| Local anaesthetics: adverse effects |
Local anaesthesia is an effective and relatively safe method of pain control, and has a very low incidence of significant adverse effects. Different anaesthetic drugs can have different complications. The adverse effects of local anaesthetics include:
central nervous system effects—light-headedness, nervousness, apprehension, euphoria, confusion, dizziness, drowsiness, blurred or double vision, twitching, tremors, convulsions and unconsciousness, difficulty swallowing, slurred speech, sensations of heat or cold
respiratory effects—respiratory depression, respiratory arrest
cardiovascular effects—bradycardia, hypotension, cardiovascular collapse, cardiac arrest
allergic responses (usually to the ester group of local anaesthetics)—cutaneous lesions, urticaria, oedema, anaphylactoid reactions
local complications
methaemoglobinaemia (prilocaine, dose-dependent).
Clinicians using local anaesthetics should be familiar with the diagnosis and management of drug-related toxicity. Other acute emergencies can arise with nerve blocks. Resuscitation drugs and equipment, including oxygen, should always be available for the immediate management of adverse reactions. An intravenous cannula should be inserted before complicated procedures.
Systemic toxicity can arise after inadvertent intravascular injection of local anaesthetic, rapid systemic absorption, excessive dose administration or impaired drug clearance.
Local anaesthetics can cause circumoral and tongue numbness, light-headedness, visual and auditory disturbances, generalised muscular twitching, loss of consciousness, seizures, coma, respiratory arrest and cardiovascular collapse.
With the majority of local anaesthetics, minor central nervous system effects are seen before seizures or the onset of cardiovascular toxicity, and can act as a warning of serious events. However, longer-acting local anaesthetic agents, notably bupivacaine, can cause cardiovascular effects before any central nervous system effect. Changes in cardiac conduction, excitability, refractory period, contractility and peripheral vascular resistance can occur at therapeutic blood concentrations. Higher concentrations lead to life-threatening atrioventricular block, ventricular arrhythmias and depressed cardiac contractility. Bupivacaine has relatively greater cardiotoxicity than levobupivacaine and ropivacaine. Due to the potential for systemic toxicity, bupivacaine, levobupivacaine and ropivacaine are contraindicated for use in intravenous regional anaesthesia (Bier block). Limited case reports suggest that infusion of fat emulsions may be of clinical benefit in managing cardiac toxicity related to long-acting amide local anaesthetics (eg bupivacaine, ropivacaine) in patients not responding to standard resuscitative measures. A number of Australian hospitals are now stocking 20% fat emulsion in ward areas that routinely administer local anaesthetics.
Allergic reactions to aminoester local anaesthetics can be due to the drug itself, or to the preservatives (eg methylparaben) or antioxidants (eg sodium metabisulfite) that are in some preparations. Cross-sensitivity can also occur, as many foods, drugs and skin preparations contain similar preservatives.
Allergy to the aminoamide local anaesthetics is very rare, but has been reported.
Positive skin testing to one local anaesthetic does not provide any information about other drugs.
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