section 7 instrumentation

7/29/2019 Section 7 Instrumentation

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SECTION 7: INSTRUMENTATION

7.1 EMG Machine Components

Nerve and muscle action potentials are recorded and displayed for analysis by the EMG machine.Basic instrumentation includes: 1) electrodes to detect the signals, 2) amplifiers to increase signalstrength, 3) filters to reduce stray signals, 4) an oscilloscope to view the signals, and 5) for nerveconduction studies an electrical stimulator is required to activate the nerve (Figure 7.01). ModernEMG instruments are based on digital computers, and the original analog signals are convertedto digital signals and placed in temporary memory storage, and can be viewed on a computerscreen or are available for further processing. Special processing capabilities include: 6) signalaveraging to sharpen waveforms, and 7) voltage trigger and delay lines to enhance viewing ofneedle EMG signals. All EMG instruments have the same basic elements, and differ only in thepositioning of the controls and the display arrangements.

7.2 Recording Electrodes

A recording electrode consists of two electrodes required to measure the potential differencesbetween to sites. The most common arrangement of the two electrodes is for one to be close to

the nerve or muscle fibers under study (the active, exploring, stigmatic or G1 electrode) to recordionic currents flowing from action potentials, and the other electrode to be placed far away (theindifferent, reference or G2 electrode) so as to be theoretically unaffected by the ionic currentsflowing from action potentials at the active site. Although the flow of ionic currents and thesubsequent changes in potentials originate at specific sites along nerves and muscles, potentialdifferences can still be detected at a distance from the source. Thus, the neutrality of theindifferent electrode is only relative, but with proper attention to electrode positioning the signal israrely contaminated to a significant degree.

There are many possible configurations of recording electrodes, and their design is based on thetype of information desired. All electrodes represent a compromise between selectivity for certainelectrical features and inclusion of all electrical activity. For electrodiagnosis two basic electrodeconfigurations are used, surface and intramuscular, with variations described below.

7.21 Surface Recording Electrodes

For recording sensory nerve action potentials (SNAP), the two electrodes are placedlongitudinally along the nerve, with the active electrode positioned to see the arriving actionpotentials and the reference electrode some distance beyond to record the retreating potentials(Figure 7.02). Because the traveling whole nerve action potential is 30 to 40 mm in length alongthe nerve, changes in the distance between the two recording electrodes affects the amplitude ofthe sensory nerve action potential. The maximal amplitude will be recorded when one electrodesees the maximal potential and the other electrode sees a zero potential. Inter electrodedistances of 30 to 40 mm are optimal, with little change occurring with greater distances (Figure7.03). Ten mm disk electrodes are used to record from sensory nerves in limbs and ringelectrodes to record from digits (Figure 7.03).

For recording muscle action potentials (CMAP), the active electrode is placed over the end plateregion and the reference over the tendon (belly tendon recording) (Figure 7.02). However, endplates within a muscle are distributed over an area (Figure 2.01), and muscles commonlyrecorded from represent a group of muscles innervated by the same nerve (median innervatedthenar muscle group, ulnar innervated hypothenar muscle group, tibial innervated intrinsicmuscles) and each muscle in the group will have its own end plate region (Figure 7.04). Theposition of the recording electrode is important. Moving the active electrode away from the endplate region changes the shape of the compound muscle action potential, leading to a loweramplitude response, and an initial positive deflection may appear if the electrode is markedly out


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of position (Figure 7.05). If all end plates were focused in one region, an electrode over this pointwould see the sum of muscle fiber action potentials leaving this site. Thus, optimal electrodeplacement is a compromise among several end plate zones. Since the maximal amplitude isrecorded when the electrode is over the aggregate end plate zone, this point cannot beanatomically defined, and will coincide with a steep rising phase of the compound muscle actionpotential. Ten mm disk electrodes are commonly used (7.02), but self-adhesive ECG electrodesare also used.

