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  • image It is important to realize that muscle relaxation does not ensure unconsciousness, amnesia, or analgesia.
  • image Depolarizing muscle relaxants act as acetylcholine (ACh) receptor agonists, whereas nondepolarizing muscle relaxants function as competitive antagonists.
  • image Because depolarizing muscle relaxants are not metabolized by acetylcholinesterase, they diffuse away from the neuromuscular junction and are hydrolyzed in the plasma and liver by another enzyme, pseudocholinesterase (nonspecific cholinesterase, plasma cholinesterase, or butyrylcholinesterase).
  • image With the exception of mivacurium, nondepolarizing agents are not significantly metabolized by either acetylcholinesterase or pseudocholinesterase. Reversal of their blockade depends on redistribution, gradual metabolism, and excretion of the relaxant by the body, or administration of specific reversal agents (eg, cholinesterase inhibitors) that inhibit acetylcholinesterase enzyme activity.
  • image Muscle relaxants owe their paralytic properties to mimicry of ACh. For example, succinylcholine consists of two joined ACh molecules.
  • image Compared with patients with low enzyme levels or heterozygous atypical enzyme in whom blockade duration is doubled or tripled, patients with homozygous atypical enzyme will have a very long blockade (eg, 4-8 h) following succinylcholine administration.
  • image Succinylcholine is considered contraindicated in the routine management of children and adolescents because of the risk of hyperkalemia, rhabdomyolysis, and cardiac arrest in children with undiagnosed myopathies.
  • image Normal muscle releases enough potassium during succinylcholine-induced depolarization to raise serum potassium by 0.5 mEq/L. Although this is usually insignificant in patients with normal baseline potassium levels, a life-threatening potassium elevation is possible in patients with burn injury, massive trauma, neurological disorders, and several other conditions.
  • image Doxacurium, pancuronium, vecuronium, and pipecuronium are partially excreted by the kidneys, and their action is prolonged in patients with renal failure.
  • image Cirrhotic liver disease and chronic renal failure often result in an increased volume of distribution and a lower plasma concentration for a given dose of water-soluble drugs, such as muscle relaxants. On the other hand, drugs dependent on hepatic or renal excretion may demonstrate prolonged clearance. Thus, depending on the drug, a greater initial dose—but smaller maintenance doses—might be required in these diseases.
  • image Atracurium and cisatracurium undergo degradation in plasma at physiological pH and temperature by organ-independent Hofmann elimination. The resulting metabolites (a monoquaternary acrylate and laudanosine) have no intrinsic neuromuscular blocking effects.
  • image Hypertension and tachycardia may occur in patients given pancuronium. These cardiovascular effects are caused by the combination of vagal blockade and catecholamine release from adrenergic nerve endings.
  • image Long-term administration of vecuronium to patients in intensive care units has resulted in prolonged neuromuscular blockade (up to several days), possibly from accumulation of its active 3-hydroxy metabolite, changing drug clearance, or the development of a polyneuropathy.
  • image Rocuronium (0.9-1.2 mg/kg) has an onset of action that approaches succinylcholine (60-90 s), making it a suitable alternative for rapid-sequence inductions, but at the cost of a much longer duration of action.

Skeletal muscle relaxation can be produced by deep inhalational anesthesia, regional nerve block, or neuromuscular blocking agents (commonly called muscle relaxants). In 1942, Harold Griffith published the results of a study using an extract of curare (a South American arrow poison) during ...

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