Different patterns of nerve stimulation
Neuromuscular function is monitored intraoperatively by evaluating the muscular response to supramaximal stimulation of a peripheral motor nerve . There are two kinds of stimulation: electrical and magnetic. Electrical nerve stimulation is used most commonly clinically. Magnetic stimulation has a theoretical advantage of not being painful and not requiring body contact. However, the bulk of the equipment and difficulty monitoring the train-of-four responses to stimulation preclude its practical use in the operating room.
The reaction of a single muscle fiber to an electrical stimulus is an all-or-none occurrence. The response of the muscle will decrease depending on the number of muscle fibers blocked in response to a neuromuscular blocking agent. The electrical stimulus applied should be 20% to 25% above that necessary for a maximal response to obtain a consistent response. This supramaximal stimulation, although painful, is possible during anesthesia . A current of uniform amplitude (20–60 mA) at a short duration (0.1–0.2 ms) is applied to a peripheral nerve and the motor response is observed. Common sites include facial nerve (facial twitch) and ulnar nerve (thumb abduction). A current of greater than 0.5 ms will cause direct muscle stimulation which is not optimal. Assessment is most commonly by tactile or visual method of elicited muscle twitches. While this is the most practical method, it is subjective and not accurate. Objective methods including electromyography, acceleromyography, and mechanomyography will give a more accurate assessment compared to tactile responses . The peripheral nerve stimulator should be able to generate 60 to 70 mA, be battery operated, and alarm if the current is not being delivered. The stimulator should be able to deliver single-twitch stimulation, TOF, and double-burst, tetanic stimulation and have a time constant to facilitate a posttetanic count .
There are five patterns of stimulation:
Single-twitch stimulation: A single supramaximal electric current is applied at a frequency ranging from 1.0 Hz (one every second) to 0.1 Hz (one every 10 s) (Fig. 24.1A).
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Train-of-four stimulation: Four stimuli at 2 Hz are applied (four stimuli in 2 s) that are repeated every 10 to 12 s if needed. The ratio of the fourth response to the first response (T4/T1 ratio) is used to assess the presence of neuromuscular blockade and its degree. In the absence of neuromuscular block, the ratio is 1. During a nondepolarizing block, the ratio decreases in proportion to the degree of the block. A depolarizing block, on the other hand, decreases all the four responses equally with TOF ratio of 1. A decrease in the TOF ratio after the administration of succinylcholine is indicative of phase II block. TOF value of 0.70 is associated with impaired respiratory muscle function, hypoxia, and aspiration in the immediate postoperative phase. Neostigmine is given only when the TOF count has returned spontaneously to three and preferably four responses. The availability of sugammadex as a reversal agent does not obviate the need for monitoring. The appropriate dose of sugammadex is adjusted according to the TOF and posttetanic stimulation responses (Fig. 24.1B).
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