Train-of-Four Stimulation
In TOF nerve stimulation, introduced by Ali and associates[14]
[15]
during the early 1970s, four supramaximal stimuli
are given every 0.5 seconds (2 Hz) ( Fig.
39-2
). When used continuously, each set (train) of stimuli normally is
repeated every 10th to 20th second. Each stimulus in the train causes the muscle
to contract, and "fade" in the response provides the basis for evaluation. That
is, dividing the amplitude of the fourth response by the amplitude of the first response
provides the TOF ratio. In the control response (the response obtained before administration
of muscle relaxant), all four responses are ideally the same: The TOF ratio is 1.0.
During a partial nondepolarizing block, the ratio decreases (fades) and is inversely
proportional to the degree of blockade. During a partial depolarizing block, no
fade occurs in the TOF response; ideally, the TOF ratio is approximately 1.0. Fade
in the TOF response after injection of succinylcholine signifies the development
of a phase II block (discussed later in the section on Depolarizing Neuromuscular
Blockade).
Figure 39-2
Pattern of electrical stimulation and evoked muscle responses
to TOF nerve stimulation before and after injection of nondepolarizing (Non-dep)
and depolarizing (Dep) neuromuscular blocking drugs (arrows).
The advantages of TOF stimulation are greatest during nondepolarizing
blockade, because the degree of block can be read directly from the TOF response
even though a preoperative value is lacking. In addition, TOF stimulation has some
advantages over tetanic stimulation: it is less painful and, unlike tetanic stimulation,
generally does not affect the degree of neuromuscular blockade.
