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HEART BLOCK

What Is Heart Block?

  • The heart has two upper chambers (atria) and two lower chambers (ventricles). In normal rhythm, the heartbeat begins in the sinoatrial (SA) node, which is the heart’s natural pacemaker in the upper right chamber. The SA node sends out an electrical signal that causes the atria to contract. This signal then passes through the atrioventricular (AV) node, through the bundle of His, and down the bundle branches (which are like the trunk of a tree). The signal then moves through the Purkinje fibers (which are like the branches of a tree) to the muscle fibers of the ventricles, causing them to contract.
  • An analogy for the normal conduction system of the heart would be to imagine a picture made up of dominos. When something pushes one of the dominos over, it causes a wave of collapsing dominos that spreads out across the picture until all the dominos are down. In this same way, all the cells in the heart receive an electrical stimulus, causing them to contract.
  • Heart block, also known as atrioventricular or AV block is when there is a “block” in one of these electrical pathways. This leads to electrical signals not being able to get through to the bottom chambers of your heart as they should.
  • An analogy for heart block would be to think of the AV node like a boom gate in the middle of a road that controls traffic. When working normally, it will allow each car to pass through from point A to point V . . . pun intended! However, an issue with the boom gate will result in a roadblock. The same applies to the AV node not allowing the electrical impulse to pass through normally. The severity of this roadblock determines the degree of the heart block. It could range from merely causing some delayed traffic through the boom gate (first-degree), to some traffic getting through and some traffic getting blocked (second-degree), or no traffic getting through at all (third-degree).

Heart Block Classification

Heart blocks are classified based on the degree to which signals from the atria reach your heart’s main pumping chambers (ventricles).

Heart Block Analogy

Think of the AV node like a boom gate in the middle of a road that controls traffic. When working normally, it will allow each car to pass through from point A to point V . . . pun intended! However, an issue with the boom gate will result in a roadblock. The same applies to the AV node not allowing the electrical impulse to pass through normally.

The severity of this roadblock determines the degree of the heart block. It could range from merely causing some delayed traffic through the boom gate (first degree), to some traffic getting through and some traffic getting blocked (second degree), or no traffic getting through at all (third degree).

First-degree Heart Block

  • All electrical signals from the atria reach the ventricles, (the impulse is not actually blocked, so the name is a misnomer), but the signal is slowed.
  • First-degree heart block rarely causes symptoms.
  • Causes of first degree AV block can include medications (beta-blockers, calcium channel blockers, digoxin, or antiarrhythmic drugs), (heart attack), chronic degenerative disease of the atrial conduction system (seen with aging), a potassium imbalance, or increased vagal tone (activity of the vagus nerve).
  • Treatment typically aims to correct the underlying cause. If it is caused by drug therapy, then we will adjust or remove the medication. If it is a potassium imbalance, then it needs to be corrected.

Second-degree Heart Block

This is the most complex form of heart block and the most diagnostically challenging. There are four categories of second-degree heart block. Type I (often referred to as Wenckebach), Type II, 2-to-1 heart block, and high-grade heart block.

  • All of these are EKG descriptions that may or may not be diagnostic of a specific site of the electrical block in the heart.
  • The site of the electrical block is what needs to be considered, along with the presence or absence of symptoms, when determining the best treatment.
  • In all types, the impulse originates in the sinus node but is conducted through the AV node intermittently. Not all electrical signals reach the ventricles. Some beats are “dropped,” resulting in a slower and sometimes irregular rhythm. This can lead to a skip in the heartbeat, which can cause dizziness and fainting.

Second-Degree AV Block Type I - Wenckebach

  • Causes can include medications (beta-blockers, calcium channel blockers, digoxin, or antiarrhythmic drugs), coronary artery disease, acute heart attack, a potassium imbalance, or increased vagal tone (activity of the vagus nerve).
  • Treatment is not typically required. Wenckebach phenomenon in the setting of bundle branch disease may require investigation to see if a pacemaker would be appropriate.

Second-Degree AV Block: Type II

  • Type II is associated with a poorer prognosis, and complete heart block may develop.
  • Causes can include age-related degenerative changes in the conduction system, new heart attack, or cardiac surgery or complications arising with cardiac catheterization. Note: Type II is not typically a result of increased vagal tone or drug effects.
  • Treatments with a pacemaker is always recommended due to the poorer prognosis for developing complete heart block.

2-to-1 Heart Block

  • In 2-to-1 heart block, every other sinus beat does not conduct to the ventricle.
  • This is most often given its own classification because it is not specific to one side or the other in the heart.
  • The causes are similar to the above types of second-degree heart block.
  • Treatment depends on symptoms, the presence of reversible causes, and the site of the electrical block (needs to be determined).

High-grade Heart Block

  • This is defined as two or more consecutively non-conducted sinus beats. Causes are similar to those listed above.
  • Treatment depends on symptoms, the presence of reversible causes, and the site of the electrical block (needs to be determined).

Third-degree (Complete) Heart Block

This is the most serious type of heart block. It is also called a complete heart block.

  • This is the most serious type of heart block. It is also called a complete heart block.
  • None of the electrical impulses from the atria reaches the ventricles.
  • When this happens, a natural, subsidiary (escape) pacemaker takes over, but this results in slow and sometimes unreliable electrical impulses to control the beat of the ventricles.
  • Most often, people with complete heart block require a pacemaker due to the unreliable nature of the escape pacemaker.

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