RECORDING OF EVOKED RESPONSES
The choice of recording method is a practical decision. Five
methods are available: measurement of evoked mechanical response of the muscle (mechanomyography
[MMG]), measurement of evoked electrical response of the muscle (electromyography
[EMG]), measurement of acceleration of the muscle response (acceleromyography [AMG]),
measurement of evoked electrical response in a piezoelectric film sensor attached
to the muscle (piezoelectric neuromuscular monitors [PZ
EMG],
and phonomyography [PMG]).
Mechanomyography
A requirement for correct and reproducible measurements of evoked
tension is that the muscle contraction be isometric. In clinical anesthesia, this
condition is most easily achieved by measuring thumb movement after application of
a resting tension of 200 to 300 g (a preload) to the thumb. When the ulnar nerve
is stimulated, the thumb (the adductor pollicis muscle) acts on a force-displacement
transducer. The force of contraction is then converted into an electrical signal,
which is amplified, displayed, and recorded. The arm and hand should be fixed rigidly,
and care should be taken to prevent over-loading of the transducer. Also, the transducer
should be placed in correct relationship to the thumb (i.e., the thumb should always
apply tension precisely along the length of the transducer). It is important to
remember that the response to nerve stimulation depends on the frequency with which
the individual stimuli are applied and that the time used to achieve a stable control
response may influence the subsequent determination of onset time and duration of
block.[53]
Generally, the reaction to supramaximal
stimulation increases during the first 8 to 12 minutes after commencement of the
stimulation. Therefore, in clinical studies, control measurements (before injection
of muscle relaxant) should not be made before the response has stabilized for 8 to
12 minutes or a 2-second or 5-second 50-Hz tetanic stimulation has been given.[54]
Even then, twitch response often recovers to 110% to 150% of the control response
after paralysis with succinylcholine. This increase in response, thought to be caused
by a change in the contractile response of the muscle, normally disappears within
15 to 25 minutes.
Although numerous methods for mechanical recording of evoked mechanical
responses exist, not all meet the criteria outlined.