7.22 Routine Intramuscular Recording Electrodes

Intramuscular electrodes for routine EMG studies are either a modified hypodermic needle(concentric electrode) or a sharpened solid wire (monopolar electrode) (Figure 7.06). The termsconcentric and monopolar refer to the location of the active recording electrode. Withconcentric electrodes the active electrode is a wire within, but insulated from, the hollow needle.The end of the needle is cut at a 15-degree angle and the active electrode surface is an ellipse.The reference electrode is the hypodermic needle (cannula). Thus with concentric electrodes theactive and reference electrodes are in fixed relationship to each other.

Monopolar electrodes are solid sharpened needles insulated except for an active recordingsurface at the tip in the shape of a cone. The reference electrode is a separate electrode, either

another needle just under the skin or a disk electrode on the surface of the skin. Thus withmonopolar electrodes active and reference electrodes can be at varying distances from eachother and can include stray signals.

Every electrode has a distance over which it can detect bioelectric signals, called the recording oruptake radius. The recording radius is limited by the attenuation of the signal due to resistance ofthe tissue. Signal amplitude is reduced by a factor equal to the square of the distance betweenthe signal and the electrode. Although concentric and monopolar electrodes differ in shape andsize of the recording surface, both electrodes records have a recording radius of 500-1,000microns. When this distance is applied to the distribution of muscle fibers within a motor unit,from seven to 15 muscle fibers of a motor unit are recorded from both electrodes.

7.23 Special Intramuscular Recording Electrodes

Special purpose electrodes are available to answer specific questions. They represent acompromise between selectivity and sensitivity.

Single fiber electrodes are hypodermic needles, but the active electrode is a wire whose exposedsurface is through a hole or port along the side of the hypodermic needle and the referenceelectrode is the cannula (Figure 7.06). The active recording surface is very small, smaller thanthe wire electrode in concentric electrodes, and has a recording radius of < 300 microns.

Accordingly, action potentials from only one or two muscle fibers of a motor unit are included.

MacroEMG Electrodes are similar to single fiber electrodes but only the terminal 15 mm of thecannula is bare and the remaining proximal portion is insulated (Figure 7.06). The macroEMGelectrode is a dual electrode: 1) The first electrode is a single fiber electrode with the active

recording electrode at the side port and the reference electrode is the cannula. 2) The secondelectrode uses the uninsulated 15 mm of the cannula as the active electrode and a separateelectrode serves as the reference electrode. The macroEMG electrode requires two amplifiers,one records single fiber action potentials from the side port electrode while the other recordsactivity from the whole motor unit from the cannula.

Near-nerve electrodes are similar to monopolar electrodes but with greater recording surfacearea. They are maneuvered under the skin to be close to a nerve and are usually to recordsensory nerve potentials.


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7.3 AMPLIFIERS

Amplifiers increase the strength of the bioelectric signals picked up by the electrodes. Actionpotentials from nerve and muscle are of low amplitude (microvolts to millivolts) and must beamplified for viewing on the oscilloscope screen. The two wire leads from the electrodes areconnected to the amplifier. The terms G1 and G2 refer to the grid inputs to vacuum tubes fromolder amplifiers, and are now more commonly used in the vocabulary of EEG and evokedpotentials. On EMG machines, G1 leads and the connectors they plug into are usually black incolor and G2 red in color. Despite the changes in nomenclature the concepts are important forunderstanding how nerve and muscle potentials are amplified.

Bioelectric signals recorded with the electrodes reflect the potential at one site relative to thepotential at the other site. Absolute voltages, based on the potential difference with respect to theearth or ground (because the earth can absorb and distribute an infinite number of ions) are notconsidered.

7.31 Differential AmplifiersA differential amplifier is used in EMG machines, and consists of two amplifiers that areinterconnected (Figure 7.07) such that the signal entering one amplifier (G2) is subtracted fromthe signal entering the other amplifier (G1). If G2 is the indifferent lead and is not influenced by

the biologic voltage change to be measured, then the output of the amplifier primarily reflectsvoltage changes at G1 (Figure 7.08). However, if G2 also partially sees the same voltage asdoes G1, the output will be reduced. The convention is for amplifiers to represent a negativevoltage at G1 as an upward deflection. Since the main extra cellular spike component of bothnerve and muscle fiber action potentials are negative, the waveform spikes trace mainly upwardpatterns.

