• First-degree heart block. The electrical impulses are slowed as they pass through the conduction system, but all of them successfully reach the ventricles. First-degree heart block rarely causes any symptoms or problems. In most cases, the source of the delay is in the atrioventricular node. Approximately one in every 1,000 people has first-degree heart block. Well-trained athletes are particularly likely to have it because their hearts pump more efficiently, requiring fewer contractions than the hearts of the general population. Heart block may also be caused by certain medications, such as beta blockers, calcium-channel blockers or digitalis (a type of inotropic drug). A change of medication often resolves the condition. 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 longer with each heartbeat until a beat is skipped entirely. The source of the interference is usually within the AV node. The condition may sometimes cause dizziness.

  • 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 because of interference from somewhere below the AV node (e.g., the bundle of His). In contrast with type I second-degree heart block, this condition is generally less common and carries a higher risk of developing into complete (third-degree) heart block. A physician may recommend an artificial 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, due to a problem that may lie anywhere between the atrioventricular node and the bundle branches, although the latter is more common. In the absence of any electrical impulses from the atria, the ventricles may generate some impulses on their own (called 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, complete heart block poses a medical emergency with potentially severe symptoms and a serious risk of the heartbeat stopping completely (cardiac arrest). If a pacemaker cannot be implanted immediately, then a temporary pacemaker wire might be used to keep the heart pumping until surgery can be performed.