What is Bradycardia?


The heart can beat too slowly for several reasons.

  • Sick Sinus Syndrome (or Sinus Node Dysfunction) is when the heart’s natural pacemaker (sinus node) sends electrical signals too slowly.
  • Heart Block is when the electrical signal is blocked before reaching the lower chambers of the heart (the ventricles).


Sinus Node Dysfunction (or Sick Sinus Syndrome) is a specific kind of slow heart rate (bradycardia). With sinus node dysfunction, the heart’s natural pacemaker (sinus node) either

  • Does not maintain a heart rate that is needed by the body, or
  • Cannot increase the heart rate when more oxygen is needed, such as when exercising.

When the sinus node does not work properly, other heart tissues may begin a heartbeat. However, the rate may be inconsistent or too slow for normal activities. With a heart rate that is too slow, not enough oxygen is pumped to the body. The lack of oxygen causes symptoms such as dizziness, extreme tiredness, shortness of breath, or fainting.

Common causes for sinus node dysfunction include congenital heart defects, illnesses, cardiac drugs, the natural aging process, or scar tissue from a heart attack. Sometimes the exact cause is unknown.

Pacemakers relieve the symptoms of sinus node dysfunction by beginning a heartbeat at the rate needed to meet the oxygen needs of the body.


Heart Block is a specific cause of bradycardia (an irregular or abnormally slow heart rate). The slow heart rate (usually less than 60 beats per minute) occurs when a heartbeat is stopped before it makes the lower heart chambers (ventricles) pump blood to the body.

Diagnostic tests such as an electrogram (ECG) assist the doctor in seeing when heart block is causing the slow heart rhythm. When the slow heart rhythm does not happen during testing, an implantable loop recorder, may be used to clarify the diagnosis. A pacemaker relieves the symptoms of bradycardia caused by heart block by ensuring that the ventricles pump at an appropriate rate.