Clinical validation of electromyography and acceleromyography as sensors for muscle relaxation

Abstract

The aim of this study was to determine which of two clinically applied methods, electromyography or acceleromyography, was less affected by external disturbances, had a higher sensitivity and which would provide the better input signal for closed loop control of muscle relaxation.
In 14 adult patients, anaesthesia was induced with intravenous opioids and propofol. The response of the thumb to ulnar nerve stimulation was recorded on the same arm. Mivacurium was used for neuromuscular blockade. Under stable conditions of relaxation, the infusion-rate was decreased and the effects of turning the hand were investigated.
Electromyography and acceleromyography both reflected the change of the infusion rate (P = 0.015 and P < 0.001, respectively). Electromyography was significantly less affected by the hand-turn (P = 0.008) than acceleromyography. While zero counts were detected with acceleromyography, electromyography could still detect at least one count in 51.1%.
Electromyography is more reliable for use in daily practice as it is less influenced by external disturbances than acceleromyography.

Full text

European Journal of Anaesthesiology 2007; 1–7
r 2007 Copyright European Society of Anaesthesiology
doi: 10.1017/S0265021506002353
Original Article
Clinical validation of electromyography and acceleromyography
as sensors for muscle relaxation
P. H a
¨
nzi
*
, D. Leibundgut
*
, R. Wessendorf
y
, R. Lauber
*
, A. M. Zbinden
*
University Hospital of Bern,
*
Department of Anaesthesiology, Murtenstrasse, Bern, Switzerland;
y
Department of
Anaesthesiology, Kreiskrankenhaus Erding, Bajuwarenstrasse, Erding, Germany
Summary
Background and objective: The aim of this study was to determine which of two clinically applied methods,
electromyography or acceleromyography, was less affected by external disturbances, had a higher sensitivity
and which would provide the better input signal for closed loop control of muscle relaxation. Methods: In 14
adult patients, anaesthesia was induced with intravenous opioids and propofol. The response of the thumb to
ulnar nerve stimulation was recorded on the same arm. Mivacurium was used for neuromuscular blockade.
Under stable conditions of relaxation, the infusion-rate was decreased and the effects of turning the hand were
investigated. Results: Electromyography and acceleromyography both reflected the change of the infusion rate
(P 5 0.015 and P , 0.001, respectively). Electromyography was significantly less affected by the hand-turn
(P 5 0.008) than acceleromyography. While zero counts were detected with acceleromyography, electro-
myography could still detect at least one count in 51.1%. Conclusions: Electromyography is more reliable for
use in daily practice as it is less influenced by external disturbances than acceleromyography.
Keywords: NEUROMUSCULAR BLOCKING AGENTS, mivacurium; NEUROMUSCULAR BLOKADE,
acceleromyography, electromyography; EQUIPMENT AND SUPPLIES.
Introduction
Mechanomyography (MMG) is general ly used as
gold standard to measure neuromuscular function
[1]. However, its clinical use in daily practice is
limited
[2]. Electromyography (EMG) gives com-
parable results to MMG and can be used as well
[1].
Subjective estimation of the extent of the train-
of-four (TOF, T4/T1) fade is poor, especially when
the TOF ratio exceeds 0.40–0.50
[3,4]. Manually
measured counts cannot be used as a continuous
input for a feedback-controlled system. However, an
advantage of the TOF is the fact that no calibration
is needed before the measurement
[5]. This reduces
induction time and avoids the problem of baseline
shift. The disadvantage is the fact that it cannot be
determined once there are only three counts left. For
this reason, we used T1% (percentage of first twitch
to initial twitch before relaxation) as reference value,
which had to be calibrated before starting the
measurements. A waiting period of 10 min was
introduced to minimize the effect of the baseline
drift.
Despite potential adverse effects of using neuro-
muscular blocking agents, there is no quantitat ive
routine monitoring of neuromuscular function
during anaesthesia
[6] using MMG or EMG as these
measurement techniques tend to be more time
consuming. Clinical estimation of neuromuscular
blockade may be biased
[13]. Acceleromyography
(AMG) was introduced in 1988 by Viby-Mogensen
as a new method for monitoring neuromuscular
function, fulfilling the basic requirements for a
Correspondence to: Daniel Leibundgut, Department of Anaesthesiology,
Section of Research, University Hospital of Bern, 3010 Bern, Switzerland.
