Complete AV block (third-degree AV block)

For the heart to contract in a coordinated manner, the electrical signal generated in the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. must reach the ventricles.

To read: The electrical system

This passage is only possible through a single checkpoint: the atrioventricular node — your “electrical gatekeeper.”

It is here that the signal must “show its passport” before continuing its path. Under normal conditions, this gatekeeper allows each impulse to pass while protecting the heart from excessively rapid rates. It ensures that the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. and ventricles work together, in the right order and at the right time.

However, this “checkpoint” can be influenced. Some factors may slightly speed up conduction, but many others tend to slow it down. When this slowing becomes more pronounced, or when some impulses no longer pass through, this is referred to as an atrioventricular block (AV block).

Different Types of Atrioventricular Block (AV Block)

AV blocks are classified according to the severity of the delay or interruption:

First-degree AV block: the signal passes, but more slowly
Second-degree AV block: some signals pass, others do not
Third-degree AV block: no signals pass through; the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. and ventricles beat independently

This section focuses on complete AV block — the form in which no impulses pass through the AV node.

Complete AV block: when the road beyond the gatekeeper is completely closed

Complete AV block is very different from first-degree or second-degree AV block.

In this situation, the AV node — the “electrical customs officer” — no longer allows any impulses from the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. to pass through.

This is not simply a delay: transmission is completely interrupted. All electrical impulses generated by the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. are blocked before reaching the ventricles.

A Backup Electrical Impulse

The ventricles, no longer receiving the electrical instruction normally sent from above, must find another solution. They rely on a new source of electrical impulses, located lower down in the conduction system, to take over.

This backup rhythm, called an “escape rhythm,” allows the heart to continue beating, but it is generally slower and much less reliable than the rhythm coordinated by the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles..

Impact on the ECG

On the electrocardiogram (ECG), this loss of communication is striking: the P waves, representing atrial activity, follow their own rhythm, while the QRS complexes, reflecting ventricular activity, follow a completely separate rhythm.

There is no longer any relationship between the two.

This complete lack of connection — a true “electrical divorce” between the upper and lower chambers of the heart — is the hallmark of complete AV block.

Possible causes

Complete AV block occurs when the conduction pathway below the AV node is damaged. Several conditions can affect this “electrical road.”

It may result from disease of the conduction system or from the natural aging of these structures, which makes them less efficient over time. In some cases, scarring or damage from previous heart disease can also interfere with signal transmission.

In more acute situations, such as a myocardial infarction affecting the anterior part of the heart, electrical communication may be abruptly interrupted.

Another possible cause is calcified aortic stenosis. Because this valve is located close to the “electrical gatekeeper” and conduction pathways, calcium deposits may, over time, irritate or compress these essential structures and impair normal signal transmission.

In all cases, this represents a true conduction problem affecting the pathway between the upper and lower chambers of the heart.

Possible symptoms

Symptoms are often more pronounced than in other types of AV block:

dizziness
significant fatigue
weakness or malaise
sensation of a very slow heart rate
episodes of confusion
possible loss of consciousness

To read: Cardiac syncope

Detection of complete AV block

Complete AV block is usually identified through an ECG, a Holter monitor, or prolonged monitoring in the context of dizziness, fainting, or syncope.

With the increasing availability of smartwatches capable of recording electrical tracings, early signs may sometimes be detected using these devices.

When the recordings are of sufficient quality, they may clearly show the dissociation between P waves and QRS complexes, raising suspicion of complete AV block.

However, even when such technology suggests this diagnosis, confirmation must always be made using medical-grade recordings, as proper interpretation and management require specialized expertise.

Management

Once diagnosed, complete AV block requires intervention, as the ventricles no longer receive signals from the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. and rely on a backup rhythm that is often too slow or unstable.

Management generally involves the implantation of a pacemaker, which ensures a regular and reliable heart rhythm.

In certain specific situations — for example, after a reversible acute event (medications, recent myocardial infarction, cardiac surgery) — the condition may be temporary. However, it still requires close monitoring in a hospital setting.