Instruction manual

7.2 BASELINE ADJUSTMENT
Before any muscle relaxant is administered, the instrument should be adjusted to provide Supramaximal Stimulation
(SMS). SMS is defined as the level at which additional stimulation current does not increase twitch response. This
setting can be approximated by adjusting the control knob to the level where any further increase in stimulus current
would not increase the level of the twitch response. Note the reading on the digital display and maintain this baseline
setting throughout the entire procedure.
7.3 TWITCH RESPONSE
The simplest test provided by the PNS is the twitch response where individual stimuli are generated at intervals of
one to ten seconds. Shortly after administering the muscle relaxant, the twitch response will start to become
depressed. At this point more than 70 percent of the receptors should be blocked. When the twitch is completely
eliminated, greater than 90 percent of the receptors are occupied by the relaxant. The twitch can be used as a
quantitative monitor by adjusting the muscle relaxant administration to maintain a faint, but perceptible muscular
contraction (twitch) in response to the PNS. This assures adequate operating conditions while avoiding excessive
relaxant administration. In the event the twitch has been abolished by an inadvertent relaxant overdose, if one waits
until the twitch reappears before administering subsequent relaxant, the incidence of failure of reversal can be
minimized.
7.4 TETANIC STIMULI
When the single twitch response has returned to normal, approximately 20 percent of the receptor pool is free.
Fortunately, the diaphragm needs fewer receptors available to respond normally than do peripheral muscles. This
can be observed clinically in that spontaneous respiration may be detected before an indirectly stimulated response.
However, a patient with 80 percent receptor block may still need to be carefully monitored. Thus, it is important to
have a means of assessing when recovery has proceeded to a more adequate level. It was to this end that the tetanic
stimuli evolved. This is the administration of 50 or 100 Hz stimuli for a period of approximately 5 seconds duration.
The higher frequency puts a greater demand on the neuromuscular synapse because as each successive stimulus
arrives at the nerve ending, it depletes the local store of transmitters so that the amount of acetylcholine available for
release by each succeeding stimulus falls. When the fraction of free receptors is also decreased, the tetanic response
does not maintain its initial intensity; it fades. The higher the rate of tetanic stimulation, the more sensitive the test.
Fade can be detected at 100 Hz when as few as 50 percent of the receptors are occupied and at 50 Hz when as few as
70 percent of the receptors are occupied. Unfortunately, tetanic stimuli are painful and are therefore of limited value
in detecting subtle neuromuscular blockade in the unanesthetized patient.
7.5 TRAIN-OF-FOUR
In the Train-Of-Four (TOF) test, the ulnar nerve is stimulated with four supramaximal stimuli 0.5 second apart and
the ratio of the fourth twitch to the first twitch is used to determine the degree of neuromuscular block. The primary
advantage of TOF is that the first response provides a built-in control for the fourth response. Built-in control is a
great convenience in the clinical setting in which factors such as patient movement can change the initial tension of
the muscle and hence the amplitude of the twitch response. A good rule of thumb is that the degree of block may be
estimated by counting the number of twitches seen following the four stimulus pulses. When only one twitch is
present, there is greater than 90 percent block. All four twitches appear when the single twitch is depressed by 75
percent. Recovery from the block occurs when all four twitches in the train are the same height. At this time, about
25 percent of the receptor pool is free. Thus, TOF is a slightly more sensitive test than the twitch.
7.6 NERVE BLOCK PROCEDURES
The PNS can be used as an aid in locating any nerve that has a motor component, but is not a substitute for
knowledge of basic anatomy and block technique. The basic technique of locating nerves using a PNS is outlined in
the following paragraphs. It is important to observe the digital display of the output current since the accuracy of
nerve location depends on how precisely the current can be set. The lower the current setting, the closer the needle
must be to the nerve to obtain a motor response.
CAUTION: Do not use the RED (HI) output jack for nerve location. Stimulation of nerves using needles requires
very low current. Use of the RED (HI) output may result in possible needle burns. The leadwire with the BLACK
plug should be attached to the BLACK (COM) jack on the rear panel of the instrument and the leadwire with the
RED plug should be attached to the YELLOW (LO) jack adjacent to the BLACK (COM) jack. Short the leadwires
together by attaching the alligator clips to each other. Turn the instrument ON and depress the TWO HZ button.
Adjust the amplitude control knob on the side of the instrument until the digital display reads 3.0 mA. This confirms
that the instrument is operating properly and also sets the current control knob to approximately the correct position.
Disconnect the alligator clips from each other and attach the negative leadwire (BLACK alligator clip) to the block
needle (BD STIMEXtm insulated needle or eqivalent). Connect the positive leadwire (RED alligator clip) to a gelled
electrode placed on the trunk of the body some distance from the block site to minimize direct stimulation of local