Practical Local Anaesthesia

Local anaesthetics produce reversible loss of sensation in a limited area usually by decreasing membrane permeability to sodium and temporarily halting nerve conduction.¹ Local anaesthetics can also be used to provide local analgesia postoperatively.

• Topical
• Infiltration anaesthesia
• Nerve blocks - can be minor or major nerves e.g. femoral nerve block
• Intravenous regional block (Bier's block)
• Haematoma blocks
• Plexus block
• Extradural and spinal anaesthesia

Topical

• e.g. EMLA cream, amethocaine eye drops, Ethyl chloride/dimethyl ether spray
• EMLA cream is commonly used in children and occasionally in some adults
• The EMLA cream is put for example, on the back of the hand before cannulation
• EMLA should be covered with a non-absorbable adhesive
• However it requires administration for at least 60 minutes to take effect
• However, studies suggest that amethocaine gel has a faster onset of action and may be superior to EMLA cream³
• Local anaesthetic eye drops usually work instantly and can allow removal of foreign bodies
• Local refrigerants e.g. ethyl chloride/dimethyl ether spray essentially freeze the skin
• Local refrigerants should be sprayed until the skin goes white and then the procedure should be performed immediately
• Local refrigerants are useful for superficial procedures such as lancing a boil

Infiltration anaesthesia

• Most commonly this is in the skin
• The skin should be prepared adequately to begin with e.g. iodine
• Inject with smallest needle first producing a bleb in the skin then the needle size can be increased and further anaesthetic infiltrated in the same area
• Always pull back on the syringe before injecting to avoid inadvertent intravenous injection
• Wait a few minutes (some say at least 5- 10 minutes) before starting the procedure
• Always check that the area is anaesthetised before starting⁴

Nerve blocks

• Can be minor or major nerves e.g. ring block or femoral nerve block
• A ring block involves anaesthetising the main nerves of the fingers or toes
• This involves injecting local anaesthetic at the base of the finger on its lateral and medial sides
• Always pull back on the syringe before injecting to avoid inadvertent intravenous injection
• This will provide anaesthesia of the whole finger for example
• Major nerve blocks and plexus blocks involve injecting fairly large volumes in to the nerve plexus e.g. brachial plexus
• The addition of midazolam may lead to anaesthesia quicker⁵
• This should only be performed in experienced hands and resuscitation facilities should be available

Haematoma blocks

• This can be used for fractures
• It involves infiltrating the fracture site with an anaesthetic e.g. lignocaine

Intravenous regional block (Bier's block)

• Provides anaesthesia for the distal arm or leg
• A cannula is inserted in a distal vein of the limb e.g. back of hand
• A tourniquet is applied to the top of the limb e.g. arm or thigh - must ensure it will not leak
• The patients blood pressure should be measured before and the tourniquet applied at least 50 mmHg above this level
• The anaesthetic is injected in the cannula
• This leads to mottling of the skin
• Then the procedure can be performed
• The tourniquet should not be released for at least 15 minutes - even if the procedure finishes beforehand as systemic absorption occurs and toxicity can ensue⁶
• This procedure should only be performed in a specialist setting by an experienced doctor
• It should not be used if the procedure is likely to take 15 minutes or less

Extradural and spinal anaesthesia

Local anaesthesia is injected in to the space between the epidural and dura. The local anaesthetic, most often lignocaine or bupivicaine leads to inhibition of conduction at the intradural nerve roots arising from the spine. Vascular absorption can vary and enhanced block can occur in the elderly and pregnant women.

On the other hand in spinal anaesthesia the anaesthetic is introduced to the cerebrospinal fluid. The effect is similar to that of extradural anaesthesia but the onset and duration of action is longer meaning that lower doses can be used.
Practically these procedures require the patient to curl up in the fetal position thus not appropriate if they have spinal disease preventing this

• The skin is cleansed
• The skin is anaesthetised by local infiltration
• A spinal needle is introduced in to an appropriate interspinous space
• For spinal anaesthesia the spinal needle is in place once CSF appears
• It can now be used to inject anaesthesia
• Epidurals (extradural) blocks are more difficult to perform
• But they are preferred to spinal blocks as they can be used for prolonged periods of time e.g. labour

Local side effects

• Allergy, redness of skin

Systemic side effects and complications

These usually result from the inadvertent administration of the anaesthetic in to the systemic circulation or rapid absorption

• CNS toxicity - results in dizziness, visual disturbances, tinnitus, generalised convulsions and eventual coma.
• Haemodynamic instability - may also occur if cardiovascular toxicity occurs
• Anaphylaxis may also occur

For extradural and spinal anaesthesia - see Important complications of anaesthesia. The main complications of spinal anaesthesia are