Local anaesthesia

Essentials

1
Local anaesthetic infiltration and nerve blocks may be used as a supplement to oral, inhaled or parenteral analgesia.
2
Also, they may be the primary method of achieving analgesia, particularly where pain is localized to a digit or within a peripheral nerve distribution region.
3
Local anaesthetic toxicity may occur with inadvertent bolus intravenous injection or by exceeding the recommended maximum safe dose. Neurological and cardiovascular effects predominate and may be fatal.
4
Resuscitation equipment should always be available when using these agents. Refractory local anaesthetic systemic toxicity with cardiovascular collapse or arrest may respond to 20% lipid emulsion therapy.
5
Intravenous regional anaesthesia with prilocaine for a Bier block is a simple, safe technique commonly used for reduction of forearm fractures, but requires two medical practitioners and specialized equipment.
6
Formal training and accreditation should occur prior to independent practice, particularly with more complex blocks, such as Bier and the femoral nerve.

Local anaesthetic infiltration and nerve blocks should be used for patients presenting to the emergency department (ED) with pain, either to supplement other analgesia or for definitive pain relief. Nerve blocks are most appropriate when the pain is localized, as in certain fractures and wounds to a digit, or within a peripheral nerve distribution region. Local anaesthesia may also be used topically, particularly in children, and prior to arterial blood gas puncture and insertion of large intravenous cannulae where, contrary to popular perception, it does not increase the likelihood of failure.

Pharmacology

Local anaesthetic agents are all weak organic bases that inactivate plasma membrane voltage-gated fast sodium channels, temporarily blocking membrane depolarization and preventing nerve impulse transmission. All are vasodilators, with the exception of ropivacaine and cocaine, hence the use of adrenaline to prolong their duration of activity and to improve safety by delaying absorption and/or by administering lower effective doses.

Amino ester and amino amide local anaesthetics

Amino esters

Local anaesthetic agents that contain an ester bond between the intermediate chain and lipophilic aromatic end (amino esters) include cocaine, procaine and amethocaine. They are poorly protein bound and undergo hydrolysis by plasma pseudocholinesterase to para-amino benzoic acid.

Amino amides

Amide-type agents that contain an amide bond between the intermediate chain and aromatic end (amino amides) include lignocaine, prilocaine, bupivacaine and ropivacaine. They are highly protein bound, much more stable and undergo hepatic metabolism.

Local anaesthetics are available in single or multidose vials, with or without dilute adrenaline at 1:200,000 (containing 5 μg adrenaline per millilitre) to prolong their duration of action.

Local anaesthetic reactions

Antioxidants, such as sodium bisulphite or metabisulphite, are added to adrenaline-containing solutions and preservative, such as methylparaben, to multidose vials and are implicated in some apparent ‘allergic’ reactions to the local anaesthetic. Other reaction mimics include vasovagal episodes, adrenergic sympathetic stimulation and anxiety-related responses.

True allergy to local anaesthetics is extremely rare at <1.0% reactions, when verified by progressive challenge testing, and is usually to the amino amides. More common are contact dermatitis or delayed local swelling (discussed later).

Duration of action

The duration of action of local anaesthetics is related to the degree of protein binding, vasoactivity, concentration and possibly pH, although the addition of adrenaline is the most practical way to prolong their effect. Table 22.2.1 gives standard maximum safe doses and duration of action of commonly used agents. Solutions containing adrenaline should not be injected near end arteries, such as in the fingers, toes, nose or penis, even though surprisingly this well-established dogma is not supported by the literature. Normal blood flow is restored to the digit within 60 to 90 minutes of inadvertent injection of local anaesthesia with adrenaline (epinephrine) at standard commercial dilutions, without any evidence of harm.

Table 22.2.1

Maximum recommended safe dose and duration of action of common local anaesthetics

Drug	Dose (mg/kg) ^a	Duration (h)
Lignocaine	3	0.5–1
Lignocaine with adrenaline	7	2–5
Bupivacaine	2	2–4
Prilocaine	6	0.5–1.5

a A 1% solution contains 10 mg/mL.

