Overview
- Pattern of varying PR intervals
- Progressive PR interval lengthening with intermittent dropped beads
- Due to a reversible conduction block at the level of the AV node (i.e. AV nodal crest or atria-AV node junction)
- Malfunctioning AV nodal cells tend to progressively fatigue until failing to conduct an impulse (i.e. injured AV node with a long refractory period); this is different to cells of the His-Purkinje system which tend to fails suddenly and unexpectedly
ECG Features
- PR interval progressively increases with each successive beat until non-conducted beat
- “Longer-longer-longer-drop, that’s a sign of Wenckebach”
- Longest PR interval is immediately before the dropped beat, while the shortest PR interval is immediately after the dropped beat
- Relatively constant PP interval, but RR interval progressively shortens with each cycle
- Greatest increase in PR interval duration is often between the first and second beats of the cycle
- Wenckebach pattern tends to repeat in P:QRS groups with ratios of X:X-2 (e.g. 3:2, 4:3, 5:4)
Causes
- Cardiac: Inferior MI, cardiac surgery (e.g. mitral valve repair, Tetralogy of Fallot repair), myocarditis (e.g. Lyme disease)
- Medications: AV nodal blocking agents (e.g. beta-blockers, calcium channel blockers, digoxin, amiodarone)
- Other: Increased vagal tone (e.g. athletes), hyperkalemia, normal variant
Clinical Significance
- Often benign with little-to-no hemodynamic instability
- Low risk of progressing to third-degree AV block
- If asymptomatic: no treatment is generally required
- If symptomatic: often responsive to atropine; rarely require permanent pacing
ECG Example
Patient: 37-year-old male with Lyme carditis
Interpretation: Sinus rhythm with second-degree AV block (Mobitz I)
– Ventricular rate: 42 BPM
– PR interval: **
– QRS duration: 88 ms
– QT/QTc interval: 486/405 ms
– P-R-T axes: +55 +17 +45
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