chapter 25. neuromuscular transmission monitoring

8/11/2019 Chapter 25. Neuromuscular Transmission Monitoring

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Chapter 25

Neuromuscular Transmission MonitoringMuscle relaxants are employed in anesthesia to provide muscle relaxat ion and/orabol ish pat ient movement . Numerous s tudies have documented enormous var iat ion

in pat ients ' responses to muscle relaxants . Disease s tates and perioperat ive

medicat ions can also modify the responses of these medicat ions ( 1 ) . The depth of

neuromuscular block (NMB) should be monitored when muscle relaxants are used

to avoid drug overdosage or underdosage and residual NMB during recovery

(2 ,3 ,4 , 5 ,6 , 7 ).

EquipmentMonitor ing the magni tude of NMB is acco mplished by del iver ing an electr ical

s t imulus near a per ipheral motor nerve and evaluat ing the evoked response of the

muscle(s) innervated by that nerve.

S t i m u l a t o r

Several s t imulators are shown in Figure 25.1 . Desirable features include

compactness, l ight weight , and

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simplici ty. Most are bat tery-operated with a means to check the bat tery s tatus.

Mounting brackets for securing the device are desirable. A st imulator may be in a

module in a mult iparameter monitor. The abi l i ty to del iver information to an

automated record ( Chapter 28 ) should be considered when choosing a s t imulator.


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View Figure

Figure 25.1 Neuromuscular stimulators. A: This simpledevice has only two patterns of stimulation: tetanus andsingle twitch. The delivered current cannot be varied and isnot displayed. Note the metal ball electrodes. (Courtesy ofProfessional Instruments, a subsidiary of Life Tech, Inc.) B: This unit has three modes of stimulation: single stimulus(twitch), tetanus, and TOF. The current is varied by using arheostat at the side, but there is no display of the current

being delivered. C: This unit has four patterns ofstimulation: single twitch (available at 0.1 and 1 Hz), TOF(which can be repeated automatically every 12 seconds),50-Hz tetanus, and DBS. It also is capable of delivering thestimulus pattern for obtaining a PTC. The selected current isdisplayed in the window. Failure to deliver this current willcause a mark to be displayed to the right of the word ERROR . Note that the connections for the lead wires are ofdifferent colors. D: This unit has three modes ofstimulation: single stimulus (which can be delivered at 0.1,1, or 2 Hz), tetanus (which is available at a frequency of 50to 100 Hz), and TOF. Stimulus current is varied by using arheostat at the side. The delivered current is displayed in awindow, to the left of which is an indicator that lights whena stimulus is being delivered. A battery status check buttonis present.

CurrentCurrent , not vol tage, is the determining factor in nerve s t imulat ion. Because skin

resis tance may change, only a s t imulator that automatical ly adjusts i ts output to

maintain a constant direct c urrent can ensure unchanging st imulat ion with changes

in skin resis tance. Wiping the

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skin with alcohol wil l remove insulat ing skin oi ls and lower the resis tance.

The force of muscle c ontract ion is proportional to the number of act ivated muscle

f ibers . I f a motor nerve is s t imulated with suff icient current , a l l of the muscle f ibers

suppl ied by that nerve wil l contract . The current required for this is cal led the

maximal current . In the cl inical set t ing, s t imuli of g reater than maximal

(supramaximal) intensi ty are used to ensure that maximal s t imulat ion is del ivered i f

resis tance increases. In the majori ty of pat ients , a current of 30 mil l iamperes (mA)


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will produce a supramaximal response when the ulnar nerve is stimulated ( 8 ) . When

the poster ior t ibial nerve is s t imulated, higher currents are needed ( 9 ). A

supramaximal current is general ly 2.5 to 3 t imes higher than the lowest currentcapable of el ici t ing an evoked response ( threshold current) ( 10 ) . Higher currents

may be needed in pat ients with edema ( 11 ,12 ) or diabetes ( 13 ). Much lower

currents (5 to 8 mA) are needed when needle electrodes are used ( 14 ).

A curre nt display is us ef ul in al er ti ng th e user to th e pos sibi l i ty of a di sconne c ti on ,

broken lead, weak bat tery, or poorly conduct ing electrodes, because these

problems wil l cause the current to be reduced. Some st imulators have an alarm to

warn when the selected current is not being delivered.

A subma ximal cu rr ent ma y be bett er f or awa ke pa t ien ts o r for thos e re cov e ri ng f ro m

anesthesia, because patient discomfort increases with the intensity of the

st imulat ing current ( 15 ,16 , 17 , 18 ). Use of a submaximal current may result in more

rel iable detect ion of residual NMB when visual or t act i le monitoring is used ( 19 ). A

submaximal current is not rel iable for general NMB monitor ing.

FrequencyThe frequency of s t imuli is usual ly expressed in Hertz (Hz) , which is cycles/second.

One Hz is one cycle/ second, and 0.1 Hz is equal to 1 s t imulus every 10 seconds.

With a nondepolar izing block, increased st imulus frequency wil l s horten the onset

t ime and prolong the durat ion of act ion ( 20 ,21 ).

WaveformThe stimulus waveform should be rectangular (square wave) and monophasic.

Biphasic waves may produce repet i t ive s t imulat ion, which can lead to

underestimation of the depth of NMB present.

DurationThe durat ion should be 300 s or less ( 20 ) . I f the durat ion of the pulse is over 0 .5

msec, a second action potential may be triggered.Stimulation Patterns

S i n g l e Tw i t c h

Single- twitch (T 1 ) s t imuli are usual ly del ivered at a f requency of 0.1 or 1 Hz. A

frequency greater than every 10 seconds is associated with a progressively

diminished response and could resul t in overest imat ing the NMB.

The control response s t rength is noted ( Fig. 25.2A ). The s t rengths of subsequent

twitches are then compared with the control and expressed as a percentage of the


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control (s ingle-pulse or - twitch depression, T 1 %, T1%, T 1 :T c ) . With both a

nondepolar izing and a depolar izing block, there wil l be progressive depression of

the response as the block develops. A decrease in temperature wil l a lso cause areduced response ( 22 ,23 ,24 ,25 ,26 ).

The single s t imulus is useful in establ ishing a supramaximal s t imulus and for

ident i fying when condi t ions sat isfactory for intubat ion have been achieved. I t can

be used ( in conjunct ion with a tetanic s t imulus) to monitor deep levels of NMB (the

posttetanic count , discussed below).

