Local Anaesthetic Agents and Techniques

Definition

Local anaesthetic agents are used clinically to produce a temporary loss of sensation to a defined area of the body where the drug is administered, which is achieved either by topical application or injection. ¹

Classification

There are two basic types that are clinically useful, esters or amides. A few structurally non-related agents also possess local anaesthetic properties.

Mechanism of Action

Local anaesthetic agents reversibly interrupt nerve impulses by blocking sodium channels, which results in a reduction of the sodium ion influx and consequently impairment of the action potential across the membrane thus stopping conduction.

Preparations

Most local anaesthetics are weak bases and poorly soluble in water so they are usually constituted in hydrochloride salts solutions. Dilute preparations are generally acidic and may additionally contain stabilisers, preservatives and fungicides.

Topical Agents

• These are applied directly to the skin, and other mucous membranes such as the eyes, ears, nose and mouth. The drugs must penetrate tissue barriers to be effective, and high concentrations of local anaesthetics are generally required for absorption through intact skin, which may be slow or unreliable. ¹
• Lidocaine/prilocaine cream (EMLA) is widely used in general practice in patients requiring skin anaesthesia prior to a minor procedure such as implant insertion, cryotherapy of genital warts, etc. It is also useful for patients (especially children) who have needle phobia and require phlebotomy.
• Tetracaine (amethocaine) – can be used as an alternative to EMLA cream. There is limited comparative information but what is available suggests that tetracaine may have the edge on EMLA cream in terms of efficacy and speed of onset².

Infiltration

• Amides e.g. lignocaine are commonly used for minor surgical procedures.
• A small gauge needle is used to inject the drug directly into the tissue around the nerve. ³ Several local anaesthetics are available in combination with adrenaline (epinephrine).
• The addition of a vasoconstrictor prolongs the anaesthetic effect. However, when using adrenaline GPs need to familiarise themselves with its contraindications, dosage requirements, and adverse effects (see Using Local Anaesthetics Safely below and monograph on adrenaline).

Field Block

Local anaesthetic is injected into muscle.

Nerve Block

This technique involves the injection of local anaesthetic around a nerve.
A new drug butyl amino-benzoate has been discovered fo use as a nerve block agent.
It is selective for certain nerve fibres and can produce minimal motor block with a duration that can extend into weeks. ¹

Epidural Anaesthetic

Local anaesthetic solutions are administered into the epidural space of the vertebrae.
The technique is a common practice to relieve the symptoms of labour.
Examples of local agents used for this procedure include ligocaine and bupivacaine.

Spinal Anaesthetic

Local anaesthetic agents are injected directly into the cerebrospinal fluid (CSF).

Intravenous Regional Anaesthetic (IVRA)

This technique involves injecting the local anaesthetic agent into a vein or a limb that has been occluded by a tourniquet cuff. Lignocaine or prilocaine are preferred agents for this procedure. ³

See individual drugs for full list of adverse effects and contraindications.

• Local agents are relatively safe but systemic or localised reactions may occur.
• Familiarise yourself with the therapeutic range and operate well below potentially toxic levels.
• Before undertaking a procedure, roughly calculate the safe dose for that particular patient, taking into account age, weight, vascularity of the area and clinical condition.
• Do not exceed your calculated safe total dose – it's easy to do inadvertently if you do several procedures in the same session.
• Do not exceed the level of your competency – i.e steer clear of anything other than local infiltration unless you have had extensive training in more advanced techniques.
• Give injections slowly and avoid inadvertent intravenous administration.
• Do not inject into inflamed or infected tissue.
• Familiarise yourself with the symptoms and signs of toxicity:
  • There may be a feeling of inebriation and lightheadedness followed by sedation, circumoral paraesthesia and twitching.
  • Convulsions can occur in severe reactions.
  • Inadvertent intravenous injection can cause convulsions and cardiovascular collapse very rapidly.
  • There is an argument for having full resuscitation facilities in any site where minor surgery and local anaesthesia is used. Oxygen, an Ambu-bag and an airway are minimum requirements.
  • Allergic reaction/anaphylaxis to amides is rare ¹, but be aware of the symptoms and signs and have everything to hand in the event of an emergency.
  • Adrenaline must be used in a low concentration (e.g. 1 in 200,000) and should not be given with a local anaesthetic injection in digits and appendages as it may cause ischaemic necrosis. When adrenaline is included the final concentration should be 1 in 200,000 (5 micrograms/mL). The total dose of adrenaline should not exceed 500 micrograms.

Lidocaine

Lidocaine (lignocaine) is the commonest used preparation in general practice minor surgery and when mixed with adrenaline has a duration of action of about 90 minutes.

Prilocaine

Prilocaine is an alternative to lidocaine with similar low toxicity. There are no meta-analyses of comparative trials. Most trials that have been done have involved dental anaesthesia or spinal anaesthesia and the findings have no relevance to minor surgery in general practice.

Procaine

Procaine isnow seldom used, as it has a shorter duration of action than lidocaine and provides less intense analgesia because of reduced spread through the tissues.

Bupivacaine, Levobupivacaine, Ropivacaine

All have similar characteristics, and are mainly used in spinal surgery.