~~ The output of the amplifier may either be displayed so that if the difference between the two inputs is negative, then the waveform moves either upward (negative up) or down- ward (positive up). The display method used is merely a convention; in this discussion we will conform to the negative-up convention.

Far-field potentials have been historically characterized by (1) potentials distributed over wide

recording areas; (2) equal latencies at all recordin points; and (3) positive p ~ l a r i t y . ' " ~ ~ . ~ ~ ~ " ~ ~ ~ , ~ ~ , ~ ~ These characterizations will be discussed as well as how far-field concepts should be revised to in- clude newer results and insights.

THE LEADING/TRAILING DIPOLE MODEL

The traditional solid angle geometry explanation of a waveform's near-field morphologic characteristics in a volume conductor may have limitations in attempting to fully elucidate a number of far- field observations.6,62 Although solid angle geometry has been the primary model for workers in far-field potentials, it is clearly inadequate because clinical data, animal experimentation, and computer stimulations demonstrate that not all far- field potentials resulting from advancing wave- fronts are positive, and amplitude diminishes minimally over distance in some The solid angle volume conductor theory works reasonable well for near-field waveform observations within a volume conductor, but a more com- prehensive theory is needed to better explain far- field potentials. The leadinghrailing (LIT) dipole model is the latest attempt to provide a simple explanation as to how far-field potentials arise in volume c~riductors."~"".~~'"O A dipole (+-) is a mathematical abstraction in which a current passes from a positive point-source to a negative point- source in a volume conducting medium (Fig. 1). For the purposes of modeling an action potential, the dipoles should be considered as a constant current source as opposed to a constant voltage generator. This is justified because the high membrane resistant to ionic currents results in little change in the current flow when the impedance that the current is driven into is changed, i.e., the characteristic of a constant-current source.54

When trying to understand the characteristics of dipole currents, it is first easiest to imagine a centrally placed dipole in a spherical homogeneous volume conductor. The potentials detectable on the surface of the sphere are dependent upon the (1) angular relationship between the dipole's axis and the point of recording; (2) magnitude of the dipolar current; (3) spacing between the positive and negative pole; (4) volume conductor's conductivity; and (5) size of the ~ p h e r e . ' ~ ~ ~ ~ , ' ~ In a sphere with a particular dipolar orientation (Fig. 2), the observed potential is most positive when the positive and negative aspects of the dipole are oriented toward the surface E-1 and E-2 electrodes, respectively. Rotating the surface electrodes 90 degrees, but maintaining the same inter-


Volume Conductor

A (j

E-2

C () E- 1

E ($ E-2

Diff Amp (E-1) - (E-2)

CRT Trace

-2

0 + v - (-v) I +2v

* a * - 0 90 180 270 380

-2

ov - ov I ov

-v - (+v) = -1v

ov - ov = ov

+ v - (-v) = +2v

+ 2 J - 0 W 180 270 380

* a -1 f - , - 0 W 180 270 380

- + 2

a w 180 270 360

0 80 100 270 380

FIGURE 2. A dipole placed in the center of a spherical volume conductor with recording electrodes E-1 and E-2 on the sphere’s surface. In (A), a maximum positive potential is recorded. As the electrodes rotate in the clockwise direction through 360 degrees (B-E), a waveform with a cosine function is described. Note that the waveform crosses the zero line when the electrodes are perpendicular to the dipole and no potential is detected. The differential amplifier (Diff Amp) portion of the figure denotes the potentials as they are observed by the recording electrodes and subtracts E-2 from E-1 . The CRT trace describes the potential recorded. An identical CRT trace would be obtained if the electrode locations had remained as in (A), but the dipole had rotated counterclockwise.

electrode separation results in no net potential difference between the electrodes so that a zero potential is recorded. A rotation of 180 degrees produces the opposite orientation of- the electrodes starting position. An additional 90 degrees (270 degrees) again produces no net potential. A complete rotation of 360 degrees results in a surface potential waveform with characteristics described by 21 cosine function of the angle between the dipole arid recording electrode (Fig. 2A- E).“”.“5 If the radius of the sphere were enlarged, the magnitude of the recorded surface potentials would decline.34

In a straight axon, the action potential is not a dipolar point-source but rather a constant-current distributed-source, spread over a finite neural segment. As a result of activated voltage-gated Na+ channels, tnoriophasic positive intracellular action potentials occur following a suprathreshold depo- larizirig pulse. For the purposes of this discussion, we will ignore the terminal hyperpolarization phase of the intracellular action potential, as it does not directly contribute to the current flows and waveforms described. A “freeze frame” depic- tion of this event is shown (Fig. 3A).2j3333337,53 This monophasic action potential can be considered


OutsMe

inside c Membrane

: :

A

Direction of Propagation

FIGURE 3. (A) lntracellular sodium and potassium conductance (gNa and gK) associated with an action potential traveling from right to left. The intracellular action potential is monophasic and positive. (B) lntraaxonal “forward and backward” longitudinally directed intracellular current flows. (C) Triphasic extracellular waveform associated with an action potential’s near-field as described by the transmembrane current. (Modified with permis- sion from Noble53 and Jewett DL, Rayner MD: Basic Concepts of Neuronal Function. Boston, Little Brown, 1984, p 156.)

distributed spatially along the axon’s longitudinal extent. The first derivative of this action potential’s spatial voltage distribution (change in voltage with respect to distance) yields the axon’s intraax- ial current flow in both directions, i.e., forward and backward local circuit currents (Fig. 3B and C ) . The peak of the action potential delineates zero forward or backward flow as there is no change in voltage and the derivative of no change is zero. Therefore, in our example of an action potential traveling from right to left (Fig. 3A), intraaxonal current flow to the left of this zero point is traveling ahead of the action potential (forward current flow), while that to the right is proceeding intraaxonally in the direction opposite action potential propagation (backward current flow ) (Fig. 3C). The forward current flow is equivalent to the

leading dipole, while the oppositely directed current flow is the trailing dipole. The integrated area under the curve representing forward current flow is equal to the area representing backward current flow and the summation of these two areas equate to the intracellular action potential, thereby supporting the concept of the equivalent dipoles, i.e., the q ~ a d r u p o l e . ~ ~ The action potential may also be characterized as a central negative sink region flanked by two positive (leading and trailing) current source regions, the traditional source-sink-source or tripole (+ - +) configuration. The second derivative of the action potential yields the triphasic extracellular potential expected in volunie conductors from propagating action potentials (Fig. 3D).’”.”4 In the far-field, the above noted two spatially distributed directional currents can be simplified to two oppositely oriented point source dipoles (+-; -+); i.e., in the far-field this siniplification provides sufficient de- tail to explain observed potentials. The action potential’s constant current generator may then be conceptualized as two dipoles. The leading dipole (LD: +-) and trailing dipole (TD: -+) are then in a backifront configuration (+--+) similar to a linear q~adrupo le . . ”~

