Atrioventricular (AV) block is one of the most important conduction abnormalities encountered on ECG. Correctly identifying the pattern—particularly distinguishing Mobitz I from Mobitz II—has major implications for prognosis, monitoring and pacemaker therapy.
The key is to systematically assess the relationship between P waves, PR intervals and QRS complexes.
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1. What is AV Block?
AV block occurs when conduction of electrical impulses from the atria to the ventricles is delayed or interrupted at the AV node, His bundle or Purkinje system.
It is broadly classified as:
1. First-degree AV block
2. Second-degree AV block
Mobitz I / Wenckebach
Mobitz II
2:1 AV block
High-grade/advanced AV block
3. Third-degree AV block / complete heart block
The ECG diagnosis depends primarily on identifying:
P-wave behavior → PR interval → conducted/dropped QRS complexes → atrial–ventricular relationship.
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2. First-Degree AV Block
ECG features
The defining feature is:
PR interval >200 ms with 1:1 AV conduction.
Every P wave is followed by a QRS complex, but conduction through the AV system is prolonged.
ECG appearance
P → prolonged PR → QRS
P → prolonged PR → QRS
P → prolonged PR → QRS
There are no dropped beats.
Important point
Although traditionally called "first-degree AV block," this is more accurately described as first-degree AV delay, because no P wave actually fails to conduct.
Causes
Common causes include:
Increased vagal tone
Ξ²-blockers
Verapamil/diltiazem
Digoxin
Antiarrhythmic drugs
Inferior MI
Myocarditis
Degenerative conduction disease
Increased AV nodal conduction time in athletes
Clinical significance
Usually benign, particularly in patients without structural heart disease.
However, marked PR prolongation, particularly ≥300 ms, can produce symptoms resembling pacemaker syndrome because atrial contraction occurs too early relative to ventricular contraction.
Management
Usually:
No treatment required.
Management should focus on the underlying cause and symptoms.
Pacemaker therapy is generally considered only when significant symptoms can be attributed to the AV delay or when there are other important conduction abnormalities.
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3. Second-Degree AV Block
In second-degree AV block, some P waves conduct and some do not.
Therefore:
P waves > QRS complexes
The major challenge is determining whether the block is Mobitz I or Mobitz II.
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4. Mobitz Type I — Wenckebach
Classic ECG pattern
The hallmark is:
Progressive PR prolongation → dropped QRS → reset
For example:
P–QRS
P–longer PR–QRS
P–even longer PR–QRS
P–no QRS
P–QRS
The PR interval progressively lengthens until one P wave fails to conduct.
The important concept
"Longer, longer, longer, drop."
This is one of the easiest ways to remember Mobitz I.
Additional ECG findings
You may see:
Grouped beating
Narrow QRS, particularly when the block is at the AV node
Shortest PR after the dropped beat
Progressive PR prolongation before the dropped beat
Site of block
Mobitz I is most commonly AV nodal.
However, an important electrophysiological caveat is that Mobitz I can occasionally be infranodal, particularly when associated with a wide QRS.
Causes
Common causes include:
Increased vagal tone
Inferior myocardial infarction
Ξ²-blockers
Calcium-channel blockers
Digoxin
Sleep
Post-procedural states
AV nodal disease
Prognosis
AV nodal Mobitz I generally has a relatively benign course and is less likely to suddenly progress to complete heart block.
Management
If asymptomatic and clearly nodal:
Usually observation is sufficient.
If symptoms are attributable to the AV block, pacing may be considered.
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5. Mobitz Type II
Mobitz II is more concerning.
ECG definition
The key feature is:
Constant PR intervals before the dropped QRS.
There is sudden failure of conduction without progressive PR prolongation.
Example:
P–QRS
P–QRS
P–QRS
P–no QRS
P–QRS
The PR intervals of conducted beats remain essentially constant.
Memory aid
Mobitz II = PR stays the same, then suddenly drops.
Or:
"Same, same, same → DROP."
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6. Why Mobitz II Is Dangerous
Mobitz II usually indicates disease below the AV node, particularly within the His-Purkinje system.
