The most important depolarizing & non-depolarizing muscle relaxants

Succinylcholine

This is a depolarizing muscle relaxant with about 45s onset of action and an action duration of < 10 min. Few doses are required to attain therapeutic effects (small Vd), and it undergoes metabolism by pseudocholinesterase into succinylmonocholine.

Useful for rapid sequence intubation with intravenous adult dose as 0.5-1.5mg/kg. In children dose is usually higher. Storage should be at about 5ºC and usage within 14 days on exposure. 

The following can prolong its duration: hypothermia; high doses; phenelzine; oral contraceptives; neostigmine; pyridostigmine; metoclopramide; cyclophosphamide; esmolol; pancuronium; echothiophate; enzyme mutation; liver disease; kidney failure; pregnancy; nondepolarizing relaxants (during phase II block).

The side effects include: bradycardia (in low doses); tachycardia (in high doses); fasciculations & post-operative myalgia; hyperkalemia; high intraocular pressure; rigidity of masseter muscle; prolonged paralysis (in reduced and atypical pseudocholinesterase); malignant hyperthermia; high intracranial pressure; histamine release.

Contra-indications due to hyperkalemia risk include: massive trauma & burns; stroke; metabolic acidosis; encephalitis; polyneuropathy; myopathies; head & spinal cord injury; Guillain-Barre syndrome; tetanus; severe intraabdominal infection.

Atracurium

A non-depolarizing benzylisoquinoline muscle relaxant that undergoes hydrolysis by nonspecific esterases. It also undergoes chemical breakdown at physiological PH and temp. respectively. Its pharmacokinetic is independent of liver or renal function.  

It comes as a solution of 10mg/ml, with an intravenous intubation dose of 0.5mg/kg. Intraoperative dose can be given initially as 0.25mg/kg with incremental dose of 0.1mg/kg every 15 min or 5-10mcg/kg/min infusion. Onset of action is about 3 min, while action duration is at an average of 37.5 min.

Side effects: histamine release; allergic reactions; hypotension and tachycardia; prolonged paralysis (in hypothermia and acidosis); bronchospasm. Not advisable for use in asthma. Storage should be at about 5ºC and usage within 14 days on exposure. 

Cisatracurium

A more potent stereoisomer of atracurium that undergoes chemical breakdown in plasma at physiological PH & temp into laudanosine and acrylate. It doesn’t alter heart rate and blood pressure. Onset of action is about 3 min, with action duration at an average of 57.5 min. Pharmacokinetic is independent of liver and kidney function.

Intubating iv dose is about 0.1-0.15mg/kg, with maintenance infusion dose of 1-2mcg/kg/min. Storage should be at about 5ºC and usage should not exceed 21 days on exposure.

Mivacurium

This is a benzylisoquinoline that undergoes metabolism by pseudocholinesterase. Like other non-depolarizing muscle relaxants, edrophonium antagonizes it effectively than neostigmine. Neostigmine effectively antagonizes acetylcholinesterase. Side effect includes histamine release.

Intubating iv dose is 0.15-0.2mg/kg, with bolus maintenance dose of 0.05mg/kg. Infusion maintenance dose averages at 10mcg/kg/min. Time of onset of action is 3 min, while action duration is about 25 min.

Pancuronium

This is a steroid structural muscle relaxant that undergoes metabolism and elimination by the liver and kidney respectively. Time of onset of action is 3 min, with action duration up to about 90 min.

It comes as a solution of 1/2mg/ml. Its intubating iv dose is 0.08-0.12mg/kg, and intraoperative bolus dose is 0.04mg/kg. There is indication of a maintenance bolus dose of 0.01mg/kg at about every 30 min.

Side effects include: ventricular arrhythmia; allergic reactions; hypertension and tachycardia.

Contraindications: aortic stenosis; coronary artery disease; hypertrophic cardiomyopathy; combinations with halothane & tricyclic antidepressants.     

Vecuronium

It has a minor structural difference with pancuronium. It undergoes metabolism in the liver and eliminated through bile and kidney. Onset of action is 3 min, and action duration is about 67.5 min.  

Risk factors for prolonged paralysis: high-dose/long-term corticosteroid use; sepsis; female gender; kidney failure; postpartum. Side effects include tolerance and receptor dysfunction from long-term use.

It’s intubating iv dose is 0.08-0.12mg/kg, and intraoperative bolus dose is 0.04mg/kg. Maintenance bolus dose of 0.01mg/kg at about every 18 min. Maintenance infusion dose at 1.5mcg/kg/min.

Rocuronium

A steroid analogue of vecuronium that undergoes no metabolism, but elimination occurs through liver and kidney (slightly). Onset of action is 1.5 min, while action duration is about 55 min. It is a rapid and effective precurarization agent before succinylcholine administration. 

Intubating iv dose is 0.45-0.9mg/kg, while maintenance bolus dose is 0.15mg/kg. Infusion maintenance dose is about 5-12mcg/kg/min.

IM injection of 1mg/kg in infants/2mg/kg in children provides adequate paralysis of vocal cord & diaphragm for intubation after 3-6min. Faster when injection site is deltoid muscle.  

Risk factors for prolonged paralysis: older patients; pregnancy; severe liver failure. It is shown to have a slight vagolytic effect. Sugammadex is effective at antagonizing rocuronium.