E-mail: daniel.leibundgut@dkf.unibe.ch; Tel: 141 31 632 27 58; Fax: 141 31
632 88 48
Accepted for publication 18 November 2006 EJA 3498

simple and reliable clinical monitoring tool [7].It
became popular because of its straightforward
and easy application
[8]. As a resear ch instrument,
it may be used after careful calibration
[8,9].
However, AMG cannot be directly compared to
MMG
[10,11] or EMG with respect to the esti-
mation of neuromuscular blockade and stimulating
patterns
[12]. Furthermore, as AMG works dyna-
mically and MMG works isometrically, it is not
possible to use the two methods simultaneously on
the same arm. AMG has been investigated and
comparisons have been made with EMG and MMG
[1,11,13–18]; however, sensitivity and robustness
to artefacts have not been investigated in AMG and
EMG so far.
For delivering short-acting newer neuromuscular
blocking agents, a continuous, accurate and reliable
signal would be useful, specially if these agents
should be given using automatic feedback control
systems
[19]. Input signals for control loops should
be discrete and of a higher resolution, compared to a
categorical signal such as a TOF value. They must
give a rapid and true measurement of what should
be controlled and should not be subjected to arte-
facts or to drifts as these are difficult to detect by an
automatic control system.
The objective of this study was to answer the
question if the AMG response to the ulnar nerve
stimulation using a common clinically established
device gives reliable, discrete input signal, which
could be routinely used. We tested EMG against
AMG using two manipulations: (a) the change
of infusion rate (lowering the dose) and (b) the
position change of the sensor hand, answering
the fact that the two measurement methods quan-
tify different effects but answer the same clinical
question.
Materials and methods
The study was approved by the Ethics Committee of
the District of Bern. Written informed consent was
obtained from each patient. Fourteen patients (nine
females and five males, mean age 41.7 yr (26–55),
mean body mass index (BMI) 24.85 kg m
22
(17.3–32.5)), ASA Class I or II, who were scheduled
for elective surgery, were included in the study. The
patients were free of any known neuromuscular
diseases, or renal and hepatic diseases, and did not
take any drugs known to interfere with neuromus-
cular transmission.
Anaesthesia and the measurement of neuro-
muscular blockade was performed in accordance
with the Good Clinical Research Practice (GCRP)
in pharmacodynamic studies of neuromuscular
blocking agents
[1] to keep bias as low as possible.
All patients were premedicated orally with 7.5
or 15 mg of midazolam or 1 mg of lorazepam
30–60 min before induction of anaesthesia. On
arrival in the oper ating theatre, an intravenous
(i.v.) catheter was inserted to administer fluids
and drugs. Pulse oximetry, electrocardiogram and
non-invasive blood pressure were monitored on the
same arm. The other arm was used for neuromus-
cular monitoring. Bispectral index was routinely
monitored. After preoxygenation, anaesthesia was
introduced with bolus doses of propofol 2%
2–2.5 mg kg
21
, fentanyl (0.1–0.2 mg) and remi-
fentanil (1–2 mgkg
21
). The trachea of the patients
was intubated without the use of neuromuscular
blocking agents. In case of problems or danger for
the patient, succinylcholine 1 mg kg
21
was used for
relaxation. Anaesthesia was maintained with an
infusion of remifentanil (100–800 mgh
21
), a target
controlled infusion of propofol 2% 1.5–8 mgmL
21
and bolus doses of fentanyl (0.05–0.1 mg). Core
temperature was monitored and maintained using
forced air warming blankets (Bair Hugger
TM
;
Augustine Medical Inc., Eden Prairie, MN, USA).
The ulnar nerve was stimulated on the forearm
with the TOF (four supramaximal square wave
pulses of 0.2 ms duration and a frequency of 2 Hz).
