Core Concept: The PR Interval Baseline
Before classifying Atrioventricular (AV) blocks, establish the precise clinical measurement standard:
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PR Interval Measurement: Measured from the onset of the P wave to the onset of the QRS complex (representing total time from atrial depolarization through AV node conduction to ventricular depolarization).
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Normal Range: $0.12\text{ s} \text{ to } 0.20\text{ s}$ (3 to 5 small boxes).
An AV block occurs when electrical impulse transmission from the atria to the ventricles is delayed, intermittently blocked, or completely disrupted at the AV node or His-Purkinje system.
■ The 4 Types of AV Block
1. 1st Degree AV Block
• Pattern: PR > 0.20 s, strictly CONSTANT. 1:1 P:QRS conduction.
• Key: Benign & asymptomatic. Caused by high vagal tone, aging, or AV nodal blockers (BB, CCB, Digoxin).
2. 2nd Degree Mobitz I (Wenckebach)
• Pattern: Progressive PR lengthening (PR1 < PR2 < PR3) until a QRS is DROPPED.
• Key: AV node fatigue. Often transient (Inferior MI / RCA). Monitor closely.
3. 2nd Degree Mobitz II
• Pattern: CONSTANT PR interval in conducted beats + Sudden DROPPED QRS.
• Key: Infranodal block (His-Purkinje). High risk of arrest! Ineffective with Atropine -> Requires Pacing/Pacemaker.
4. 3rd Degree AV Block (Complete Heart Block)
• Pattern: Complete AV Dissociation. P-P regular, R-R regular, but completely independent.
• Key: Medical Emergency! Severe hypotension & syncope -> Emergent Pacing & Permanent Pacemaker.
| AV Block Type | PR Interval Behavior | Conduction Ratio | Primary Anatomic Location | Clinical Management |
|---|---|---|---|---|
| 1st Degree | Prolonged (> 0.20 s), Constant | 1 : 1 | AV Node | Monitor / Adjust meds |
| 2nd Degree Mobitz I | Progressive lengthening before drop | Variable | AV Node | Monitor / Treat underlying cause |
| 2nd Degree Mobitz II | Constant in conducted beats | Intermittent drop (2:1, 3:1) | His-Purkinje System | Pacing / Permanent Pacemaker |
| 3rd Degree (Complete) | Independent (AV Dissociation) | None (Independent rates) | AV Node or His-Purkinje | Emergent Pacing / Permanent Pacemaker |
■ Board Clinical Pearls
1. Atropine Response: Works on AV node (Vagus). Effective for 1st Degree & Mobitz I. Ineffective for Mobitz II & 3rd Degree.
2. MI Association:
• Inferior MI (RCA) -> AV Node Ischemia -> Mobitz I (Wenckebach)
• Anterior MI (LAD) -> Septal Infarction -> Mobitz II or 3rd Degree Block