There are several disadvantages associated with using single twitch. There needs

to be a control . I t cannot dis t inguish between a depolar izing and nondepolar izing

block. Most important ly, the response 's return to control level does no t guarantee

that ful l recovery from NMB has occurred.

T r a i n - o f - f o u r

Train-of-four (TOF, T 4 , T 4 /T 1 ) consists of four s ingle pulses of equal intensi ty

del ivered at intervals of 0.5 seconds (2 Hz) ( Fig. 25.2B ) ( 27 ) . TOF should not be

repeated more frequent ly than every 10 to 12 seconds ( 4 ). Many modern

st imulators do not al low the T OF to be repeated more of ten. Use of TOF ev ery 10

seconds wil l resul t in a shorter onset t ime for NMB than i f i t i s used every 20

seconds ( 21 ,28 ).

With the control response (before any relaxant has been given) , a l l four responses

are the same. The pat tern seen with a depolar izing block differs f rom that of a

nondepolar izing block ( Fig. 25.2B ). With a par t ial depolar izing block, there is an

equal depression of al l four twitches. With a nondepolarizing block, there is

progressive depression of height with each twitch (fade) . As the block is deepened,

the fourth twitch wil l be el iminated f i rs t , then the third, and so on ( Fig. 25.3 ).

Counting the number of twitches (train-of-four-count or TOFC) permits quantitative

assessment of a nondepolar izing block. Wi th recovery or reversal of a

nondepolar izing block, the TOFC increases unt i l there are four responses, thenfade decreases.

The t rain-of-four rat io (T r , T 4 rat io, T 4 :T 1 , T r %, TR%, TOF rat io, TOFR) is the rat io

of the ampli tude of the fourth response to that of the f i rs t , expressed as a

percentage or a fraction. It provides an estimation of the degree of nondepolarizing

NMB. In the absence of nondepolar izing block, the TOFR is approximately 1

(100%). The deeper the block, the lower the TOFR ( Fig. 25.3 ) . Since determining


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the TOFR requires that four twitches be present , i t c annot be used to monitor a

deep block.

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View Figure

Figure 25.2 Patterns of stimulation and response. A: Single-stimulus stimulation at 1 Hz (1 stimulus/second).The height of the control twitches are noted. With either adepolarizing or a nondepolarizing block, twitch height isdecreased. B: Train-of-four stimulation. Four successivesingle stimuli are delivered with 0.5-second intervals. With

a nondepolarizing block, there will be progressivedepression of the response with each stimulus (fade). With adepolarizing block, the responses will be depressed equally.C, D: Double-burst stimulation. Three stimuli are deliveredat 50 Hz, followed 0.75 seconds later by two or threesimilar stimuli. There will be depression of the response tothe second burst with a nondepolarizing block. Note theincreased height of the response to the first burst comparedwith that seen with TOF stimulation. TW, time weight TOF,train of four; DBS, double-burst stimulation.

View Figure

Figure 25.3 Onset and progressive deepening ofnondepolarizing block using train-of-four stimulation.When there is no NMB present, all four responses are equal.With onset of the block, there is progressive depression oftwitch height with each twitch (fade). As the block

progresses, the last twitch is lost and the TOFC is less than4. TOFR, train-of-four ratio; TOFC, train-of-four count.

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Accurate as ses sme nt of th e TOFR ma y not requ i re a supra ma xima l s t imul us ( 15 ).

Test ing at 10 mA above the lowest current at which four responses can be el ici ted

may provide values that are consistent with those of s upramaximal test ing ( 29 ).The TOF pat tern has several advantages. I t i s a more sensi t ive indicator of residual

NMB than the s ingle twitch. A control is not necessary. I t can dis t inguish between a

depolar izing and a nondepolar izing block and is of v alue in detect ing and fol lowing

the development of a phase II block fol lowing succinylchol ine administ rat ion.

The main disadvantage of TOF is i ts poor performance at both ex tremes of NMB,

deep relaxat ion or near complete recovery ( 4 , 30 ,31 ,32 , 33 , 34 , 35 ) . Tact i le or visual

observat ion of the TOFR is of l i t t le value above a rat io of 0.40.5.

Te t a n u sTetanus is a rapidly repeated (e.g. , 50, 100 or even 200 Hz) stimulus. In the

absence of NMB, this c auses sustained contract ion of the s t imulated muscles . With

a depolar izing block, the response wil l be depressed in ampli tude but sus tained.

With a nondepolar izing block, the response is depressed in ampli tude and the

contract ion is not sustained (fade or decrement) . With profound NMB, there is no

response. Fade af ter 50 Hz tetanic s t imulat ion is a more sensi t ive index of NMB

than single twitch but not suff icient ly sensi t ive to be used for assessing adequate

recovery ( 36 ) . Studies differ on the s ignif icance of fade af ter 100 Hz ( 36 ,37 ) .

The most commonly used frequency is 50 Hz, because i t s t resses the

neuromuscular junct ion to the same extent as a maximal voluntary effor t . Fade may

not be seen a t lower frequencies when a s ignif icant nondepolar izing block is

present . Use of 100 Hz a l lows more s ensi t ivi ty in evaluat ing residual paralysis ( 37 )

and is more useful in monitor ing profound NMB ( 38 ) .

The durat ion of the tetanic s t imulus is important because i t affects fade. The

standard durat ion is 5 seconds. Tetanic s t imulat ion should not be repeated more

often than every 2 minutes ( 39 ,40 ). Some newer stimulators l imit how frequently it

can be used.Post- tetanic faci l i ta t ion (potent iat ion, PTF) is a temporary increase in response to

st imulat ion fol lowing a tetanic s t imulus. I t i s seen with a nondepolar izing, but not a

depolar izing, block ( 39 ,41 ) . I t i s maximal at around 3 seconds and lasts up to 2

minutes.

When the NMB is so profound that there is no response to s ingle twitch or TOF

st imulat ion, i t may be possible to est imate NMB by using the posttetanic count

(PTC) ( 42 ) . This is performed by administer ing a tetanic s t imulus of 50 Hz for 5


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seconds. After a 3-second pause, s ingle- twitch s t imuli are appl ied at 1 Hz, and the

number of (posttetanic) responses is counted. The number of twitches el ici ted

increases as the depth of NMB decreases. The t ime to appearance of the f i rs ttwitch in a TOF is inversely related to the number of posttetanic twitches present

(43 ,44 ,45 , 46 , 47 ,48 ,49 ) . An ev en deeper block can be monitored by count ing the

number of responses fol lowing 100-Hz tetanus ( 38 ).