’The leadingltrailing (LIT) dipole model has a number of pertinent aspects. ( 1 ) A far-field potential will be produced when the dipole moments of the L/’T dipoles are not equal. This inequality of dipole moments may result when various properties of the volume conductor or the axon itself (curved pathways or termination) alter the symmetry or balance between the L/T dipole mo- mentsY2 (2) In the L/T dipole model, the far-field potential is proportional to both the dipole current and the dipole separation, the product of which represents the dipole moment.“) In support of this obsrevation, the far-field potential was shown in humans to be proportional to the magnitude of the action potential in the volume conduc-

(3) An action potential has a fixed duration. As a result, the dipole’s longitudinal extent along the surface of the nerve can be calculated by the relationship NCV (nerve conduction velocity) = d(spatia1 distance)/t(action potential duration). A straight neural segment of sufficient length to en- compass both the leading and trailing dipoles produces the expected triphasic waveform with a near-field recording montage.46 A far-field montage, however, records no potential from a straight axon during any instants when the full action potential is presented along the axon.’? This is because the L/T dipole moments are equal and


opposite, with the result that any potentials generated by one dipole are canceled by the other dipole, which is of equal magnitude, but oppositely directed. (4) A number of volume conductor segments in the hurnan (extremities and digits) approach the geometry of a cylinder possessing unique properties compared with a sphere with respect to whether the potentials within the volume conductor decline with distance. That is, a far-field potential's magnitude declines inversely, though not cqually. as the recording location ex- pands radially in a cylinder or sphere, respectively, from the generating source, assuming a centrally located generator. However, along the cylinder's loiigitudinal direction, about 1.9 times the cylinder's radius the entire cylinder becomes equipoteritial with a magnitude that declines niini- mally if at all throughout the volume's remaining

Additionally, this potential appears extent," ,2?,6(t

essentially instantaneously along the cylinder's en- fhis observation may be better

understood i f one considers familiar analogies. An example is a small glass tube filled with a concluct- ing solution, i.e.. a micropipette or wick electrode. Placing this r-ecorcling device in a muscle or nerve cell will record the intracellular potential without a change in magnitude or latency irrespective of the length, provided the input impedance of the amplifier is significantly higher than that of the mi- cropipet te, especially as the length (impedance) increases. Similarly, the ECG is the same whether it is recorded at the wrist or elbo dipole is confined within a cylindrical volume conductor segment of' relatively small size compared with an adjoining larger one, little i f any current flows into the larger cyliiider.'2."5 Said in another way, a cylinder such as an insulated wire can transmit a potential from one point to another even though no significant current is moving. We can now apply the L/'I dipole model to specific examples developed to simulate far-field potentials observed in clinically relevanc experiments.

tire length, 1":3:3.60

Volumetric ChangeslCut Nerve Ends. An afferent sensory volley beginning in a digit passes first through a larger and flatter hand volume, then into a cylindrical and expanding forearm and arm, progressing into the torso, and the volume being constricted again at the neck. 'The action potential must then travel through the narrow foramen magnum and finally enter the sphe- roidal cranium. This pathway contains a number of transition zones separating various volume

conductors: metacarpoplialangeal joint, wrist, el- bow, axilla, spinal column entry, foramen magnum, etc. Any of these boundary zones may produce an imbalance in the action potential's LIT dipoles.

To experimentally simulate the noted volume conductor changes, the sciatic nerve of a frog can be suspended in Ringer's solution and placed in a small cylindrical container (syringe) closed at one end with an appreciable portion of the nerve extending beyond the opening. 'I'his small cylinder can then be placed in a much larger hemicylindri- cal tub filled with Ringer's solution. An action potential may be produced within the nerve located in the smaller container which propagates f'roni

bounctary zone will exist at the transition between the t w o size volumes at the opening of the syringe. T w o recording electrodes are l o c atecl in the tub such that E-1 is placed axially beyond the nerve's termination and E-2 is situated 0 1 1 the wall of the larger container at the region where the nerve exits the syringe. In this montage, neither electrode is located on the nerve o r close enough to detect its near-field and constitutes a far-field recording. When the nerve is excited, two far-field waveforms are observed. The first is a monophasic positive potential that coincides with the emergence of the action potential from the syringe into the tub. A second monophasic waveform with a ncga- tive polarity is observed to occur simultaneously with the arrival of the action potential at the end of the nerve. T w o far-field potentials are generated, one at the transition between volume sizes and the other at the nerve's termination.

Careful consideration of the dipole model gives a reasonable conceptualization of what is occurring when the action potentila encounters a change in the traversed compartmental volumes (Fig. 4A-C) or arrives at the end of a nerve (Fig. 4D and E). Consider a neural impulse proceeding from a small region to a relatively larger one. Ini- tially, the L/T dipoles are completely confined within the small container and the current associated with the dipoles does not enter the larger compartment. 'The two recording electrodes do not detect any electrical activity and the cathode ray tube (CRT) trace remains flat (Fig. 4A). As the leading dipole begins to exit the same cylinder and enter the larger volume, an imbalance in dipole moments is created between the leading and trailing dipoles. In effect, a potential difference exists at the boundary between the two compartments. In the given example, the two electrodes

the snialler into the larger container. "x' A


record a different voltage from the emerging dipole (LD), and differential amplification produces an electronic subtraction. The reason a different potential is recorded by E-1 and E-2 is because of the way in which the voltage decreases radially compared to axially at the boundary zone within a cylindrical volume conductor. Specifically, the recorded voltage is less at the cylinder's inner wall at the region of the dipole imbalance compared to greater than 1.9 cylindrical radii along its longitudinal extent.21,2296" An initial positive deflection of the CRT trace is observed coincident with the emergence of the leading dipole. The peak of the potential develops when the leading dipole is completely out of the smaller cylinder denoting the maximum potential associated with the greatest imbalance of dipole moments.

