Class lb

Class lb drugs are useful for the prevention and treatment of premature ventricular contractions, ventricular tachycardia and ventricular fibrillation, particularly associated with ischaemia.

Lidocaine is the first choice drug for ventricular arrhythmias resistant to DC cardioversion. It decreases normal and abnormal automaticity and decreases action potential and refractory period duration. The threshold for ventricular fibrillation is raised, but it has minimal haemodynamic effects. The antiarrhythmic properties of lidocaine are enhanced by hypoxaemia, acidosis and hyperkalemia, so that it is particularly effective in ischaemic cells, e.g. after acute myocardial infarction, during cardiac surgery, or in arrhythmias associated with digitalis toxicity. The usual loading dose is 50-100 mg, followed by a continuous infusion, titrated to response. Lidocaine redistributes rapidly after i.v. injection and it may be necessary to repeat the loading dose after 5-10 min. Close adjustment of the infusion rate is required to avoid CNS toxicity (confusion, dysarthria, tremor, numbness, dizziness and convulsions). Cardiotoxic effects (bradycardia, hypotension, asystole) occur at higher doses and are potentiated by hypoxaemia, acidosis and hypercapnia. It has a short half-life (less than 2 h), is 70% bound to plasma proteins and is metabolized in the liver. Clearance is decreased if hepatic blood flow is decreased (e.g. in the elderly, those with congestive cardiac failure, after myocardial infarction), and also by (3- blockers, cimetidine and liver disease. In these circumstances, the dose should be reduced by 50%. It is less effective in the presence of hypokalaemia.

Mexiletine is a longer-acting, orally effective lidocaine analogue, which is well absorbed orally, with peak plasma concentrations after 2 h and an elimination half-life of 10-15 h. It undergoes hepatic metabolism and is excreted via the kidneys. Hypotension, bradycardia and heart block may occur after i.v. administration. Most frequent adverse effects involve the CNS and include tremors, nystagmus, confusion, speech disturbances, tinnitus, paraesthesiae and convulsions. Gastrointestinal effects are also common during oral treatment.

Tocainide is another lidocaine analogue that may be given orally or parenterally. It is used only in patients with poor LV function in whom other drugs have been unsuccessful or are con-traindicated, because it frequentiy causes blood dyscrasias; other adverse effects resemble those of mexiletine. Arrhythmias unresponsive to lidocaine are unlikely to respond to tocainide.

Phenytoin has effects similar to those of lidocaine, but uniquely accelerates intraventricular conduction and may suppress re-entrant arrhythmias. It was formerly used for the treatment of digoxin-induced ventricular arrhythmias, but has been superseded.

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