Antidromic Tachycardia

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Contents 1 Things you need for Initiation of True or “classic” Antidromic Tachycardia. 2 Other Initiation of Antidromic Tachycardia 3 EP substrate of Antidromic Tachycardia 4 AVN reentry versus Antidromic Tachycardia 5 Characteristics of AVNRT with an innocent bypass tract 6 Methods to exclude AVNRT as a cause of pre-excited tachycardia

Things you need for Initiation of True or “classic” Antidromic Tachycardia.

Image:Initiation AVRT.tif

1. Intact conduction over the bypass tract
2. antegrade block in the AV node or HPS
3. intact retrograde conduction over the HPS and AVN, which assumes recovery of excitability of the AVN for retrograde conduction following partial antegrade penetration
4. Most commonly seen in LL pathways due to prolonged V-H time

Other Initiation of Antidromic Tachycardia

1. An APD blocks in the node and conducts down the AP and returns via a different AP.
2. An APD conducts down the AP and slow pathway causing 1:2 conduction

EP substrate of Antidromic Tachycardia

1. Effect of AVN refractoriness on VA conduction. RV extrastimulus has to be delivered >200ms for the AV node to recover retrograde conduction. All patients have retrograde 1:1 of 360 msec.
2. Bypass tract is usually free wall, because lateral pathways provide additional 50 msec delay. The delay from the atrial insertion site to the retrograde his deflection must be in the order of 150msec. It is always important to prove that the recorded his potential is retrogradely activated; confirmed by a fixed V-H interval during incremental atrial pacing or a similar V-H during pacing at the ventricular insertion site. Additionally retrograde nodal conduction must be excellent with 1:1 conduction at PCL of 300ms.

AVN reentry versus Antidromic Tachycardia

One could distinguish by analyzing HA intervals, because the location of the his potentials would depend on whether or not the were antegrade or retrograde.

HA intervals < 70 msec are never seen in Antidromic tachycardias. Analyze the H-A interval during documented VA conduction over the normal pathway during V pacing versus during antidromic tachycardia.

HA[pacing] – HA[ART]

In AVNRT the difference is always > 0 msec, while in preexcited tachycardias it is always < 0 msec.

Characteristics of AVNRT with an innocent bypass tract

1. Initiation of AVNRT with appearance of preexcitation dependent on HV prolongation or 2:1 block below the His
2. HA RVP> HA SVT
3. Initiation of preexcited tachycardia by VPC with HA>HA SVT despite longer H-H preceeding initiation of SVT.
4. Further support is the 1:2 response with atrial prematures which conduct down the fast with V, up the slow with A then back down the pathway
5. Most clear cut proof would be demonstration of AVNRT following spontaneous or drug induced block in the BT with the same HA interval, atrial activation sequence, and CL

Methods to exclude AVNRT as a cause of pre-excited tachycardia

Image:PAC on His.tif

1. Exact Atrial and Ventricular capture by APC when the AVJ is depolarized
2. Failure of entrainment by Atrial Pacing or Ventricular Pacing to influence VA during tachycardia
3. HA during tachycardia > HA RVP