In electrocardiography, during a cardiac cycle

Cardiac cycle

The cardiac cycle is a term referring to all or any of the events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next. The frequency of the cardiac cycle is described by the heart rate. Each beat of the heart involves five major stages...

, once an action potential is initiated, there is a period of time that a new action potential cannot be initiated. This is termed the effective refractory period (ERP) of the tissue. This period is also termed the absolute refractory period (ARP).
During this period, depolarization on adjacent cardiac muscles does not produce a new depolarization in the current cell as it has to refract back to phase 4 of the action potential before a new action potential can activate it. ERP acts as a protective mechanism and keeps the heart rate in check and prevents arrhythmias and coordinates muscle contraction. Anti-arrhytmic agents used for arrhythmias usually prolong the ERP. For the treatment of Atrial fibrillation

Atrial fibrillation

Atrial fibrillation is the most common cardiac arrhythmia . It is a common cause of irregular heart beat, identified clinically by taking a pulse. Chaotic electrical activity in the two upper chambers of the heart result in the muscle fibrillating , instead of achieving coordinated contraction...

, it is a problem that the prolongation of the ERP by these agents also affects the ventricles, which can induce other types of arrhythmias.