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Anaesth Intensive Care 2012; 40: 690-696

Comparison of evoked electromyography in three muscles

of the hand during recovery from non-depolarising

neuromuscular blockade

S. PHILLIPS*, P A. STEWARTf, N. FREELANDER:]:, G. HELLER§

Department of Anaesthesia Sydney Adventist Hospital Sydney New South W ales Australia

SUMMARY

The evoked electromyographic responses to supramaximal train of four stimulation of three muscles, all

innervated by the ulnar nerve, were compared during recovery from non-depolarising neuromuscular

blockade. The abductor digiti minimi was the most resistant to neuromuscular blockade {P


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E M G IN MUSCLES OF THE ULNAR NERVE

69 1

monitor integrates the EMG response curve to

represent the depth of NMB and this is presented

numerically and graphically.

The ulnar nerve is most frequently used to

monitor NMB, probably because of its easy

accessibility . This study compared the evoked EMG

in three muscles, all innervated by the ulnar nerve,

during recovery from neuromuscular blockade; the

abductor digiti minimi (ADM), adductor pollicis

(AP) and the first dorsal interosseous (FDI). The

primary outcomes of repeatability, sensitivity,

bias and limits of agreement were compared in

an effort to provide some recommendations for

clinical practice .

MATERIALS AND METHODS

The study had approval of the Human Research

and Ethics Committee of our Institution (EC00141:

project 2011/020). Written informed consent was

obtained from all participants.

Patients over 18 years of age who would require

NMB for their surgical procedure were eligible

for recruitmen t if an arm was to be available for

electrode placement, able to be kept warm, not be

used for blood pressure recording or have neuro-

muscular disease.

The anaesthetist in charge of each case made

all the clinical decisions. A research assistant

applied the electrodes and performed all data

recordings. The temperature of the study arm

was maintained above 32°C by use of a forced air

warmer and warm intravenous fluids '*.

The Datex Ohmeda GE Healthcare Neuro-

muscular Transmission Monitor-EMG was used

throughout the study. Red Dot™ Ag/AgCl paediatric

micropore™ backed electrodes (3M™) with a

diameter of 7 mm were applied to skin that had

been degreased with alcohol and shaved if

necessary to reduce impedance. The stimulating

negative (brown) electrode was positioned 1 cm

proximal to the wrist skin crease over the ulnar

nerve and the positive (white) 3 to 5 cm more

proximal. The black earth electrode was placed at

the proximal wrist crease. These three electrodes

were not moved during the study period. The

sensing green electrode was placed over the belly

of the muscle being studied and the red indifferent

electrode over its tendon insertion (Figures 1 to

3). An acceptable signal had a smooth, two-phase

proflle with a well defined initial upward deflection

not influenced by stimulus artefact, and returned

FIGURE 1:

Electrodes positioned to monitor abductor

digiti minimi.

FIGURE 2:

Electrodes positioned to monitor adductor pollicis.

FIGURE 3:

Electrodes positioned to monitor first dorsal

interosseous.

After induction of general anaesthesia, the

supramaximal stimulation current was established

using the automatic function of the Neuro-

muscular Transmission Monitor-EMG before the

administration of any NMB. This function applies

a steadily increasing single stimulus until no further

increase in twitch height is elicited, and then adds

15% of the stimulus to ensure tha t it is supra-

maximal. Graphical representation of both signal

and the response was checked to ensure adequate

electrode placement. The standard TOF stimulation,

at 2 Hz with 200 /.tsec square wave pulses, repeated

every 20 seconds was used throughout the study.

Three recordings of the TOF were made for

each muscle during spontaneous recovery from


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692

S. PH ILLIP S, P. A. STEWART ET AL

next muscle. The stimulating and earth electrodes

were not moved. A recording cycle comprised

nine recordings within a three minute period. After

the administration of neostigmine to reverse NMB,

only one recording from each muscle was made

before recording from the next muscle because of

the more of rapid rate of recovery.

The sequence in which the muscle groups

were measured was consistent within each patient's

data set but was randomised between patients

to remove any bias due to the ordering of the

sequence.

Each recording cycle was analysed independently

and as such each patient acted as their own control.

The effect of potentially confounding variables

such as type or dose of NMB, administration of

NMB agonists e.g. aminoglycosides, speed of

recovery, administration of extra doses of NMB

are equal for each muscle at each study point, and

are so controlled for each recording cycle.

