Heart block refers to an abnormality in cardiac conduction, which may be characterized by prolonged conduction time and partial or total interruption of conduction. Conduction block can be seen in physiologic as well as pathologic conditions. Physiologic conditions, such as blocked atrial premature and disturbed atrioventricular septation, are considered to be physiologic conduction block. The majority of conduction blocks are seen in pathologic conditions, which may include sinoatrial block, atrioventricular block, atrioventricular block, and intraventricular block, and often occur in patients with various types of organic heart disease. In addition, conduction block can occur in patients with vagal hypertonia, or sick sinus node syndrome of unknown cause. Severe types of block, with associated symptoms such as blackouts and syncope, require treatment such as artificial pacemaker placement.