A s ign if icant di sadvan tage of te tan ic s t imul a ti on is tha t i t i s ver y painf ul and sho uld

be avoided in the conscious pat ient .

D o u b l e - b u r s t S t i m u l a t i o n

Double-burst s t imulat ion (DBS, mini tetanus) consists of two short sequences of 50

Hz tetanic stimuli separated by 750 msec. The two most commonly used are DBS 3 , 3 and DBS 3 , 2 . DBS 3 , 3 consis ts of three 0.2-msec impulses at 50 Hz, fol lowed 750

msec later by an ident ical burst ( Fig. 25.2C ). DBS 3 ,2 consis ts of three impulses

followed by two such impulses 750 msec later ( Fig. 25.2D ). Another permutation of

DBS is DBS 3 , 3 80-40, which is three s t imuli at 80 Hz fol lowed 750 msec later by

three s t imuli at 40 Hz. A modif ied DBS consist ing of f i rs t two st imuli of 0.3 ms

durat ion at 50 Hz and then two st imuli of 0.2 ms durat ion at 50 Hz has also been

used ( 50 ).

The pr imary use of DBS has been to detect residual NMB. Studies show that fade

(response to the second burst weaker than that to the f i rs t ) is more readi ly detected

with DBS than TOF using visual or tact i le monitor ing ( 19 ,30 , 31 , 32 , 33 , 51 ,52 ) . I t a lso

has been used for int raoperat ive assessment of NMB ( 53 ) . DBS and TOF have a

close relat ionship over a wide range of NMB ( 4 , 54 ,55 ) . Another use of DBS is to

assess deep block, s ince the f i rs t twi tch in double burst can be detected at deeper

block levels than the f i rs t twi tch in TOF ( 53 ,56 ,57 , 58 ) .

DBS causes more discomfort to the awake pat ient than TOF st imulat ion but less

than tetanic s t imulat ion ( 16 ) . I t can be used at submaximal currents . This causes

less discomfort in the a wake pat ient and, in most cases, is more rel iable thantest ing with supramaximal s t imuli ( 10 ).

DBS should not be repeated at intervals of less than 12 seconds ( 32 ) . Caut ion

should be used when switching between double-burst and TOF stimulation ( 59 ) . Up

to 92 seconds may be required before the responses are stabilized.

E l e c t r o d e s


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Stimulat ion is achieved by placing two electrodes along a nerve and passing a

current through them. St imulat ion can be carr ied out ei ther t ranscutaneously using

surface electrodes or percutaneously with needle electrodes.

Types

S u r f a c e E l e c t r o d e s

Surface (gel , patch, pad) electrodes have adhesive surrounding a gel led foam pad

in contact with a metal disc with a knob for at tachment to the electr ical lead. They

are readi ly avai lable, easi ly appl ied,

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disposable, self-adhering, and comfortable. The electrodes can be those usually

used to monitor the electrocardiographic tracing. The electrode-skin resistance

decreases with a large conduct ing area, as do sk in burns and pain. However, a

large conduct ing area may make i t di ff icul t to obtain supramaximal s t imulat ion and

may st imulate mult iple nerves, so i t may be bet ter to use pediatr ic electrodes. The

best resul ts are obtained i f the skin is properly c leansed and rubbed with a n

abrasive ( 20 ).

There are electrodes special ly designed for per ipheral nerve s t imulat ion. These

have a d ifferent thickness than electrocardiogram (ECG) electrodes and chemicalbuffers to maintain skin surface pH.

M e t a l E l e c t r o d e s

Some st imulators are suppl ied with two metal bal ls or plates s paced about 1 inch

apart, which attach directly to the stimulator ( Fig. 25.1A ). These are convenient to

use but may not make good contact. Burns have been reported with their use ( 60 ).

N e e d l e E l e c t r o d e s

Needle electrodes may be useful when supramaximal s t imulat ion cannot be

achieved by using surface electrodes. This usual ly occurs when the skin is

thickened, cold, or edematous and in obese, hypothyroid, diabet ic , or renal fai lure

pat ients ( 20 ,61 ).

Addi t io na l com pl ic at io ns (b rok en nee dles , in fec ti on , burn s , and ne rv e da ma ge) are

associated with their use. Needle electrodes carry a greater r isk of direct muscle

st imulat ion than surface electrodes ( 62 ).

Polarity


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View Figure

Figure 25.4 For tactile evaluation of thumb adduction, thehand is supine and a slight preload is applied. (Picturecourtesy of Biometer.)

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M e c h a n o m y o g r a p h y

The mechanomyogram (MMG) ut i l izes a force-displacement t ransducer, such as a

st rain gauge, at tached to a f inger or o ther par t of the body that can be rest rained

by a preload and wil l move when st imulated. The t ransducer converts the

contract i le force into an electr ical s ignal , which is amplif ied and displayed on a

monitor screen or recorded on a chart . Single- twitch h eight , response to tetanic

st imulat ion, and the T 4 ratio can be accurately measured by using an MMG ( 75 ) .

Using the MMG entai ls a n umber of diff icul t ies . These devices are cumbersome and

diff icul t to set up for s table and accurate measurements ( 76 ) . Proper t ransducer

orientat ion, isometr ic condi t ions, and appl icat ion of a s table preload are required

(77 ). Maintenance of muscle temperature within limits is important for accurateresul ts . Mechanomyography is rarely used cl inical ly but is regarded as the gold

standard for scient i f ic measurement of neuromuscular response ( 5 , 78 ) .

A c c e l e r o m y o g r a p h y

With acceleromyography (ACG, AMG), a thin piezoelectric transducer or a small

aluminum rod with electrodes on both s ides is f ixed to the moving part ( 79 , 80 ) ( Fig.

25.5 ). When the part moves, a voltage which is proportional to the acceleration of

the moving part is generated. This me thod requires unrestr icted movement of the


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muscle being st imulated. An elast ic preload can be appl ied to return the moving

part to i ts or iginal posi t ion.

ACG can be us ed to as ses s NMB at th e hand wi th the pati en t' s arm tuc ked a t th eside as long as the thumb can move freely. A protect ive device can be used to

allow thumb motion while protecting the hand and forearm ( 81 ).

