Electromyography
Evoked EMG records the compound action potentials produced by
stimulation of a peripheral nerve. The compound action potential is a high-speed
event that for many years could only be picked up by means of a preamplifier
and a storage oscilloscope. Modern neuromuscular transmission analyzers are able
to make on-line electronic analyses and graphic presentations of the EMG response.
The evoked EMG response is most often obtained from muscles innervated
by the ulnar or the median nerves. Stimulating electrodes are applied as in force
measurements. Although both surface and needle electrodes may be used for recording,
no advantage is obtained by using the latter. Most often, the evoked EMG is obtained
from the thenar or hypothenar eminence of the hand or from the first dorsal interosseous
muscle of the hand, preferably with the active electrode over the motor point of
the muscle ( Fig. 39-12
).
The signal picked up by the analyzer is processed by an amplifier, a rectifier,
and an electronic integrator. The results are displayed either as a percentage of
control or as a TOF ratio.
Two new sites for recording the electromyography response have
been introduced: the larynx and the diaphragm.[39]
[55]
[56]
[57]
[58]
Using a non-invasive disposable laryngeal
electrode
attached to the tracheal tube and placed
Figure 39-12
Electrode placement for stimulation of the ulnar nerve
and for recording of the compound action potential from three sites of the hand.
A, Abductor digiti minimi muscle (in the hypothenar
eminence). B, Adductor pollicis muscle (in the thenar
eminence). C, First dorsal interosseus muscle. (Courtesy
of Datex-Ohmeda, Helsinki, Finland.)
between the vocal cords, it is possible to monitor onset of neuromuscular block in
the laryngeal muscles.[55]
[56]
So far, however, the method is mainly of interest in clinical research when investigating
onset times of the laryngeal muscles.[59]
In paravertebral
surface diaphragmal electromyography the recording electrodes are placed on the right
of the vertebrae T12/L1 or L1/L2 for EMG monitoring of the response of the right
diaphragmatic crux to transcutaneous stimulation of the right phrenic nerve at the
neck.[55]
[57]
[58]
As is the case with surface laryngeal EMG, surface diaphragmal EMG is mainly of
interest in clinical research, because of the difficulties connected with the stimulation
of the phrenic nerve transcutaneously at the neck.[59]
Evoked electrical and mechanical responses represent different
physiologic events. Evoked EMG records changes in electrical activity of one or
more muscles, whereas evoked MMG records changes associated with excitation-contraction
coupling and the contraction of the muscle as well. For these reasons, the results
obtained with these
methods may differ.[60]
[61]
Although evoked EMG responses generally correlate well with evoked mechanical responses,
[62]
marked differences may occur, especially in
the response to succinylcholine and in the TOF ratio during recovery from a nondepolarizing
block.[60]
[61]
[62]
In theory, recording of evoked EMG responses has several advantages
over recording of evoked mechanical responses. Equipment for measuring evoked EMG
responses is easier to set up, the response reflects only those factors influencing
neuromuscular transmission, and the response can be obtained from muscles not accessible
to mechanical recording. However, evoked EMG does entail some difficulties. Although
good recordings are possible in most patients, results are not always reliable.
For one thing, improper placement of electrodes may result in inadequate pickup of
the compound EMG signal. If the neuromuscular transmission analyzer does not allow
observation of the actual waveform of the compound EMG, determining the optimal placement
of the electrodes is difficult. Another source of unreliable results may be that
fixation of the hand with a preload on the thumb may be more important than generally
appreciated,[62]
[63]
as changes in the position of the electrodes in relationship to the muscle may affect
the EMG response. In addition, direct muscle stimulation sometimes occurs. If muscles
close to the stimulating electrodes are stimulated directly, the recording electrodes
may pick up an electrical signal even though neuromuscular transmission is completely
blocked. Another difficulty is that the EMG response often does not return to control
value. Whether this situation is the result of technical problems, inadequate fixation
of the hand, or changes in temperature is unknown ( Fig.
39-13
). Finally, the
Figure 39-13
Evoked electromyographic printout from a Relaxograph.
Initially, single-twitch stimulation was given at 0.1 Hz, and vecuronium (70 µg/kg)
was given intravenously for tracheal intubation. After approximately 5 minutes,
the mode of stimulation was changed to TOF stimulation every 60 seconds. At a twitch
height (first twitch in TOF response) of approximately 30% of control (marker 1),
1 mg of vecuronium was given intravenously. At marker 2, 1 mg of neostigmine was
given intravenously, preceded by 2 mg of glycopyrrolate. The printout also illustrates
the common problem of failure of the electromyographic response to return to control
level. (Courtesy of Datex-Ohmeda, Helsinki, Finland.)
evoked EMG response is very sensitive to electrical interference, such as that caused
by diathermy.
