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 text focuses on second-degree AV block and explains its two main types.
Second-degree AV block
Second-degree atrioventricular block occurs when the transmission of the electrical signal between the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. and ventricles becomes inconsistent.
Unlike first-degree AV block — where all impulses eventually pass through the AV node — here, some impulses are conducted while others are not.
Two distinct mechanisms can be observed:
- Type 1 (Mobitz I)
- Type 2 (Mobitz II)
Type 1 – Mobitz I (Wenckebach)
In Mobitz I, the atrioventricular node — our “electrical gatekeeper” — gradually slows its conduction.
A gatekeeper that “gets tired” and progressively slows down
From one heartbeat to the next, the time required for the impulse to pass increases progressively.
Eventually, when the delay becomes too long, one impulse is not transmitted: a P wave appears on the ECG without an associated QRS complex. The atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. contract, but the ventricles are not stimulated.
Then the cycle repeats: conduction resumes, the delay lengthens again, and another dropped beat occurs.
This cyclical pattern is typical of an AV node that is influenced but structurally normal.
Influences on the AV node (Mobitz I)
Mobitz I is almost always related to external influences acting on a healthy AV node.
-Nervous system influence (parasympathetic)
The parasympathetic nervous system slows conduction through the AV node.
- In adolescents or young adults, increased vagal (parasympathetic) tone often explains this type of block
- During vasovagal episodes, nausea, pain, or abrupt cessation of effort, a transient slowing may lead to temporary Mobitz I
-Medication effects
Certain medications naturally slow conduction through the AV node:
- beta-blockers
- some calcium channel blockers
- other agents that affect conduction
Usually without symptoms
Most often, there are no symptoms.
When present, they are generally mild:
- brief sensation of a missed beat
- feeling of a pause
- transient fatigue
- lightheadedness
Incidental discovery
Mobitz I is often discovered incidentally:
- during an ECG performed for another reason
- on a Holter monitor
- or during an exercise test
An important point: when the heart rate increases or sympathetic tone rises, AV conduction improves. Mobitz I often disappears under these conditions, with normalization of conduction.
This typically indicates a normal physiological response rather than disease of the cardiac conduction system.
Follow Up
Mobitz I is generally benign.
Minimal follow-up is sufficient in most cases, especially when associated with:
- vagal influence
- adolescence
- known medication effects
Type 2 - Mobitz II
The customs officer is not the problem: it is his exit route that is damaged.
Mobitz II is very different in nature from Mobitz I.
Here, the AV node functions normally. The conduction time between the atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. and ventricles remains constant: the PR interval on the ECG does not become progressively longer.
Then, without warning, an impulse is suddenly blocked. A P wave appears, but no QRS complex follows. The atriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. do their job, but the ventricles are not stimulated.
Unlike Mobitz I, this block is not preceded by progressive slowing. Instead, the problem occurs just after the impulse has passed through the AV node, along the electrical pathway that carries the signal toward the ventricles.
This location is called infranodal. To use our analogy, the problem is therefore in the “driveway,” the customs officer’s exit route.
It is an abnormality of the electrical system itself, rather than the temporary influence of an external factor.
The Most Common Causes
This problem may be caused by:
- aging of the conduction system;
- electrical scarring;
- disease of the electrical system.
The system then becomes unable to conduct impulses consistently, even though the AV node is functioning normally.
Possible symptoms
Symptoms may be more pronounced:
- dizziness
- significant fatigue
- sensation of skipped beats
- episodes of weakness
- sometimes brief loss of consciousness
To read: Cardiac syncope
Detection of Mobitz II
Often identified through:
- ECG
- Holter monitoring
- or prolonged monitoring in the context of dizziness or fainting
Follow Up
Closer clinical monitoring is recommended. Holter monitoring may be repeated periodically.
This condition carries a risk of progression to a more advanced AV block.
To read: Holter monitoring
Possible treatments for Mobitz II
Depending on the situation:
- specialized evaluation
- close follow-up
- and in many cases, a pacemaker may be considered to stabilize conduction
Important recommendation
Urgent medical evaluation is recommended in the event of sudden loss of consciousness. In such situations, pacemaker implantation may be indicated.
Simple summary
- Mobitz I:
- progressive slowing at the AV node
- often benign
- related to vagal or medication influences
- symptoms mild or absent
- Mobitz II:
- disease of the conduction system
- sudden blockage
- more serious condition
- may require a pacemaker
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