ADVERSE-EFFECTS AND DRUG-INTERACTIONS ASSOCIATED WITH LOCAL AND REGIONAL ANESTHESIA

Systemic and localised adverse effects of local anaesthetic drugs usua lly occur because of excessive dosage, rapid absorption or inadvertent intravascular injection. Small children are more prone than adults to methaemoglobinaemia, and the combination of sulfonamides and prilocai ne, even when correctly administered, should be avoided in this age gr oup. The incidence of true allergy to local anaesthetics is rare. All local anaesthetics can cause CNS toxicity and cardiovascular toxicity if their plasma concentrations are increased by accidental intravenous injection or an absolute overdose, Excitation of the CNS may be manif ested by numbness of the tongue and perioral area, and restlessness, w hich may progress to seizures, respiratory failure and coma. Bupivacai ne is the local anaesthetic most frequently associated with seizures, Treatment of CNS toxicity includes maintaining adequate ventilation an d oxygenation, and controlling seizures with the administration of thi opental sodium or benzodiazepines. Cardiovascular toxicity generally b egins after signs of CNS toxicity have occurred. Bupivacaine and etido caine appear to be more cardiotoxic than most other commonly used loca l anaesthetics. Sudden onset of profound bradycardia and asystole duri ng neuraxial blockade is of great concern and the mechanism(s) remains largely unknown. Treatment of cardiovascular toxicity depends on the severity of effects. Cardiac arrest caused by local anaesthetics shoul d be treated with cardiopulmonary resuscitation procedures, but bupiva caine-induced dysrhythmias may be refractory to treatment. Many recent reports of permanent neurological complications involved patients who had received continuous spinal anaesthesia through a microcatheter. I njection of local anaesthetic through microcatheters and possibly smal l-gauge spinal needles results in poor CSF mixing and accumulation of high concentrations of local anaesthetic in the areas of the lumbosacr al nerve roots. In contrast to bupivacaine, the hyperbaric lidocaine ( lignocaine) formulation carries a substantial risk of neurotoxicity wh en given intrathecally. Drugs altering plasma cholinesterase activity have the potential to decrease hydrolysis of ester-type local anaesthe tics. Drugs inhibiting hepatic microsomal enzymes, such as cimetidine, may allow the accumulation of unexpectedly high (possibly toxic) bloo d concentrations of lidocaine. Reduction of hepatic blood flow by drug s or hypotension will decrease the hepatic clearance of amide local an aesthetics. Special caution must be exercised in patients taking digox in, calcium antagonists and/or beta-blockers.