Heart block is an abnormal heart rhythm where the heart beats too slowly (bradycardia). In this condition, the electrical signals that tell the heart to contract are partially or totally blocked between the upper chambers (atria) and the lower chambers (ventricles). For this reason, it is also called atrioventricular block (AV block).
What is Heart Block?
A normal heartbeat begins as an electrical signal that comes from the heart’s master pacemaker, the sinoatrial (SA) node, located in the right atrium. The electrical signal travels through the atria and reaches another natural pacemaker called the atrioventricular (AV node). As the signal continues and crosses to the ventricles, it passes through a bundle of tissue called the AV bundle, also called the bundle of His. The bundle divides into thin, wire-like structures called bundle branches that extend into the right and left ventricles. The electrical signal travels down the bundle branches to thin fibers. Lastly, these fibers send out the signal to the muscles of the ventricles, causing them to contract and pump blood into the arteries. Heart block occurs when the signal is disrupted or slows down at any point on this electrical path.
Types of Heart Block
- 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. Well-trained athletes may have this. Medications can contribute to the condition. No treatment is generally necessary for first-degree heart block.
- Second-degree heart block (Type I) – The electrical impulses are delayed further and further with each heartbeat until a beat is skipped entirely. It sometimes causes dizziness and/or other symptoms. People normal heart rhythms may sometimes have this when they are sleeping.
- Second-degree heart block (Type II)v– With this condition, some of the electrical impulses are unable to reach the ventricles. This condition is less common than Type I, but is generally more serious. In some cases, a pacemaker is implanted to treat the abnormally slow heartbeat that may result from this condition.
- Third-degree heart block – With this condition, also called complete heart block, none of the electrical impulses can reach the ventricles. When the ventricles (lower chambers) do not receive electrical impulses from the atria (upper chambers), they may generate some impulses on their own, called functional or ventricular escape beats. Ventricular escape beats, natural backup signals, usually are very slow, however, and cannot generate the signals necessary to maintain full functioning of the heart muscle.
- Bundle Branch Block – With this condition, the electrical impulses are slowed or blocked as they travel through specialized conducting tissue in the ventricles.
Symptoms of Heart Block
Some people with heart block will not experience any symptoms. Others will have symptoms that may include the following:
- Fainting (syncope)
- Dizziness (spinning sensation)
- Chest pain
- Shortness of breath
Risk factors for Heart Block
Some medical conditions increase the risk for developing heart block. These medical conditions include:
- Heart failure
- Prior heart attack
- Valve abnormalities
- Some medications or exposure to toxic substances
- Other prior illnesses