Antiarrhythmic Drugs and the Vaughan-Williams Classification

Antiarrhythmic Drugs and the Vaughan-Williams Classification

1. Definition, scope & clinical burden

• An arrhythmia (dysrhythmia) is an abnormality of the rate, rhythm, or site of origin of the cardiac impulse, or an abnormality of impulse conduction (Padmaja 7e Ch.29, p.374).
• Antiarrhythmic drugs are agents used to prevent or terminate irregularities of cardiac rhythm; only a minority of arrhythmias actually need drug treatment (KDT 8e Ch.39, p.570).
• Arrhythmias are common: they occur in up to 25% of patients on digitalis, ~50% of anaesthetised patients, and >80% of patients with acute MI; nearly 3 of 4 acute-MI patients show some rhythm disturbance (Katzung 16e Ch.14, p.235; KDT 8e Ch.39, p.570).
• Arrhythmias are the most important cause of sudden cardiac death; ventricular fibrillation (VF) is the single most common mechanism of sudden death (KDT 8e Ch.39, p.570).
• Antiarrhythmic drug therapy has two goals — termination of an ongoing arrhythmia, or prevention/prophylaxis of an arrhythmia (G&G 14e Ch.34, p.667).
• Paradox / central caveat: antiarrhythmic drugs not only suppress arrhythmias but can also cause them (proarrhythmia), even during long-term therapy — so prescribing requires a precise diagnosis, exclusion of precipitating factors, a reasonable expectation of benefit, and risk minimisation (G&G 14e Ch.34, p.667).
• Precipitating/exacerbating factors: ischaemia/hypoxia, acidosis–alkalosis, electrolyte disturbance (especially hypokalaemia, hypomagnesaemia), excess catecholamines, autonomic influences, drug toxicity (digitalis, antiarrhythmics themselves), fibre stretch (heart failure), mechanical injury, and scarred/diseased tissue (Katzung 16e Ch.14, p.236; KDT 8e Ch.39, p.570).