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Heart Block

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Heart block is a heart rhythm where the heart beats slower than normal (bradycardia).  In this condition, the electrical impulses that inform the heart to contract are totally or partially blocked between the atria and the ventricles.   This is why heart block is also called atrioventricular block (AV block).


heart block image

What is Heart Block?

In normal hearts, a natural pacemaker that tells the heart when to beat is present.   The master pacemaker called the sinoatrial (SA), or sinus node is located in the right atrium and acts like a spark plug that fires in a rhythmic, regular pattern to regulate the heart’s rhythm.   The electrical signal then reaches another natural pacemaker called the atrioventricular node (AV node).   As the signal continues and crosses to the ventricles, it passes through the bundle of His.   The bundle divides into wire-like thin structures called bundle branches that extend into the left and right ventricles.   The electrical signal travels down the bundle branches to thin fibers.  Heart block occurs when the passage of electricity from the top of the heart to the bottom of the heart is interrupted or delayed.


The Types of Heart Block

  • First-degree heart block:  The electrical impulses slow down as they pass through the conduction system, but the signals all eventually and successfully reach the ventricles.   First-degree heart block infrequently causes problems or symptoms.  Trained athletes may have first-degree heart block.   Some medications are known to cause this condition and treatment is generally not needed.


  • Second-degree heart block (Type I): The electrical signals with each heartbeat are delayed further and further until an impulse fails to reach to the ventricles.   Sometimes this causes dizziness and/or shortness of breath.   Even individuals with a normal conduction system may sometimes have Type I second degree heart block while sleeping


  • Second-degree heart block (Type II):  With Type II, some of the electrical impulses are unable to reach the ventricles.  T his condition is not as common as Type I, but is more serious.   Usually, a cardiologist will recommend a pacemaker to treat type II second degree heart block, because it frequently progresses to third degree heart block.


  • Third-degree heart block: With third degree heart block or complete heart block, no electrical impulses from the atria reach the ventricles.   When the ventricles don’t receive electrical impulses from the atria, they may generate some impulses on their own, called ventricular escape beats or junctional beats.   Junctional beats are the heart’s naturally occurring backup system but are usually quite slow.   Patients frequently feel poorly in complete heart block, with fatigue and lightheadedness.


  • Bundle Branch Block – In bundle branch block, the electrical signals are blocked or slowed as they make their way through the specialized conducting tissue in either of the two ventricles.



Symptoms of Heart Block

Some individuals with heart block won’t experience symptoms where others may feel the following:


  • Dizziness or lightheadedness
  • Shortness of breath
  • Fainting (a.k.a. syncope)
  • Chest pain



Risk factors for Heart Block

There are some medical conditions that can increase the risk for developing heart block, such as:


  • A prior heart attack
  • Heart valve surgery
  • Lyme disease
  • Aging
  • Heart failure
  • Abnormal heart valves
  • Some medications
  • Aging



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