Patrick J. Tchou, MD, talks about blocked impulses in the heart

What is heart block?

The heartbeat is created by an electrical signal that starts in the heart’s upper right chamber (right atrium). The signal is produced in an area of specialized cells in the atrium called the sinus node.

The electrical signal moves down through the heart to the atrioventricular (AV) node, another cluster of specialized cells that is located in the center of the heart between the atria and ventricles. The AV node is sometimes referred to as an electrical relay station because its function is to slow the electrical current before it passes to the lower chambers of the heart (ventricles). From the AV node, the electrical current travels to the ventricles along special fibers embedded in the heart walls. When the current arrives in the ventricles, they contract and pump blood out to the body.

In people with heart block, also called AV block, the electrical signal that controls the heartbeat is partially or completely blocked from reaching the ventricles.

What are the different types of heart block?

Heart block is classified as first-, second- or third-degree, depending on the extent of electrical signal impairment.

In first-degree heart block, the electrical impulse moves more slowly than normal through the AV node but it still conducts each signal. This condition is common in highly trained athletes. It can also be caused by drugs, particularly those that slow electrical impulse conduction through the AV node, such as beta-blockers, diltiazem, verapamil, digoxin and amiodarone.

Second-degree heart block is broken down into two categories: Type I and Type II.

Type I heart block (also called Mobitz Type I or Wenckebach's AV block) is the less serious form of second-degree heart block. In this condition, the electrical signal goes slower and slower until the heart actually skips a beat.

In patients with Type II heart block (also called Mobitz Type II), some of the electrical signals do not reach the ventricles, and the pattern is irregular. Individuals with this type of heart block may have a heartbeat that is slower than normal. The area that is blocked is lower in the conduction system and is often associated with more severe conduction disease.

In patients with third-degree (complete) heart block, the electrical signal is not sent from the atria to the ventricles. The heart compensates by producing electrical signals from a specialized pacemaker area in the ventricles. These signals make the heart contract and pump blood, but at a rate that is much slower than normal.

What are the symptoms of heart block?

First-degree heart block often does not cause symptoms. It may be detected during a routine electrocardiogram (ECG/EKG), but the patient’s heart rate and rhythm are usually normal.

Symptoms of second- and third-degree heart block include fainting, dizziness, fatigue, shortness of breath and chest pain. In third-degree heart block, the symptoms reflect the severity of the slow heart rate. In some cases this may be dangerous and need immediate medical attention.

What causes acquired heart block?

Acquired heart block has many possible causes, including heart attack (the most common cause), heart disease, an enlarged heart (cardiomyopathy), heart failure and rheumatic fever. Sometimes heart block occurs as a result of injury to the heart during open heart surgery, as a side effect of some drugs, or after exposure to a toxin.

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