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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
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ih
15
.
A
a
- « - FDI
- A -
AP
. . 4 - .
AD M
Time,
minutes
o
^ o
en
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E M G IN MUSCLES OF THE ULNAR NERVE
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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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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E M G IN MUSCLES OF THE ULNAR NERVE
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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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