The recording electrodes for EMG were set up fol-
lowing the manufacturer guid elines at the adductor
pollicis muscle. We compared EMG (Datex-
Ohmeda AS/3; Helsinki, Finland) with AMG
(TOF-Watch SX, Organon Teknika; Boxtel, The
Netherlands), both devices being installed on the
same stimulus-electrodes of the arm of the patient
using an electrical switch. The electrodes (Ag/AgCl-
ECG electrodes for children, recording diameter of
10 mm; REF 1008, Nessler Medizinaltechnik,
Innsbruck, Austria) were stuck to the skin, which
had been cleaned beforehand. Temperature of the
stimulated hand was measured with a surface elec-
trode (TOF-Watch SX) and ke pt constantly above
328C
[1]. The acceleration transducer was attached
to the flexor-side of the thumb over the distal
interphalangeal joint. The thumb was able to move
freely while hand and arm were fixed with the splint
from Organon-Teknika on a rigid board
[10,20].
The counts (T1–T4), T1% (T1/Tref * 100) and the
TOF were continuously measured by computer,
separately for each device (TOF-Watch SX-Monitor,
software version 1.2 by Organon-Teknika), for
TOF-Watch SX and Labview (National Instruments
Corporation, Austin, TX, USA) and for the EMG-
Monitor (Datex-Ohmenda, Helsinki, Finland).
After induction, the AMG was calibrated according
to the instructions of the manufacturer using its
automatic start-up procedure. A period of at least
10 min was allowed for baseline drift of the nerval
2 P. H a ¨nzi et al
r 2007 Copyright European Society of Anaesthesiology, European Journal of Anaesthesiology, 1–7

responses by using the 0.1 Hz mode (supramaximal
stimuli with square wave pulses of 0.2 ms duration
every 10 s). A second calibration of the AMG was
performed in a similar fashion. EMG was set up and
also calibrated according to the instructions of the
manufacturer. Supramaximal current was deter-
mined separately for each device. The T1% of EMG
was used as reference value for all measurements.
After the stabilization phase and second calibration,
both devices were started with a time difference of
30 s and then the TOF was measured every minute
for each device.
Patients were normoventilated and end-tidal CO
2
was kept constant. A syringe pump (Asena GH;
Alaris Medical Systems, Basingstoke, Hampshire,
UK) was used to apply a continuous infusion of
mivacurium chloride (mivacron
s
; Glaxo Wellcome
Gmbh & Co, Zeneca Gmbh) with an initial rate
of 0.2–0.3 mg kg
21
h
21
. Once T1% of the EMG
was stable (criteria for stabilization of T1%: ampli-
tude within 15%, during 15 min), the infusion rate
of mivacurium was lowered to 0.05 or 0.1 mg
kg
21
h
21
. After this change, we waited again for
signal stabilization using the above-mentioned cri-
teria, then turned the sensor-hand by 908 from
a vertical position with the thumb up to a palmar-
side-down position (hand turn) (Fig. 1; first trace).
Mean T1% over 5 min before lowering the miva-
curium dose (5HD), mean T1% over 5 min before
hand turn (5LDb) and mean T1% over 5 min
after hand turn (5LDa) were used for comparing
these events.
Statistical analysis
To measure the reaction to lowering the mivacurium
dose, HD was compared with the LDb of each
device. The effect of turning the hand was shown by
comparing LDb with LDa. To compare AMG with
EMG for both effects, lowering the dose or changing
the hand position, the differences of the mean T1%
before the event to the mean T1% after the event
were compared using t-test (normality test passed) or
U-test (normality test failed). To compare EMG and
AMG for their potential to be used in situations
with high neuromuscular blockade, no-twitch
response measurements of one method were com-
pared to twitch responses of the other, and vice versa.
To compare the overall deviation of EMG-counts to
AMG-counts, the L1-Norm value was used:
L
1
5
Dt
T
X
n
i51
jEMGðiÞAMGðiÞj
Dt 5 sampling time.
T 5 total time of observation of one period.
n 5 total number of measurements during obser-
vation period.
The differences were considered as statistically sig-
nificant when P , 0.05 (F
0.95
).
Results
Seventeen patients were selected to participate, of
which 14 are presented in the results. Three patients
dropped out because of surgical complications
during operation, the duration of the operation or
because of inadequate depth of relaxation with
respect to the surgical demands. The average
temperature of the sensor-hands was 33.78C
(31.5–35.68C). The mean BMI was 24.85 kg m
22
(range: 17.3–32.5 kg m
22
); three of the patients
were obese (BMIs of 30.7, 31.2 and 32.5 kg m
22
).
