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Muscle relaxation is part of a ''balanced anaesthetic technique''. Most anaesthetic agents produce a mild-moderate amount of muscle relaxation and often this is not sufficient. Increased muscle relaxation can be produced by increasing anaesthetic depth, the use of local anaesthetic techniques, or the use of centrally or peripherally acting muscle relaxants. However, it is important to realise that muscle relaxants have no anaesthetic or analgesic effect themselves and so should never be used alone.
Muscle relaxation is part of a ''balanced anaesthetic technique''. Most anaesthetic agents produce a mild-moderate amount of muscle relaxation and often this is not sufficient. Increased muscle relaxation can be produced by increasing anaesthetic depth, the use of local anaesthetic techniques, or the use of centrally or peripherally acting muscle relaxants. However, it is important to realise that muscle relaxants have no anaesthetic or analgesic effect themselves and so should never be used alone.
'''Neuromuscular blocking agents'''(NMBA) are peripherally acting muscle relaxants. They can be classified as '''depolarising''' or '''non-depolarising''' depending on whether they are competitive or not.
'''Neuromuscular blocking agents'''(NMBA) are peripherally acting muscle relaxants. They can be classified as [[Neuromuscular Blockers#Depolarising Neuromuscular Blockers|'''depolarising''']] or '''non-depolarising''' depending on whether they are competitive or not.
==Neuromuscular Transmission==
==Neuromuscular Transmission==
*Replacement of dislocations and fractures.
*Replacement of dislocations and fractures.
*Intraocular procedures.
*Intraocular procedures.
==Depolarising Neuromuscular Blockers==
'''Depolarising agents''' are non-competive neuromuscular blocking agents. '''Suxamethonium''' is currently the only depolarising agent in clinical use. It is ultra short acting wih rapid onset of action.
===Mechanism of Action===
Suxamethonium (succinylcholine) comprises of 2 acetylcholine (ACh) molecules bound together. Binding to post synaptic receptors, suxamethonium generating an action potential. However, it is not broken down by acetylcholinesterases and so remains bound to the receptor. It is this binding that prevents normal transmission at the site. Therefore, it produces an initial muscle contraction followed by a prolonged period of muscle relaxation. It relies on a fall in concentration of the agent in the blood before it unbinds to travel down the concentration gradient back into the circulation. It is here that is broken down by ''pseudocholinesterase'' enzyme.
===Pharmacological Considerations===
Suxmethonium is adminstered intravenously with muscle relaxation seen 30-60s after administration. Duration of action is variable between species, lasting approximately 5 minutes in cats but 20-30 minutes in dogs. This is thought to be due to the amount of pseudocholinesterase enzyme present.
===Contraindications and Side Effects===
*Prolonged duration of actions may be seen in patients with liver disease, renal disease or malnutrition due to reduced levels of pseudocholinesterase.
*Cardiovascular effects such as variations in heart rate and blood pressure may be seen due to the structural similarity of suxamethaonium to ACh.
*Potassium levels may increase, which is normally insignificant in a healthy patient. However, patients with denervation this efflux may cause potentially fatal arrhythmias.
*Should not be used in patients with penetrating eye injuries due to the increases in intraocular pressure seen.
*Malignant hyperthermia trigger.
*Muscle soreness and myoglobinuria.
===Clinical Uses===
Suxamethaonium is used to aid endotracheal intubation of cats.
==Drugs in this Class==
==Drugs in this Class==