succinylcholine

Suxamethonium chloride
Systematic (IUPAC) name
2,2'-[(1,4-dioxobutane-1,4-diyl)bis(oxy)]bis (N,N,N-trimethylethanaminium)
Identifiers
CAS number	306-40-1
ATC code	M03AB01
PubChem	5314
DrugBank	APRD00159
ChemSpider	21080
Chemical data
Formula	C14H30N2O4
Mol. mass	290.399 g/mol
Pharmacokinetic data
Bioavailability	NA
Metabolism	By pseudocholinesterase, to succinylmonocholine and choline
Half life	?
Excretion	Renal (10%)
Therapeutic considerations
Pregnancy cat.	A(AU) C(US)
Legal status	POM(UK) ℞-only(US)
Routes	Intravenous, Intramuscular
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Suxamethonium chloride (also known as succinylcholine) is a medication used to induce muscle relaxation, usually to make endotracheal intubation possible. Suxamethonium is sold under the trade names Anectine and Scoline.

Suxamethonium acts as a depolarizing neuromuscular blocker. It imitates the action of acetylcholine at the neuromuscular junction, acting on muscle type nicotinic receptors, but it is degraded not by acetylcholinesterase but by butyrylcholinesterase, a plasma cholinesterase. This hydrolysis by butyrylcholinesterase is much slower than that of acetylcholine by acetylcholinesterase.

Chemistry

Suxamethonium is a white crystalline substance, it is odourless; solutions have a pH of about 4, the dihydrate melts about 160 °C, the anhydrous melts at about 190 °C; it is highly soluble in water (1 gram in about 1 mL), soluble in alcohol (1 gram in about 350 mL), slightly soluble in chloroform, and practically insoluble in ether. Suxamethonium is a hygroscopic compound.^[1] The compound consists of two acetylcholine molecules that are linked by their acetyl groups.

History

It has been in use since the pharmacological properties of succinylcholine were discovered around 1950 by K.H. Ginzel, H Klupp, and Gerhard Werner in Vienna, Austria.

Effects

There are two phases to the blocking effect of suxamethonium; Phase 1 block is the principal paralytic effect.

Phase 1 block

Binding of suxamethonium to the nicotinic acetylcholine receptor results in opening of the receptor's monovalent cation channel; a disorganized depolarization of the motor end plate occurs and calcium is released from the sarcoplasmic reticulum.

In the normal muscle, following depolarization, acetylcholine releases from the receptor and is rapidly hydrolyzed by acetylcholinesterase and the muscle cell is able to 'reset' ready for the next signal.

Suxamethonium has a longer duration of effect than acetylcholine and is not hydrolyzed by acetylcholinesterase. It does not allow the muscle cell to reset, by keeping the membrane potential above threshold. When acetylcholine binds to an already depolarized receptor it cannot cause further depolarization.

Calcium is removed from the muscle cell cytosol independent of repolarization (depolarization signaling and muscle contraction are independent processes). As the calcium is taken up by the sarcoplasmic reticulum, the muscle relaxes. This explains muscle flaccidity rather than tetany following fasciculation.

Phase 2 block

Following infusion or repeated doses of suxamethonium, phase 2 block may occur. The receptor blockade takes on characteristics of a non-depolarising neuromuscular block (i.e., fades in response to nerve stimulation, however unlike depolarizing block in Phase 1 it cannot be reversed).

Organophosphorus poisoning, An example of the phases of blocking

Phase 1

At the phase 1 block, the membrane is fully depolarised and the muscle fibre is therefore inexcitable. This is a phase of 'sustained depolarization'. This can occur with organophosphorus poisoning. The organophosphorus compound (for example dyflos) is an anticholinesterase. This will cause a decrease in the function in Acetylcholine esterase, which leaves the post-synaptic cell with more ACh available to activate it. The constant depolarization causes a phase 1 block.

In pharmacological terms, the Phase 1 block is potentiated by the anticholinesterase, and can be antagonized (opposed) by competitive blockers of this.

In this case, the poisonous organophosphorus compound can be antagonized (opposed) by atropine (which effectively reduces the available ACh) and pralidoxime.

Phase 2

If the ACh receptor is 'held open' for an extended period of time, or if the concentration of the blocker is excessive (this could be vastly elevated levels of ACh from organophosphorus poisoning) then the phase two block begins.