7.32 Common Mode Rejection Ratio

Stray electromagnetic voltages in the environment can be large in amplitude and could distort thebioelectric signals. The most common stray signal comes from alternating current (AC) current inthe room, and can be readily recognized by its 60 Hz frequency. An advantage of the differentialamplifier is that it can cancel out most stray 60 Hz noise. Since both G1 and G2 inputs will be

influenced similarly, the 60 Hz noise in one input will be inverted and cancel with the noise in theother input (Figure 7.09). The ability of a differential amplifier to reject signals common to bothinputs is called the common mode rejection ratio. However, the two component amplifiers withinthe differential amplifier (Figure 7.07) are not absolutely identical in their gains, resulting in somedegree of unequal subtraction of the common signals. The rejection ratio is a measure ofamplifier mismatch, and high values are preferable, e.g., 10,000 to 1. When stray 60 Hz signalsare of very high amplitude some part of the common signal will be present and the bioelectricsignal will be superimposed on it. A ground electrode is important because it ties the amplifier toearth and helps reduce stray noise.

7.33 Electrode and Amplifier Impedance

Electrodes and amplifiers offer resistance or impedance to current flow which could affect signals,

although these issues have usually been handled by the equipment companies . The voltagesfrom nerve and muscle fiber action potentials are conveyed from tissue to oscilloscope byelectronic circuits consisting of electrodes and amplifiers. By Ohms law, current and voltage arerelated to each other by the factor of resistance. Resistive elements impede the flow of current,but there are two types of resistance. Resistance is unaffected (remains constant) when thefrequency of the voltage in the signal changes. Impedance is resistance that is affected (variesin value) when the frequency changes. Since action potentials and other bioelectric signals varywith time, the resistive factor of interest is impedance.


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Impedance is important at two sites in the current flow from the tissue, through the electrode andinto the amplifier. For a given voltage from the tissue, there will be a voltage drop (loss) acrossthe electrode proportional to the electrode impedance, and a drop across the amplifierproportional to the amplifier impedance. These two voltage drops must sum to equal the voltagefrom the tissue. Accordingly, the impedance of the amplifier must be higher than that of theelectrode so that more of the voltage drop occurs across the amplifier, and hence is amplified anddisplayed.

When comparing two electrodes, the one with the higher impedance will be more susceptible tostray signals. This is because a stray voltage will induce a larger current across an electrode ofhigh impedance. As a practical example, monopolar electrodes have higher impedance thanconcentric electrodes CHECK.

7.4 Filters

It would be ideal to record pure nerve and muscle fiber action potential waveforms, but manyfactors, including the impedance properties of the electrodes, the potentials from other activenerves and muscle, and the stray voltages in the atmosphere contribute to contaminate signalswith unwanted waveform components. Filters can reduce unwanted components in thewaveform, but have the risk of distorting the signal. Accordingly, filters represent a compromise

and filter values must be selected with an understanding of the components that make upwaveforms and how filters affect them.

7.41 Waveform ComponentsAny waveform can be decomposed into a number of sine waves of different frequencies andamplitudes and phase relationships. One mathematical technique to identify the constituentwaveforms is called Fourier analysis. Theoretically, an infinite number of appropriate sine wavesare required to regenerate a given waveform, but a good reproduction of a waveform can beconstructed by combining the most influential five to seven different sine waves (Figure 7.10).This indicates that there are relatively few principal components in any waveform such as theSNAP, CMAP and MUAP.

7.42 Filter Function

Filters exert their influence by reducing the principal sine wave components in a waveform basedon their frequency. A filter attenuates (reduces) the amplitude of component sine waves. Thedegree or amount of attenuation varies from 0% (no effect on the waveform) to 100% (nocontribution to the waveform). Filtering is accomplished by selecting the frequency component tobe reduced. There are filter values for high and low frequency components.

