Chapter 12. Cholinesterase Inhibitors & Other Pharmacologic Antagonists to Neuromuscular Blocking Agents

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  The primary clinical use of cholinesterase inhibitors, also called anticholinesterases, is to reverse nondepolarizing muscle blockade.
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  Acetylcholine is the neurotransmitter for the entire parasympathetic nervous system (parasympathetic ganglions and effector cells), parts of the sympathetic nervous system (sympathetic ganglions, adrenal medulla, and sweat glands), some neurons in the central nervous system, and somatic nerves innervating skeletal muscle.
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  Neuromuscular transmission is blocked when nondepolarizing muscle relaxants compete with acetylcholine to bind to nicotinic cholinergic receptors. The cholinesterase inhibitors indirectly increase the amount of acetylcholine available to compete with the nondepolarizing agent, thereby reestablishing neuromuscular transmission.
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  In excessive doses, acetylcholinesterase inhibitors can paradoxically potentiate a nondepolarizing neuromuscular blockade. In addition, these drugs prolong the depolarization blockade of succinylcholine.
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  Any prolongation of action of a nondepolarizing muscle relaxant from renal or hepatic insufficiency will probably be accompanied by a corresponding increase in the duration of action of a cholinesterase inhibitor.
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  The time required to fully reverse a nondepolarizing block depends on several factors, including the choice and dose of cholinesterase inhibitor administered, the muscle relaxant being antagonized, and the extent of the blockade before reversal.
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  A reversal agent should be routinely given to patients who have received nondepolarizing muscle relaxants unless full reversal can be demonstrated or the postoperative plan includes continued intubation and ventilation.
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  In monitoring a patient’s recovery from neuromuscular blockade, the suggested end points are sustained tetanus for 5 sec in response to a 100-Hz stimulus in anesthetized patients or sustained head lift in awake patients. If neither of these end points is achieved, the patient should remain intubated and ventilation should be continued.
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  Sugammadex exerts its effects by forming tight complexes in a 1:1 ratio with steroidal neuromuscular blocking agents.
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  Cysteine causes inactivation of gantacurium via metabolic degradation and adduct formation.

Incomplete reversal of neuromuscular blocking agents and residual post-procedure paralysis are associated with morbidity; therefore, careful evaluation of neuromuscular blockade and appropriate pharmacologic antagonism are strongly recommended whenever muscle relaxants are administered.

The primary clinical use of cholinesterase inhibitors, also called anticholinesterases, is to reverse nondepolarizing muscle blockade. Some of these agents are also used to diagnose and treat myasthenia gravis. More recently, newer agents, such as cyclodextrins and cysteine, with superior ability to reverse neuromuscular blockade from specific agents, are being investigated with promising results. This chapter reviews cholinergic pharmacology and mechanisms of acetylcholinesterase inhibition and presents the clinical pharmacology of commonly used cholinesterase inhibitors (neostigmine, edrophonium, pyridostigmine, and physostigmine). It concludes with a brief description and mechanisms of action of some unique reversal agents.

The term cholinergic refers to the effects of the neurotransmitter acetylcholine, as opposed to the adrenergic effects of noradrenaline (norepinephrine). Acetylcholine is synthesized in the nerve terminal by the enzyme cholineacetyltransferase, which catalyzes the reaction between acetylcoenzyme A and choline (Figure 12-1). After its release, acetylcholine is rapidly hydrolyzed by acetylcholinesterase (true cholinesterase) into acetate and choline.