BASIC PHARMACOLOGY OF REVERSAL AGENTS

Clinical limitations of acetylcholinesterase antagonists

Administration of a nondepolarizing neuromuscular blocking drug (muscle relaxant) is a standard practice in many anesthetic scenarios. These muscle relaxants work by competitive antagonism of the neurotransmitter acetylcholine at nicotinic cholinergic receptors within the neuromuscular junction of skeletal muscle (Martyn et al; Neuromuscular physiology and pharmacology, anesthesia. Edited by RD Miller, Philadelphia, Churchill Livingstone, 2000). At the conclusion of the procedure for which the muscle relaxant was administrated, there will be a greater or lesser degree of residual muscle weakness. It is usually necessary to pharmacologically reverse this residual weakness to restore full function to the patient's muscles. Indeed, failure to reverse neuromuscular block is associated with increased perioperative morbidity and mortality (Arbous et al; Impact of anesthesia management characteristics on severe morbidity and mortality. Anesthesiology 2005;102:257-268; quiz 491-2). Pharmacologic reversal currently relies on the administration of an anticholinesterase drug, which decreases the metabolism of acetylcholine at the neuromuscular junction and allows its concentration to increase and hopefully overcome the effect of the muscle relaxant. This approach to reversal has significant limitations; the mechanism of reversal is indirect, the efficacy is limited and unpredictable, and undesirable autonomic responses occur. This review will address these limitations.

Treatment of anticholinergic-induced ileus with neostigmine

A 33-year-old man with a history of recreational benztropine abuse presented to the emergency department with confusion, abdominal pain, and distention. An abdominal radiograph revealed gross fecal loading. He was initially treated with intravenous fluids and opiate analgesia. Subsequently, a diagnosis of anticholinergic poisoning was made, based on tachycardia, delirium, dry mucosa, and reduced bowel sounds. Treatment with tacrine reversed the delirium, and a history of repeated benztropine use was obtained. Persistent ileus was treated with repeated doses of neostigmine, and gastrointestinal motility returned with prompt defecation. Neostigmine appears to be useful in reversing ileus caused by anticholinergic drug overdose. Theoretically, it may be useful in reversing anticholinergic ileus resulting from acute drug overdose, allowing or enhancing decontamination, but the safety and potential efficacy of neostigmine in this scenario have not been established.

Comparison of the effects of neostigmine and edrophonium on the duration of action of suxamethonium

Rapid sequence induction of anaesthesia necessitating the use of suxamethonium may occasionally be needed soon after antagonism of neuromuscular block with anticholinesterase agents. The onset and duration of action of 1 mg kg-1 of suxamethonium was recorded in groups of 10 patients each, 5 or 10 min after the administration of edrophonium 1 mg kg-1 or neostigmine 40 micrograms kg-1 given for the antagonism of atracurium-induced neuromuscular block. Plasma cholinesterase activity was measured before, and 5 and 10 min after the administration of the anticholinesterases. A further 10 patients received suxamethonium 1 mg kg-1 without prior atracurium or anticholinesterase administration to serve as controls. The onset of action of suxamethonium was significantly prolonged when administered 5 min after both anticholinesterases, compared to the control group (P < 0.01). Recovery of suxamethonium block was delayed significantly after neostigmine, compared to both the edrophonium and the control groups (P < 0.05-0.001). Plasma cholinesterase activity was significantly reduced with the use of neostigmine but not with edrophonium (P < 0.001).

Dose-responses for edrophonium during antagonism of vecuronium block in young and older adult patients

The dose-response relationship for edrophonium during antagonism of vecuronium-induced neuromuscular blockade was studied in two groups of adult patients of mean (SD) age 35 (10.0) years (n = 42) and 77 (5.4) years (n = 42) respectively. Neuromuscular block was monitored by recording the force of contraction of the adductor pollicis muscle following train-of-four stimulation. Six patients in each age group received 0.1, 0.3, 0.5, 0.7, 1.0, or 1.5 mg.kg-1 of edrophonium, or normal saline at 10% recovery of T1 (first response in the train-of-four) after a single dose of vecuronium 0.08 mg.kg-1. The train-of-four ratios were recorded continuously over the next 10 min and the values at 1 min intervals from 5 min onwards were used to construct the dose-response curves. The dose-response curves showed no significant difference between the two age groups except at 10 min. The estimated dose of edrophonium required for attaining a train-of-four ratio of 0.7 at 10 min was 0.9 and 1.3 mg.kg-1 in the younger and older groups, respectively (p < 0.05).