THE NERVE STIMULATOR
Although many nerve stimulators are commercially available, not
all meet the basic requirements for clinical use.
Figure 39-6
Relationship between the rate of muscle response to stimulation
of the tracheal carina and the degree of neuromuscular blockade of peripheral muscles,
as evaluated by using post-tetanic count. The subjects were 25 patients anesthetized
with thiopental, nitrous oxide, and fentanyl who were given vecuronium (0.1 mg/kg)
for tracheal intubation. For comparison, the first response to TOF stimulation usually
occurs when PTC is approximately 10 (range, 6 to 16). The carina was stimulated
with a soft sterile rubber suction catheter introduced via the endotracheal tube.
The total response consisted of mild responses plus severe response. A mild response
was said to occur if stimulation of the carina induced only slight bucking that did
not interfere with surgery. A severe response was said to occur if stimulation elicited
bucking that interfered with surgery and required intervention. Elimination of severe
responses requires an intense neuromuscular blockade; PTC must be less than 2 to
3, and elimination of all reactions requires that PTC be 0. (From Fernando
PUE, Viby-Mogensen J, Bonsu AK, et al: Relationship between post-tetanic count and
response to carinal stimulation during vecuronium-induced neuromuscular blockade.
Acta Anaesthesiol Scand 31:593, 1987. Copyright 1987, Munksgaard International
Publishers, Ltd. Copenhagen, Denmark.)
Figure 39-7
Pattern of electrical stimulation and evoked muscle responses
to TOF nerve stimulation and double-burst nerve stimulation (i.e., three impulses
in each of two tetanic bursts, DBS3,3
) before injection of muscle relaxants
(control) and during recovery from nondepolarizing neuromuscular blockade. TOF ratio
is the amplitude of the fourth response to TOF divided by the amplitude of the first
response. DBS3,3
ratio is the amplitude of the second response to DBS3,3
divided by the amplitude of the first response. (See text for further explanation.)
The stimulus should produce a monophasic and rectangular waveform, and the length
of the pulse should not exceed 0.2 to 0.3 msec. A pulse exceeding 0.5 msec may stimulate
the muscle directly or cause repetitive firing. Stimulation at a constant current
is preferable to stimulation at a constant voltage because current is the determinant
of nerve stimulation. Also, for safety reasons, the
Figure 39-8
Fade detectable by feel in the response to TOF and double
burst stimulation (DBS3,3
) in relation to the true TOF ratio, as measured
mechanically. The axis indicates the percentage of instances in which fade can be
felt at a given TOF ratio.[28]
[29]
It appears that it is not possible to exclude residual neuromuscular block by any
of the methods, whether a TOF ratio of 0.7 or 0.9 is taken to reflect adequate recovery
of neuromuscular function. (See text for further explanation.)
nerve stimulator should be battery-operated, include a battery check, and be able
to generate 60 to 70 mA, but not more than 80 mA. Many commercially available stimulators
can deliver only 25 to 50 mA and provide a constant current only when skin resistance
ranges from 0 Ω to 2.5 kΩ. This deficiency is a disadvantage; during
cooling, skin resistance may increase to approximately 5 kΩ, which may cause
the current delivered to the nerve to fall below the supramaximal level, leading
to a decrease in the response to stimulation. As a result, the anesthesiologist
may misjudge the degree of neuromuscular blockade. Ideally, the nerve stimulator
should have a built-in warning system or a current level display that alerts the
user when the current selected is not delivered to the nerve.
The ideal nerve stimulator should have other features as well.
The polarity of the electrodes should be indicated. Also, the apparatus should
be capable of delivering the following modes of stimulation: TOF (as both a single
train and in a repetitive mode, with TOF stimulation being given every 10 to 20 seconds);
single-twitch stimulations at 0.1 and 1.0 Hz; and tetanic stimulation at 50 Hz.
In addition, the stimulator should have a built-in time constant system to facilitate
post-tetanic count. Tetanic stimulus should last 5 seconds and be followed 3 seconds
later by the first post-tetanic stimulus. If the nerve stimulator does not allow
objective measurement of the response to TOF stimulation, at least one DBS mode should
be available, preferably DBS3,3
. Single-twitch stimulation at 1 Hz is
useful during initiation of monitoring because it shortens the time necessary to
determine supramaximal stimulation. Most investigators agree that there is no need
for tetanus at 100 or 200 Hz because 50-Hz tetanic stimulation stresses neuromuscular
function to the same extent as does a maximal voluntary effort.
Furthermore, in contrast to 100- and 200-Hz stimulation, 50-Hz tetanic stimulation
does not cause fatigue (fade) in nonparalyzed muscle.
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