Here the nicotinic receptors undergo desensitization and eventual closure. They are no longer receptive to the substance which was previously opening them. This in pharmacological terms means previous agonists are beginning to behave like antagonists (blockers). Here, in the phase 2 block, the resting membrane potential of the cell is restored, but the muscle remains paralyzed because the receptors are temporarily unreceptive to any activation.

Medical uses

Its medical uses are limited to short-term muscle relaxation in anesthesia and intensive care, usually for facilitation of endotracheal intubation. Despite its adverse effects, including life threatening malignant hyperthermia, hyperkalaemia and anaphylaxis, it is perennially popular in emergency medicine because it arguably has the fastest onset and shortest duration of action of all muscle relaxants. The former is a major point of consideration in the context of trauma care, where endotracheal intubation may need to be completed very quickly. The latter means that, should attempts at endotracheal intubation fail and the patient cannot be ventilated, there is a prospect for neuromuscular recovery and the onset of spontaneous breathing before hypoxaemia occurs.

Suxamethonium is also commonly used as the sole muscle relaxant during electroconvulsive therapy, favoured for its short duration of action.

The recent arrival of the cyclodextrin sugammadex may well render suxamethonium obsolete. Sugammadex can be used to 'instantly' reverse the effects of longer-acting muscle relaxants, particularly rocuronium. This means that rocuronium can be given in sufficiently high dose to work quickly, and then reliably reversed when necessary, all without the unwelcome side effects of suxamethonium.

Suxamethonium is quickly degraded by plasma butyrylcholinesterase and the duration of effect is usually in the range of a few minutes. When plasma levels of butyrylcholinesterase are greatly diminished or an atypical form is present (an otherwise harmless inherited disorder), paralysis may last much longer.

Side effects

Side effects include muscle pains, acute rhabdomyolysis with hyperkalemia, transient ocular hypertension, constipation^[2] and changes in cardiac rhythm including bradycardia, cardiac arrest, and ventricular dysrhythmias. In patients with neuromuscular disease or burns, a single injection of suxamethonium can lead to massive release of potassium from skeletal muscles with cardiac arrest. Conditions having susceptibility to suxamethonium induced hyperkalemia are burns, closed head injury, acidosis, Guillain–Barr syndrome, cerebral stroke, drowning, severe intraabdominal sepsis, stroke, massive trauma, myopathy, and tetanus.

Suxamethonium does not produce unconsciousness or anesthesia, and its effects may cause considerable psychological distress while simultaneously making it impossible for a patient to communicate. For these reasons, administration of the drug to a conscious patient is strongly contraindicated, except in necessary emergency situations.

Hyperkalemia

The side effect of hyperkalaemia happens because the acetylcholine receptor is propped open, allowing continued flow of potassium ions into the extracellular fluid. A typical increase of potassium ion serum concentration on administration of suxamethonium is 0.5 mmol per litre, whereas the normal range of potassium is 3.5 to 5 mmol per litre. The increase is transient in normal patients. Hyperkalaemia does not generally result in adverse effects below a concentration of 6.5 to 7 mmol per litre.

Severe hyperkalemia will cause changes in cardiac electrophysiology, which, if severe, can result in asystole.

Death

This drug has occasionally been used as a paralyzing agent for executions by lethal injection, although pancuronium bromide is the preferred agent today because of its longer duration of effect and its absence of fasciculations as a side effect. It has also been used for murder by Dr. Carl Coppolino.^[3] Suxamethonium was the drug used to murder Nevada State Controller Kathy Augustine^[4], and was used by surgical technician Kim Hricko in the 1998 murder of her husband Steve.^[5]

See also

• Malignant hyperthermia, a rare and potentially fatal side effect of suxamethonium.
• Anesthetics

References

1. ^ Gennaro, Alfonso. Remington: The Science and Practice of Pharmacy, 20th ed. Lippincot, Wiliams and Wilkins, 2000:1336.
2. ^ DiPiro, Joseph, et al. Pharmacotherapy: A Pathophysiologic Approach. 6th ed. McGraw-Hill, 2005:685.
3. ^ Tracing the Untraceable - TIME
4. ^ "Official's husband gets life for her murder". Seattle Times. June 30, 2007. http://seattletimes.nwsource.com/html/nationworld/2003768941_ndig30.html. Retrieved 2008-05-15. 
5. ^ "Maryland Legal Briefs, 11/23/04". http://findarticles.com/p/articles/mi_qn4183/is_20041123/ai_n10064075. Retrieved 2008-09-28.