Succinylcholine was used due to expected difficult
intubation in three patients.
The stimulation power of both devices after
searching for the supramaximal current for each
sensor was significantly different (P , 0.001, using
a U-test): the median current of AMG was 60 mA
(range 55–60 mA) while the median current of
EMG was 37.5 mA (range 23–64 mA), despite the
fact that the same stimulating electrodes wer e used.
The mivacurium dose was not lowered according
to a scheme but based on the experience of the
anaesthetist and depending on the phase of surgery.
Figure 1 shows a sample recording of one experi-
ment, with inaccurate high T1% values at the
beginning of mivacurium infusion.
On lowering the dose (mean of
0.086 mg kg
21
h
21
), both devices reacted compar-
ably in the expected direction. Both EMG and
AMG show a significant change of mean T1%
between the phase before lowering the dose to the
phase afterwards (P 5 0.015 and P 5 0.004,
respectively, Table 1).
The hand turn disturbed the EMG signal (P 5
0.863) much less than the AMG signal (P 5 0.007)
(Table 1). For control, we compared two phases
under stable conditions that showed no significant
differences (AMG: P 5 0.953 and EMG: P 5 0.972)
(Fig. 2). The comparison of EMG and AMG showed
no significant difference with respect to lowering
the dose (P 5 0.306) compared to turning the hand
(P 5 0.008) (Table 2). The difference of mean T1%
before and after hand turn was significantly
(P 5 0.008) smaller for EMG (20.26%) than for
AMG (210.01%) (Fig. 3).
In situations with hig h neuromuscular blockade,
a comparison of the twitch response records between
EMG and AMG is as follows: in measurements
where EMG recorded no answers, AMG detected
EMG and AMG validation 3
r 2007 Copyright European Society of Anaesthesiology, European Journal of Anaesthesiology, 1–7

one or more count in 30% of the non-twitch
response periods; whereas where AMG recorded no
answers, EMG detected one or more counts in 51%
of the non-twitch response periods (Fig. 4). The
overall deviation of EMG counts to AMG counts
calculated as mean L1 was 0.56 (0.47).
20 40 60 80 100 120 140 160 180 200
T1%
0
20
40
60
80
100
120
EMG
AMG
20 40 60 80 100 120 140 160 180 200
EMG counts
0
1
2
3
4
TOF ratio (%)TOF ratio (%)
0
20
40
60
80
100
120
20 40 60 80 100 120 140 160 180 200
Infusion rate [mg
kg
–1
h
–1
]
0.0
0.1
0.2
0.3
0.4
Time
(
min
)
20 40 60 80 100 120 140 160 180 200
AMG counts
0
1
2
3
4
0
20
40
60
80
100
120
140
Counts
TOF ratio
Lower the
Infusion rate
Hand turn
Counts
TOF ratio
Figure 1.
Sample trial data traces. The vertical lines mark the events for lowering the mivacurium dose and the hand turn. The first trace shows the
mivacurium infusion rate. The second, the T1% values for both EMG and AMG. The last two traces show the TOF ratio and the count
values for EMG and the bottom one for AMG.
4 P. H a ¨nzi et al
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Discussion
This study compared the AMG and EMG sensors
with respect to accuracy and artefact tolerance, with
the objective of evaluating which sensor is more
suitable to integrate in a feedback-controlled system
for the application of muscle relaxants. AMG, in
contrast to EMG, was more affected by external
disturbances such as movement and was less sensi-
tive at high degrees of neuromuscular blockade. The
difference between the measurements of AMG and
EMG may be caused by the fact that these devices
do not measure the same physiological phenomenon
Table 1. Influence of lowering the mivacurium-infusion-rate (HD vs. LDb) and of hand turn (LDb vs. LDa) on EMG and AMG.
T1% HD (SD) LDb (SD) Lda (SD) P value
AMG Lowering mivacurium-rate 10.053 (7.269) 32.933 (27.058) 0.004
**
Hand turn 32.933 (27.058) 42.947 (26.814) 0.007
*
EMG Lowering mivacurium-rate 21.934 (16.950) 39.049 (19.861) 0.015
**
Hand turn 39.049 (19.861) 39.305 (18.625) 0.863
T1% values are given as mean (standard deviation).
