Muscle relaxants neuromuscular blockers

Isoflurane is a respiratory depressant (71). At concentrations which are associated with surgical levels of anesthesia, there is Htde or no depression of myocardial function. In experimental animals, isoflurane is the safest of the oral clinical agents (72). Cardiac output is maintained despite a decrease in stroke volume. This is usually because of an increase in heart rate. The decrease in blood pressure can be used to produce "deHberate hypotension" necessary for some intracranial procedures (73). This agent produces less sensitization of the human heart to epinephrine relative to the other inhaled anesthetics. Isoflurane potentiates the action of neuromuscular blockers and when used alone can produce sufficient muscle relaxation (74). Of all the inhaled agents currently in use, isoflurane is metabolized to the least extent (75). Unlike halothane, isoflurane does not appear to produce Hver injury and unlike methoxyflurane, isoflurane is not associated with renal toxicity. 

Halothane exerts a pronounced hypotensive effect, to which a negative inotropic effect contributes. Enflurane and isoflurane cause less circulatory depression. Halothane sensitizes the myocardium to catecholamines (caution serious tachyarrhythmias or ventricular fibrillation may accompany use of catecholamines as antihypotensives or toco-lytics). This effect is much less pronounced with enflurane and isoflurane. Unlike halothane, enflurane and isoflurane have a muscle-relaxant effect that is additive with that of nondepolarizing neuromuscular blockers. 

Isoflurane, an isomer of enflurane, together with sevoflurane are the most commonly used inhalation anesthetics in humans. Isoflurane does not sensitize the myocardium to catecholamines, has muscle relaxing action so less neuromuscular blocker is required and causes less hepatotoxicity and renal toxicity than halothane. 

Skeletal muscle relaxation (this requirement is currently met with the aid of skeletal muscle blockers used in conjunction with the anesthetic [see Neuromuscular Blockers, later]). 

Drugs discussed in this chapter are used to decrease muscle excitability and contraction via an effect at the spinal cord level, at the neuromuscular junction, or within the muscle cell itself. Some texts also classify neuromuscular junction blockers such as curare and succinylcholine as skeletal muscle relaxants. However, these drugs are more appropriately classified as skeletal muscle paralytics because they eliminate muscle contraction by blocking transmission at the myoneural synapse. This type of skeletal muscle paralysis is used primarily during general anesthesia using neuromuscular blockers as an adjunct in surgery was discussed in Chapter 11. Skeletal muscle relaxants do not typically prevent muscle contraction they only attempt to normalize muscle excitability to decrease pain and improve motor function. 

General anesthetics are rarely given alone. In addition to the analgesic agents just mentioned, benzodiazepines (midazolam, Versed diazepam, Valium ) are commonly used as adjuncts for the relief of anxiety, amnesia, and sedation prior to induction of anesthesia. Neuromuscular blockers (e.g., succinylcholine or pancuronium) can also be administered during the induction of anesthesia to relax skeletal muscles. 

Clinical use of muscle relaxants. Among the available neuromuscular blockers, succinylcholine displays the fastest onset of action. The patient can be intubated as early as 30-60 seconds after intravenous injection ( rapid sequence intubation ), which is important in emergency situations with an increased risk of aspiration (e.g., ileus, full stomach, head trauma). Postoperative muscle pain due to succinylcholine can be prevented by preinjection of a small dose of a nondepolarizing blocker ( precurarization ). In combination with propofol p. 218), rocuronium (p.184) creates intubation conditions comparable to those obtained with succinylcholine. 

Skeletal muscle relaxants fall into three major categories those that reduce spasticity, those that cause neuromuscular blockade and those that work at the cellular level. Spasmolytic agents (e.g. metho-carbamol, guaifenesin) act centrally whereas neuromuscular blockers (e.g. succinylcholine (suxamethonium), pancuronium, atracurium) act at the neuromuscular end plate to produce muscular relaxation. Dantrolene falls into the third category and acts within the muscle cell itself to produce relaxation. 

Muscle relaxants Non-depolarizing neuromuscular blockers Short-lasting muscle paralysis Pancuronium, atracurium... 

Atracurium - muscle relaxant (non-depolarizing neuromuscular blocker) adjunct to... 

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