Filter circuits are made from resistors and capacitors and there are a number of different filtercircuits that can be constructed, and Butterworth filters are the type most commonly used in EMGinstruments. Although a filter setting is designated by a specific frequency value (eg, 10 Hz,10,000 Hz, etc), a filter functions to attenuate the signal over a range of frequencies that arecentered at the designated frequency. Thus, for any particular filter setting there is a frequency-attenuation curve (Figure 7.11). For low frequency filters, there will be a 33% reduction of thesignal at the nominal frequency with greater attenuation at lower frequencies and less attenuation

at higher frequencies. For high frequency filters, there will be a33% attenuation at the nominalfrequency with greater attenuation at higher frequencies and less at lower frequencies. Note,there is alternative nomenclature "high frequency" filters are also called "low pass" filters, and"low frequency" filters can be considered to be "high pass" filters. The 33% reduction, whenexpressed as on a log scale, is also called a 3 decibel (db) attenuation.

There is a special filter available on many EMG instruments, called a notch filter, that selectivelyfilters out 60 Hz noise (Figure 7.11). These filters are designed to mainly reduces 60 Hz, but theyalso attenuate to lesser degrees other frequencies above and below 60 Hz.


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7.42 Frequency Components of Waveforms and NoiseFilter settings are selected to reduce noise but not change the signal waveform to appreciabledegrees. The SNAP and MUAP waveforms have the highest frequency component waveformsand the CMAP the lowest (Figure 7.12). If high frequency filter settings were lowered during therecording of SNAP and MUAP, the steep rise time and high amplitude of the waveform would bereduced. The CMAP waveform has lower frequency components, and If low frequency filtersettings were raised during the recording of CMAP, the amplitude and duration would be reduced.

The most common noise is 60 Hz, but it is usually possible to reduce it by positioning of theground electrode, and a 60 Hz notch filter is rarely necessary (Figure 7.13). There is a very highfrequency noise due primarily to inherent electronic and bioelectric factors. This is observed mostcommonly during sensory studies due to the high display sensitivities required . There is also avery low frequency noise due to inadvertent body movements such as the patients respirationand cardiac movements and movements of the electromyographers hand holding the stimulatorand EMG needle during the study.

7.43 Filter SettingsSensory nerve conduction studies are recorded at high gain (20 V) and the SNAP is susceptibleto high frequency noise, and the high frequency filter is lowered to 2K Hz to reduce it. There arevery few low frequency components in the SNAP, and the low frequency filter is set to 20 Hz to

eliminate slow movement-related voltage changes.

Motor nerve conduction studies are recorded at low display settings (5 mV) and the CMAP ismuch less affected by noise. There are lower frequency components in the CMAP and the lowfrequency filter is set low (2 Hz) so as not to exclude them. There are few high frequencycomponents in the CMAP, but the high frequency filter is traditionally set high (10K Hz).

Needle EMG studies are recorded at a variety of display settings, from 20 V/div for assessmentof abnormal spontaneous activity to 2 mV/div for assessment of MUAP recruitment. Abnormalspontaneous activity (positive waves and fibrillation potentials) and the MUAP have a range ofcomponent frequencies. The filter settings are therefore broad, from 20 Hz low frequency valuesto 10K Hz high frequency values. There is a special situation when it is desirable to filter outmost low frequency components, and that is when making jitter and fiber density measurements

with a single fiber electrode. Under these circumstances, the low frequency filter is set to 500 Hzwhich intentionally distorts the low frequency components and brings out the high frequencysharp components at the expense of the low frequency components.

Sudomotor responses are a measure of small fiber autonomic nerve function. They measurevery slow potential changes (~0.1 Hz) due to changes in skin moisture (sweat). Accordingly, thelow filter setting is set to the lowest possible value.

Common filter settings are listed in Table 7.1.

7.5 Waveform Display

An oscilloscope is a device that displays waveforms. It consists of a screen that forms an X-Y

graph, with time on the X-axis and voltage on the Y-axis (Figure 7.13). The screen has graphmarkings that form either a square or rectangular grid, and markings are called divisions. Whilethe original oscilloscope was based on an electron beam moving back and forth, currentoscilloscopes in EMG machines are computer screens that emulate oscilloscopes.

