What are the types of atrioventricular (AV) heart blocks and how are they differentiated on electrocardiogram (ECG)?

Heart Blocks: Classification and Differentiation

Heart blocks are classified into first-degree, second-degree (Mobitz Type I and Type II), and third-degree (complete) based on ECG patterns of AV conduction, with differentiation primarily determined by PR interval behavior, QRS width, and the presence or absence of conducted P waves. 1, 2

First-Degree AV Block

• Every atrial impulse conducts to the ventricles but with a prolonged PR interval >200 ms 3, 1
• This represents a delay rather than true "block" since all impulses eventually conduct 1, 4
• When QRS is narrow (<120 ms), the delay is almost always in the AV node; when QRS is wide, the delay may be in either the AV node or His-Purkinje system 3, 5
• Only His bundle electrography can precisely localize the site of delay when QRS is wide 3, 5
• Profound first-degree block (PR >300 ms) can cause pacemaker syndrome-like symptoms due to loss of AV synchrony 1, 4

Second-Degree AV Block

Mobitz Type I (Wenckebach)

• Progressive lengthening of the PR interval occurs until a P wave fails to conduct; the PR interval then shortens after the blocked beat and the cycle repeats 3, 2
• The PR prolongation may be subtle in the last cycles before the blocked P wave and requires comparison with the shortest PR interval (usually after the block) 3
• Narrow-QRS Type I is almost always AV nodal in location and carries a benign prognosis 2, 5
• Wide-QRS Type I requires electrophysiology study to determine if the block is infranodal, which would mandate pacing 2
• Common and benign in athletes and during sleep; monitoring generally not required for asymptomatic patients 3

Mobitz Type II

• The PR interval remains constant before and after the blocked P wave; any PR shortening after the block excludes Type II 3, 2
• Type II block is invariably infranodal (His-Purkinje system) and most often presents with bundle branch block pattern (wide QRS) 2, 6
• Permanent pacemaker implantation is mandatory for all Type II block regardless of symptoms due to high risk of progression to complete heart block 2, 6
• Early studies demonstrated that infranodal block can progress rapidly and unpredictably with risk of sudden death 3

2:1 AV Block

• Cannot be classified as Type I or Type II based on PR behavior alone since there are no consecutive conducted beats to compare 3, 2
• Narrow QRS suggests nodal level (better prognosis); wide QRS suggests infranodal level (worse prognosis) 2, 7
• Electrophysiology study is often required to definitively localize the block and guide pacing decisions 2

Advanced (High-Grade) AV Block

• Multiple consecutive P waves are blocked without complete AV dissociation 3, 2
• Generally indicates infranodal disease and warrants permanent pacing 2

Third-Degree (Complete) AV Block

• No atrial impulses conduct to the ventricles; complete AV dissociation exists with an independent escape rhythm 3, 8
• Narrow QRS escape rhythm (40-60 bpm) indicates junctional or high His-Purkinje origin; wide QRS escape rhythm (20-40 bpm) indicates ventricular origin below the His bundle 5, 8
• Complete heart block with narrow QRS may have block in the AV node or within the His bundle; wide QRS escape usually indicates infra-Hisian block 3, 5
• Permanent pacing improves survival, especially in symptomatic patients 3, 8
• Complete heart block caused by AV nodal disease has a stable junctional escape and is not immediately life-threatening, whereas infranodal block is more dangerous 3

Key Differentiation Algorithm

Step 1: Assess P Wave Conduction

• All P waves conducted with prolonged PR → First-degree block 1
• Some P waves blocked → Second-degree block 3
• No P waves conducted with AV dissociation → Third-degree block 8

Step 2: For Second-Degree Block, Examine PR Interval Behavior

• Progressive PR lengthening before block → Mobitz Type I 2
• Constant PR before and after block → Mobitz Type II 2
• Only one conducted beat visible (2:1 pattern) → Cannot classify; proceed to Step 3 2

Step 3: Assess QRS Width for Anatomic Localization

• Narrow QRS (<120 ms) → Usually AV nodal (better prognosis) 2, 5
• Wide QRS (≥120 ms) → Likely infranodal (worse prognosis, higher pacing indication) 2, 5

Step 4: Consider Clinical Context

• Type II block, wide-QRS Type I, advanced block, and symptomatic complete block require permanent pacing 2
• Narrow-QRS Type I in asymptomatic patients requires observation only 2
• Reversible causes (drugs, electrolytes, Lyme disease, acute MI) should be excluded before permanent pacing 2

Critical Prognostic Distinctions

• The anatomic site of block (nodal vs. infranodal) is more important for prognosis than the degree of block 3, 2
• Untreated chronic infranodal second-degree block has poor prognosis with frequent progression to complete block and syncope 3, 2
• AV nodal blocks generally have benign prognosis dependent on underlying heart disease 3
• Intra-Hisian block has uncertain prognosis but commonly manifests with heart failure and syncope 3, 2

Common Pitfalls

• Do not assume 2:1 block is benign Type I without assessing QRS width and clinical context 2
• Do not delay pacing in Type II block waiting for symptoms—progression can be sudden and fatal 2, 6
• Sleep-related AV block during obstructive sleep apnea does not require pacing unless symptomatic while awake 2
• Nonconducted premature atrial contractions and atrial tachycardia with block can mimic second-degree AV block 6