Practical local anaesthesia

Types of local anaesthesia¹

Local anaesthetic drugs act by causing a reversible block to conduction along nerve fibres. Their applications include:

• Topical.

• Infiltration anaesthesia.

• Nerve blocks. Can be minor or major nerves - eg, femoral nerve block.

• Intravenous regional block (Bier's block).

• Plexus block.

• Extradural and spinal anaesthesia.

Local anaesthetics may also be used for postoperative pain relief, thereby reducing the need for analgesics such as opioids.

Different local anaesthetics²

Bupivacaine:

• Longer duration of action than other local anaesthetics.

• Slow onset of action, taking up to 30 minutes for full effect.

• Often used in lumbar epidural blockade and is particularly suitable for continuous epidural analgesia in labour, or for postoperative pain relief.

• It is the principal drug used for spinal anaesthesia. Hyperbaric solutions containing glucose may be used for spinal block.

Levobupivacaine:

• An isomer of bupivacaine, has anaesthetic and analgesic properties similar to bupivacaine hydrochloride, but is thought to have fewer adverse effects.

Lidocaine:

• Effectively absorbed from mucous membranes and is a useful surface anaesthetic in concentrations up to 10%.

• Except for surface anaesthesia and dental anaesthesia, solutions should not usually exceed 1% in strength.

• The duration of the block (with adrenaline/epinephrine) is about 90 minutes.

Prilocaine:

• Low toxicity which is similar to lidocaine.

• A hyperbaric solution of prilocaine hydrochloride (containing glucose) may be used for spinal anaesthesia.

Ropivacaine:

• An amide-type local anaesthetic agent similar to bupivacaine hydrochloride.

• Less cardiotoxic than bupivacaine, but also less potent.

Tetracaine:

• Para-aminobenzoic acid ester.

• Effective for topical application. A 4% gel is indicated for anaesthesia before venepuncture or venous cannulation.

• Rapidly absorbed from mucous membranes and should never be applied to inflamed, traumatised, or highly vascular surfaces.

• Should never be used to provide anaesthesia for bronchoscopy or cystoscopy because lidocaine hydrochloride is a safer alternative.

Administration by injection

Prior to administration, all injectable medicines must be drawn directly from their original ampoule or container into a syringe and should never be decanted into gallipots or open containers. This is to avoid the risk of medicines being confused with other substances, eg, skin disinfectants, and to reduce the risk of contamination.

Avoid accidental intravascular injection. Local anaesthetic injections should be given slowly in order to detect inadvertent intravascular administration.

When prolonged analgesia is required, a long-acting local anaesthetic is preferred to minimise the likelihood of cumulative systemic toxicity.

Local anaesthesia around the oral cavity may impair swallowing and therefore increases the risk of aspiration.

Epidural anaesthesia is commonly used during surgery, often combined with general anaesthesia, because of its protective effect against the stress response of surgery. It is often used when good postoperative pain relief is essential.

Vasoconstrictors in combination with local anaesthetics

Local anaesthetics cause dilatation of blood vessels. The addition of a vasoconstrictor such as adrenaline/epinephrine diminishes local blood flow, slowing the rate of absorption and prolonging the anaesthetic effect.

Avoid inadvertent intravenous administration of a preparation containing adrenaline/epinephrine. The use of adrenaline/epinephrine with a local anaesthetic injection in digits or appendages [unlicensed use] may be associated with a risk of ischaemic necrosis though the combination is used in some circumstances (such as hand surgery that uses the Wide-awake Local Anaesthesia No Tourniquet (WALANT) technique).

Adrenaline/epinephrine must be used in a low concentration when administered with a local anaesthetic. Care must also be taken to calculate a safe maximum dose of local anaesthetic when using combination products.

In patients with severe hypertension or unstable cardiac rhythm, the use of adrenaline/epinephrine with a local anaesthetic may be hazardous. For these patients an anaesthetic without adrenaline/epinephrine should be used.

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 within a minute after a few seconds of discomfort on application. Their numbing effect can then allow removal of foreign bodies.

• Local refrigerants (eg, 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 - eg, 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 - eg, 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 into the nerve plexus - eg, 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 - eg, lidocaine.

• It should only be performed by experienced specialists.

Intravenous regional block (Bier's block)

• Provides anaesthesia for the distal arm or leg.⁶

• A cannula is inserted in a distal vein of the limb - eg, the back of the hand.

• A tourniquet is applied to the top of the limb - eg, 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.⁷

• 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 (ie 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).

• Injection of anaesthetic.

• Epidural (extradural) blocks are more difficult to perform. However, they are preferred to spinal blocks, as they can be used for prolonged periods of time - eg, labour.

Side-effects of local anaesthesia¹

Local side-effects

• Pain - this can be reduced by using a smaller needle, pre-warming 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.

Severe local anaesthetic-induced cardiovascular toxicity

After injection of a bolus of local anaesthetic, toxicity may develop at any time in the following hour. In the event of signs of toxicity during injection, the administration of the local anaesthetic must be stopped immediately.

Cardiovascular status must be assessed and cardiopulmonary resuscitation procedures must be followed. Lidocaine must not be used as anti-arrhythmic therapy.

If the patient does not respond rapidly to standard procedures, 20% lipid emulsion such as Intralipid® [unlicensed indication] should be given intravenously, followed by an infusion.

Standard cardiopulmonary resuscitation must be maintained throughout lipid emulsion treatment.

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