ECG Mastery: Blue Belt online course: Become an ECG expert. Similar mechanisms exist for the other types of AVNRT.This most common type of re-entrant circuit is termed Slow-Fast AVNRT.The short cycle length is responsible for the rapid heart rate.This creates a circus movement whereby the impulse continually cycles around the two pathways, activating the Bundle of His anterogradely and the atria retrogradely (3).By the time the premature impulse reaches the end of the slow pathway, the fast pathway is no longer refractory, and the impulse is permitted to recycle retrogradely up the fast pathway (2).However, if a premature atrial contraction (PAC) arrives while the fast pathway is still refractory, the electrical impulse will be directed solely down the slow pathway (1).The impulse transmitted down the fast pathway enters the distal end of the slow pathway and the two impulses cancel each other out During normal sinus rhythm, electrical impulses travel down both pathways simultaneously.Mechanism of re-entry in “slow-fast” AVNRT:ġ) A premature atrial contraction (PAC) arrives while the fast pathway is still refractory, and is directed down the slow pathwayĢ) The ERP in the fast pathway ends, and the PAC impulse travels retrogradely up the fast pathwayģ) The impulse continually cycles around the two pathways Initiation of re-entry In comparison to AVRT, which involves an anatomical re-entry circuit (Bundle of Kent), in AVNRT there is a functional re-entry circuit within the AV node.
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