The beam of the oscilloscope sweeps across the screen from left to right at a constant rate. Unitsfor the X-axis are in milliseconds per division (msec/div). Synonyms for the X-axis include "timebase" and "sweep speed". The beam can sweep across in two modes: 1) In the "free run" or"continuous sweep" modes, when the beam reaches the right side of the screen, it immediatelyreturns to the left side of the screen and starts a new sweep, and thus runs freely or sweeps


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continuously. The return or fly-back of the beam, although not instantaneous, can be consideredso in comparison to the sweep time. This allows one to look for and observe events in real timeor as they happen. Most needle EMG studies use this mode to observe events as they occur intime. 2) In the "triggered mode", the beam sits at the left side of the screen out of sight until it istriggered to sweep across. It immediately returns to the left side but does not sweep again until itis again triggered. This mode permits timing the sweep to capture an event. The timing can beset to catch the waveform when the sweep is in the mid portion of the screen. All nerveconduction studies use the triggered mode pressing the stimulus button simultaneously sendsthe stimulus to the patient to activate the nerve and to the oscilloscope to trigger the sweep. Thetriggered mode is also useful during the needle examination to capture motor unit actionpotentials for examination in greater detail.

7.52 Amplitude

Units for the Y-axis are in portions of a volt per division (mV/div or V/div). Synonyms for the Y-axis include "gain", "amplitude", "volts", and "sensitivity". Gain refers to the amplification by theamplifier, and sensitivity refers to the settings for the display. Although technically they refer todifferent operations, practically they are equivalent, and which operation is accomplished bychanging the settings varies among EMG machines.

Amplitude settings are adjusted to observe the whole waveform, with increasing the settings forlow amplitude signals and decreasing it for high amplitude signals. There is an order ofmagnitude range of amplitude settings for the various waveforms (Figure 7..14). Biologic andelectronic noise has an amplitude of ~4-5 V, and it is difficult to extract biologic signals < 5Vwithout using averaging techniques.

7.53 Display Settings for Nerve Conduction Studies

The goal is to display waveforms optimally, that is, maximally spread out in time and voltageacross the grid. Most nerve conduction and EMG waveforms fall into limited ranges of time andvoltage and appropriate X- and Y-axis values are easily determined. Most new EMG machinesautomatically select the appropriate display settings. At times it is advantageous to change thesettings to observe certain phenomena.

Time base: Distal latency values from stimulating at the wrist or ankle for normal sensory andmotor nerve conduction studies are up to 6 msec, and most proximal latency values fromstimulating at the elbow and knee are up to 13 msec. X-axis settings of 2msec/div provide 20msec of total viewing time, which is usually adequate to observe the waveforms (Figure 7.15).With stimulation at more proximal sites, such as the axilla and from Erb's point, latencies mayexceed 20 msec, requiring a change to 5 msec/div. However, some neuropathies have aprominent component of demyelination resulting in very slow conduction velocities, and distallatencies may be longer than 20 msec, and the sweep speed should be increased to 5 msec/divwhen a demyelinating neuropathy is a consideration.

F-wave latencies are 28 msec or greater for upper extremity nerves and 48 msec or greater forlower extremity nerves. Sweep speeds must be adjusted accordingly, to 5 msec/div for upper

and 10 msec/div for lower extremity nerves.

Amplitude: Normal sensory nerve action potential (SNAP) amplitudes range from 6 to 30microvolts (V) and a sensitivity of 10 v/div is appropriate. Midpalmar sensory responses mayreach amplitudes greater 100 v, and adjustments must be made accordingly. Very low SNAPresponses,


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of very low amplitude in pathologic states, and the display sensitivity should be increased to 500V/div is axonal pathology is suspected.

F-wave amplitudes are 100 to 500 v, and setting the gain or sensitivity to 200 v/div isreasonable.

