Heart Block or Atrioventricular (AV) Block

Atrioventricular (AV) block

Heart block describes a group of abnormally slow heart rhythms with different degrees of blockage of the electrical signal from the top chamber of the heart (atrium) to the bottom chamber of the heart (ventricle).

1st degree heart block (1st degree AV block) describes an increase in the length of time it takes for the signal to pass from the atrium to the ventricle. This heart rhythm can have a genetic cause or may be due to medications. Sometimes, it is found in athletes. This condition rarely causes symptoms or problems.

2nd degree heart block (2nd degree AV block) comes in two forms:

Mobitz Type I or Type I second degree heart block (also called Wenckebach): describes a beat-to-beat increase in the length of time that the signal takes to get from the atrium to the ventricle. Eventually, this results in a missed beat from the ventricle. Sometimes, this heart rhythm is normal and can occur during sleep, but other times it can cause symptoms such as lightheadedness or dizziness.

Mobitz Type II or Type II second-degree heart block : describes missed beats in the ventricle, although the length of time the signal takes to get from the atrium to the ventricle remains normal. This heart rhythm is uncommon and can cause such symptoms as lightheadedness, dizziness and/or passing out, or it can occur without symptoms.

3rd degree heart block (3rd degree AV block) is also called a complete heart block. With this condition, signals from the atrium do not reach the ventricle at all. Some people have the ability to generate signals from the bottom chamber regardless of the lack of signal from the top chamber (an “escape” rhythm), but the signal is often very slow and does not stimulate adequate heart muscle pumping function. This heart rhythm can be life threatening in those who do not have an underlying “escape” rhythm.


For over a century, a leader in patient care, medical education and research, with expertise in virtually every specialty of medicine and surgery.

About BWH