Anaesthesiology
Regional Anaesthesia
Regional anaesthesia for MBBS and NEET-PG: local anaesthetic pharmacology and doses, LA systemic toxicity and lipid rescue, spinal, epidural and brachial plexus blocks, mapped to NMC codes AS5.
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Regional Anaesthesia
Regional anaesthesia for MBBS and NEET-PG: local anaesthetic pharmacology and doses, LA systemic toxicity and lipid rescue, spinal, epidural and brachial plexus blocks, mapped to NMC codes AS5.
This chapter covers anaesthesia produced by blocking nerves rather than the whole brain, from the pharmacology of local anaesthetics to spinal, epidural and peripheral nerve blocks. It emphasises safe dosing, the recognition and lipid-emulsion treatment of local anaesthetic toxicity, and the physiology and complications of central neuraxial blocks.
High-yield: Regional Anaesthesia
- Local anaesthetics block voltage-gated sodium channels from inside the axon, preventing the propagation of the action potential.
- Small, myelinated and rapidly firing fibres are blocked first, so autonomic and pain fibres are lost before touch and motor function.
- The maximum safe dose of plain lignocaine is about 3 milligrams per kilogram, rising to about 7 milligrams per kilogram with added adrenaline.
- The maximum safe dose of bupivacaine is about 2 milligrams per kilogram, and it is the most cardiotoxic of the common local anaesthetics.
- Adrenaline is added to prolong the block and reduce systemic absorption, but must be avoided near end-arteries such as the fingers, toes and penis.
- Local anaesthetic systemic toxicity begins with perioral numbness, tinnitus and a metallic taste, then progresses to seizures and cardiovascular collapse.
- The specific treatment of local anaesthetic systemic toxicity, especially from bupivacaine, is intravenous 20% lipid emulsion (Intralipid).
- Spinal (subarachnoid) anaesthesia injects a small dose into the cerebrospinal fluid below the end of the spinal cord to give a rapid, dense block.
- In an adult the spinal cord ends at about the level of L1 to L2, so a spinal is placed at the L3 to L4 or L4 to L5 interspace to avoid it.
- Hyperbaric 0.5% bupivacaine is the standard spinal agent, and the level of block is adjusted by patient position and baricity.
- Hypotension and bradycardia after a spinal are due to sympathetic blockade and are treated with fluids and a vasopressor.
- A post-dural-puncture headache is postural, worse on sitting, and is treated with an epidural blood patch when severe.
- Epidural anaesthesia places a catheter in the epidural space for a titratable, segmental and prolonged block used widely in labour analgesia.
- Brachial plexus blocks anaesthetise the upper limb, and ultrasound guidance improves success and reduces the risk of nerve or vessel injury.
Local anaesthetic maximum doses
- **Lignocaine plain:** About 3 mg/kg. With adrenaline about 7 mg/kg.
- **Bupivacaine:** About 2 mg/kg. Most cardiotoxic; standard hyperbaric agent for spinal anaesthesia.
- **Toxicity signs:** Perioral numbness, tinnitus, metallic taste, then seizures and cardiovascular collapse.
- **Toxicity treatment:** Stop injection, airway and oxygen, intravenous 20% lipid emulsion.
NMC competencies in this chapter
- **AS5.1:** Principles and indications for regional anaesthesia
- **AS5.2:** Correlative anatomy of the brachial plexus and neuraxial spaces
- **AS5.3:** Principles and techniques of peripheral nerve blocks
- **AS5.4:** Pharmacology of local anaesthetics and adjuvants
- **AS5.5:** Caudal and epidural anaesthesia: principles and technique
- **AS5.6:** Common blocks in surgery: spinal, epidural and brachial plexus
Frequently Asked Questions
How do local anaesthetics work?
They block voltage-gated sodium channels from within the nerve, stopping the action potential. Small, fast-firing autonomic and pain fibres are blocked first, before touch and motor fibres.
What is the maximum safe dose of lignocaine?
About 3 milligrams per kilogram for plain lignocaine, rising to about 7 milligrams per kilogram when adrenaline is added because absorption is slowed. Doses should always be calculated by weight.
How is local anaesthetic systemic toxicity treated?
Stop the injection, secure the airway and give oxygen, and treat seizures. The specific antidote is intravenous 20% lipid emulsion, which is especially important for bupivacaine toxicity.
Why does a spinal cause hypotension?
The block interrupts sympathetic outflow, causing vasodilatation and, with a high block, bradycardia. It is managed with intravenous fluids, a vasopressor and, where needed, atropine.
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