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 is initiated by an electrical signal that comes from the heart’s natural pacemaker, the sinoatrial (SA) node, located at the top of the right atrium. The electrical signal travels through the atria and reaches the atrioventricular (AV) node. After crossing the AV node, the electrical signal passes through the bundle of His. This bundle then 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 and eventually reach the muscle cells of the ventricles, causing them to contract and pump blood to the body. Heart block occurs when this passage of electricity from top to bottom of the heart is delayed or interrupted.
Types of Heart Block
- First-degree heart block – The electrical impulses are slowed as they pass through the conduction system, but they all successfully reach the ventricles. First-degree heart block rarely causes symptoms or problems. Well-trained athletes may have first-degree heart block. Some medications can also cause this condition. No treatment is generally needed for first-degree heart block.
- Second-degree heart block (Type I) – The electrical impulses are delayed further and further with each subsequent heartbeat until a beat fails to reach to the ventricles entirely. This type of block most often is physiologic and is seen with high vagal tone and during sleep. It rarely causes symptoms. It sometimes causes dizziness and/or other symptoms.
- Second-degree heart block (Type II) – With this condition, some of the electrical impulses are unable to reach the ventricles. This condition is less common than Type I, and is more serious. Usually, your doctor will recommend a pacemaker to treat type II second degree heart block, as it frequently progresses to third degree heart block.
- Third-degree heart block – With this condition, also called complete heart block, none of the electrical impulses from the atria 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 junctional or ventricular escape beats. Ventricular escape beats, the heart’s naturally occurring backups, are usually very slow. Patients frequently feel fatigue, lightheadedness, and decreased stamina in complete heart block. Patients are usually treated by implanting a permanent pacemaker.
- Bundle Branch Block – With this condition, the electrical impulses are slowed or blocked as they travel through the specialized conducting tissue in one of the two 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 Lightheadedness
- Shortness of breath
- Decline in exercise capacity
Risk factors for Heart Block
Some medical conditions increase the risk for developing heart block. These medical conditions include:
- Heart failure
- Prior heart attack
- Heart valve abnormalities
- Heart valve surgery
- Congenital heart diseases
- Some medications or exposure to toxic substances
- Lyme disease