Induce the uninducible and stop the unstoppable

• how to induce typical AVNRT
• what the induction is telling you about the mechanism of tachycardia
• how to quickly and safely terminate arrhythmia

Induction and termination of AV node dependent arrhythmia are essential parts of any EP study. It is both crucial for getting the right diagnosis, uneventful procedure and assessment of intraprocedural success.
 
To induce AV nodal reentrant tachycardia (AVNRT) during an electrophysiology study, the following steps are typically followed:

• Baseline assessment: Begin with a thorough baseline assessment to understand the patient’s intrinsic conduction system. Pay attention to AV node characteristic like AH HV intervals and Wenckebach point (see Module 1) as well as to AV conduction pattern (concentric vs eccentric).
• Incremental pacing: Incremental atrial pacing is done to gradually increase the heart rate, which can lead to AVNRT initiation after observed activation of slow pathway. Sometime (especially at the beginning of your career) it is hard to recognize if the tachycardia is already ongoing so pay close attention to AV interval and if you see previous V colliding with your current A (pacing stimulus) you can pause stimulation for a brief moment and see if the tachycardia is already ongoing.
• Atrial programmed stimulation: Introduce a premature atrial beat after a series of regular beats. The premature beat should be delivered with decreasing coupling intervals. This can help you to observe jump and possibly initiate AVNRT. Note that atrial induction is most typical for typical AVNRT while initiation from ventricle let you suspect a fast-slow (atypical) form.
• Adding pharmacological modulation:  if the arrhythmia is not easily inducible, an infusion of isoproterenol (or other beta-agonist that increases sympathetic activity) can be administered. This drug lowers the threshold for AVNRT by increasing the heart rate and shortening the refractory period of the slow pathway. Paradoxically, in some cases characterized by super conducting fast pathway a lowering of sympathetic activity (i.e with very mild sedation) will improve inducibility as worse activity of fast pathway “gives space” for slow pathway conduction.     

To terminate AVNRT during EP study, several approaches can be used:

• Atrial overdrive pacing: Pacing the atrium at a rate faster than the tachycardia can interrupt the reentrant circuit.  To achieve that you need to reach AV block that will terminate tachycardia immediately.  
• Ventricular overdrive pacing: In some cases, pacing the ventricle faster than the tachycardia rate can cause retrograde conduction to reach the AV node during its refractory period, breaking the reentrant circuit. However, this is rather unpleasant to patient and pacing ventricle at high rate can be hemodynamically problematic.