Succinylcholine (SCh) remains a useful muscle relaxant because of its ultra-rapid onset and short-duration neuromuscular blocking properties, which cannot be duplicated by any of the available nondepolarizing muscle relaxants.

1. Neuromuscular Effects. SCh binds to postsynaptic nicotinic receptors, where it exhibits ACh-like activity. SCh also binds to extrajunctional receptors and presynaptic receptors.
   1. The net effect of SCh-induced depolarization is uncoordinated skeletal muscle activity that manifests clinically as fasciculations.
   2. SCh predictably increases masseter muscle tone (this may be responsible for poor intubating conditions), and masseter muscle spasm may be associated with malignant hyperthermia. It is likely that increased masseter muscle tone is mediated by ACh receptors because it is blocked by nondepolarizing drugs.
   3. Nonparalyzing doses of nondepolarizing drugs (pretreatment) block visible evidence of SCh-induced depolarization, suggesting that presynaptic receptors are principally involved in the production of fasciculations.
   4. The blocking effect of SCh at the NMJ is probably attributable to desensitization (prolonged exposure to an agonist leads to a state characterized by a lack of responsiveness of the receptors).
2. Characteristics of Depolarizing Blockade
   1. SCh initially produces features characterized as phase I block (Table 20-2: Characteristics of Phase I Depolarizing Blockade).
   2. Phase II block develops after prolonged exposure to SCh or high doses of SCh and has characteristics of a nondepolarizing neuromuscular blockade (Table 20-3: Characteristics of The Nondepolarizing Neuromuscular Blockade). The onset of phase II block coincides with the appearance of tachyphylaxis to the effects of SCh.
3. Pharmacology of Succinylcholine
   1. SCh is rapidly hydrolyzed (the elimination half-life of <1 minute) by plasma cholinesterase (pseudocholinesterase).
   2. The dose producing 95% blockade (ED₉₅) at the adductor pollicis is 0.3 to 0.5 mg/kg.
   3. The time until full recovery at the adductor pollicis muscle is dose dependent and reaches 10 to 12 minutes after a dose of 1 mg/kg (Fig. 20-3: Duration of action of neuromuscular blockade with 1 mg/kg of succinylcholine (Sux) and 1.2 mg/kg of rocuronium followed 3 minutes later by 16 mg/kg of sugammadex (Roc-sug)).
4. Side Effects (Table 20-4: Side Effects of Succinylcholine)
5. Clinical Uses
   1. The principal indication for SCh is to facilitate tracheal intubation (1 mg/kg intravenous [IV] is the usual dose, which is increased to 1.5–2.0 mg/kg IV if pretreatment is used).
   2. SCh is especially indicated for “rapid sequence induction” when a patient presents with a full stomach and the possibility of aspiration of gastric contents (fast onset and brief duration allowing return of spontaneous breathing).
   3. SCh (4 mg/kg intramuscular) is the only effective NMBD in children with difficult intravenous access and provides adequate intubating conditions in about 4 minutes.

Neuromuscular Blocking Agents

1. Physiology and Pharmacology
2. Neuromuscular Blocking Agents
3. Depolarizing Blocking Drugs: Succinylcholine
4. Nondepolarizing Drugs
5. Drug Interactions
6. Altered Responses to Neuromuscular Blocking Agents
7. Monitoring Neuromuscular Blockade
8. Reversal of Neuromuscular Blockade