The following demographic data were collected:

age,

gender, body mass index, study arm, dominant

arm and American Society of Anesthesiologists

physical status .

An a priori sample size calculation estimated

the number of patients required to detect a 10%

difference in TOF between muscles, with a Type

1 error of 0.05 and a power of 0.9 to be 22. A

10% difference in TO F was selected to be the

smallest difference that was clinically significant.

Recovery TOF/time curves were constructed

and compared for each patient. The readings

were taken sequentially in triplets, so the three

muscles were never monitored at exactly the same

time.

Consequently, for comparison of recovery at

the different muscles at a particular time point, it

was necessary to approximate the recovery curves

and interpolate at the specified time. Spline

curves were used for smooth approximation and

interpolation. In order to compare readings at

a target TOF ratio, the time at which the readings

at ADM were closest to the target was chosen

as the target time. Readings were then interpolated

on the ADM, FDI and AP spline curves to get

the interpolated readings at the target TOF

ratio. This is illustrated in Figure 4. In order

to construct the spline curve for a muscle in a

stable manner, at least five readings were needed

at that muscle.

The resistance to NMB was assessed by com-

paring the number of times each muscle recovered

first using the chi-square test. The internal

consistency or repeatability of each muscle, before

NMB antagonism, was determined by comparing

the triplets of data at each time period in each

patient to determine the coefficient of repeatability'^

The three muscles were then compared pairwise,

using the Bland-Altman plot to display the bias and

limits of agreement *. Bias is defined as the mean of

the difference between two measurements, e.g. in

the comparison between TOF measured at ADM

and AP muscles, bias=2[TOF ADM-TOF AP]/n.

Precision is the standard deviation of the differences.

The limits of agreement are the range enclosed

by ±1.96 standard deviations. These were examined

across all TOF's during recovery, and then separately

at T OF 0.25, 0.5, 0.75 and 0.9.

RESULTS

A total of 38 patients were recruited, four were

intraoperative arm repositioning

xcluded after

Patient 30 Patient 36

8-

S-

s H

o

6

—^

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ih

15

.

A

a

- « - FDI

- A -

AP

. . 4 - .

AD M

Time,

minutes

o

^ o

en


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693

precluded access for electrode rotation. Thirty-

one patients had sufficient data (more than five

readings) for ADM :FDI com parison, 29 for ADM:AP

comparison and 30 for FDI:AP comparison. Data

from 25 patients at TOF of >0.9 were available

for analysis at that level. The demographic details

are shown in Table 1.

The ADM was the most resistant muscle to NMB,

recovering first 84 of the time

P


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69 4

S. PHIL LIPS , P. A. STEWART ET AL

T LE 2

Bias

and limits

of agreem ent for ADM -AP

ADM-FDI and AP-FDI

Muscle comparison

A D M - A P

A D M - F D I

AP- FD I

Bias

0.11

0.10

0.02

For overall TOFs

Limits of agreement

-0.13-0.35

-0.21-0.41

-0.37-0.41

Bias

0.10

0.09

0.01

A t T OF 0.9

Limits of agreement

-0.10-0.30

-0.10-0.28

-0.27-0.28

A D M = a b d u c t o r digi t i minimi, AP=adduc tor

pollicis,

FDI=firs t dorsa l in te rosseous,

T O F = t r a i n

of four.

ADM FDI

K ADM AP

« FDI AP

Li

O

T O F 0 . 2 5

T O F 0 .5 TO F 0 .7 5 TO F 0 .9

F I G U R E 8: TOF difference for all muscles at TOF 0.25, 0.50,

0.75 and 0.9. TOF=train of four, ADM=abductor digiti tninimi,

FDI=fi rs t dorsal in te rosseous, AP =ad duc tor

pollicis.

in the different muscle groups. The ADM is the most

resistant muscle - that is it recovers more quickly

to a TOF of 0.9. The relative resistance of the

ADM to the muscles required for airway patency

has not been directly determined. However, the

relative resistance of the AP compared to the

laryngeal and upper airway muscles has been

studied . The ADM is more resistant than the AP

and FDI, and so it must be noted that it will recover

before the all airway muscles are at full strength.

The EM G at ADM is the most internally

consistent with a repeatabilify coefficient of 4.4%.

However, all three muscles are similar and within

the clinically acceptable range.