Most s tudies show a fai r ly c lose relat ionship between TOFRs measured by ACG

and the MMG ( 29 ,80 , 82 , 83 ,84 ,85 ,86 ,87 ,88 ,89 ,90 , 91 ) or electromyography (EMG)

(75 ,85 ,92 , 93 ), although the results are not interchangeable. Some studies show

poor correlat ion ( 94 , 95 ). In awake patients, the results are affected by extra

movements to which the thumb may be subjected, leading to poor repeatabili ty ( 96 ).

Accelero metry is eas y and con venien t to us e, relat iv e inexpe ns ive, and can be

interfaced with a computer. I t does not require a preload. I t gives more accurate

resul ts than visual or tact i le evaluat ion ( 68 ,92 ).

K i n e m y o g r a p h y

Kinemyography (KMG) ut i l izes a bending sensor that is placed between the thumb

and foref inger ( Fig. 25.6 ) . The core of the sensor is a piezoelectr ic mater ial ( 97 ).

Movement is determined by the change in shape of the mater ial when i t i s bent by

adductor pol l icis muscle contract ion. When the piezoelectr ic mater ial changes

shape, the electr ical charge in the mater ial is redist r ibuted, and this leads to an

electron f low to balance the charges. This f low is measured as a potent ial change

that is proportional to the amount of dis tor t ion. The hand need not be immobil ized

since the position and direction of the thumb do not affect the measurement as long

as the thumb is able to move freely. This device is in a module that can be

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added to a multipurpose monitor ( Fig. 25.7 ). The results of the neuromuscular

test ing are displayed on the monitor screen. This technology can measure TOF,

double burst , and single twitch.


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View Figure

Figure 25.5 Accelerography. The piezoelectric wafer isattached to the moving part-in this case, the thumb. Whenthe thumb moves, an electrical signal proportional to theacceleration is produced. The monitor allows determinationof single-twitch depression, TOF count or ratio and/or thePTC. Responses can be displayed by using the printer.(Courtesy of Biometer International A/S.)

View Figure

Figure 25.6 Sensor for kinemyography. The sensor issecured with tape.

KMG has been compared with mechanomyography ( 98 ,99 ) . There was a greement

as to the time to intubation and recovery, but KMG lagged behind the MMG in

determining recovery from NMB.

P i ez o e l ec t r i c F i lm

This method uses a disposable piezoelectr ic f i lm ( 10 0 ) . This is placed so that i t

spans a movable joint ( 10 1 ) . Muscle movement f rom evoked st imulat ion bends the

f i lm and generates a vol tage that is proport ional to the amount of bending. I t has


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record. Most have alarms for funct ioning errors , loose connect ions, increased skin

resis tance, absence of supramaximal s t imulat ion, and the l ike. Most show the EMG

waveform and automatical ly adjust the gain so that i t occupies the ful l s cale.

View Figure

Figure 25.7 The TOF count and ratio are shown on themonitor. The scale at the bottom shows the frequency ofstimulation (every 20 seconds) and how much time haselapsed since the last stimulus. This information comesfrom a Kinemyograph.

View Figure

Figure 25.8 Electromyography monitor. The T 1%, TOFR,and TOFC can be measured and are displayed in the boxes

to the right of the printer. Responses can be recorded byusing the printer. A T 1% high alarm is present. TOFstimulation is performed automatically every 20 seconds.(Courtesy of Datex Medical Instrumentation, Inc.)

With a nondepolarizing NMB, the action potential amplitude is decreased, and there

is fade with TOF. Frequently, the ampli tude does not return to 100% of control with

recovery, al though the TOFR wil l equal approximately 100%. Different hand

posi t ions may affect the resul ts ( 11 2 ) .

A num ber of s tu dies comp aring EMG an d MMG hav e bee n pu b lishe d

(86 ,113 , 114 ,115 ,116 ,117 ,118 ,119 ,120 ,121 ,122 ,123 , 124 ) . With a nondepolar izing


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superci l i i muscle can be monitored by placing the microphone above the medial

portion of the eyebrow ( 142 , 14 3 , 14 4 ,14 5 ) . The muscles of the larynx can be

monitored by placing the sensor in the vest ibular fold just la teral to the vocal cords(110 , 14 6 ,147 ).

Studies comparing phonomyography, ACG, and mechanomyography by using hand

and corrugator superci l i i muscles show some agreement , a l though the resul ts are

not interchangeable ( 99 , 14 0 ,14 1 ,14 3 ,14 4 ,145 ,146 ).

The phonomyogram is easy to use and can be used on a number of different

muscles . I t provides a s table basel ine with relat ively few dis turbances from ar t i facts

(145 ) . Data c an be t ransferred to an automated anesthesia record.

Since this method monitors lo w frequency sounds, ar t i facts are possible. Vessel

pulsat ions can cause small waves in the basel ine. Electrosurgery uni ts may cause

interference. The mic rophone may come off the ski n.

Choice of Monitoring SiteThe si te of s t imulat ion should be away from the surgical f ield. I f visual or tact i le

monitor ing is to be used, the locat ion must be accessible to the anesthesia

provider. I f a muscle in an arm or l eg is used, the blood pressure should be

measured on a different extremity. An ar ter iovenous shunt does not contraindicate

that arm being used to monitor NMB ( 14 8 ) . I f the pat ient has an upper-motor-

neuron lesion, a nerve in an affected (paret ic) extremity should not be used,

because i t may falsely show resis tance to nondepolar izing drugs ( 14 9 ,15 0 ) . I f

possible, the nerve s t imulator electrodes should be placed on a d ifferent extremity

from the pulse oximeter probe to avoid ar t i facts ( 15 1 ,152 ,153 ).

U l n a r N e r v e

The ulnar nerve is most commonly used, and the adductor pollicis (thumb) muscle

is most commonly monitored. Because this muscle is on the s ide of the arm

opposi te the s i te of s t imulat ion, there is l i t t le direct muscle s t imulat ion. However,residual NMB may be easier to detect tact i lely by using the index f inger ( 71 ) .

The ulnar nerve can be s t imulated at the elbow, wrist , or hand ( Figs. 25.9 , 25.10 ) .

