Tetanic Stimulation
Tetanic stimulation consists of very rapid (e.g., 30-, 50-, or
100-Hz) delivery of electrical stimuli. The most commonly used pattern in clinical
practice is 50-Hz stimulation given for 5 seconds, although some investigators have
advocated the use of 50-, 100-, and even 200-Hz stimulation for 1 second. During
normal neuromuscular transmission and a pure depolarizing block, the muscle response
to 50-Hz tetanic stimulation for 5 seconds is sustained. During a nondepolarizing
block and a phase II block after injection of succinylcholine, the response will
not be sustained (i.e., fade occurs) ( Fig.
39-3
).
Fade in response to tetanic stimulation is normally considered
a presynaptic event; the traditional explanation is that at the start of tetanic
stimulation, large amounts of acetylcholine are released from immediately available
stores in the nerve terminal. As these stores become depleted, the rate of acetylcholine
release decreases until equilibrium between mobilization and synthesis of acetylcholine
is achieved. Despite this equilibrium, the muscle response caused by tetanic stimulation
of the nerve at, for example, 50 Hz, is maintained (given normal neuromuscular transmission)
simply because the release of acetylcholine is many times greater than the amount
necessary to evoke a response. When the "margin of safety"[16]
of the postsynaptic membrane (i.e., the number of free cholinergic receptors) is
reduced by a nondepolarizing neuromuscular blocking agent, the decrease in release
of acetylcholine during tetanic stimulation produces fade. In
Figure 39-3
Pattern of stimulation and evoked muscle responses to
tetanic (50-Hz) nerve stimulation for 5 seconds (Te) and post-tetanic stimulation
(1.0-Hz) twitch. Stimulation was applied before injection of neuromuscular blocking
drugs and during moderate nondepolarizing and depolarizing blocks. Note fade in
the response to tetanic stimulation, plus post-tetanic facilitation of transmission
during nondepolarizing blockade. During depolarizing blockade, the tetanic response
is well sustained and no post-tetanic facilitation of transmission occurs.
addition to blocking the postsynaptic receptors, nondepolarizing neuromuscular blocking
drugs may also impair the mobilization of acetylcholine within the nerve terminal.
This effect may contribute to the fade in the response to tetanic (and TOF) stimulation.
The degree of fade depends primarily on the degree of neuromuscular blockade. Fade
also depends on the frequency (Hz) and the length (seconds) of stimulation and on
how often tetanic stimuli are applied. Unless these variables are kept constant,
results from different studies using tetanic stimulation cannot be compared.
During partial nondepolarizing blockade, tetanic nerve stimulation
is followed by a post-tetanic increase in twitch tension (i.e., post-tetanic facilitation
[PTF] of transmission) (see Fig. 39-3
).
This event occurs because the increase in mobilization and synthesis of acetylcholine
caused by tetanic stimulation continues for some time after discontinuation of stimulation.
The degree and duration of PTF depend on the degree of neuromuscular blockade, with
PTF usually disappearing within 60 seconds of tetanic stimulation. PTF is evident
in electromyographic, acceleromyographic, and mechanical recordings during a partial
nondepolarizing neuromuscular blockade. In contrast, post-tetanic twitch potentiation,
which sometimes occurs in mechanical recordings before any neuromuscular blocking
drug has been given, is a muscular phenomenon that is not accompanied by an increase
in the compound muscle action potential.
Tetanic stimulation has several disadvantages. It is very painful
and therefore normally not acceptable to the unanesthetized patient. Furthermore,
especially in the late phase of neuromuscular recovery, tetanic stimulations may
produce a lasting antagonism of neuromuscular blockade in the stimulated muscle,
such that the response of the tested site may no longer be representative of other
muscle groups.[17]
[18]
Traditionally, tetanic stimulation has been used to evaluate residual
neuromuscular blockade. However, except in connection with the technique of post-tetanic
count (see later), tetanic stimulation has very little place in everyday clinical
anesthesia. If the response to nerve stimulation is recorded, all the information
required can be obtained from the response to TOF nerve stimulation. In contrast,
if the response to nerve stimulation is evaluated only by feel[19]
or by eye (Viby-Mogensen and colleagues, unpublished observation), even experienced
observers are unable to judge the response of tetanic stimulation with sufficient
certainty to exclude residual neuromuscular blockade.