Adverse effects

Systemic toxicity

Systemic toxicity occurs after unrecognized rapid intravenous or intra-arterial injection or by exceeding the recommended safe maximum dose. Symptoms and signs of toxicity are related to plasma drug levels and progress from circumoral tingling, dizziness, tinnitus and visual disturbance to muscular twitching, confusion, convulsions, coma and apnoea. Cardiovascular effects are also seen with high plasma levels, including bradycardia, hypotension and cardiovascular collapse ultimately with ventricular fibrillation or asystole, which are all exacerbated by associated hypoxia. See Box 22.2.1 for the features of local anaesthetic toxicity related to increasing plasma levels.

Box 22.2.1

Features of systemic local anaesthetic toxicity (in order of increasing plasma levels)

Circumoral tingling
Dizziness
Tinnitus
Visual disturbance
Muscular twitching
Confusion
Convulsions
Coma
Apnoea
Cardiovascular collapse (highest plasma levels)

Management of systemic toxicity

The management of systemic toxicity includes immediate cessation of the drug, summoning help, airway maintenance, supplemental oxygen and incremental doses of an intravenous benzodiazepine, such as midazolam 0.05 to 0.1 mg/kg, for seizures. Major reactions may require endotracheal intubation, fluids and cautious use of vasopressors and inotropes, as high doses can impede resuscitation in toxic cardiomyopathy. Refractory arrhythmias with cardiovascular collapse from local anaesthetic systemic toxicity (LAST) may respond best to intravenous 20% lipid emulsion 1.5 mL/kg bolus followed by 0.25 mL/kg/min for roughly 10 minutes following the recovery of vital signs.

As adverse reactions occur immediately or within minutes after local anaesthetic use, medical expertise, resuscitation equipment and monitoring facilities must always be readily available.

Other reactions

Other adverse reactions to local anaesthetics involve allergy, including contact dermatitis, and rarely anaphylaxis predominantly to the amino amides, catecholamine effects from added adrenaline, vasovagal reactions when the patient is upright (such as during a dental procedure), cytotoxic delayed wound healing, malignant hyperthermia from amino amide use and methaemoglobinaemia due to prilocaine or benzocaine ( Box 22.2.2 ).

Box 22.2.2

Adverse reactions to local anaesthetics (other than systemic toxicity)

Allergy:
- Amides >> esters
- Additives, such as methylparaben, sodium metabisulphite
Catecholamine effects from added adrenaline
Vasovagal
Delayed wound healing
Malignant hyperthermia
Methaemoglobinaemia—prilocaine, benzocaine

Topical anaesthesia

Some agents such as EMLA (eutectic mixture of local anaesthetics including 2.5% lignocaine and 2.5% prilocaine) are used topically, particularly to decrease the pain of insertion of cannulae or for lumbar puncture and suprapubic catheter insertion in children. EMLA takes up to 1 hour for maximal effect and, paradoxically, is a venoconstrictor making vessel puncture harder. A potentially superior alternative for cannula insertion is 4% amethocaine (AnGel), as this has a quicker onset and is a vasodilator, although operator experience in cannulation is likely to be of more relevance.

Likewise, a mixture of 1:1000 adrenaline, 4% lignocaine and 0.5% amethocaine, with the acronym ALA (or known as LET in North America standing for lidocaine, epinephrine and tetracaine) up to 0.1 mL/kg, may be used inside small wounds instead of, or to reduce the pain of, injecting local anaesthetic prior to closure, again in children or adolescents.

Specific nerve blocks

The following nerve blocks are contraindicated in uncooperative patients, those with local sepsis in the injection zone and in the rare patient with true local anaesthetic allergy. Care must be taken not to exceed the recommended maximum local anaesthetic doses (see Table 22.2.1 ), and monitoring facilities, resuscitation equipment and medical expertise must be available at all times.

Formal training and accreditation should occur prior to independent practice, particularly with the more complex blocks, such as Bier and femoral nerve.

Digital nerve block (‘ring block’)

Indications

Wound debridement, suturing, drainage of infection, fracture or dislocation reduction around the nail, fingertip and distal finger or toe.

Contraindications

Local sepsis, Raynaud phenomenon and peripheral vascular disease.

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Essentials