St imulat ion at the wrist wi l l produce thumb adduct ion and f inger f lexion. St imulat ion

at the elbow produces hand adduction as well . If an MMG or electromyogram is

used for measuring the response, the s t imulat ing electrodes should be placed at

the wrist to l imit hand motion.


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View Figure

Figure 25.9 Placement of electrodes for ulnar nervestimulation. A: The electrodes are placed along the ulnaraspect of the distal forearm. B: The electrodes are placedover the sulcus of the medial epicondyle of the humerus.

At the wr is t , the two elec trode s s ho uld be plac ed a lo ng the me d ia l aspec t of th e

distal forearm, approximately 2 cm prox imal to the proximal wris t skin crease with

the negat ive electrode dis tal ( 18 ) ( Fig. 25.9A ). There, the ulnar nerve is superf icial .

Al te rn a te l y, the po si ti ve el ec tro de may be pl ac ed on the dors a l s ide of th e wr i s t

(Fig. 25.10 ) . At the elbow, the electrodes should be placed over the sulcus of the

medial epicondyle of the humerus ( Fig. 25.9B ). Caution must be exercised to

ensure that the electrodes do not cause ulnar nerve compression ( 15 4 ). The

electrodes may also be placed on the hand with the negative electrode on the palm

between the base of the thumb and the second finger and the positive electrode in

the same posi t ion on the dorsal s ide of the hand ( 15 5 ).


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View Figure

Figure 25.10 Alternate placement of electrodes for ulnarnerve stimulation. The negative electrode is placed alongthe ulnar aspect of the ventral side of the wrist. The positiveelectrode is placed on the dorsal side.

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View Figure

Figure 25.11 Sites for electrodes for electromyographymonitoring with ulnar nerve stimulation and recording fromthe dorsal interosseous muscle. The active receivingelectrode is placed in the web between the index finger andthe thumb and the reference electrode, at the base of thesecond finger. Ref, reference electrode; AR, activereceiving electrode; G, grounding electrode; N, negative-stimulating electrode; P, positive-stimulating electrode.

When EMG monitor ing is used, the recording electrodes can be placed over the

hypothenar, thenar, or dorsal interosseous muscle. The electr ical resis tance of the


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palm skin may v ary because of sweat product ion and may be increased in manual

workers ( 156 ) . The dorsum of the hand is less affected than the palm in both

respects , so the dorsal interosseous muscle may be preferred. To record thereact ion of the dorsal interosseous muscle, the act ive receiving electrode is placed

in the web between the index finger and the thumb and the other electrode at the

base of the second finger ( Fig. 25.11 ) . Surface electrodes are s imple to f ix here,

easy to maintain in position, and seldom are disturbed by hand movements ( 15 7 ).

For the hypothenar EMG, both electrodes are placed on the palmar side over the

hypothenar eminence or the act ive electrode is placed on the hypothenar eminence

and the other below the second l ine on the r ing f inger or at the base of the dorsum

of the f i f th f inger ( Fig. 25.12 ) ( 15 8 ,159 ). If the thenar muscle EMG is recorded,

electrodes are placed on the thenar eminence and the proximal phalanx of the

middle or index f inger or the lateral s ide of the base of the thumb ( Fig. 25.13 ) .

Abduct io n of th e thu mb wi th a cons ta nt p re tensio n wi l l br in g th e mu scl es closer to

the skin and minimize movement ( 14 ,109 ).

View Figure

Figure 25.12 Placement of electrodes for electromyographymonitoring from the hypothenar eminence. The activeelectrode is placed over the hypothenar eminence. Thereference electrode may be placed more distally on thehypothenar eminence, below the second line on the ringfinger or at the base of the fifth finger as shown. Ref,reference electrode; AR, active receiving electrode; G,grounding electrode; N, negative-stimulating electrode; P,

positive-stimulating electrode.

For tact i le assessment , the thumb should be held in s l ight abduct ion and the

observer 's f ingert ips placed over the dis tal phalanx in the direct ion of movement

(160 ) ( Fig. 25.4 ) . Preloading the thumb with a rubber band may i mprove visual


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the lateral head of the gastrocnemius muscle ( 166 ) . The use of this muscle may

cause significant leg movement, which may distract the surgeon ( 78 ) .

P o s t e r i o r T i b i a l N er v eTo stimulate the posterior t ibial nerve, electrodes are placed behind the medial

maleolus and anter ior to the Achi l les tendon at the ankle ( Fig. 25.14 ). Stimulation

causes plantar f lexion of the foot and big toe. ACG can be used at this s i te

(9 ,167 ,168 , 16 9 ) . I f EMG monitor ing is used, the receiving electrodes are placed on

the f lexor hal lucis brevis on the plantar surface of the foot or on the intermetatarsal

muscles with the reference electrode on the big toe ( Fig. 25.15 ) .

The poster ior t ibial nerve s i te offers many advantages. I t i s especial ly useful in

chi ldren, when i t i s di ff icul t to f ind room on the arm because of other monitors orinvasive l ines, and when the hand is inaccessible or for o ther reasons such as

amputation, burns, infection, or head and neck procedures ( 170 ).

View Figure

Figure 25.14 Placement of electrodes for stimulating the posterior tibial nerve. The negative electrode is placed behind the medial malleolus, anterior to the Achilles tendon.The positive electrode is placed just proximal to thenegative electrode. Stimulation causes plantar flexion of thegreat toe.

Compared with the ulnar nerve, the poster ior t ibial nerve displays a lag t ime with aslower onset of relaxat ion ( 16 9 ,171 , 17 2 ) . Most s tudies show l i t t le difference in the

time to recovery from the neuromuscular relaxation ( 170 , 17 1 ,17 2 ,173 , 174 , 17 5 ). The

probabi l i ty of tact i le detect ion of fade in response to TOF or DBS is less at the

great toe than at the thumb ( 73 ).

P e r o n e a l N e r v e

To st imulate the peroneal ( lateral popl i teal) nerve, electrodes are placed on the

lateral aspect of the knee ( Fig. 25.16 ) . I t may be necessary to t ry different posi t ions


22/74

to achieve the best response ( 17 6 ,17 7 ) . St imulat ion causes dorsi f lexion of the foot .

Compared with the ulnar nerve, the peroneal nerve shows a s lower onset of

relaxat ion and the muscles show greater resis tance to NMB ( 177 ) .

