Types of local anaesthesia

• 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.

Different local anaesthetics¹

Practical application of local anaesthesia

Topical

• Examples include EMLA® cream, tetracaine hydrochloride 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® cream should be covered with a non-absorbable adhesive.
• However, post-administration it requires at least 60 minutes to take effect.
• Studies suggest that tetracaine hydrochloride gel has a faster onset of action and may be superior to EMLA® cream.³
• 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. They are also useful for cannulation in children and adults if there is no time to wait for EMLA® to work.

Infiltration anaesthesia

In all cases of infiltration anaesthesia avoid inadvertent intravascular injection.

• Most commonly, this is into the skin.
• The skin should be prepared adequately to begin with, e.g. with iodine.
• Inject with the smallest needle, first producing a bleb in the skin; then the needle size can be increased and further anaesthetic infiltrated in the same area.
• Wait a few minutes (some say at least 5-10 minutes) before starting the procedure.
• Always check that the area is anaesthetised before starting.⁴

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.
• 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 quicker anaesthesia.⁵
• 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. lidocaine.
• It should only be performed by experienced specialists.

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. the back of hand.
• A tourniquet is applied to the top of the limb e.g. the arm or thigh, usually in the form of an inflated blood pressure cuff. It is essential that the cuff does not leak and this can be helped by having a second inflated cuff on the arm. There should also be another member of staff on hand whose only job is to maintain the cuff pressure throughout the procedure.
• The patient's blood pressure should be measured before and the cuff pressure is set at least 50 mm Hg 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 ensue.⁶
• 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

Epidural anaesthesia involves injecting anaesthetic agent into the epidural space (i.e. the space outside the dura mater). The local anaesthetic, most often lidocaine or bupivacaine, 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 in pregnant women.

On the other hand, in spinal anaesthesia the anaesthetic is introduced to the cerebrospinal fluid (CSF). The effect is similar to that of extradural anaesthesia but the onset and duration of action are longer, meaning that lower doses can be used.

Practically, these procedures require the patient to curl up in the fetal position and thus are not appropriate in the presence of spinal disease. The procedure involves:

• Antisepsis of the skin.
• The skin being anaesthetised by local infiltration.
• A spinal needle being introduced into an appropriate interspinous space.
• For spinal anaesthesia, the spinal needle being secured in place (which is once CSF appears).
• It can now be used to inject anaesthesia.
• Epidural (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.

Side-effects of local anaesthesia⁷

Local side-effects

• Pain - this can be reduced by using a smaller needle, prewarming the local anaesthetic, buffering with sodium bicarbonate and injecting very slowly.
• Allergy, redness of skin.

Systemic side-effects and complications

These usually result from the inadvertent administration of the anaesthetic into the systemic circulation or from rapid absorption:

• CNS toxicity - results in dizziness, visual disturbances, tinnitus, generalised convulsions and eventual coma. Circumoral paraesthesiae is a common early neurotoxic sign.
• Haemodynamic instability - may also occur if cardiovascular toxicity occurs. Intravenous lipid emulsion may be a useful antidote for refractory cardiovascular collapse.
• Anaphylaxis may also occur.

For extradural and spinal anaesthesia - see separate article Important Complications of Anaesthesia. The main complications of spinal anaesthesia are:

Document references

1. Update in Anaesthesia The Pharmacology of Local Anaesthetic Agents
2. Krunic AL, Wang LC, Soltani K, et al; Digital anesthesia with epinephrine: an old myth revisited. J Am Acad Dermatol. 2004 Nov;51(5):755-9. [abstract]
3. Boyd R Jacobs M EMLA or amethocaine (tetracaine) for topical analgesia in children Best Evidence Topics Oct 2004
4. Quaba O, Huntley JS, Bahia H, et al; A users guide for reducing the pain of local anaesthetic administration. Emerg Med J. 2005 Mar;22(3):188-9. [abstract]
5. Jarbo K, Batra YK, Panda NB; Brachial plexus block with midazolam and bupivacaine improves analgesia. Can J Anaesth. 2005 Oct;52(8):822-6. [abstract]
6. Cobb AG, Houghton GR; Local anaesthetic infiltration versus Bier's block for Colles' fractures. Br Med J (Clin Res Ed). 1985 Dec 14;291(6510):1683-4.
7. British National Formulary