Anesthesia Student Survival Guide.pdf - Index of

50 ● AnesthesiA student survivAl Guide
antagonizing acetylcholine from binding at the postsynaptic nicotinic receptor
in the neuromuscular junction. The result of this competitive antagonism is an
inhibition of junctional depolarization.
Onset time and duration of action are as follows: rocuronium < vecuronium
< cisatracurium < pancuronium. Because of their longer durations of
action as compared to succinylcholine, NMBs are commonly used to maintain
muscle relaxation during surgery. NMBs are also used to facilitate intubation,
but the time to achieve equivalent and ideal intubating conditions is significantly
longer than with succinylcholine.
Reversal of NMBs is accomplished by the administration of an acetylcholinesterase
inhibitor (e.g. neostigmine), which prevents breakdown of acetylcholine
at the neuromuscular junction. The subsequent excess of acetylcholine
can then out-compete the NMB for junctional binding, and allow for muscle
depolarization. An anticholinergic, such as glycopyrrolate, must be simultaneously
administered to prevent muscarinic overactivity such as severe bradycardia,
asystole, or bronchospasm.
Most of the commonly used NMBs are metabolized to some degree, but they
rely mainly on biliary and renal excretion for termination of action. Cisatracurium is
the exception, as it is degraded in the plasma (Hoffman elimination). Cisatracurium
is therefore commonly used in patients who have renal or hepatic dysfunction.
Side effects of NMBs are rare, with tachycardia (pancuronium), and
hypotension (cisatracurium) being the most frequently encountered. Allergic
reactions to anesthetics are rare, but are most commonly from NMBs.
Acetylcholinesterase Inhibitors
Neostigmine and edrophonium are acetylcholinesterase inhibitors that are
primarily used to reverse neuromuscular blockade. They work by preventing
breakdown of acetylcholine at the neuromuscular junction, thereby allowing
the competitive inhibition of nondepolarizing NMBs to be overcome. Major
side effects are bradycardia and excessive salivation. These are due to sudden
and substantial increases in acetylcholine concentrations. Concomitant
administration of an anticholinergic (such as glycopyrrolate) is required to prevent
these side effects.
A “cholinergic crisis” may result from an overdose of acetylcholinesterase
inhibitors or when the agent is given without a concomitant anticholinergic
drug. Symptoms include bradycardia, bronchospasm, vomiting, miosis, and
muscle weakness. Many nerve gases used in warfare are acetylcholinesterase
inhibitors that can lead to a severe cholinergic crisis.