7.54 Display Settings for Needle EMG Studies

Time base: The needle EMG study is commonly viewed in a free run or real time mode. It ispractical to use a 10 msec/div sweep speed because at 5 msec/div successive tracessuperimpose over and it is difficult to discern activity clearly, and at 20 sec/div there are gaps inthe trace as it moves along (Figure 7.16). An additional advantage of using 10 sec/div sweepspeeds is that the discharge frequency of motor unit action potentials (MUAP) can be estimated(see Section X).

MUAPs can be isolated and viewed in greater detail using a voltage trigger and delay line (seeSection X). Under these conditions, a sweep speed of 2 to 5 msec/div give maximal detail.

Amplitude: The needle EMG is performed in two stages. The first is assessment of abnormalspontaneous activity performed in the free run mode. Abnormal spontaneous activity, in the form

of p-waves and fibrillation potentials, has a wide amplitude range, from 5 to over 200 v. In orderto detect low amplitude abnormal spontaneous activity it may be necessary to use 20 v/div. Thesecond is assessment of MUAP recruitment patterns and general MUAP waveformconfigurations. The amplitude of normal MUAPs varies among muscles, and ranges from 800 to1500 v. It is reasonable to slowly increase the sensitivity in steps, until the MUAPs occupy thewhole screen, as opposed to immediately changing to 200 v/div. In the triggered mode, thesensitivity should be adjusted to view the MUAP maximally.

Common time and voltage sensitivity settings are in Table 7.2.

7.55 Raster and Superimposed Sweeps

Sweeps can be displayed individually, and rewritten until the appropriate waveform is obtained

and then saved. Alternatively, the same (or similar) sweeps can be saved consecutively as araster (Figure 7.17). This is routinely used for nerve conduction studies where waveforms fromstimulating at different sites along the nerve can be compared. It can also be used during needleEMG studies to observe MUAP waveform instability. The same waveforms can be combinedtogether in a superimposed mode for direct comparison (Figure 7.17).

7.56-Trigger and Delay Line

In the free run sweep mode during needle EMG, waveform activity is observed continuously.Usually, there are multiple motor units discharging, sometimes present at the same time andsuperimposing on themselves, making separated observation difficult. Further, the sweep speedis usually relatively slow (10 msec/div) and details of the waveforms are difficult to discern. Thereare two electronic features of the EMG machine that can be used to capture separate motor unit

waveforms and display them in greater detail.

There is an adjustable voltage trigger that can be set so that when the voltage of the waveformexceeds the set value the subsequent portions of the waveform occurring later in time can bedisplayed on a second oscilloscope screen (Figure 7.19). The voltage threshold can be set foreither negative or positive portions of the waveform. It is therefore possible to adjust the triggerthreshold up (negative on the screen) or down (positive on the screen) to detect a motor unitwaveform and isolate it from other motor units if some component exceeds voltage levels ofsurrounding motor units.


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One problem is that components of the isolated waveform that occurred before the voltagecrossed the trigger threshold will not be captured (Figure 7.19). This can be resolved by use ofthe delay line. EMG machines have a memory module (Figure 7.01) that stores a limited timesegment of data. Thus, at the time that the voltage crosses the trigger, the machine can retrievethe earlier components of the waveform. Generally it is sufficient to look back in time 4 to 8 msecto see the whole motor unit waveform. It is also possible to increase the time base of thecaptured motor unit to see it in greater detail.

7.57 Waveform Averaging

When a biologic signal is contaminated by random noise, signal averaging can reduce some ofthe variability due to noise and provide a more accurate waveform. The principle of averaging isbased on the fact that the portion of the signal attributed to noise will be random whereas thebiologic signal will be invariant. Thus, if the signal is averaged the random changes will averageto zero and the biologic changes will be preserved (Figure 7.20). Averaging is useful to delineatelow amplitude sensory signals and to clarify the onset and termination portions of MUAPwaveforms.

