Spirochetes Invade the AV Node
Borrelia burgdorferi spreads hematogenously and colonizes cardiac tissue.
- ~3 wks: Onset after tick bite (placeholder figure)
- ~1-10%: Cardiac involvement (placeholder figure)
- AV node: Site of predilection (placeholder figure)
- young adult: Typical age (placeholder figure)
Mechanism of invasion
Spirochetes trigger local inflammation in nodal tissue.
PR Interval Prolongation Begins
Conduction delay through the inflamed AV node lengthens the PR interval.
- 120-200ms: Normal PR (placeholder figure)
- >200ms: First-degree PR (placeholder figure)
- often none: Symptoms (placeholder figure)
- variable: Progression risk (placeholder figure)
ECG correlate
PR prolongs but every P wave still conducts.
Intermittent Conduction Failure
Some atrial impulses fail to reach the ventricles.
- I or II: Mobitz type (placeholder figure)
- intermittent: Dropped beats (placeholder figure)
- telemetry: Monitoring (placeholder figure)
- often: Reversibility (placeholder figure)
Wenckebach vs Mobitz II
Progressive vs fixed-ratio dropped beats.
Third-Degree Complete Heart Block
Atrial and ventricular activity become fully dissociated.
- complete: AV dissociation (placeholder figure)
- junctional/ventricular: Escape rhythm (placeholder figure)
- syncope risk: Symptoms (placeholder figure)
- high: Urgency (placeholder figure)
Hemodynamic consequence
Independent chamber rates reduce cardiac output.
Temporary Pacemaker Placement
High-grade block prompts temporary pacing pending antibiotic response.
- high-grade block: Indication (placeholder figure)
- transvenous temp: Device type (placeholder figure)
- days: Duration (placeholder figure)
- rare: Permanent pacer need (placeholder figure)
Bridge to recovery
Pacing supports rhythm while antibiotics clear infection.
Block usually resolves fully with antibiotic treatment.