Local Analgesia Overview
Introduction
Local analgesia/anaesthesia is used in patient to desensitise a particular region of the body using techniques such as local infiltration, epidural, paravertebral and specific nerve blocks, and intravenous regional anaesthesia. It can be used as a diagnostic aid, for example lameness investigations in horses, peri- and post-operative analgesia, possibly reducing the depth of anaesthesia required to perform a procedure, as well as anaesthesia for minor surgical procedures, for example paravertebral nerve blocks are used for standing surgeries in cattle. Local anaesthetic agents are used, but in some cases it may be necessary to use a sedative agent to calm the animal, such as in the case of standing surgeries. Techniques are often easy to learn and do not require any expensive equipment.
Mechanism of Nerve Blocks
Many local analgesics exist as both a charged or uncharged form, both of which are important in the local blockade. The block is produced by a blockade of sodium ions during action potential generation. As the drug concentration increases, there is a reduced spike potential slowing conductance, until eventually the threshold is not reached, and conduction is therefore blocked.
Local Anaesthetic Agents
There are a variety of drugs available to use for local analgesia/anaesthesia. They vary in duration of action, but all act in a similar way. Lipid solubility determines the potency of the agent, while the protein binding ability is thought to determine the duration of action. The preparation is also important, as some preprations cannot be used for certain blocks or in certain regions of the body, for example for epidurals, only preservative free preparations can be used.
Care should be taken when using local anaesthetic agents that doses are kept as low as possible to avoid toxicity problems. It is also important to take care when using agents with adrenaline in the preparation as this causes severe vasoconstriction which if used in a region with end artery supply for example, it may lead to necrosis of that area.
Agents include Lidocaine, Mepivicaine and Bupivicaine among others.
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