The conduction system can deteriorate suddenly, resulting in:
Mobitz II → high-grade AV block → complete heart block
The ventricular escape rhythm may also be slow and unreliable.
Therefore, Mobitz II should be taken much more seriously than typical nodal Mobitz I.
ECG clues suggesting infranodal disease
Look for:
Wide QRS
Bundle-branch block
Bifascicular block
Alternating bundle-branch block
Structural heart disease
However, a narrow QRS does not completely exclude infranodal disease.
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7. 2:1 AV Block — An Important Pitfall
In 2:1 AV block:
One P wave conducts → one P wave does not conduct.
So the ECG shows:
P–QRS → P–DROP → P–QRS → P–DROP
The problem is that you cannot reliably classify this as Mobitz I or Mobitz II from the surface ECG alone because there is only one conducted PR interval between blocked P waves.
Therefore:
Never automatically call 2:1 AV block "Mobitz II."
The level of block should be determined using the clinical context, QRS morphology and, when necessary, additional testing/EPS.
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8. High-Grade / Advanced AV Block
High-grade AV block occurs when ≥2 consecutive P waves fail to conduct, while some AV conduction is still present.
For example:
P–QRS → P–QRS → P–P–DROP–DROP → P–QRS
This is different from complete heart block because there is still some evidence of AV conduction.
High-grade AV block is generally considered clinically significant and often reflects advanced His-Purkinje disease.
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9. Third-Degree AV Block — Complete Heart Block
This is the most severe form.
ECG hallmark
There is:
Complete AV dissociation.
The atria and ventricles depolarize independently.
What does this mean?
P waves occur at their own rate.
QRS complexes occur at their own escape rhythm.
There is no consistent relationship between P waves and QRS complexes.
Typical ECG pattern
P P P P P
QRS QRS QRS
The PR interval changes continuously because the atria and ventricles are not electrically linked.
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10. Escape Rhythm in Complete Heart Block
The ventricular escape rhythm is extremely important.
Junctional escape
Usually:
Narrow QRS
Rate approximately 40–60/min
This suggests an escape originating around the AV junction/His region.
Ventricular escape
Usually:
Wide QRS
Rate approximately 20–40/min
This represents a more distal escape and is generally less reliable.
A slow, wide-complex escape in complete heart block is particularly concerning.
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11. How to Differentiate Mobitz I vs Mobitz II
Feature Mobitz I Mobitz II
PR interval Progressively increases Constant
Dropped QRS Yes Yes
Pattern Progressive PR prolongation → drop Sudden drop
Typical site AV node His-Purkinje
QRS Often narrow Often wide
Prognosis Usually more benign Potentially dangerous
Risk of progression Lower if nodal High
Pacemaker Usually symptom-driven Generally indicated if not reversible
The ACC/AHA/HRS guideline recommends permanent pacing for acquired Mobitz II, high-grade AV block or complete AV block when not due to a reversible or physiologic cause, regardless of symptoms.
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12. The Most Important ECG Approach
When you see suspected AV block, use a systematic approach.
Step 1 — Find the P waves
Ask:
Are P waves present and regular?
Don't mistake T waves or artifact for P waves.
Step 2 — Determine the atrial rate
Is the atrial rhythm regular?
Step 3 — Look at every PR interval
Ask:
Is PR constant or progressively increasing?
Step 4 — Look for dropped QRS complexes
Does a P wave occur without a subsequent QRS?
Step 5 — Determine the pattern
PR progressively longer → drop = Mobitz I
PR constant → sudden drop = Mobitz II
Step 6 — Assess the QRS
A wide QRS increases suspicion for infranodal disease, although it is not diagnostic by itself.
Step 7 — Look for AV dissociation
If P waves and QRS complexes have independent rhythms:
Complete heart block.
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13. First-Degree vs Second-Degree vs Complete Heart Block
A simple way to remember:
First degree
Everything conducts, but slowly.
P → QRS
P → QRS
P → QRS
Second degree
Some conduct, some don't.
P → QRS
P → QRS
P → DROP
Complete heart block
Nothing conducts from atria to ventricles.
P P P P P
QRS QRS QRS
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14. Causes of AV Block
Think of causes under several categories.
