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Normal Heart Rhythm

Normal Heart Rhythm

Sinus Node Sends Sequential Electrical Impulse

Every heartbeat begins in the right atrium, one of four chambers in your heart. In the right atrium, your heart's pacemaker, the sinoatrial (SA) or sinus node, sends an electrical signal causing the top chambers (the two atria) to contract, allowing the two bottom chambers (the ventricles) to fill with blood.

Electrical Signal Travels Across the AV Node

That signal then travels almost instantaneously across the atrioventricular (AV) node into the ventricles. This causes the ventricles to contract, pushing blood to the lungs and rest of your body.


Sinus Node Malfunction or Electrical Impulse Blocked

In bradycardia, either the pacemaker (sinus node) is not working properly, or the electrical signal is not getting through for some other reason, causing the slow rhythm.

Heartbeat: Anatomy of the Heart

Sinus Node Problem

The most common cause of bradycardia is sinus node dysfunction. The signal may be blocked, paused or be sending alternating fast and slow signals. When sending alternating fast and slow signals, it is called tachycardia-bradycardia syndrome.

AV Node Problem

The atrioventricular (AV) node connects the sinus node to the ventricles. When this signal is disrupted, it can result in bradycardia. Sometimes called heart block, AV node malfunction is common in people with heart disease.

Bundle Branch Block

There are several types of AV node malfunction. One is called bundle branch block. Bradyarrhythmias also can happen as a result of severe bundle branch block. Bundle branch block is a condition in which the electrical signal traveling down either or both of the bundle branches -- bundles of nerve tissue -- is delayed or blocked. When this occurs, the ventricles don't contract at exactly the same time, as is normally the case, and the heart has to work harder to pump blood.

The cause of bundle branch block may be an existing heart condition.

  • Metabolic problems such as hypothermia
  • Damage to the heart from heart attack or heart disease
  • Another part of the body taking over as pacemaker
  • A normal response to deep relaxation
  • A normal response to being in excellent physical shape

Contact Information

Cardiovascular Medicine
Division of the CardioVascular Institute
Beth Israel Deaconess Medical Center
330 Brookline Avenue
Boston, MA 02215