Expansion
Two pathways, unidirectional block and slow conduction
Expansion
Three conditions must coexist:
- Two anatomically or functionally distinct pathways joined at both ends
- Unidirectional block in one pathway
- Slow conduction in the other, so that the blocked pathway has time to recover excitability before the impulse returns
The impulse then circulates continuously, driving the chamber faster than the sinoatrial node.
Examples map onto the mechanism directly: atrioventricular nodal re-entrant tachycardia uses fast and slow pathways within the node; Wolff-Parkinson-White uses the accessory bundle of Kent as the second limb; atrial flutter uses a macro-re-entrant circuit around the tricuspid annulus; and scar-related ventricular tachycardia uses surviving muscle strands within infarcted tissue.
Treatment follows: slow or block one limb (adenosine, beta blockers), prolong refractoriness (amiodarone), or ablate the circuit.