The manner in which differential amplification

Volume Conductor

A

B

C

D

E

E-2 E-1

E-2

0 E-2 E-1

E-2

I E-2

processes these individual dipole far-field potentials may be observed in Figure 4. Recall that, at some distance from the boundary zone proportional to the radius, the detected potential is larger than that recorded radially at the volume conductor's edge directly aligned with the boundary zone6" (Fig. 4B). A negative potential is recorded in Figure 4B by the E-2 electrode because it is aligned with the current sink at the moment pictured. Just previous to this time period, it would have recorded a lesser potential because the dipole moment in the larger volume would be less (the length of the dipole in the volume being smaller). Contrary to what one may anticipate, a monopole is a theoretical abstraction and cannot be generated by a propagating action potential. As the action potential just begins to enter the larger container, a small positive monopole is not de-

Diff Amp (E-1) - (E-2)

LD 0 - 0 TD + o - 0

0 - 0 = 0

LD 4 - (-1)

TD + o - 0

4 - (-1) = 5

LD 0 - 0 TD + O - 0

0 - 0 = 0

LD 0 - 0 TD + ( - 4 - 4}

-4 - 4 = -8

LD 0 - 0 TD + o - 0

0 - 0 = 0

CRT Trace

-J FIGURE 4. An action potential represented by a leadinghrailing dipole is shown propagating along a nerve suspended in a volume conductor with two different cylindrical volumes. The E-1 electrode is placed axially beyond the nerve and E-2 is located on the inner surface of the large container at the junction between the two volumes. The differential amplifier (Diff Amp) denotes what the electrodes record with respect to the leading and trailing dipoles. The data obtained from the E-2 electrode for the leading (LD) and trailing dipoles (TD) is summated and then subtracted from the summated data of E-1 for both dipoles. The electrical result of this process is then displayed by the CRT trace. The leading and trailing dipoles are assigned arbitrary values of magnitude as recorded by the electrodes in the far-field. Recall that the radially displaced E-2 detects a smaller potential than the axially located E-1 . A monophasic positive and negative potential are produced at the volumetric transition zone and nerve's termination, respectively.

Far-Field Potentials MUSCLE 8, NERVE March 1993 243

tected. A dipole representing a portion of the leading edge of the action potential with an associated small dipole moment, however, is the potential recorded by the electrodes. As the trailing dipole emerges from the small container, a balance in dipole moments begins to be reestablished with a resultant decline in the far-field potential’s magnitude. The amplifier records a decline in the difference between the IJT dipoles and the CRT trace returns to baseline, The end result of an action potential crossing the boundary between the two cylindrical volumes is a monophasic positive far-field potential. ’The amplitude of the far-field potential generated is dependent upon the location of the two electrodes. If the E- l electrode is maintained axially with the nerve but the E-2 electrode is displaced further from the bouritiary zone, the far-field potential magnitude should decline. For exarnple, locating the E-2 electrode more than 1.9 rxlii2’~22~‘i‘’ from the boundary zone eliminates the far-field potential as the cylinder is equipotential at this point and E-1 arid E-2 record the same information resulting in no net potential difference.

As the action potential reaches the end of the nerve, there is i i o longer neural tissue to sustain the forward moving intracellular currents of the leading dipole. A decline in the leading dipole now creates an imbalance in the dipole moments with an accompanying far-field potential (Fig. 4D and E). In this instance, the trailing and not leading dipole is dominant, and the far-field potential reflects the trailing dipole’s leading portion (negative) with the C R T trace displaying a negative waveform. The negative potential’s amplitude is maximum when only the trailing dipole is present. This is because the leading dipole is absent and, therefore, incapable of balancing any aspect of the trailing dipole, i.e., maximum dipolar asymmetry. When the trailing dipole encounters the nerve’s termination, i t too declines with a return of the CRT trace to baseline. A monophasic negative far- field potential results as the action potential encounters the end of the nerve. Cutting the nerve so as to shorten the conducting distance in the larger cylinder nioves the negative potential closer to the positive waveform. When the nerve is short enough, a tiphasic positivehegative potential is observed. 3‘his may be one explanation of how clinically observed biphasic far-field potentials may arise; i.e., axcms extending a short distance beyond an anatomic size

By reversing the direction of action potential propagation in the above experiment, the poten-

tials’ polarity remains unchanged provided the same recording electrode configuration is maintained. Initiating depolarization at the end of the nerve in the large cylinder produces a negative nionophasic potential from the single dipole, now LD, as the action potential starts to propagate. Similarly, a monophasic positive waveform is again produced at the size boundary as only the T D is now in the large volume. Interchanging the E-1 and E-2 electrode connections to the amplifier, however, does result in detecting a potential of the opposite sign, because the amplifier sub- stracts E-2 from E-1. ‘The polarity of far-field potentials is, therefore, not dependent upon the direction of neural propagation, but on the predominant dipole, the position of the two recording electrodes relative to that dipole, and the amplifier connections. These generalizations are supported clinically by noting essentially the same far-field potentials but of opposite polarity with different recording niontages.20,~‘ ,~t’ ,~~ This observation dispels the idea that action potentials ap- proaching the E-1 recording electrode always produces a positive far-field potential.

