The electrical system of the heart conduction system is a complex network of cells and fibers through which electrical impulses travel at lightning speed through the heart, triggering a heartbeat. The impulses are first sent out by the sinoatrial node (sinus node or S-A node), located in the top of the upper–right chamber of the heart (the right atrium). From there, the impulses spread through the atria and to the atrioventricular node (A-V node), where they are transmitted to the lower chambers of the heart, the ventricles, via the bundle branches. Once the impulses reach the ventricles, they cause the chambers to contract and pump out blood in a routine and consistent manner.

If impulses are sent from the sinoatrial node at a slow rate, or if the impulses are delayed as they travel through the conduction system, the heartbeat will be slow. The impulses may even be blocked altogether, which will lead to a complete stopping of the heart (complete heart block) unless treatment is received immediately.

In many cases, a temporarily slow heartbeat is not medically significant by itself. For instance, sinus bradycardia is a normal response to deep relaxation or being in excellent physical shape. Bradycardia may also be caused by:

• Aging-related degeneration of the heart’s electrical conduction system.
  
  
• Certain medications, such as those to treat arrhythmias, high blood pressure (e.g., beta blockers, calcium channel blockers) or heart failure (digoxin). Once these medications have been reduced or discontinued, the bradycardia will usually resolve on its own.
  
  
• Protease inhibitors, a class of medication typically used to treat HIV infection.
  
  
• Coronary artery disease.
  
  
• Disturbances in metabolism (e.g., low thyroid levels) or electrolyte balance (e.g., high or low potassium levels).
  
  
• Some heart conditions that are inherited or present at birth (congenital heart defects).

Some types of persistent bradycardia, however, can cause increasingly worse symptoms and even death if left untreated.