7.6 Nerve Stimulation

Electrical stimulation of a nerve is performed to assess conduction velocity and estimate thenumber of functioning nerve fibers. Two poles of a stimulating electrode are applied to the skinover the nerve (percutaneous stimulation). Square wave pulses of current are delivered. Boththe duration and the amplitude of the pulses can be adjusted pulse duration ranges from 0.01 to1.0 msec and amplitude from 0 to 100 mA. The two poles of the electrode are the anode(positive) and cathode (negative). The pulse of current passes through the skin and nervecoverings. For a single nerve fiber, the current causing an excess of positive charges under theanode resulting in hyperpolarization of the nerve fiber membrane, and a subsequentdepolarization of the memebrane under the cathode (Figure 7.21). When the degree ofdepolarization at a node of Ranvier is sufficient to open voltage-gated sodium channels in thenerve membrane, an action potential is generated and propagates in both directions along thefiber. At the anode, the action potential will reach a region of hyperpolarization and may die out.Thus it is important to orient the stimulating electrode such that the cathode is closer to the

recording electrodes to prevent anodal block of impulses.

Electrical stimulation is applied to a whole nerve, activating all nerve fibers. However,measurements of timing (distal latency, conduction velocity, F-wave latency) assess only thefastest conducting fibers. The duration of the response provides some measure of theconduction velocities of the remaining fibers. The amplitude of the response is an estimate of thenumber of nerve fibers contributing to the response.

The goal during nerve conduction studies is to use the least current that activates all fibers in thenerve. However, in the context of clinical nerve conduction studies the term supramaximalstimulation current is 120% of that needed to achieve a maximal response (Figure 7.18).Currents above supramaximal results in anodal jumping which shortens the latency, and possibleactivation of activate neighboring nerves. It is rarely necessary to use stimulation currents >50

mA in amplitude or >0.1 msec in duration, except in pathological conductions.


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SECTION 7: INSTRUMENTATION FIGURES

Figure 7.01Components of an EMG machine: differential amplifier, filters, digital-analog converter, computerwith software programs, oscilloscope (display), and electrical stimulator.

Figure 7.02Disk and ring electrodes used to record SNAP from nerve and CMAP from muscle.


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Figure 7.03Change in SNAP amplitude with different distances between recording and reference electrodes.Left: Change in distance between ring electrodes. Right: Change in distance between diskelectrodes.

Figure 7.04Composite drawing of muscles in the thenar eminence. The motor point of each muscle is uniqueto that muscle. A: Abductor pollicis. B: Flexor pollicis brevis. C: Opponens pollicis. Theoptimum position of the recording electrode for all muscles is determined empirically.


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Figure 7.05Change in CMAP waveform with different recording electrode positions over a muscle. Byconvention, deflection below baseline is positive and deflection above baseline is negative.Waveform with initial positive deflection indicates recording electrode some distance away from

motor point. Waveforms with initial negative deflection indicate proper recording electrodeposition, but optimum electrode position noted by highest amplitude and steepest slope to thewaveform.

Figure 7.06

Intramuscular electrodes showing arrangement of active and reference electrodes. A: Concentricelectrode with central active and cannula reference electrodes. B: Monopolar electrode withactive conical tip electrode and separate reference electrode. C: Single fiber electrode with smallactive electrode at side port and reference cannula. D: Macro electrode with large cannula asactive electrode and separate reference electrode. A single fiber electrode acts as separateelectrode to mark the motor unit.


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Figure 7.07Diagram of differential amplifier. A: Internal arrangement consisting of two amplifiers connected

in inverse relationship. B: Merged into one amplifier.

Figure 7.08Operation of differential amplifier. A: Active (G1) input receives biologic negative voltage andreference (G2) receives zero voltage, resulting in output showing full biologic voltage. B: Activeinput receives biologic voltage, but reference also receives partial biologic voltage, resulting inoutput showing reduced voltage.


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Figure 7.09Common mode rejection feature of differential amplifier. Active input receives biologic signalsuperimposed on 60 Hz sine noise and reference input receives only 60 Hz sine wave noise,resulting in rejection of common 60 Hz noise.

Figure 7.10Constituent sine waveforms identified by Fourier analysis. A: Original square waveform can bedecomposed into component sine waves that can be recombined to approach original waveform(only three components shown). B: Similar decomposition and recombination for more complexwaveform. Waveforms to the right represent partial recombinations.