A crushed nerve end behaves quite differently than a cut nerve end with respect to far-field potential generation. The crushed nerve end is a familiar nerve preparation and was initially used to determine the intracellular action potential prior to the development of intracellular microelec- trades.* When a forceps or some other instrument is used to crush the terination of a nerve, an E-1 electrode located directly on the crushed portion of the nerve records a negative potential which approximates the intracellular resting potential when E-2 is placed on a healthy aspect of the nerve. This is the so-called demarcation potential or “killed end” potential and appears because this localized portion of membrane is disrupted and allows one to essentially record the intracellular potential, i.e., an extracellularly directed negative current flow is established.8” Should an action potential propagate toward the crushed region, only the forward directed intracellular positive source current is capable of entering this aspect of the nerve as this is now primarily a passive current which originated at the voltage-dependent sodium-gated negative sink. This positive current exits through the disrupted membrane coniplet- ing the local circuit current and produces a positive potential at E-1 (E-2 placed off the nerve), i.e., a positive near-field potential. ?’he negative-current sink, however, cannot enter the crushed region of nerve because the voltage-dependent so-


dium gates are nonfunctional. As a result, the action potential propagates up to but not into the crushed segment of nerve. Because the negative sink dissipates upon reaching the crushed membrane, the associated local circuit currents, forward and backward, also cease. The previously described positive deflection returns to baseline producing a positive near-field “killed end” potential. If a far-field instead of near-field electrode montage is utilized, e.g., E-1 and E-2 some distance from the nerve, a far-field potential is not detected.12 This is because the dipole moments are always balanced as the negative sink disap- pears when encountering the crushed nerve segment with a concomitant and synchronous dissipa- tion of the leading and trailing source currents. A crushed nerve preparation, therefore, generates a positive near-field “killed end” potential but not a far-field potential when the above noted recording montage is used. Contrast this outcome with the well-demarcated cut end of a nerve producing a far-field potential. A far-field potential is detected because the forward traveling intraaxonal current must stop at the end of the nerve. The associated local circuit current ceases, temporarily leaving intact voltage gates and a functional negative sink. Transiently, the trailing dipole is no longer balanced by the leading dipole and the ensuing dipolar imbalance generates a far-field potential. Shortly thereafter, the negative sink encounters the absence of neural tissue with the action potential’s trailing dipole dissipating and the CRT re- turning to baseline.

Directional Changes. Abducting the arm, or flex- ing and extending the metacarpophalangeal .joints produces respective alterations in the observed SEP and peripheral SNAP far-field potentials.’ ’*“ Computer simulations and animal studies have shown that changing the direction of neural propagation can result in changes in far-field waveforms. 12,33,60 It is important to recognize, however, that the experiments of directly bending the nerve without altering the surrounding volume conductor is not necessarily comparable with bending the volume conductor along with the nerve. A change in the direction of action potential propagation clearly generates far-field potentials, but this may not be the same mechanism of far-field potential production with alteration of the body’s joint angles. The L/T dipole model can be utilized to visualize far-field potential production with bending a nerve back on itself for example, to reverse the action potential volley.

A nerve may be located in a cylindrical volume conductor with the midportion angled back 180 degrees (Fig. 5). E-1 and E-2 are positioned greater than 1.9 times the cylinder’s radius and placed axially away from the end of the nerve with respect to the nerve bending back on itself. A neural impulse can then be introduced at one end of the nerve. Initially, the propagating L/T dipoles are on a straight length of nerve and the leading dipole moment’s potential is canceled by the trailing dipole moment’s potential (Fig. 5A). As the leading dipole begins to turn the corner, the potentials from the leading and trailing dipole moments no longer cancel each other and a far-field potential begins to be produced. Continued neural propagation creates a situation where the peak of the action potential reaches the bend, in which both dipoles are symmetrically aligned with respect to the two electrodes, and the potentials from both dipoles are summaated (Fig. 5B). Fi- nally, both the leading and trailing dipoles are again on a straight segment of nerve and a far- field potential is no longer detected (Fig. 5C). This sequence of events results in a monophasic negative far-field potential. The far-field potential’s magnitude is dependent upon the angle to which the nerve is bent, with the largest potential occurring if the nerve is bent 180 degrees. An angle less than 180 degrees will result in a negative far-field potential with a comparatively decreased amplitude. 12s32 This relationship follows a cosine function.22 Interchanging E-1 and E-2 results in a positive far-field potential for reasons already discussed. Again, the polarity of the far-field potential is subject to the location of the recording electrodes with respect to the imbalance of dipole moments.

Resistivity Changes. A cylindrical container can be constructed with two adjoining compartments of different electrical conductivity.22 The recipro- cal of conductivity is the resistivity or resistance per unit volume the medium has to the movement of current. It is important to recognize what occurs to the voltage gradient of the constant current L/T dipole in media with different resistivities with a constant dipole size. For example, assume that the media of one half of the cylinder has twice the resistivity (half the conductivity) of the media in the other half. The net current associated with the action potential in either compartment is identical because the action potential is predominantly a constant current source. In the higher resistivity half of the cylinder, however, the


A

B

C

Volume Conductor

E-1

E-2

E-1

E-2

E-1

Diff Amp (E-I) - (E-2)

0 - 0 = 0

LD -4 - 4

TD + (-4 - 4 )

-8 - 8 = -16

CRT Trace

i E-2

FIGURE 5. Action potential (LTT dipoles) propagating along a neural segment with a 180-degree bend and two electrodes (E-1 and E-2) located opposite the bend. (A) Action potential completely contained on a longitudinal nerve segment with the centrally located leading and trailing dipoles yielding no far-field potential when E-1 and E-2 are located greater than 1.9 times the cylinder’s radius from the boundary zone. (B) Neural propagation produces a situation where the leading (LD) and trailing dipoles (TD) are aligned symmetrically about the bend in the nerve. A monophasic negative far-field potential is generated. (C) Continued neural propagation then results in the two dipoles on a straight neural segment with a dipolar balance again achieved at the two electrodes

current lines spread further away from the nerve than in the low resistivity half. Thus, the spatial distribution o f the isopotential lines are not the same between the two media. When the leading dipole of‘ the action potential crosses into the compartment with half’ the resistance, the spatial gradient of isopotential lines shift closer to the nerve. ‘This isopotential gradient shift has altered the potential field for the leading dipole, while the trailing dipole’s potential field distribution is unchanged as it is in the higher resistance half of the cylinder. The recording electrodes, therefore, will record different potential values when the LD and T D are separated by a boundary compared with the situation in which both dipoles are located on the same side of the boundary. The resistivity of the media also af’feects the recorded near-field potential. Because of the altered isopotential line distribution, near-held recordings in the compartment with tlic higher resistance will show a waveform with a greater amplitude than that in the lower resistivity aspect of the cylinder for comparable elect1 ode locations in space between the

