Automaticity, abnormal arrhythmias caused

O Cardiac arrhythmias may be caused by abnormal impulse formation (automaticity), abnormal impulse conduction (reentry), or both. 

Actions Lidocaine, a local anesthetic, shortens phase 3 repoiarization and decreases the duration of the action potential (Figure 17.8). Unlike quinidine, which suppresses arrhythmias caused by increased normal automaticity, lidocaine suppresses arrhythmias caused by abnormal automaticity. Lidocaine, like quinidine, abolishes ventricular reentry. 

This is a class IB drug used primarily for the emergency treatment of ventricular arrhythmias. It has little effect on sinus node automaticity but depresses normal and abnormal forms of automaticity in Purkinje fibres. It is generally ineffective against supraventricular and accessory pathway-induced (e.g. WPW syndrome) arrhythmias. Lidocaine is relatively safe and free from adverse cardiovascular side effects. It causes minimal cardiodepression, although high doses can cause heart block. The most common side effect is a dose-related CNS toxicity. It is given intravenously as a bolus of 1 mg-kg-1 followed by an infusion of 20-50 pg-kg-l-min-1. 

Altered rate of automatic discharge or abnormality of the mechanism by which an impulse is generated from a centre in the nodes or conducting tissue, is one cause of cardiac arrhythmia, e.g. atrial fibrillation, flutter or tachycardia. 

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