View Figure

Figure 25.15 Electromyography monitoring using the posterior tibial nerve. The active receiving electrode is placed over the flexor hallucis brevis and the referenceelectrode, on the big toe. Ref, reference electrode; AR,active receiving electrode; G, grounding electrode; N,negative-stimulating electrode; P, positive-stimulatingelectrode.

P.817


23/74

View Figure

Figure 25.16 Electrode placement for stimulating the peroneal (lateral popliteal) nerve. The electrodes are placed lateral to the neck of thefibula. Stimulation causes dorsiflexion of the foot.

M u s c u l ar B r a n c h o f t h e F em o r a l Ne r v eThe muscular branch of the femoral nerve can be stimulated and movement in the

vastus medial is muscle evaluated. This muscle can be used to monitor

neuromuscular funct ion in the prone pat ient . When compared with the adductor

pol l icis muscle, the onset of NMB and recovery were qu icker ( 17 8 ) .

F a c i a l N er v e

The facial nerve, which enervates the muscles around the eye, is one of the easier

muscles to s t imulate and observe. I t i s most useful for detect ing the onset of


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should not be used to assess recovery from NMB because the responses may show

complete recovery while s ignif icant NMB is s t i l l present ( 18 2 ,18 3 , 184 , 18 7 ,188 ,189 ) .

M a n d i b u l a r N er v eThe mandibular nerve, a branch of the trigeminal, supplies the masseter muscle. It

can be s t imulated by placing the negat ive electrode anter ior and infer ior to the

zygomatic arch and by placing the posi t ive electrode on the forehead. St imulat ion

causes the jaw to close. The onset of NMB in this muscle is faster than in the hand

muscles ( 19 0 ,191 ) . In adul ts , this muscle is more sensi t ive to both depolar izing and

nondepolarizing drugs than the hand muscles ( 19 0 ,192 ) . In chi ldren, the sensi t ivi ty

may be equal ( 19 1 ).

S p i n a l A c c e s s o r y N er v eThe spinal accessory nerve can be s t imulated by p lacing the electrodes over the

depression between the ramus of the mandible and the mastoid process/

s ternocleidomastoid muscle ( 19 3 ). Stimulation causes the sternomastoid and

trapezius muscles to contract .

P.818

This can cause shoulder and thorax movement with transmission to the abdomen

(194 ) .

R e c u r r e n t L a r y n g e al N er v e

The recurrent laryngeal nerve innervates most of the intr insic muscles of the larynx

(110 ) . I t c an be s t imulated percutaneously by using two electrodes between the

notch between the thyroid and the cr icoid car t i lages ( 11 0 , 195 ). The response can

be measured by placing the t racheal tube cuff between the vocal cords and

measuring pressure changes within the cuff ( 195 ) or by using phonomyography with

the microphone placed in the vest ibular fold lateral to the vocal cords ( 14 6 ). EMG

in the larynx can be accomplished by using a special ized t racheal tube with

incorporated wire electrodes ( 196 ) or an electrode attached to the tube and placed

between the vocal cords ( 13 0 ).

Use

B e f o r e In d u c t i o n


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Prior to anesthesia induction, the stimulator should be connected to electrodes that

are posi t ioned over the selected nerve. I f EMG monitor ing is to be used, the

receiving electrodes should be placed at least 15 minutes before induct ion.Electrode s i tes should be dry and free of excessive hair or scar t issue or other

lesions. The skin should be thoroughly cleansed by using a so lvent such as alcohol ,

then completely dr ied and rubbed br iskly with a gauze pad u nt i l a s l ight redness is

vis ible .

The electrodes should be checked to ver i fy that the gel is moist . I t i s important to

avoid spreading the gel or ov erlapping adhesive while placing the electrodes. A gel

br idge between the electrodes can short-ci rcui t them and lead to poor s t imulat ion.

Af ter th e lea ds a re at tache d to th e e le c tr od e , a pie ce of ta pe shoul d be pl aced ov er

the leads to p revent movement . I t i s good pract ice to create a loop to prevent

electrode displacement ( Fig. 25.18 ).

I n d u c t i o n

During induct ion, the neuromuscular s t imulator can be used to determine the onset

time of NMB, detect unusual sensitivity to relaxants, and determine whether or not

the pat ient is suff icient ly relaxed for t racheal intubat ion.

Af ter in duc ti on of an es the si a bu t bef ore ad mi nis te r ing any mu sc le re la xan ts , the

st imulator should be turned ON and set to del iver s ingle- twitch s t imuli a t 0.1 Hz.

Applyin g s ti mu lati on mo re freq uentl y wi ll ma ke i t ap pear as i f th e t ime of on set of

NMB is shorter ( 197 , 19 8 , 19 9 ) . The output of the s t imulator should be increased

unt i l the response does not increase with increasing current , then increased 10% to

20%. If maximal s t imulat ion is not achieved with a current of 50 to 70 mA, the

electrodes should be checked for proper placement. I f maximal s t imulat ion s t i l l

cannot be achieved, needle electrodes should be used.

Special needle electrodes are avai lable commercial ly, but ordinary inject ion

needles can be used. They should be short and thin. The needles should be placed

subcutaneously. Inserting them deeper may produce direct muscle excitation and/or

cause damage to the nerve. The angle of inser t ion should be paral lel to the nerve.

There should be at least a few cent imeters between the needles. They should be

fixed in place with tape. The lead should be attached to the shaft of the needle

unless the needle has a metal hub.

Correct EMG electrode placement should be ver i f ied by observing the qual i ty of the

evoked waveform, which should approximate a s ine wave. The gain control should

be adjusted so that the waveform occupies the ful l s cale.


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I n t u b a t i o n

Complete relaxat ion of the jaw, laryngeal and pharyngeal muscles , and diaphragm

is needed for excel lent intubat ing condi t ions and to reduce the r isk of t rauma. I t

should be kept in mind that the response to intubat ion is a funct ion of both

muscular block and the level of anesthesia . I t

P.819

is possible to intubate a pat ient with less- than-complete paralysis i f a suff icient

depth of anesthesia is present ( 200 ) .

View Figure

Figure 25.18 Electrodes in place. Creating loops andsecuring the wires with tape will decrease the likelihood thatthe wires will be pulled off the electrodes.

The onset of NMB wil l be faster in central ly located muscles such as the

diaphragm, facial , laryngeal , and jaw muscles than per ipheral muscles such as the

adductor pol l icis ( 11 0 ,19 0 , 201 , 20 2 ,20 3 ,204 , 205 ,206 ,207 ,20 8 , 20 9 ,210 ).

