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What is the mechanism of AVNRT?

Asked by Anonymous 

AV node contains 2 pathways:

1. Alpha pathway (Slow):

  • Slow conduction
  • Short refractory period
2. Beta pathway (Fast):
  • Fast conduction
  • Long refractory period
Normal condition: Anterograde conduction occurs down the fast pathway

Atrial extrasystole:
  1. An extrasystole during the long refractory pathway of fast (beta) pathway propagates exclusively down the slow pathway
  2. By the time the impulse reaches down the distal end of fast pathway, it is no longer refractory and it conducts retrogradely
  3. A re-entrant circuit is formed:
    • Anterograde conduction down the slow pathway
    • Retrograde conduction up the fast pathway
    • Almost simultaneous activation of atria and ventricles