Discordant responses of narrow QRS tachycardia to atrial extra stimuli timed at different coupling intervals. What is the mechanism?

Unusual response to His‐refractory atrial premature complex: What is the mechanism?

“…This is not a challenge although technically could conceivably be a nodo‐fascicular NQT 11,12 . In AVNRT, an AES delivered when the His is refractory can conduct to the ventricle by the slow pathway 13‐17 . Indeed, a single AES by itself cannot always differentiate the atypical AVNRT from the AT because a very early AES may result in termination of an AT and even an early AES may not terminate an atypical AVNRT with an upper common pathway 11,18,19 .…”

Interesting response of slow regular narrow QRS tachycardia to atrial extra stimulus

“…During AVNRT, a late coupled AES may not affect the immediate tachycardia beat but can conduct anterogradely over the slow AV nodal pathway to affect the next following beat of tachycardia resulting in either advancement or delay of the next His or termination of the tachycardia . Similarly, AVRT may also utilize an anterograde slow AV nodal pathway and AES, by conducting through it, might well terminate the tachycardia by blocking in the anterograde slow pathway.…”