Neuromuscular blocking agents

image of Neuromuscular blocking agents
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Many drugs used for induction or maintenance of anaesthesia provide a degree of skeletal muscle relaxation, but in general this is only mild to moderate at a surgical plane of anaesthesia. This chapter looks at different ways to achieve more profound muscle relaxation, neuromuscular blocking agents (NMBAs) and their mechanism, monitoring the neuromuscular junction, clinical assessment of recovery from neuromuscular blockade, monitoring adequacy of anaesthesia, individual agents, interactions with other drugs, and more.

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16.1 Microanatomy of the neuromuscular junction. ACh = acetylcholine. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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16.2 Summary of events leading to normal muscle contraction.
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16.3 (a) Peripheral nerve stimulator with needle electrodes. (b) Placement of needle electrodes over the ulnar nerve on the medial aspect of the elbow. (c) Stimulation of the peroneal nerve can be achieved by electrode placement over the lateral head of the fibula. (d) Needle electrodes placed over the dorsal buccal branch of the facial nerve.
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16.4 Train-of-four (TOF) stimulation pattern. (a) Normal animal, no neuromuscular blockade. All four twitches in the TOF are present and of equal strength. (b) Onset of non-depolarizing blockade. Fade is present in the TOF but all twitches are still perceptible. (c) Neuromuscular blockade is now more profound: the fourth twitch in the TOF (T) has disappeared and the remaining three twitches are weaker.
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16.5 Double-burst stimulation (DBS). Each burst comprises three stimuli at a frequency of 50 Hz with the second burst following 750 milliseconds after the first. In the absence of neuromuscular blockade, this produces two distinct muscle twitches.
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16.7 Acceleromyography using stimulation of the ulnar nerve, with the transducer attached to the palmar aspect of the metacarpus. Note that the leg is fixed in place using tape across the distal antebrachium: this is essential to ensure accuracy of the technique. In this case, the dog was recovering from rocuronium-induced neuromuscular blockade, and the acceleromyograph is displaying a TOF ratio of 90% (0.9), indicating that neuromuscular function has recovered sufficiently to allow endotracheal extubation.
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