Altered Impulse Conduction
Abnormal conduction of impulses within the heart can lead to arrhythmias. The most common pathophysiologic mechanism for abnormal conduction results from localized or regional depolarization due to hypoxia caused by impaired coronary blood flow. Depolarization decreases the action potential amplitude and rate of depolarization (phase 0 slope is decreased), both of which decrease the velocity of action potential conduction or completely stop the conduction of action potentials (i.e., conduction block). Conduction blocks can also be caused at the AV node by excessive vagal activation or because of drugs that reduce conduction such as beta-blockers and calcium-channel blockers.
When the conduction block is not a complete AV block (e.g., left bundle branch block), electrical impulses can travel along alternate conduction pathways to depolarize the ventricles. When this occurs, it takes longer for the ventricles to depolarize. This is manifested as an increase in the duration of the QRS complex, and a change in its shape. Sometimes, the abnormal conduction pathways can cause a self-perpetuating, circular movement of electrical activation. This is termed reentry and is a major cause of ventricular and supraventricular tachycardias.
RK Revised 04/06/07