The heart is comprised of four chambers: two upper chambers (atria) and two lower chambers (ventricles). The
sinoatrial (SA) node, located near the top of the right atrium, produces electrical signals that are sent to the
atrioventricular (AV) node.
The AV node then sends the signals to the ventricles, which are the primary pumping chambers of the heart. When the heart is functioning well, the electrical signals are transmitted smoothly from the atria to the ventricles, causing rhythmic muscle contractions that pump blood to the rest of the body.
Heart block occurs when the electrical signals from the sinoatrial node are too slow. A heart block does not mean that blood is being blocked from the heart.
There are three types of heart block, ranging from mild to serious:
- First-degree heart block—This is the mildest form of heart block. In this case, the electrical signals from the SA node move more slowly than normal to the AV node, but all signals reach the ventricles. There are usually no symptoms, and heartbeat and rhythm are normal. This type of heart block is often found in well-trained athletes.
Second-degree heart block—A second-degree heart block means that some of the electrical signals are not reaching the ventricles. This causes “dropped beats.” There are two types of second-degree heart block:
- Type I second-degree heart block (also called Mobitz Type I or Wenckebach’s AV block)—The electrical signals become increasingly delayed with each heartbeat, ultimately causing a beat to be missed.
- Type II second-degree heart block (also called Mobitz Type II)—In this type of heart block, some of the electrical signals do not reach the ventricles. This is less common, but more serious.
- Third-degree, or complete, heart block—This is the most serious type of heart block. In this condition, no electrical signals are able to reach the ventricles. The ventricles compensate by contracting on their own, but at a much slower rate than is safe for the heart to maintain full function.
Third-degree heart block is extremely serious and requires immediate care from your doctor. First- and second-degree heart block should be diagnosed by your physician, who will help you determine the best course of treatment, if any.
Anatomy of the Heart
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