The sinus node is the heart's natural pacemaker. A heartbeat begins when the sinus node starts electrical signals in the upper right chamber of the heart (right atrium). Electrical signals from the sinus node travel through a group of cells called the atrioventricular node (AV node) before moving to the ventricle.
Heartblock is when the AV node does not let all electrical signals reach the ventricles. This causes a slow or unsteady heartbeat and does not allow enough blood to circulate through the body. Heart block can cause an abnormally low heart rate (bradycardia) with symptoms such as dizziness, extreme fatigue, shortness of breath, or fainting spells. A pacemaker compensates for the impaired AV node by restoring the heart rate to a level that meets the demands of everyday living.