There have been multiple attempts to categorize antiarrhythmic medications; the most widely accepted system is known as the Vaughan Williams classification. Although this system is not perfect, it is used to sort the various antiarrhythmic medications based on their mechanism of action. Some of the drugs exhibit multiple modes of action, however, and may belong in more than one class. The class system includes:

• Class I agents. These drugs are sodium channel blockers that slow electrical conduction in the heart. They are used to treat rapid heart rhythms that originate in the ventricles (e.g., supraventricular tachycardia, ventricle tachycardia, ventricular fibrillation). Class I is further broken down into three subclasses:
  
  

• 1a (quinidine, procainamide and disopyramide). Procainamide and quinidine can be used to convert rapid heart rhythms in the atria (atrial fibrillation) to a normal heart rhythm and help keep patients from having these arrhythmias.
  
  

• 1b (lidocaine, mexiletine, tocainide and phenytoin). These drugs  work with rapid ventricular rhythms, including ventricular tachycardia and ventricular fibrillation. They have no effect on atrial arrhythmias.
  
  

• 1c (ecainide, flecainide, moricizine and propafenone). These drugs are used for life-threatening ventricular tachycardia or ventricular fibrillation, as well as atrial fibrillation.
  
  
• Class II agents. Class II agents are conventional beta blockers. These medications reduce the workload of the heart by blocking certain hormones from binding with beta receptors in the heart, thus keeping a rapid heartbeat from being triggered. These drugs can also reduce a patient's blood pressure and heart rate. This class of of drugs includes propranolol, metoprolol, atenoiol and betaxolol. Beta blockers given after heart surgery have shown to be useful in helping to prevent atrial fibrillation, which occurs in 25 to 40 percent of patients.
  
  
• Class III agents. These drugs block potassium channels in the heart. Drugs in this class include amiodarone, azimilide, bretylium, clofilium, dofetilide, tedisamil, ibutilide, sematilide and sotalol. Sotalol is used in the treatment of ventricular arrhythmias (e.g., ventricular tachycardia). Amiodarone is used in the treatment of ventricular tachycardia or ventricular fibrillation.
  
  
• Class IV agents. Class IV agents are calcium channel blockers. This class includes vetapamil and diltaizem. These medications reduce the workload of the heart and slow its rhythm by blocking calcium ions from signaling the blood vessels to constrict or tighten.
  
  
• Class V agents. This class was created for drugs that did not fit into the other classifications. Class V antiarrhythmic agents include adenosine and digoxin (a kind of inotrope).

Certain antiarrhythmics, such as adenosine, beta blockers and calcium channel blockers, may be used in the hospital during an emergency to quickly restore a tachyarrhythmia to a normal rhythm.

One of the issues with antiarrhythmics is a relatively high incidence of side effects, including arrhythmia. Because of this, the proper choice and dosing of antiarrhythmic medication may require time while the physician and the patient experiment with different medications to see which works the best. Physicians choose which type of antiarrhythmic to prescribe based on each patient’s medical history and current symptoms/conditions.