Drugs
Ξ²-blockers
Verapamil
Diltiazem
Digoxin
Some antiarrhythmic drugs
Ischemia
Inferior MI → commonly nodal AV block
Anterior MI → more concerning for extensive His-Purkinje disease
Degenerative conduction disease
Age-related fibrosis
Lev/Lenègre disease
Inflammatory/infiltrative disease
Myocarditis
Sarcoidosis
Amyloidosis
Lyme disease
Metabolic
Hyperkalemia
Hypoxia
Severe metabolic disturbances
Increased vagal tone
Sleep
Athletes
Vasovagal episodes
Carotid sinus stimulation
Iatrogenic
Cardiac surgery
TAVI
Septal procedures
Catheter ablation
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15. AV Block in Acute MI
The clinical context matters greatly.
Inferior MI is commonly associated with nodal AV block and Mobitz I. This often improves with reperfusion or resolution of ischemia.
Anterior MI, particularly when associated with new bundle-branch disease and advanced AV block, raises much greater concern for extensive His-Purkinje injury.
In acute MI, persistent Mobitz II, high-grade AV block or third-degree AV block—particularly when infranodal—can require pacing.
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16. When Is a Pacemaker Required?
The most important principle is:
Reversible cause first
Before committing to permanent pacing, assess for reversible or physiologic causes such as:
Drug toxicity
Electrolyte abnormalities
Acute ischemia
Increased vagal tone
Other reversible conditions
For acquired Mobitz II, high-grade AV block or complete AV block not attributable to a reversible or physiologic cause, permanent pacing is recommended regardless of symptoms.
For first-degree AV block and typical AV nodal Mobitz I, pacing is generally symptom-driven.
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17. Important ECG Pitfalls
Pitfall 1: Calling every dropped beat Mobitz II
A dropped QRS alone does not equal Mobitz II.
Always examine the preceding PR intervals.
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Pitfall 2: Calling 2:1 block Mobitz II
You generally cannot distinguish Mobitz I from Mobitz II when every alternate P wave is blocked using the surface ECG alone.
Determine the likely level of block.
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Pitfall 3: Assuming narrow QRS means benign
Most nodal blocks are narrow, but infranodal disease can occasionally present with a narrow QRS.
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Pitfall 4: Missing concealed P waves
A nonconducted P wave may be hidden within:
T wave
ST segment
Previous QRS/T complex
Careful rhythm-strip analysis is essential.
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Pitfall 5: Mistaking AV dissociation for complete heart block
AV dissociation does not always equal complete AV block.
For example, an accelerated junctional rhythm can produce AV dissociation without complete AV block.
The key question is:
Is there evidence of independent atrial and ventricular rhythms with failure of AV conduction?
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18. High-Yield Exam Pearls
First-degree AV block
PR >200 ms + every P conducts.
Mobitz I
Progressive PR prolongation → dropped QRS.
Mobitz II
Constant PR → sudden dropped QRS.
2:1 AV block
Cannot reliably classify as Mobitz I or II from the surface ECG alone.
High-grade AV block
≥2 consecutive nonconducted P waves with some AV conduction remaining.
Complete heart block
No relationship between P waves and QRS complexes.
Wide QRS + Mobitz II
Think:
Infranodal disease → high risk of progression.
Mobitz II / high-grade / complete AV block
Think:
Pacemaker—after excluding reversible causes.
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Conclusion
The diagnosis of AV block becomes much easier when you stop looking at the ECG as a collection of individual beats and instead examine the relationship between P waves, PR intervals and QRS complexes.
The simplest framework is:
First degree:
All P waves conduct, PR prolonged.
Mobitz I:
PR gets longer → QRS drops.
Mobitz II:
PR stays constant → QRS suddenly drops.
Complete heart block:
P waves and QRS complexes have no relationship.
Recognizing these patterns is not merely an ECG exercise—it directly influences the urgency of monitoring, the need to investigate reversible causes and the decision regarding permanent pacing.
Key guideline references
2018 ACC/AHA/HRS Guideline on Bradycardia and Cardiac Conduction Delay
2021 ESC Guidelines on Cardiac Pacing and CRT
ESC guidance on AV block in acute myocardial infarction
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