**
Indicates a significant value for lowering the infusion rate.
*
Indicates a significant value
for AMG for the hand turn. EMG: electromyography; AMG: acceleromyography.
90
80
80
70
60
50
40
30
20
10
70
60
50
T1%
T1%
40
30
20
10
0
AMG phase 1 AMG phase 2 EMG phase 1 EMG phase 2
Figure 2.
Comparison of two phases (each 5 min) under stable
conditions for AMG (left) and EMG (right). The
box plots show median (notched), 25% and 75%
quartiles, range of data and outliers (for values
beyond 1.5 times the inter quartile range of upper or
lower quartile).
Table 2. Differences of T1% values of EMG and AMG at
lowering mivacurium-infusion rate and at hand turn.
DEMG (SD) DAMG (SD) P values
Lowering dose 217.12 (23.98) 222.88 (26.20) 0.306
Hand turn 20.26 (5.65) 210.01 (12.30) 0.008
*
Values are given as mean (standard deviation).
*
Indicates a
significant difference between EMG and AMG for the hand turn.
EMG: electromyography; AMG: acceleromyography.
Counts
1 2 3 4 Invalid
Percent EMG/AMG twitches of
no-twitch responses of AMG/EMG (%)
0
5
10
15
20
25
30
35
40
45
EMG twitches (no-twitch response of AMG)
AMG twitches (no-twitch response of EMG)
Figure 4.
No twitch response of EMG compared to the twitch response of
AMG (shaded) and reverse (black). Number of twitch responses
outside the valid range of 0 to 4 counts are summarized under
‘Invalid’.
10
0
− 10
T1%
− 20
− 30
− 40
EMG AMG
Figure 3.
Comparison of the two devices for hand turn. Mean T1% before
the hand turn minus mean T1% after the turn. The box plots
show median (notched), 25% and 75% quartiles, range of data
and outliers (for values beyond 1.5 tim es the inter quartile range
of upper or lower quartile).
EMG and AMG validation 5
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[21]. AMG and EMG are preferred in the clinical
routine to MMG because they are easier to use.
We followed the guidelines for GCRP in phar-
macodynamic studies of neuromuscular blocking
agents
[1]. We allowed a period of 10 min for
stabilization of the baseline, as according to earlier
studies the most significant drift occurs during this
time
[22].TOFratiosmaygoupto1.3withoutany
neuromuscular blocking agent on board as also
reported in other studies
[23], even though the
thumb was able to move freely with the hand totally
fixed to a board. Non-relaxed thumbs might not
return to the starting point and might not move in
one direction only. Previous studies using AMG and
EMG showed that the measurement of neuromus-
cular blockade cannot be compared between both
arms of one individual
[10]. For that reason, the
patient’s response to neuromuscular stimulation was
determined on the same arm, stimulating with the
same electrodes for both devices. To take into con-
sideration that EMG and AMG measure different
physical effects, supramaximal stimulation current is
different as well. So far, no further data on supra-
maximal stimulation current while comparing dif-
ferent methods have been published. AMG and EMG
recorded on the same limb are published by Kopman
and colleagues
[24] andshowedanoverestimationof
AMG TOF values compared to EMG TOF.
In operations where full relaxation is required,
such as brain or ophthalmic surgery, any sudden
movement by the patients may result in complica-
tions. It is therefore essential to use a system that
provides reliable and highly sensitive measure-
ments. The L1-norm showed a difference between
the two devices during the observed periods of half
a count: EMG sensors showed higher sensitivity and
seemed to be more reliable.
In conclusion, these findings suggest that in
situations where reliable monitoring is essential such
as in cases where short acting drugs are applied,
feedback control is used, or in especially critical
operations, EMG should be given preference over
AMG despite the fact that its installation is more
time-consuming. EMG, as a more accurate method,
is preferable for research, while AMG, as a simpler
and stable method, is preferable for routine practice.
Acknowledgments
Stiftung zur Foerderung der wissenschaftlichen
Forschung an der Universitaet Bern.
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