Local anaesthesia

What are local anaesthetics?

Local anaesthetics (LAs) are topical or injected agents that induce a reversible loss of sensation in a part of the body while allowing patients to remain alert and conscious.

LAs are lipophilic molecules that block voltage-gated sodium channels in the nerve membrane, preventing depolarisation and therefore the transmission of action potentials (nerve impulses).

Depending on the concentration and dose, local anaesthetics can block all forms of sensation (eg, pain, temperature, touch, pressure) along with autonomic and motor functions.

LAs can be broadly classified as esters (para-aminobenzoic acid (PABA)–based) or amides (non-PABA-based) according to their intermediate chain.

Ester local anaesthetics include:

• Benzocaine
• Chloroprocaine
• Cocaine
• Procaine
• Proparacaine
• Tetracaine
• Amylocaine
• Oxybuprocaine.

Amide local anaesthetics include:

• Articaine
• Bupivacaine
• Dibucaine
• Etidocaine
• Levobupivacaine
• Lidocaine (also known as lignocaine)
• Mepivacaine
• Prilocaine
• Ropivacaine
• Sameridine
• Tonicaine
• Cinchocaine.

What are local anaesthetics used for?

Local anaesthetics are often used in dermatological, surgical, and dental procedures to minimise pain and discomfort. It can be administered in various ways.

Local anaesthesia

• Topical 
  • Comes in multiple formulations eg, cream, patch, spray, mouthwash
  • Used to numb superficial pain eg, minor skin injuries, sore throat, haemorrhoids, before venepuncture or cannulation
  • Oral or nasal sprays can be used before endoscopy to inhibit the gag reflex 
• Dermal/subcutaneous infiltration 
  • Used for minor procedures eg, biopsy or excision of a skin lesion
• Tumescent anaesthesia
  • Specialised technique that injects large volumes of diluted LA and adrenaline (epinephrine) into subcutaneous fat until the tissue is swollen and firm
  • Used for liposuction and other dermatosurgical procedures involving larger areas of skin

Regional anaesthesia

• Peripheral nerve block
  • LA is injected close to a major peripheral nerve or nerve plexus to anaesthetise large areas supplied by that nerve eg, brachial plexus block for arm surgery.
• Neuraxial anaesthesia 
  • LA is injected into the subarachnoid space (spinal anaesthesia) or epidural space (epidural anaesthesia) to target the spinal cord or nerve roots.

What are the contraindications and cautions for local anaesthetics? 

The exact contraindications and precautions for local anaesthetic use will vary according to the procedure.

Contraindications:

• Allergy to the local anaesthetic or any of its excipients
• Active infection at the injection site — risks spreading the infection to deeper healthy tissue
• Use of LA containing adrenaline at end-artery sites eg, ears, nose, digits, penis, wrist, and ankle.

Precautions:

• Bleeding disorder or anticoagulant use 
• Congestive heart failure
• Partial or complete heart block
• Severe hepatic or renal insufficiency.

Tell me more about local anaesthetics

Local anaesthetics exist in equilibrium between ionised and non-ionised forms, determined by the drug’s pKA and local tissue pH.

The non-ionised form is lipid-soluble and readily crosses the phospholipid membrane. Within the axoplasm, the slightly more acidic environment causes the molecule to re-ionise. 
It is the ionised form that binds the voltage-gated sodium channel at its intracellular face, thereby inhibiting sodium ion influx and action potential propagation.

Dosages and durations

Local anaesthetic solution is frequently combined with adrenaline, a vasoconstrictor. This prolongs the duration of action, provides haemostasis (bleeding control), and reduces the risk of toxicity from systemic absorption.

The dosages and durations, with and without vasoconstrictors, for commonly used local anaesthetics are outlined in Tables 1 and 2, respectively.

Table 1. Maximum dosages for local anaesthetic agents

Table 2. Maximum dosages for local anaesthetic agents with the addition of a vasoconstrictor, such as adrenaline

Alkalinised (buffered) local anaesthetics

Alkalinisation (buffering) involves adding an alkali (eg, sodium bicarbonate) to the LA solution immediately before injection to raise its pH.

This offers two major advantages:

• Less stinging pain during infiltration as the pH of the solution is brought closer to physiological pH
• Faster onset of action as more LA molecules exist in the non-ionised form that can cross nerve membranes. 