In Figure (iA, the LIT dipoles are initially

two regiolls.“2:’2,~”’

within one half of the cylinder containing a uniform resistivity, with no detectable far-field potential because of the balance of dipole moments. Also, the recording electrodes are greater than 1.9 times the cylinder’s radius. As the leading dipole enters the lower resistance portion of the cylinder, an imbalance in the dipole moments results and a far-field potential develops. The peak of the waveform occurs when the leading and trailing dipoles are in media of two different resistances (Fig. 6B). Continued neural propagation in this case results in equal dipole moments when both dipoles are within media of the same resistivity, with a decline in the magnitude of the far-field potential to zero (Fig. 6C). In the given example, when proceeding from a high to low resistance volume conductor with E-1 in the low-resistance aspect, a riegative monophasic far-field potential will be produced. Reversing the resistivities will change the polarity of the far-field potential while the direction of propagation does not alter the observed polarity. Again, E-1 and E-2 are located greater than 1.9 times the cylinder’s radius from the boundary zone between the two resistivity compartments yielding the given far-field potential magnitude.


A

B

C

Volume Conductor

E-1 ("i 0 +--+ 3

\ ? \J E-2

E-1

+ -,'- + \ 0 a + -,'- + I

E-2

E-1

Diff Amp CRT Trace (E-1) - (E-2)

LD 0 - 0 TD + O - 0

0 - 0 = 0

LD 4 - (-4)

TD + ( -8 - -4 - 4 = -8

LD 0 - 0 TD + 0 - 0

0 - 0 = 0

I

E-2

FIGURE 6. A cylindrical volume conductor of two resistivities. The volume left of the dotted line has a resistance twice (one-half the conductance) that of the volume to the right. A monophasic negative far-field potential results when an action potential propagates from left to right and crosses the boundary between the two resistances. The action potential's magnitude is the same as that for previous examples.

STIMULATORS AS FAR-FIELD GENERATORS

The typical constant current or voltage stimulator used in routine nerve conduction evaluations may be considered a dipole generator. With respect to volume conduction theory, the stimulator generates a dipole moment similar to those described above. Indeed, in far-field volume conductor experiments, constant current stimulators have been utilized to model far-field potentials in the hope of better understanding far-field potential production in the body.lg-2y.s3 Individuals performing nerve stimulation are quite familiar with far-field potentials, because the stimulus artifact detected at a distance from the site of stimulation is essentially a far-field potential. Recall that the stimulus artifact always precedes the desired waveform because as a far-field potential, it propagates throughout the body's volume conductor almost instantaneously (actually at the speed of electro- magnetic waves in the media).

EXPERIMENTAL AND CLINICAL FAR-FIELD POTENTIAL OBSERVATIONS

Nakanishi passed the sciatic nerve of a bullfrog through a series of removable partitions with different size openings in a container of Ringer's so- l~t ion.""~ ' Communication through all compartments was achieved through the partitions' small openings. These openings created a constriction in the volume conductor altering the size of the median through which the action potential and its accompanying current field lines propagated. When the recording electrodes were placed in the first and last locations (far-field recording), and the nerve was activated, far-field potentials were observed that directly corresponded to the number of partitions the action potential traversed. These findings suggested that every time the action potential passed a size change in the volume conductor, a waveform was generated and later supported by additional investigations. 12*23,34 Al-


though Nakanishi",52 proposed that the intercompartmental constriction produced an increase in the volume conductor's impedance through which the action potential propagated, the far- field potentials were more likely generated because the constriction caused an asymmetry with respect to the action potential's leading and trailing dipoles.

I n addition to alterations in compartmental openings, a change in the anatomical orientation of the nerve or a nerve's branch point was also postulated to p1-0duce fir-field potentials." Clinical investigations of' far-field generator sources, therefore, shifted from exclusively neural sources (clus- ters of nuclei or synaptic activity) and traveling dipoles (ascending action potential volleys) to the symmetry of action potential current fields. The application c ) f ' classic volume conductor theory to a dipole in an infinite homogeneous and isotropic medium needed to he modified.'" A traveling t r i p l e

Olpolrr Rrcordlng

(source- sink- source current distribution)"6 propagating in a nonhomogeneous and anisotropic en- vironment was constructed.""-'"'

A series of referential peripheral sensory nerve recordings in the upper extremity provided chal- lenging insights into the relationship between the traveling action potential and the surrounding volume conductor.'"),",-l~~,~~~ Kimura et al.'" placed a number of electrodes from the forearm into the distal portion of the second digit along the course of the radial nerve. A single electrode was also placed on the fifth digit. 'I'he radial nerve was activated proximal to the forearm recording electrodes. Serial electrodes along the radial nerve could be arranged in a sequential near-field bipolar montage, or each individually referenced to the fifth digit in a near-field-type referential montage. Bipolar recordings demonstrated the expected biphasic sensory nerve action potential (SNAP) with increasing linear latencies confirming

( -a) - v

(4)- v

(-1)- v

( ( + l ) - v

(+2) - v

(4)- v

(ts)- v

( t q - v

(4- v

( t a ) - v

(+I))- V

1.Omr R r f r t m t h l Rrcordlng

FIGURE 7. Sensory nerve action potential across the hand along the second digit in a normal subject recorded antidromically after stimulation of the superficial sensory branch of the radial nerve 10 cm proximal to the radial styloid. In a bipolar recording (left), the initial peaks, N1 demonstrated a progressive latency increase and reduction in amplitude with no response beyond -1. In a referential recording (right), biphasic peaks, PI-NI and PII-NII (arrows pointing down) showed a stationary latency irrespective of the recording site along the digit. (Reproduced with permi~sion.~')


a traveling neural volley. 'Ihe referential montage also revealed the anticipated triphasic potential propagating along the course of the radial nerve (Fig. 7). Of particular importance in the referential recording is the appearance of positive and negative waveforms (far-field potentials) with constant latency at locations coincident with and beyond the propagating action potential. By corre- lating the initial appearance of the far-field potential with the anatomic location of the action potential at that instant, the body's .junctional segment producing the far-field potentials could be identified.41244 This localization technique identified the forearm/hand (wrist), and metacarpophalangeal regions as the sites of origin for the observed far-field potentials. Because the axon is continuous, the probable mechanism producing the detected far-field potentials is the effect of the volume conductor size variations at these regions, or some alteration in the axon. A size change in the axon and/or in the volume conductor's geometry, impedance, or some other property had to have occurred at the two anatomic partitions described above. These findings have also been documented in the median nerve for the first three digits and in other body Initial computer simulations of the above described clinical experiment could only document the positive far- field potentials6 More sophisticated computer- modeling techniques suggested that both negative and positive far-field potentials could indeed be generated by specific volume conductor conditions. 'O

Another finding was an alteration in the far- field potential with a change in the joint angle through which the propagating action potential traveled. Abducting the arm altered the latency and morphology of the first scalp recorded SEP far-field potential (€9). ''2"b Far-field waveform morphology and latency alterations were also observed to occur in peripheral nerve far-field recordin s with flexion of the metacarpophalarigeal joints.l' Waveform alterations were documented in both SEP cephalic recordings and peripheral nerve far-field observations with respect to body segment positioning.

