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==Mechanism of Action==
==Mechanism of Action==
Local anaethetic drugs reversibly interfere with action potential generation and conduction in the neurons around which they are administered. To reach the neuronal plasma membrane where they act, local anaethetic drugs must first enter the nerve sheath. Only molecules lacking ionic charge may do this, and so local anaesthetic agents work more effectively in an alkaline pH when charge is neutral. Once inside the sheath, the drug gains charge and can then bind to voltage-gated Na<sup>+</sup> channels, preventing depolarisation of the cell. Local anaesthetics also infiltrate and change the composition of the cell membrane to take effect. However, they do NOT alter resting membrane potential.
* Local anaethetic drugs reversibly interfere with action potential generation and conduction in the neurons around which they are administered. To reach the neuronal plasma membrane where they act, local anaethetic drugs must first enter the nerve sheath. Only molecules lacking ionic charge may do this, and so local anaesthetic agents work more effectively in an alkaline pH when charge is neutral. Once inside the sheath, the drug gains charge and can then bind to voltage-gated Na<sup>+</sup> channels, preventing depolarisation of the cell. Local anaesthetics also infiltrate and change the composition of the cell membrane to take effect. However, they do NOT alter resting membrane potential.
Small diameter nerve fibres are blocked before large fibres by local anaesthetics, and fibres which are myelinated are blocked before those which are unmyelinated. Therefore, A-delta fibres are blocked before C fibres, and so the sensation of pain is eliminated before that of gentle touch.
* Small diameter nerve fibres are blocked before large fibres by local anaesthetics, and fibres which are myelinated are blocked before those which are unmyelinated. Therefore, A-delta fibres are blocked before C fibres, and so the sensation of pain is eliminated before that of gentle touch.
The blocking effects of local anaethetics is more effective in neurons which are firing. This is because action potentials cause channels in the nerve sheath to cycle between open, resting inactive conformations. The drugspenetrate the sheath through open channels, and bind most readily to inactivated channels.
* The blocking effects of local anaethetics is more effective in neurons which are firing. This is because action potentials cause channels in the nerve sheath to cycle between open, resting inactive conformations. The drugspenetrate the sheath through open channels, and bind most readily to inactivated channels.
* Local anaesthetic drugs also cause vasodilation. To prevent this effect leading to increased systemic absorption of drug, vasoconstrictors (e.g. adrenaline) are commonly added to preparations.
==Pharmacokinetic Considerations==
==Pharmacokinetic Considerations==
==Side Effects and Contraindications==
==Side Effects and Contraindications==
Local anaesthetics may have undesirable effects on the CNS. These include termors, convulsions and respiratory depression. The cardiovasular system may also be adversely affected, with drug administration leading to reduced cardiac contractility, vasodilation and hypotension. [[#Bupivicaine|bupivicaine]] is particularly cardiotoxic, but stereoisomerism exists. The cardio- and neuro-toxic effects of dextrobupivicaine are greatly increased compared to that of levobupivicaine. Commercially available preparations of [[#Bupivicaine|bupivicaine]] are a 50:50 mixture of the two forms of the drug.
Local anaesthetic drugs are known to reduce epithelial repair, and so it it wise to administer them away from the site of surgical incision so that healing is not impaired. Inadvertent intravenous administration of these drugs can result in tissue irritation and allergic reactions.
==Drugs in this Group==
==Drugs in this Group==
===Lidocaine===
Lidocaine is an amide-linked local anaesthetic, which aslo has anti-arrhythmic properties. The time to onset of action is 3-10mins, after which the drug's effects last 60-90mins. The duration of action may be prolonged by the addition of adrenaline to the preparation. Lidocaine is a very cheap drug, and is a component of EMLA cream.
===Bupivacaine===
Bupivacaine is also an amide-linked local anaesthetic, but with a duration of action of around 8 hours, it is longer acting than lidocaine. It also has a longer time to onset (20-30mins). It is very cardiotoxic, casuing asystole by decreasing the force of contraction, and therefore must NOT be given intravenously.
===Mepivicaine===
Another amide-linked drug, mepivicaine has the same potency as lidocaine but is less irritant and vasodilatory. The time to onset is only 5-10 mins, and the effects last 90-180 mins. However, the drug is expensive, and is not formulated with a preservative and so the vial must be discarded once the seal is broken.
===Procaine===
Procaine is ester-linked, and so may cause allergic reactions to the PABA metabolite. It has an onset of action of 15-20 minutes and a duration of action of 45-60 minutes and may be added to penicillin to reduce pain on injection.
===Proxymetacaine===
Proxymetacaine is amide-linked. It has a 10 second onset, with 10-20 min duration. It may be used on the conjunctival sac, but is toxic to corneal epithelium with long-term use.
===EMLA===
"EMLA" stands for Eutectic Mix of Local Anaesthetic, and is formulated as a cream containing [[#Lidocain|lidocaine]] and prilocaine. It is used for topical anaesthesia of the skin, for example before venupuncture. It must be applied 30-60mins before the anticipated insult to allow absorption and effect to take place.
{{Learning
|full text = [http://www.cabi.org/cabdirect/FullTextPDF/2008/20083097773.pdf ''' Review of the analgesic, prokinetic, and anti-inflammatory uses of IV lidocaine.''' Mudge, M. C.; Green, E. M. ; American Association of Equine Practitioners (AAEP), Lexington, USA, Proceedings of the 53rd Annual Convention of the American Association of Equine Practitioners, Orlando, Florida, USA, 1-5 December, 2007, 2007, pp 245-248, 26 ref.]
|Vetstream = [https://www.vetstream.com/canis/search?s=analgesia Analgesia]
}}
[[Category:Anaesthetic Drugs]]