The AP and FDI show excellent agreement,

with a bias of only 2%. The AP and FDI have 11%

and 10% bias respectively compared with the

ADM. The Bland-Altman plot compares

measurement techniques against each other when

the true value is not known. It does not identify

the true value. Hence, while AP and FDI agree

with each other, it must be noted that this is

not the same as being the most accurate. The

limits of agreement (1.96 standard deviations

muscle comparisons. When stratified for different

levels of TOF, at a TOF of 0.9 the limits of

agreement were m uch smaller for the A DM . Hence,

we have concluded that EM G recordings at the

ADM are more precise. We can speculate why

the ADM is the most resistant and repeatable for

EM G . Differences in regional blood flow, muscle

temperature, density and fype of receptors and

muscle fibre composition may exist. The size

and depth of each muscle may affect the

qualify of readings made with surface electrodes,

especially so when movement of the finge rs

due to nerve stimulation is not restricted. ADM is

a relatively superficial and larger muscle compared

with the AP small and deep, or the FDI superficial,

but very small, and may give better recordings

with surface electrodes . This may explain the

wider limits of agreement observed at AP and FDI.

Direct muscle stimulation, theoretically possible

in the hypothenar eminence, is unlikely at currents

below 70 mA with a stimulus duration less than

300Ai,seconds' - .

RNMB not only impairs the muscles of airway

patency more than the muscles of respiration,

but also inhibits the hypoxic pulmonary drive by

an effect on the carotid body chemoreceptors'.

Patients with RNMB are at risk of both aspiration

and hypoxia. The relative sensitivities of the

different muscles to NMB are shown in Table

318.21,22 •jj ĝ rela tive resis tance of the diaphragm,

1.5 to 2 times that of the AP, is well recognised and

the extreme sensitivify of the extra-ocular muscles

has been utilised in eye surgery. These relative

resistances must be remembered in clinical

practice to ensure safe and full recovery from NMB.

Many different fypes of NMFM stimulation

have been used. We chose to use TOF because

it provides its own control and does not require

a period of calibration or stabilisation. It is

recommended as the most suitable technique for

objective monitoring in recent papers for assessment

of RNMB .

We chose not to influence anaesthetist's choice of


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695

T A BL E

3

Increasing sensitivity of muscles to neurom uscular blockade

Diaphragm

Corrugator superci l i i

Laryngeal adductors

Orbicttlaris occuli

Abdomitial rectus

Abductor digi t i minimi

Adductor pollicis = first dorsal

It i terosseous

Getiioglossus muscles

Masseter

Pharyngeal

Note: "Increasing" is frotn top to bottom.

have been shown to have varying effect on

different muscles. However, these relaxants were

not in fact used in these patients.

CONCLUSION

There is wide inter-patient variability in response

to NMB. Ensuring full recovery of neuromuscular

function is essential to patient safety when NMB

is used. This is only possible when quantitative

monitoring confirms a TOF of >0.9 after NMB"-^"\

The EMG of the ulnar nerve is a clinically useful

and valid monitor for this situation. EMG monitoring

has the advantage of not being influenced by

restriction of movement and has applicability to

many muscle sites.

The EMG-ADM will recover earlier than the

EMG-AP or EMG-FDI and is more precise.

An EMG TOF ratio of 0.9 at ADM has

narrower limits of agreement than the EMG at

AP and FDI, and is equivalent to an EMG TOF of

0.8 at AP of FDI. The relative resistance of

ADM, FDI and AP to NMB compared with the

airway muscles must be emphasised.

ACKNOWLEDGMENTS

This study was supported by research grants from

the Australasian Research Institute and the Jackson

Rees Grant of th Australian Society of Anaesthe tists.

The Neuromuscular Transmission Monitor-EMG

monitors were loaned for the duration of the study

by Datex Ohmeda GE Healthcare. The authors wish

to thank the anaesthetists, surgeons and patients of

the Sydney Adventist Hospital for participating.

Short listed and presented for the Gilbert Troup

Prize at the Australian Society of Anaesthetists,

National Scientific Congress, Sydney, New South

Disclosures

Dr Paul Stewart received honoraria from

Schering Plough Pty Ltd and was a member of

their Medical Advisory Board in 2009. Drs Paul

Stewart and Stephanie Phillips received an

unrestricted educational grant from Merck Sharp

and Dohme in

2011.

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