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Figure 7.11Filter frequency curves. X-axis: Filter frequency from low (left) to high (right) values. Y-axis:

Amplitude of filtered response based on original (unfiltered) amplitude. Low frequency (highpass) filter curve at the left, and high frequency (low pass) filter curve at the right. Nomenclatureis to designate filter value where the signal is reduced by 30%. 60 Hz notch filter curve in themiddle.


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Figure 7.12Sine wave components of SNAP, CMAP and MUAP waveforms. Below each waveform are aseries of sine waves of different frequencies and amplitudes that represent the dominantcomponent frequencies.


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Figure 7.13Oscilloscope grids. Top: Square grid with X-axis indicating time (msec/div) and Y-axis indicatingvoltage (mV or V/div). Bottom: Rectangular grid.


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Figure 7.14Scale of waveform amplitudes with range of normal values. Noise: < 5V. SNAP: 6 to 50 V.CMAP: 2000 to 20000 V (2 to 20 mV). MUAP: 600 to 2000 V (.6 to 2 mV).

Figure 7.15

Optimizing X-axis and Y-axis settings in nerve conduction studies. A: X-axis settings. Top: 5msec/div, trace compressed. Middle: 2 msec/dib, usual sweep speed. Lower: 1 msec/dib, tracetoo spread out. B: Y-axis settings. Top: 2 mV/div, trace amplitude exceeds display capabilitiesand waveform is clipped. Middle: 5 mV/div, usual setting. Bottom: 10 mV/dib, trace compressed.


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Figure 7.16Effect of different X-axis sweep speeds during needle EMG study. A: 5 msec/div with over lap ofsuccessive sweeps. B: 10 msec/div with separation of sweeps. C: 20 msec/div with xx

Figure 7.17Raster and superimposed waveform display. Top four waveforms in raster format bottom fibersuperimposed display of the four waveforms. A: CMAP showing progression of latencies andwaveforms with stimulation at successively more proximal sites. B: MUAP showing variability ofsatellite potential.


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Figure 7.19Voltage trigger and delay line. A: Sweep in free run mode. Horizontal line at left indicatesvoltage trigger level. B: B1 shows sweep that occurs after waveform has crossed trigger voltagelevel. However, the early portion of the waveform is lost. B2 and B3 shows the effects of thedelay line (memory) allowing successively earlier portions of the waveform to be displayed. C:Waveform in A, now captured by the voltage trigger and delay line and displayed at 2 msec/div.


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Figure7.20:Effects of averaging. A: SNAP. 1 and 2: Very low amplitude signal (20 V/div). 3: SNAPpresent but contaminated by noise (5 V/div). 4: Signal averaged 6 times resulting in more clearinflection points. B: MUAP. 1: Averaged waveform with clear boundaries for marking (verticallines). 2: Superimposed waveforms those in bold were used in the average, those in light were

not. Note the variability of waveforms in light that would have contaminated the waveform.


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Figure 7.21Ion currents involved in electrical stimulation of a nerve. Anode: Membrane hyperpolarization.Cathode: Membrane depolarization, and if sufficient to exceed a threshold an action potential isgenerated.


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Table 7.1Common filter settings for electrodiagnostic studies.

Mode Low frequency (High pass) High frequency (Low pass)Sensory nerve studies 20 Hz 2K HzMotor nerve studies 2 Hz 10K HzSudomotor potential studies 0.2 Hz 1.5K HzNeedle EMG studies 20 Hz 10K HzSingle fiber needle studies 500 Hz 10K Hz

Table 7.2Common time and voltage sensitivity settings for electrodiagnostic studies.

Mode Time base VoltageSensory nerve studies 2 msec/div 10 V/divMotor nerve studies 2 msec/div 5 mV/divF-wave studies 5 or 10 msec/div 200 V/divSudomotor potential studies 1 sec/div 500 V/divNeedle EMG studies 10 msec/div 20 to 200 V/div

Single fiber needle studies 1 msec/div 200 V/div

section 7 instrumentation

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