This way or the other way around: antidromic or orthodromic

• why ANRT can be broad and narrow
• how to diagnose antidromic AVRT
• what is the best ablation approach

In Antidromic AVRT, the impulse travels anterogradely (from atria to ventricles) through the accessory pathway and retrogradely (from ventricles back to atria) through the AV node, creating a continuous loop. Thus, a broad complex tachycardia with maximum preexcitation is characteristic for this tachycardia. Such tachycardia is regular what differentiates it from atrial fibrillation triggering fast, broad and irregular (i.e. FBI) ventricle response.
Antidromic AVRT occurs rarely and can be observed in 5-10% of all accessory pathway (AP) related procedures and is more frequent in male population. In patients with antidromic AVRT you can also expect multiple APs. Antidromic AVRT can be easily mistaken for ventricular tachycardia due to the wide QRS complex. Differentiation is critical as the management strategies differ. During EP study antidromic AVRT is characterized by:

• Preexcitation at some pacing rate as well as VA conduction via AV node (concentric) are requirements for such arrhythmia
• Typical induction can be performed by right ventricular pacing causing retrograde conduction via AV node (located near pacing side) and closing the loop via AP.
• Mapping approach is very similar to orthodromic AVRT as it involves allocation of the earliest breakthrough point between atrium and ventricle. It can be conducted during ongoing tachycardia or in stable atrial pacing. Both those approaches have advantages as well as limitations. Mapping of earliest ventricle signal during ongoing tachycardia might be quite stressful for the patient and requires very good inducibility or sustainability but it provides most adequate mapping results as the ventricle activation is caused solely by AP.   In opposite, a regular atrial pacing is in most cases tolerated very well. Also, it does not depend on stable and lasting tachycardia. However, if the pacing rate is not high enough to provide maximal preexcitation, we are mapping a fusion of the AP and AV node conduction so the map is not that precise. Thus, a detailed inspection of the region of interest at higher pace rate is necessary.
• Ablation of AP is identical as in case of orthodromic AVRT and depends on catheter specification. Most cases 30-40W for 60s provides a sufficient lesion. If there is no response (termination or loss of preexcitation) within 10-20s after starting ablation its probably a sign to burn elsewhere. If you decide to map and ablate during ongoing tachycardia it is advisable to keep the same heart rate (either by inhibited pacing or starting pacing with similar CL at the moment of termination) in order to prevent catheter from dislodgment caused by long diastole.