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Abstract:
Lignocaine is widely used as a local anaesthetic and antiarrhythmic drug. It is commonly administered to patients with acute myocardial infarction as prophylaxis for ventricular fibrillation, although its efficacy in preventing primary ventricular fibrillation is still debated. Toxicity, sometimes with serious clinical consequence, is not uncommom and is usually related to overdosage. Blood lignocaine concentrations correlate roughly with antiarrhythmic and toxic effects and might be useful as an end point for monitoring prophylactic therapy. Administration of lignocaine as a local anaesthetic may result in blood lignocaine concentration in the antiarrhythmic or even toxic ranges. Expected peak levels for various routes of local anaesthesia are tabulated so that 'safe' total doses can be calculated. Intramuscular injection of high doses results in sustained therapeutic levels but is often associated with early minor toxicity. Lignocaine is eliminated primarily by hepatic metabolism, which appears to be limited by liver perfusion. Active metabolites may contribute to therapeutic and/or toxic effects. Disease states such as cardiac failure or drugs that alter hepatic blood flow may significantly affect lignocaine clearance. Pharmacokinetic studies in man show wide variability in drug disposition between patients, even when cardiac and hepatic status is considered, making specific dosing recommendations a problem. With intravenous injection, multicompartment kinetics is observed, with an initial rapid decline phase and initial decline in antiarrhythmic activity due to redistribution. With constant infusion, steady state concentrations of lignocaine are seen after 3 to 4 hours in normal subjects and after 8 to 10 hours in patients with myocardial infarction without circulatory insufficiency. In patients with cardiac failure, blood lignocaine concentration may continue to rise for 24 to 48 hours. In the presence of cardiac failure, decreased volumes of distribution and clearance require reduction in loading and maintenance doses. Lignocaine clearance is reduced in patients with liver disease and appears to be a sensitive index of liver dysfunction. A dosing algorithm for treatment of patients with myocardial infarction is presented.