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 — the “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 first-degree AV block.
First-degree AV block
First-degree atrioventricular block is the simplest and most common form of conduction disturbance between the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. and the ventricles.
In this situation, the electrical signal originating in the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. still reaches the ventricles, but it takes slightly longer than usual to pass through the atrioventricular node, our “electrical gatekeeper.”
Each impulse successfully crosses this checkpoint: nothing is blocked, but conduction is slowed.
On the electrocardiogram (ECG), this appears as a prolongation of the PR interval — that is, the delay between the P wave and the beginning of the QRS complex.
It is therefore not a true blockage, but rather a delay in conduction. This condition differs from second- or third-degree AV block, where some — or all — impulses fail to pass through the AV node.
A single electrical pathway influenced by multiple factors
The AV node, the only pathway connecting the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. and ventricles, can be influenced by several factors.
-Nervous system influence
Electrical conduction through the AV node is modulated by the balance between the sympathetic nervous system (which facilitates conduction) and the parasympathetic system (which slows it).
- In adolescents, increased parasympathetic tone often explains a slightly longer conduction time
- During nausea or vomiting, stronger parasympathetic stimulation may lead to discomfort, weakness, or a feeling of near-fainting. This may be associated with a temporary slowing of conduction and a transient first-degree AV block
-Medication effects
Certain medications slow electrical conduction through the AV node. The most common include:
Involvement of the conduction system
More rarely, the slowing is due to a problem within the electrical system itself: the “pathway” is affected, and this alteration slows the transmission of the signal to the ventricles.
No symptoms
This condition generally does not cause any symptoms.
The heart remains well coordinated: the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. and ventricles continue to contract in the proper sequence, with a regular rhythm.
Incidental finding
First-degree AV block is most often discovered incidentally, during an ECG performed for another reason.
Follow Up
In most cases, no specific follow-up is required, as this is a benign finding.
When the cause of the delay is not clear, periodic ECG monitoring may be suggested.
Important recommendation
Urgent medical evaluation is recommended in the event of sudden loss of consciousness.
In very rare situations, if worsening conduction is observed, implantation of a pacemaker may become necessary.
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