Bradycardia is a general term that describes a number of conditions in which the heart beats at an unusually slow rate (fewer than 50 to 60 beats per minute). The different types of bradycardia include:

• Sinus bradycardia. An unusually slow heartbeat due to normal causes (e.g., deep relaxation or excellent fitness). It may be present in athletes but can also occur in patients with heart disease (e.g., those experiencing cardiac ischemia) or as a reaction to a variety of medications (e.g., digoxin, beta blockers or calcium channel blockers).
  
  
• Sick sinus syndrome (also known as sinus node dysfunction). A condition in which the heart’s natural pacemaker (the sinoatrial node) malfunctions, causing an irregular heartbeat. Patients with sick sinus syndrome may have a slow heartbeat (bradycardia), a rapid heartbeat (tachycardia), or heartbeats that alternate between fast and slow (brady-tachy syndrome or tachy-brady syndrome). Sick sinus syndrome can lead to dizziness, fatigue, weakness or fainting (syncope). It is more common in elderly people and may be caused by a degeneration of the heart’s electrical conduction system. In children, it is often caused by surgery.
  
  
• Heart block (also known as atrioventricular block or AV block). A condition in which electrical impulses are slowed or blocked as they travel from the atria, through the atrioventricular node (AV node) and into the ventricles. The result is a delayed or complete lack of electrical communication between the upper chambers of the heart (atria) and the lower chambers of the heart (ventricles). Both the symptoms and treatments for heart block depend on its degree of severity:
  
  
• First-degree heart block (also known as first-degree AV block). The electrical impulses are slowed as they pass through the AV node, but all of them successfully reach the ventricles. First-degree heart block rarely causes any symptoms or problems, and is a common finding in well-trained athletes. A change of medication often resolves the condition in non-athletes. No other treatments are generally necessary.
  
  
• Type I second-degree heart block (also known as Mobitz Type I second-degree AV block or Wenckebach AV block). The electrical impulses are delayed with each heartbeat until a beat is skipped entirely. The condition rarely causes dizziness and/or other symptoms. In such cases, implantation of a pacemaker may be required.
  
  
• Type II second-degree heart block (also known as Mobitz Type II second-degree AV block). Some of the electrical impulses are unable to reach the ventricles, usually because of an underlying disease. In contrast with type I second-degree heart block, this condition generally carries more risk and is less common. A physician may recommend a pacemaker for the abnormally slow heartbeat (bradycardia) associated with this condition.
  
  
• Third-degree heart block (also known as complete heart block or complete AV block). None of the electrical impulses can reach the ventricles, usually as a result of underlying disease or certain medications. In the absence of any electrical impulses from the atria, the ventricles may generate some impulses on their own ventricular escape beats via secondary impulse generators. However, these natural “backups” are usually very slow and are generally unable to sustain the full functioning of the heart muscle. Therefore, this condition poses a medical emergency with potentially severe symptoms and a serious risk of the heartbeat stopping altogether (cardiac arrest). If a pacemaker cannot be implanted immediately, then a temporary pacemaker wire might be used to keep the heart stimulated until surgery can be performed.
  
  
• Bundle branch block. A condition in which the electrical impulses traveling through the heart are slowed or blocked completely from traveling along their normal route through the ventricles. The condition tends to produce no symptoms, although it may be a marker of risk for developing Heart block. In patients with heart failure, the presence of a bundle branch block may be associated with increased symptoms, in which case, depending on the location of the defect(s), a biventricular pacemaker may be beneficial. A person may be diagnosed with one of several specific types of bundle branch block. They include:
  
  
• Left bundle branch block (LBBB). Defects are present in both the anterior and posterior divisions (fascicles) of the left bundle branch. If only the anterior division is involved, then people will be diagnosed with an anterior fascicular block (also known as an anterior divisional block or left anterior hemiblock). If only the posterior division is involved, then people will be diagnosed with a posterior fascicular block (also known as a posterior divisional block or left posterior hemiblock).
  
  
• Right bundle branch block (RBBB), in which the defect occurs in the right bundle branch. Both LBBB and RBBB may be complete or incomplete. 
  
  
• Bifascicular block, in which defects occur in two of the three divisions of the bundle branches. More specifically, people with this condition may be diagnosed with “RBBB with left anterior hemiblock,” “RBBB with left posterior hemiblock” or LBBB.
  
  
• Trifascicular block, in which there is a form of bifascicular block (as described above) as well as a first-degree heart block.