The diaphragm, eye muscles , and most laryngeal muscles are more resis tant to

nondepolar izing relaxants than are per ipheral muscles ( 211 , 21 2 ). The diaphragm isresis tant to succinylchol ine, though the laryngeal muscles are sensi t ive to i t . The

masseter muscle is relat ively sensi t ive to both nondepolar izing and depolar izing

relaxants ( 192 ,21 3 ). It often reacts with increased tone instead of relaxation to

succinylchol ine, par t icular ly in chi ldren.

Monitoring the response of the eye muscles will reflect the time of onset and the

level of NMB at the ai rway musculature more closely than monitor ing per ipheral

muscles , which wil l underest imate the rate of onset of NMB in the ai rway


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musculature and may overest imate the degree of block

(163 , 17 9 ,202 , 214 , 21 5 , 21 6 ,217 , 218 ).

If the facial nerve cannot be used, a per ipheral nerve wil l suff ice in most cases. Inthe majori ty of pat ients , disappearance of the adductor pol l icis response is

associated with good to excellent intubating conditions. If the electromyographic

responses are being monitored, monitor ing at the hypothenar eminence may be

preferable ( 157 ).

Whatever nerve is used, i t i s recommended that s ingle twitch at 0.1 Hz be used and

that the cl inician wai t unt i l a response is barely percept ible before at tempting

laryngoscopy and intubation. More rapid stimulation may accelerate the onset of

block at the s t imulated s i te ( 198 ,19 9 ) . Double burst has been used as a n indicator

of opt imal condi t ions for t racheal intubat ion ( 219 ).

The response to s t imulat ion wil l usual ly disappear for a v ar iable per iod of t ime,

then appear and increase progressively to ful l recovery. Addi t ional relaxants should

not be given unt i l there is evidence of some recovery to make sure that the pat ient

does not have an abnormal response. However, i t i s not necessary to wai t for

complete recovery before giving addi t ional relaxants .

E le c t r o c o n v u l s i v e T h e r ap y

A comm on erro r in elec tr oc onv ul s ive therap y is del iv eri ng the el ec tri cal s timu lus

prematurely ( 220 ) . I t i s recommended that a s ingle s t imulus be appl ied at 1 Hz to

the poster ior t ibial nerve ( 22 1 ) . When there is c omplete abol i t ion of response, the

electroconvulsive therapy should be appl ied.

M a i n t e n a n c e

During maintenance, the stimulator can be used to ti trate the relaxant dosage to the

needs of the operat ive procedure so both under- and overdosage are avoided. Too

deep an NMB may make i t di ff icul t to reverse the relaxant at the terminat ion of the

anesthet ic . Underdosage may resul t in inadequate relaxat ion or undesirable pat ientmovement . In a s tudy of closed c laims against anesthesiologists , eye injur ies

const i tuted 3% of claims ( 22 2 ). Patient movement during anesthesia was the

mechanism of injury in 30% of those cases. Peripheral nerve stimulators were not

used in any pat ients who made claims for movement under anesthesia .

The degree of NMB required during a surgical procedure depends on many factors,

including the type of surgery, the anesthet ic technique, and the depth of

anesthesia . I t i s important to prevent cool ing of the monitor ing si te to avoid


29/74

impaired nerve conduct ion or increased skin resis tance, which may resul t in

overestimation of the degree of NMB ( 26 , 223 , 22 4 ) .

I t i s important to correlate the react ion to nerve s t imulat ion with the pat ient 'scl inical condi t ion because there may be a discrepancy between the degree of

relaxat ion of the monitored muscles and that of the muscles at the s i te of surgery.

If the surgeon bel ieves that relaxat ion is inadequate, the anesthesia provider

should confirm that the depth of anesthesia is s uff icient and the degree of NMB is

adequate. I t should be confirmed that the s t imulator is working properly. I f i t does

not display the del ivered current , e lectrodes may be placed on the user 's arm and a

low current used to confirm proper funct ion.

TOF is c ommonly regarded as the most useful pat tern for monitor ing NMB during

maintenance. Supramaximal currents are t radi t ional ly used. A submaximal current

may be used, but this is controversial ( 15 , 18 , 19 , 30 ,10 4 ,225 ,22 6 ) . The goal for most

cases in which abdominal muscle relaxat ion is required should be to maintain at

least one response to TOF st imulat ion in a per ipheral nerve ( 227 ,228 ) . I f no

response is present , fur ther administ rat ion of relaxants is not indicated. I f two

responses are present , abdominal relaxat ion may be adequate using balanced

anesthesia ( 229 ). Presence of three twitches is usually associated with adequate

relaxat ion i f a volat i le anesthet ic agent is used. Deeper levels of NMB may be

required for upper abdominal or chest surgery or i f diaphragmatic paralysis isneeded. I f the facial muscles are used, a t least one twitch should be added to the

mentioned recommendations.

Muscle relaxants are sometimes administered in cases such as eye surgery or laser

surgery on the vocal co rds to guarantee that movement does not occur. To ensure

total diaphragmatic paralysis , the NMB should be so intense that there is no

response to posttetanic

P.820

st imulat ion ( i .e . , the PTC is 0) ( 230 ,231 ) . One approach is to give a bolus of a

short-act ing muscle relaxant when the PTC is 1 ( 23 2 ) . Al ternat ively, the twitch

response at a resis tant muscle such as the orbicular is ocul i may be monitored and

a dose of relaxant given as soon as there is any response.

R e c o v e r y a n d R ev e r s a l

At the end of a p ro cedu re, a s t imul ato r al lows the an es th es ia prov ider to de termi ne

whether or not the block is reversible and adjust the dose of reve rsal agent , i f


30/74

required, to the pat ient 's requirements ( 233 ) . Numerous s tudies have shown that

some pat ients enter ing the postanesthesia care uni t have an unacceptable lev el of

block(69 ,234 , 235 ,236 ,237 ,238 ,239 ,240 ,241 ,242 ,243 ,244 , 245 ,246 ,24 7 ,248 ,24 9 , 25 0 ,25 1 ,2

52 ,253 ) . A nerve s t imulator may detect residual NMB, which could lead to l i fe-

threatening c omplicat ions ( 74 , 254 , 25 5 , 25 6 ,257 , 258 ).