Two additional observations regarding far- field potentials were noted. Following the generation of a far-field potential at a specific location, the waveform appeared with relatively the same

This implied latency at all recording sites. that the far-field potential extended almost instantaneously throughout the remaining longitudinal portions of the cylindrical volume conductor. The

12,41,5 I ,65

amplitude of the far-field potential, however, seemed to be dependent upon the propagating action potential's magn i t~de .~ ' An increase in the stimulus produced increases in the SNAP'S amplitude until all the axons were activated. There was a direct correlation between the SNAP amplitude and far-field potential magnitudes.

The spatial extent of the far-field potential was surprising. An experiment connecting two individuals together- at the arm with conductive electrolyte demonstrated that the far-field effects were detected in the second individual with little decre- ment in latency or amplitude.65 This finding illus- trates that the body, as a volume conductor, is sirn- ilar to a wick (electrolyte containing) electrode arid can be used to detect potential differences irrespective of its length.

In the peri heral nerve referential recordings, Kimura et aL4' noted positive and negative waveforms in the hand. Kameyama et aL3" and Ya- mada et al."4.6'" found preferentially negative far- field recordings in the arm with median nerve wrist stimulation in a referential montage. Utiliz- ing different recording montages (altering the location of the two recording electrodes), they were able to record the two sides of the potential difference created at a change in the volume conductor. These investigations clearly demonstrated that the original assumption that far-field potentials only have a positive polarity was incorrect. Both positive anti negative far-field potentials are now known to exist and depend upon the recording morita e as related to the dipole genera-

As previously stated, far-field potentials do not necessarily reflect electrical activity closely associated with the anatomical location of the recording electrodes. In patients with brachial plexus lesions, multiple sclerosis, and other central nervous system diseases, far-field potentials have been used diagnostically. 1,42527 A cautionary note, however, has been expressed regarding this practice. Changes in the anatomic orientation of a body segment can alter far-field potential morphology and/or latency that could be interpreted as pathol- ogy. Also, because of the possible anatomic vari- ability among individuals, SEP far-field potentials may not be stable enough to localize lesions affect- ing the nervous system.

tor. I 2 , 1 ~ , 1 8 , 4 l . 6 0

20.42.66

SOMATOSENSORY-EVOKED POTENTIAL FAR-FIELD GENERATORS

Advances in instrumentation produced reliable recordings for cortica somatosensory-evoked poten-


A) PZ-Hmd

B) SCalpR

C) TSK

D) Tll-K

E) GlutewK

_I 1P.V 2ms

1 Oms

FIGURE 8. (A) Far-field SEP recording following median nerve wrist stimulation. Four positive far-field potentials are recorded: P9, P I 1, P13, and P14. (B-E) Stimulation of the tibia1 nerve with a cortical referential montage [knee (K) is reference site] demonstrating far-field potentials and their development from the gluteal fold along the spinal column to scalp: P17, P24, P27, and P31. The lower latency/amplitude scale pertains to traces (B- E). (Modified with

tials (SEPs). 'l'he utilization of cortical E-1 arid noncortic.al 1.:-2 electrode locations (referential montage: E-2 opposite to side stimulated) in humans wi(h median nerve wrist stimulation revealed a series of tar-field potentials. Specifically, four sequential positive peaks preceding the cortical potential were consistently recorded with relatively the same peak latency over wide portions o f the cranium, thus meeting the initially develo ed criteria of' far-field potentials (Fig. 8A). 'I'he anatomic origin of specific neural generators producing these four SEP far-field potentials, however, remain to bc identified to optimize their diagnostic utility.

Following median nerve stimulation at the wrist, an SEP far-field potential P9I4 demonstrates a consistent positive peak at approximately 9 ms with a rctercritial (E-2 electrode to the o posite wrist) scalp recording montage (Fig. XA)."Addi- tional electrode locations at the axilla and Erb's point revealed that this far-field potential occurred in time after the corresponding axillary

4.H- , o . : ~ l , ( ; l

near-field potential, but prior to an Erb's point near-field potential.' A generator in the proximal axilla was postulated to produce the 1'9 waveform. As there were no known synapses or nuclei in this region, the traveling afferent volley itself was clearly the generator.8 It was suggested that the current field associated with the traveling neural volley may be affected by a change in the anatomical orientation of the brachial plexus or by the surrounding volume conductor itself (armithorax junction).".(53 The latter refers to changes in the electric field where the arm enters the thorax due to a geometric change in the volume conductor."' Close inspection of the P9 waveform revealed it was bimodal (PYaIPYb) in over 70%) of subjects.'ifi 1 he P9a waveform was postulated to arise within the axillary region, and the P9b potential appeared to originate just distal to Erh's point. I t was further suggested that P9a may he henerated bv an axial orientation change of the neural tissues, and P9b by a size differential in adjoining voluriie conductor segments (armithorax).

