What is an AV block?

AV block is a rhythm disorder that makes the heart beat too slowly. It is caused by problems with the way electrical signals are passed on in the heart, and may occur temporarily or all the time.

A healthy heart beats in time to its main pacemaker: the sinus node in the right atrium. At rest, it sends 60 to 80 electrical signals per minute, which spread across both atria. The AV node receives the individual signals and sends them along the conduction system, eventually causing the ventricles to contract.

In AV block, there are problems with the way these signals are passed on to the ventricles. Sometimes these are major problems and sometimes they are only minor.

In mild (first-degree) AV block, the signals from the sinus node are passed on only slightly later than usual. The heart still beats rhythmically and at normal speed, so it's typically not noticeable.

In second-degree AV block, the signals aren't passed on at all for short amounts of time. As a result, not all of the signals from the atria arrive at the ventricles. The heart regularly skips a beat – for instance, every fourth beat.

This can lead to symptoms such as palpitations or dizziness, and a pacemaker may be needed.

In the most severe form (known as third-degree AV block), the signals are completely blocked. None of the signals from the sinus node arrive in the ventricles. Instead, the AV node takes on the role of the pacemaker.

But then the heart only beats very slowly, at a rate of 40 beats per minute. As a result, the heart can no longer supply itself or other organs with enough blood.

The possible effects of this include heart failure, fainting spells and cardiac arrest. People with complete AV block typically need to have a pacemaker.

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