Even in the presence of normal SA nodal function, ectopic pacemakers can generate extra beats. Early afterdepolarizations occur during Phase 2 or 3 and can lead to a salvo of several rapid, or a prolonged series of, depolarizations and beats. This form of triggered activity is more likely to occur when the action potential duration is increased. Delayed afterdepolarizations occur after Phase 3 when the action potential is fully repolarized. Altered Impulse Conduction Abnormal conduction of impulses within the heart can lead to arrhythmias.
Conduction blocks can cause either bradycardia or tachycardia.
When the AV node is blocked, for example, impulses originating in the SA node cannot enter the ventricle. When this occurs, a pacemaker site within the AV node, Bundle of His, or more distal site may become the dominant pacemaker for the heart. Reentry Conduction blocks can also result in tachyarrhythmias. This mechanism may account for most tachyarrhythmias found in patients. Reentry can take place within a small local region or it can occur, for example, between the atria and ventricles.