' Ihe second positive potential P 1 1 occurred approximately 1 1 ms following median nerve stirnulation with a cortical- referential montage (Fig. 8A). A detailed analysis of P1 1's latency and calcu- lation of the conduction velocity allowed only enough time for the traveling action potential t o reach the root entry zone of the spinal cord sug- gesting that P11 could not arise from the nucleus cuneatus or medial lemniscus.:'"."" The P11 potential, therefore, may arise because the action potential volley alters it direction of travel or encounters an alteration of the volume conductor upon entcr- ing the spinal cord. The third positive potential followed median nerve excitation by about 1 3 ms (P 13; Fig. 8A). Nerve conduction velocity consid- erations in humans suggested that the waveform originated from a traveling action potential volley within the cervical spinal cord."8 T h e site of origin based on nerve conduction velocity, however, is only as good as the assumed proximal neural and central time of conduction. Small errors in conduction velocity can result in different presumed sites of waveform generation, particularly when considering closely spaced anatomic structures or neural orientation changes. An inconsistently observed fourth positive far-field waveform (P 14) was hypothesized to originate from the medial lemniscus, cerebellum, various cerebellar connections, or thalamocortical radiations (Fig. 8A). ','"'' Serial lesions in the central nervous system of'ani- nials concluded that thalamic radiations played a significant role in the production of the fourth

_ _

250 Far-Field Potentials MUSCLE 8, NERVE March 1993

positive waveform.27 Central nervous system lesions in humans, however, suggested P14 arose caudal to the thalamus or possibly in the mid- brain.Z7,50 The value of serial ablation techniques in localizing far-field potentials, however, must be questioned as cut neural ends can generate far- field potentials thus confusing the site of far-field potential origin in intact preparations." The exact site of origin for the P9-Pl4 far-field potentials thus remains controversial.

Stimulation of the tibia1 nerve in the lower extremity also gives rise to a number of far-field o

These waveforms are believed to originate from: (1) just distal to the sacral plexus; (2) entry of the impulse into the conus medullaris; (3) rostra1 spinal cord; and (4) the brainstem, respectively.

Somatosensory-evoked potential far-field potentials have demonstrated a relative resistance to latency or morphology alteration following volatile or intravenous anesthetic admini~tration.""."~ This is in contrast to the susceptibility of near-field cortical SEPs to the depth of anesthesia.55 Far-field potentials arising from the interaction of an action potential and the volume conductor would appear to be consistent with their insensitivity to anesthet- ics provided the primary generator of these potentials is also not affected. A few minor changes, however, have been noted in far-field potentials to some anesthetic agents."' The reason for these alterations may be secondary to a small effect of the anesthetic on the far-field generator, to tempera- ture changes under anesthesia, or to some other unidentified cause.

tentials (P17, P24, P27, and P31; Fig. 8B). 'l7.4!&,

FAR-FIELD POTENTIALS IN MUSCLE

A consistent theory of far-field potentials in a volume conductor should apply equally well to both neural and muscle tissues. Indeed, computer simulations and in vivo studies su gest that far-field potentials also arise in muscle. '7,18,2'~ Specifically, as an asymmetrically induced action potential (direct muscle stimulation bypassing the neuromus- cular junction) reaches the musculotendonous junctions, the leading dipole will begin to dissipate prior to the trailing dipole, and far-field electrodes will detect a difference in the potentials from the dipoles resulting in a far-field potential. This occurrence is similar to a neural impulse reaching the end of a nerve with a resulting far- field potential. Experimentally, far-field potentials in muscle can be rather easily demonstrated.

The biceps muscle provides a good example of a clinical preparation that yields demonstrable far-

A &

FIGURE 9. Far-field potential recorded from the biceps muscle activated with an intramuscular needle cathode. (A) Two far-field potentials recorded at a radial styloid (E-1) and patella (E-2) far- field montage. (B) Relocating the intramuscular cathode closer to the proximal musculotendonous junction. (C) Bipolar recording with E-1 and E-2 placed at the wrist showing an elimination of the far-field potential.

field potential^.'^^'^ Locating E-1 on the radial styloid ipsilateral to the investigated muscle and E-2 on the contralateral patella establishes a far-field referential recording montage. Placing a fine mo- nopolar needle electrode within the biceps muscle several centimeters proximal to the distal musculotendonous junction with a large surface anode about the triceps region forms the stimulating montage. Combining enough current to produce a minimal muscle twitch with several hundred av- erages is sufficient to result in two far-field potentials (Fig. 9).

In the above noted recording montage, an initial negative and subsequent positive far-field potential is observed (Fig. 9A). This is the expected result if one recalls the leadingltrailing dipole model. The stimulating pulse induces an action potential that propagates both toward the origin (proximal arm) and insertion (distal arm/proximal forearm) of the biceps muscle (Fig. 10A-E). The E- 1 recording electrode initially observes the dipolar moment imbalance associated with the action potential traveling toward the distal muscular insertion where the leading dipole is first extinguished. The trailing dipole with its negative portion facing E-1 results in a monophasic negative far-field potential.

Similarly, the other action potential travels toward the proximal musculotendonous junction and its leading dipole is initially extinguished but after a relatively longer latency because the action potential has traveled farther. The trailing dipole is recorded as positive because the positive portion


A

B

C

D

E

F

G

H

Muscle

E-2

1, 11 +---•

1 1 + - - +

E-2

4

I E-2

E-2

1 + - - + + - - +

E-2

+- 1 I + - - +

;",+--+ 4

E-2

1 I-+ I

E-2

E-1

1

1

1

1

E-1

E-1

E-1

i1 E-1

1

1 E-1

E-1

CRT forming two back-to-back dipoles. l'he result is that there is no far-field potential detected when the two action potentials propagate in opposite directions.

CONCLUSION

Clinical studies with evoked potentials have made observations that challenged the accepted theories of far-field potential production. Peripheral recording montages can also produce far-field potentials similar to those observed in central nervous system recordings. I t is reasonable to assume that one far-field generator theory should explain both phenomena. A careful analysis of where the traveling action potential was when a far-field potential was generated revealed foi ir- possibilities: (1) a change in the impedance of the surrounding medium; (2) volumetric alteration i n the volume conductor; ( 3 ) directional changes in neural propagation; and/or (4) the termination of excitable tissue. Clinical, animal, and mathematical investigations support all of the agove mechanisms for

-

-

-

1 h

+ A

inducing far-field potentials. AtLtlitionally, the leading/trailing dipole model appears to consis-