Several cautions apply to buffered solutions:

• Raising the pH too high may cause the drug to precipitate (crystallise) out of solution, rendering it unsuitable for injection.
  • Long-acting or amino-amide anaesthetics (eg, bupivacaine and ropivacaine) have a narrower buffering window and tolerate small additions of alkali before precipitating. 
  • Precipitation tends to increase over time, hence why buffered solutions should be prepared fresh and are not suitable for infusion.
• Adrenaline degrades faster in buffered solutions due to the higher pH.

Allergy to local anaesthetics

Allergy to local anaesthetics is more often in reaction to added preservatives (eg, methylparaben and metabisulphites) rather than the LA itself. Preservative-free solutions are available to minimise this risk.

True hypersensitivity to local anaesthetics is almost always a delayed (type IV) hypersensitivity reaction and requires prior sensitisation.

• Para-aminobenzoic acid or PABA [[Allergy to PABA]], the breakdown product of ester LAs, is known to be allergenic. Cross-reactivity between ester LAs is also common (see our page: Allergy to benzocaine). 
• Hypersensitivity to amide LAs is much less common, and cross-reactivity is often unpredictable and not well established. 

Upon re-exposure, patients will develop localised contact dermatitis or swelling within 12-24 hours of administration. In the oral mucosa, LA allergy can present as diffuse facial swelling that may mimic urticaria or angioedema.

Patch testing can be used to identify the cause of delayed-type hypersensitivity and the presence of cross-reactivity.

Conversely, immunoglobulin E-mediated (type I) hypersensitivity reactions, such as anaphylaxis, are exceedingly rare. 

What are the benefits of local anaesthetics?

• Generally well tolerated
• Avoids the risks of general anaesthesia (eg, airway manipulation, cognitive impairment), providing a safer option for patients with significant comorbidities
• Accessible in a clinic setting, as it typically does not require specific monitoring or the presence of an anaesthetist

What are the disadvantages of local anaesthetics?

• Use of needles during infiltration can cause anxiety and discomfort.
• LA is unsuitable for extensive surgeries because the surgical field is limited by the maximum safe LA dose.
• Patients remain conscious during procedures using local anaesthesia only; therefore, appropriate patient selection and counselling are required.
• Effectiveness is reduced in infected tissues due to the acidic environment and increased blood flow, requiring higher doses to achieve the desired effect.

What are the side effects and risks of local anaesthetics?

• Pain during infiltration due to tissue distension and the acidity of the LA solution
• Tachycardia, palpitations, and anxiety due to adrenaline added to the solution

Local anaesthetic systemic toxicity (LAST)

LA is relatively safe when administered at appropriate doses. Systemic toxicity, however, is a rare complication and usually occurs due to inadvertent intravascular infiltration. This risk can be mitigated by aspirating the syringe before injection.

Risk factors for LAST:

• Patient factors
  • Extremes of age ie, infants, young children, and elderly patients
  • States of increased cardiac output (eg, pregnancy) — absorption from the injection site occurs at a faster rate
  • Hepatic impairment 
• Injection into highly vascularised sites
• High doses of local anaesthetic.

The clinical symptoms and signs of LAST are broadly classified into central nervous system (CNS) and cardiovascular toxicity. Early CNS symptoms and manifestations as a result of excitation are:

• Tinnitus
• Perioral numbness
• Metallic taste
• Altered mental status
• Muscle twitching
• Seizures.

As toxicity progresses, CNS depression may occur, causing sedation and respiratory depression.

Cardiovascular signs may occur independently or in addition to CNS signs. Cardiac signs can include:

• Tachycardia
• Hypertension
• Bradycardia
• Hypotension.

In severe cases, toxicity may progress to cardiac arrhythmias (palpitations) and asystole (cardiac arrest).

Approved datasheets are the official source of information for medicines, including approved uses, doses, and safety information. Check the individual datasheet in your country for information about medicines.
  
We suggest you refer to your national drug approval agency such as the Australian Therapeutic Goods Administration (TGA), US Food and Drug Administration (FDA), UK Medicines and Healthcare products regulatory agency (MHRA) / emc, and NZ Medsafe, or a national or state-approved formulary eg, the New Zealand Formulary (NZF) and New Zealand Formulary for Children (NZFC) and the British National Formulary (BNF) and British National Formulary for Children (BNFC).