While the use of antiarrhythmic agents to suppress atrial arrhythmias (atrial fibrillation and atrial flutter) is still in practice, it is unclear whether suppression of atrial arrhythmias will prolong life.
In the past, it was believed that following myocardial infarction (heart attack), suppression of ventricular arrhythmias would prolong life. However, large clinical trials found that suppression of these arrhythmias would paradoxically increase mortality, which may happen due to the proarrhythmic effect these drugs may have (CAST trial).
In individuals with atrial fibrillation, antiarrhythmics are still used to suppress arrhythmias. This is often done to relieve the symptoms that may be associated with the loss of the atrial component to ventricular filling (atrial kick) that is due to atrial fibrillation or flutter.
In individuals with ventricular arrhythmias, antiarrhythmic agents are often still in use to suppress arrhythmias. In this case, the patient may have frequent arrhythmic events or be at high risk for ventricular arrhythmias. Antiarrhythmic agents may be considered the first-line therapy in the prevention of sudden death in certain forms of structural heart disease, and failure of these agents to suppress arrhythmias may lead to implantation of an implantable cardioverter-defibrillator (ICD).
Many attempts have been made to classify antiarrhythmic agents. The problem arises from the fact that many of the antiarrhythmic agents have multiple modes of action, making any classification imprecise.
There are five main classes in the Vaughan Williams classification of antiarrhythmic agents:
|(Na+) channel block (intermediate association/dissociation)|
|Ib||(Na+) channel block (fast association/dissociation)|
|Ic||(Na+) channel block (slow association/dissociation)|
|K+ channel blocker Sotalol is also a beta blocker|
|IV||Ca2+ channel block|
The class I antiarrhythmic agents interfere with the sodium channel. Class I agents are grouped by what effect they have on the Na+ channel, and what effect they have on cardiac action potentials. Class 1 agents are called Membrane Stabilizing agents. The 'stabilizing' is the word used to describe the decrease of excitogenicity of the plasma membrane which is brought about by these agents. Also noteworthy is that a few class 2 agents like propranolol also have a membrane stabilizing effect.
Class III agents predominantly block the potassium channels, thereby prolonging repolarization. Since these agents do not affect the sodium channel, conduction velocity is not decreased. The prolongation of the action potential duration and refractory period, combined with the maintenance of normal conduction velocity, prevent re-entrant arrhythmias. (The re-entrant rhythm is less likely to interact with tissue that has become refractory).
The Cardiac Arrhythmia Suppression Trial (CAST trial) was a medical study that demonstrated that some class 1 anti-arrhythemic drugs were not helpful to people suffering from ventricular premature depolarization. Specifically, for populations of recent heart attack survivors, the study observed lower mortality in a control population than in populations treated with encainide, flecainide or moricizine (all class 1c drugs)
Other major trials examining antiarrhythmic drugs include Antiarrhythmics Versus Implantable Defibrillators (AVID), and the Atrial Fibrillation Follow-Up Investigation of Rhythm Management (AFFIRM), which compared antiarrhythmic drugs with drugs which controlled inappropriately rapid and irregular heart rate. In both trials, the arrhythmic drugs were less effective than the alternative.
Mnemonic for Class I-IV agents: SoBe PoCa (SOBE as in South Beach or the drink, POCA as in Polka)
Also - remembering that of all anti-arrhythmics "some block potassium channels" can help you: Class I "Some" = S = Sodium Class II "Block" = B =Beta blockers Class III "Potassium" = Potassium channel blockers Class IV "Channels" = C =Calcium channel blockers
Sodium channel blockers, Beta blockers, Potassium channel blockers, Calcium channel blockers
To remember the Class I agents: Class IA “Double Quarter Pounder” (Disopyramide, Quinidine, Procainamide), Class IB “Lettuce, Tomato, Mayo” (Lidocaine, Tocainide, Mexilitine), Class IC “More Fries Please” (Moricizine, Flecainide, Propafenone).
To remember the Class III agents: "A Big Dog Is Scary" (Amiodarone, Bretylium, Dofetilide, Ibutilide, Sotalol). Dr. Hrometz 2008, Ohio Northern University
To remember calcium channel blockers: Calcium channel blockers are "Very Nice Drugs" (verapamil, nifedipine, diltiazem)
Antiarrhythmic drugs for atrial fibrillation: Do we need better use, better drugs or a randomized trial of ablation as primary therapy?
Sep 28, 2004; A trial fibrillation (AF) is the most common cardiac arrhythmia seen in clinical practice, and it contributes substantially to...