When relaxat ion is no longer required, administ rat ion of NMB drugs should be

discont inued. As recovery progresses, the responses to TOF wil l progressively

appear, then fade wil l disappear. The ease of reversing a nondepolar izing block is

inversely related to the degree of block at the t ime of reversal ( 6 ,259 ) . I f the f i rs t

twitch (T 1 ) is present , i t can be est imated how quickly the block can be reversed.

The t ime depends on the relaxant that has been used.

Recovery is governed by the sensi t ivi ty of the muscle and rate that the drug

disappears f rom the plasma. I t i s best to use a per ipheral muscle to monitor

recovery, because i ts complete recovery would indicate that residual muscular

weakness contr ibuting to problems with ai rway patency or respirat ion is u nl ikely

(110 , 18 8 ,202 , 205 , 26 0 , 26 1 ,262 ) . The probabi l i ty of detect ing fade by using the

index f inger is greater than i f the thumb or great toe is used ( 71 , 73 ).

In the past , many invest igators thought that a T OFR of 0.7 was adequate ( 4 , 26 3 ).

However, a normal response to hypoxemia, protect ion from pulmonarycomplicat ions, and absence of heaviness of the eyel ids, visual dis turbances,

diff icul ty swal lowing, or pat ient anxiety may require a higher rat io

(4 ,264 ,265 , 26 6 ,267 ,26 8 ,26 9 ,27 0 ,271 ,27 2 ,27 3 ). Most investigators now recommend

that the TOFR at the adductor pol l icis be at least 90% measured by

mechanomyography before extubation ( 24 8 ,266 ,275 , 276 ) . This is probably most

rel iably accomplished by using ACG and achieving a TOFR at least 90% of the

basel ine ( 68 ,91 , 92 ,254 , 257 , 27 7 ,278 ,279 , 280 ) . I f EMG monitor ing is being used,

residual anesthet ic effects usual ly p revent the return of T 1 to the preanesthetic

reference level, but the TOFR should exceed 90% ( 281 ).

Residual NMB cannot be rel iably detected by using TOF st imulat ion i f visual and/or

tact i le monitor ing is used ( 19 ) . Detect ion may be somewhat bet ter when using DBS

(30 ,31 ,52 , 28 2 ) . Both may be more rel iable at detect ing fade at lower currents ( 19 ).

Cl inical cr i ter ia in an awake pat ient have been used to ascer tain whether the return

of muscle s t rength is adequate. These include the abi l i ty to (a) open the eyes for 5

seconds and not experience diplopia, (b) sustain tongue protrusion, (c) sustain

head l i f t for at least 5 s econds, (d) sustain hand gr ip, (e) sustain leg l i f t ing in


31/74

chi ldren, ( f ) cough effect ively, and (g) swal low. A more sensi t ive test may be the

abi l i ty to resis t removing a tongue blade from clenched teeth ( 268 ) . Cl inical cr i ter ia

in an asleep pat ient include an adequate t idal volume and an inspiratory force of atleast 25 cm H 2 O negat ive pressure. Subject ing the pat ient to negat ive inspiratory

pressure can cause pulmonary edema. These cl inical c r i ter ia do not exclude

cl inical ly s ignif icant residual paralysis ( 24 8 ,272 ,283 ).

P o s t o p e r at i v e Pe r i o d

Even i f a nerve s t imulator has not been used during an operat ion, i t can be used

postoperatively. I f the pat ient is not ful ly anesthet ized, i t i s preferable to use less

than supramaximal s t imulat ion ( 15 ,29 ,284 ). This decreases the discomfort

associated with s t imulat ion and may improve the visual assessment accuracy ( 30 ).

L o n g - t er m M u s c l e Re la x an t In f u s i o n s

Long-term muscle relaxants infusions are sometimes used in cr i t ical care areas.

NMB monitoring should be used to avoid overdosage

(285 , 28 6 ,287 , 288 , 28 9 , 29 0 ,291 , 292 ) . A number of factors unique to the cr i t ical care

set t ing affect the response to NMB drugs ( 12 , 28 6 ) . Prolonged paralysis is

sometimes se en despi te monitor ing ( 293 , 29 4 ) .

N er v e L o c a t i o n

A perip he ral nerv e s timu la to r may be us ed to lo cate nerv es f or re gional b lo ck ( 295 ).

The current needed is far below that needed for monitoring NMB. Stimulators with

different current outputs for both funct ions are avai lable ( 296 , 297 ).

Hazards

B u r n s

Burns have been reported when using a s t imulator with metal bal l e lectrodes ( 29 8 ) .

Needle electrodes may be associated with local t issue burns from electrosurgical

uni ts because they provide good contact with minimal resis tance for exi t of high-

frequency current over a smal l area of sk in ( 299 ) . Severe burns resul t ing in

permanent loss of hand funct ion caused by a nerve s t imulator have been reported

(300 ) .

P.821

N e r v e D am a g e


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The pressure of an electrode on a nerve can resul t in palsy ( 154 ). Thumb

paresthesias were reported in pat ients whose muscular funct ion was monitored by

using an MMG ( 301 ) . Nerve damage can resul t f rom intraneural placement of aneedle electrode.

C o m p l i c a t i o n s A s s o c i a t ed w i t h N ee d l e E l ec t r o d e s

Complicat ions associated with needle electrodes include infect ion, bleeding, and

pain.

P a i n

Pat ient discomfort wil l be reduced by using lower currents and avoiding tetanic or

double-burst stimulation when the patient is not fully anesthetized ( 16 ,18 ).

E l ec t r i c a l I n t e r f e r e n c e

The use of a nerve s t imulator may cause changes in the ECG tracing or interfere

with an implanted pacemaker ( 302 ,303 , 30 4 ,305 ).

In c o r r e c t In f o r m a t io n

With some st imulators , when the bat ter ies are low, only three pulses are generated

during TOF st imulat ion ( 306 ) . This could lead to incorrect interpretat ion of the

degree of NMB.

A pote nt iall y con fus in g user interfac e on a neu rom uscular tr ans mis sion mod ule ha s

been reported ( 307 ) . The module provided a bar graph visual indicat ion of the four

responses to TOF stimulation. However, if the responses were greater than 120% of

the control response, the bar graph representat ions were chopped off . As a resul t ,

a l l four t witches could appear to be of the same height when the TOF rat io was

below 100%.

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