FIGURE 10. Diagrammatic presentation of asymmetric muscle action potential induction in muscle tissue. (A) Muscle tissue (horizontal lines) pictured with two recording electrodes beyond the proximal and distal musculotendonous junctions (vertical lines). (B) Action potential induced in muscle with needle stimulator closer to the E-1 electrode location. (C) Action potential begins to expand bidirectionally along the muscle membrane. (D) Two complete action potentials formed heading in opposite directions. (E) One of the action potentials encounters the termination of excitable tissue with its leading dipole extinguished first. A negative far-field potential is formed. (F) The trailing dipole of the action potential on the right dissipates (CRT settles back to baseline), while the other action potential has not yet encoun- tered tendon. (G) The leading dipole of the second action potential reaches its tendon yielding a positive far-field potential. (H) The trailing dipole of the second action potential is now completely absent and the CRT again returns to baseline.

of the trailing dipole is oriented toward the E-1 electrode (Fig. 10F-H). 'The result is a positive monophasic far-field potential. Relocating the intramuscular cathode closer to the proximal musculotendonous junction results in a reversal of the far-field potentials' polarity (Fig. SB). A bipolar wrist recording detects nc) far-field potentials as the potentials from the leading and trailing dipoles cancel in this montage (Fig. 9C). Note in Fig- ures 9'4 and B there is no LD from the point of stimulation when the action potentials are initiated by the cathode and begin to propagate because there are two I D S facing in opposite directions

v v - _ _ tently explain all of the experimental far-field observations. Polarity is primarily dependent upon the location of the two recording electrodes with respect to the dipolar imbalance across a boundary zone.

The preceding explanations of far-field potentials with respect to alterations in volume conductor size, directional changes, excitable tissue termination, and resistance alterations are understandably simple compared to the complexities of the human body. In reality, one may expect a combination of factors occurring simultaneously. The action potential volley could approach a boundary zone entering a compartment of not only higher or lower resistance, but also of a different volume during a change in the nerve's orientation or its ending. Ad- ditionally, other elements of the volume conductor or action potential that are not fully appreciated could influence the production of far-field potentials. Exactly which volume conductor factors are predominantly responsible for far-field potential generation requires more experimentation.

REFERENCES

1 . Anziska B, Cracco RQ, Cook AW, Feld EW: Somatosen- sory far-field potentials: studies in normal subjects and patients with multiple sclerosis. Elpctroenrpphologr Clzn Npnro- physiol 1 978;45 : 602 - 6 10.

2. Brazier MAB: Electrical Actzvity of thP Nrroous S y s t ~ ~ r i . Lon- don, Pitman Medical, 1977.


3 . Clark J , Plonsey R: The extracellular potential field of the single active nerve fiber in a volume conductor. BiophysJ

4. Cracco RQ: The initial positive potential of the human scalp recorded somatosensory evoked response. Electroen- cephalyy Clin Neurophyszol 1972;32:623-629.

5. Cracco RQ, Anziska BJ, Cracco JB, Vas GA, Rossini PM, Maccabee PJ: Short-latency somatosensory evoked potentials to median and perorieal nerve stimulation: studies in normal subjects and patients with neurologic disease. Ann NY Acad Sci 1982;388:412-425.

6. Cunnirigham K, Halliday AM, Jones SJ: Simulation of ‘stationary’ SAP and SEP phenomena by 2-dimensional potential field modelling. Electroencephaloffr Clzn Neurophysiol 1986;65:416-428.

7. Davis SL, Aminoff MJ, Panitch HS: Clinical correlations of serial somatosensory evoked potentials in multiple sclerosis. Neurology 1985;35:359-365.

8. Desniedt JE, Cheron G: Central somatosensory conduction in man: neural generators and interpeak latencies of the far-field components recorded from neck and right or left scalp and earlobes. Elrctroencrphalogr Clin Neurophy.sio1 1980;50:382-403.

9. Desniedt J E , Cheron G: Prevertebral (oesophageal) recording of subcortical somatosensory evoked potentials in man: the spinal P I 3 component arid the dual nature of the spinal generators. Electroelccrphalogr Clin Nrurophysiol 198 1 ; 52:257-275.

10. Desmedt JE, Cheron G: Non-cephalic reference recording of early somatosensory potentials to finger stirnulation in adult or aging normal man: differentiation of widespread N18 arid contralateral N20 from the prerolaiidic P22 arid N30 components. Electrorncrphulogr Clin Neurophy.~iol 198 1;52:553-570.

11. Iksmedt J E , Nguyen T H , Carnieliet J: Unexpected latency shifts on the stationary P9 somatosensory evoked potential far-field with changes in shoulder position. Electroencephu- 10g7 Clin Neurophysid 1 983;56:628- 634.

12. Deupress DL, Jewett DL: Far-field potentials due to action potentials traversing curved nerves, reaching cut nerve ends, and crossing boundaries between cylindrical volumes. Electroencephalogr Clin Neuropliysiol 1988;70:355- 362.

13. de Weerd JPC: Volume coriductiori arid electrornyogra- phy. in Notermans SLH (ed): Current Practice of Clinical Electromyography. Amsterdam, Elsevier, 1984, pp 9-28.

14. Donchin E, Callaway E, Cooper R, Desmedt ,JE, Goff. WR, Hillyard SA, Sutton S: Publication criteria for studies of evoked potentials (EP) in marl, in Uesrriedt JE (ed): Propess in Clznical Neurophysiolog. Easel, Karger, 1977, vol 1, pp 1-11.

15. Dumitru D, Walsh NE: Practical instrumentation and common sources of error. Am J Phys Med Rehabil 1988;67:55- 65.

16. Dumitru D, Walsh NE: Electrophysiologic instrumentation, in Dutnitru D (ed): Clinical Elrctrodiagnoszs; Physical Medicine arid Rehabilitation State of the Art Reviews. Philadephia, Hanley & Helfus, 1989, vol 3, no 4, pp 683- 699.

17. Dumitru D, King JC: Far-field potentials in muscle. Muscle Nerve 199 1 ; I4:98 1 - 989.

18. Dumitru D, King JC: Far-field potentials in muscle: a quantitative investigation. Arch Phy Mud Rehabil 1992; 73:270-274.

19. Dumitru D, King JC: Far-field potentials in circular volumes. Muscle Nerve 1992;15:101- 105.

20. Dumitru D, King JC: Far-field potentials in circular volumes: the effect of different sizes arid intercompartmental openings. Mwcle Nerve 1992; 15:949-959.

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