Last updated November 16, 2025
Introduction
Second-degree AV block occurs when some atrial impulses (P waves) conduct to the ventricles, and others do not resulting in intermittently dropped QRS complexes. There are two classic types of second degree av block:
- Mobitz Type I (Wenckebach)
- Mobitz Type II
Electrophysiology: Location matters
Block Type | Anatomical Site | Conduction tissue behavior | Clinical implication |
Mobitz I (Wenckebach) | AV node | Progressive fatigue → dropped beat | Usually benign, may not need pacing |
Mobitz II | His–Purkinje system | Sudden conduction failure | High risk |
- The diseased AV node has decremental conduction, the more it’s stimulated, the slower it conducts.
- The His–Purkinje system does not have decremental properties, it either conducts normally or it fails abruptly.
- Because AV nodal disease is usually functional and reversible, Mobitz I tends to be benign; His–Purkinje disease is structural and progressive, so Mobitz II has a high risk of deterioration to complete heart block
Mobitz Type I (Wenckebach)
Progressive prolongation of the PR interval until a P wave fails to conduct leading to dropped QRS complex
ECG Findings
Feature | Finding |
PR interval | Progressively lengthens beat-to-beat |
Dropped beat | Occurs after longest PR |
R-R interval | Pattern of grouped beating |
QRS width | Usually narrow (normal His-Purkinje conduction) |
Ratio | Commonly 3:2, 4:3, 5:4, etc. |
Pathophysiology
- Impaired AV nodal conduction due to functional fatigue.
- May be vagally mediated (sleep, young athletes).
- Can be medication-related (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, other AV nodal–blocking agents).
ECG Example: Mobitz I
Mobitz Type 2
Intermittent failure of His-Purkinje conduction resulting in non-conducted P waves without prior PR prolongation.
ECG Findings
Feature | Finding |
PR interval | Constant |
Dropped beats | Abrupt and unpredictable |
QRS width | Often wide (bundle branch involvement) |
Rhythm | May be regular or irregular depending on conduction ratio. In a fixed-ratio conduction (e.g., 3:1), the ventricular rate can appear regular even though multiple P waves are non-conducted.” |
Pathophysiology
- Structural disease of the His-Purkinje system.
- Unstable rhythm with high risk of progression to complete heart block.
ECG Example: Mobitz II
2:1 AV Block - How to tell Type 1 or Type 2?
When you have 2:1 conduction, every other P wave is blocked. Because every other beat is dropped, you cannot see PR interval progression, and you cannot see constant PR with random drops, so you cannot directly classify it as Mobitz I or Mobitz II from the ratio alone.
Some clues that may help distinguish the two types when it presents as a 2:1 are listed below:
Feature | Suggests Mobitz I | Suggests Mobitz II |
QRS width | Narrow | Wide |
PR on conducted beats | Long | Normal |
Clinical setting | Vagal / meds | Structural heart disease / ischemia |
Response to atropine | May Improve | No change, may worsen |
ECG Example: 2:1 2nd degree av block, narrow qrs, HR ~50s
ECG Example: 2:1 2nd degree av block, wide qrs, HR 30-40s
Wide QRS in a 2:1 block = treat as Mobitz II until proven otherwise. Why? AV nodal disease preserves intraventricular conduction → the QRS is usually narrow. His–Purkinje disease affects the ventricular conduction system itself → damage there often produces a wide QRS.
Grouped beats and differential diagnosis
“Grouped beats” means QRS complexes appear in repeating clusters separated by pauses, but may not always indicate 2nd degree av block. Consider the alternative diagnoses:
Mimic | Distinguishing Feature | Key Clue on ECG |
SA Exit Block | Impulse fails to leave the SA node → no P wave at all | Pause equals a multiple of the normal P-P interval |
Sinus Pause/Arrest | Sinus node stops firing entirely | Pause not a multiple of P-P; “pause too long”. Sinus pause is < 3s, arrest ≥ 3s. |
Atrial Premature Beat (Blocked PAC) | Early P wave (often deforming or buried in the T wave) that reaches the AV node while it is still refractory → that P wave does not conduct → appears like a “dropped” QRS. | Careful zoom shows a premature, morphologically different P wave riding on the T wave; the pause is usually non-compensatory (total pause < 2× basic P–P interval) |
Bigeminy (Atrial or Ventricular) | Alternating normal + premature beat | Coupled pattern; compensatory pause after PVC |
Sinus Arrhythmia | Variable P-P and R-R intervals | P-P interval variable |
High-grade AV Block | 2+ dropped P waves consecutively | More than one non-conducted P |
Example: Sinus rhythm becoming 2:1 SA Exit Block
Example: Sinus bradycardia with sinus pause
Example: Atrial Bigeminy
Example: Blocked PAC - note superimposed PAC in T wave
Example: Ventricular bigeminy
Example: Sinus arrhythmia
Example: High grade av block
Basic Algorithm to AV Blocks
Below is a basic approach to 2nd degree av nodal diagnosis when evaluating a rhythm with grouped QRS beats.
Step 1: Evaluate the P waves
- Are there P waves?
- Do they have similar morphology in each lead?
- Is the P wave axis normal? (Up in II, III, avF and down in aVR)
- If all yes - this is likely a sinus rhythm and proceed to step 2
- If no - consider alternative diagnosis (see differential diagnosis section).
Step 2: Evaluate the PR interval
- Is the PR interval fixed? consider Mobitz 2 or high grade av block
- Is the PR interval progressively getting longer? consider Mobitz 1
- Is the PR interval varying with no consistent relationship between P waves and QRS complexes? consider 3rd degree av block
Step 3: What is the P:QRS ratio?
- 1:1 with dropped beats - consider SA Block
- P > QRS by 1 (eg, 2:1, 3:2, 4:3, 5:4) - consider 2nd degree av block
- P > QRS by 2 or more (3:1, 4:1, etc) - consider high grade av block
Step 4: if P:QRS ratio is 2:1
- Is the QRS wide? consider mobitz 2
- is the QRS narrow? consider mobitz 1 but assume mobitz 2 if unsure
Practice
ECG Example
ECG Example
ECG Example
ECG Example
Related Posts and Practice
- ECG Exercise 14 - Identify the AV Block
- ECG Exercise 15 - Premature Contraction
- ECG Exercise 16 - Bradycardias
- ECG Topic - PR interval
- ECG Topic - 1st degree AV Block
Sources
- Garcia TB. Arrhythmia Recognition: The Art of Interpretation. 2nd ed. Jones & Bartlett Learning; 2015.
- Garcia TB. 12-Lead ECG: The Art of Interpretation. 2nd ed. Jones & Bartlett Learning; 2013.
- Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice. 6th ed. Saunders Elsevier; 2008.
- Brady WJ, et al. Critical Decisions in Emergency and Acute Care Electrocardiography. Wiley-Blackwell; 2009.
- Davila E. The ECG. 2024.
- Burns E, Buttner R. AV Block. Life in the Fast Lane (LITFL). Updated 2024.
- emDocs. AV block: diagnosis and management in the ED. emDocs.net; 2024.
- Klein LR. REBEL EM: AV conduction disturbances. 2024.
This post is for education and not medical advice.