Differentiate AVNRT from Orthodromic AVRT

The differentiation of atrioventricular nodal reentrant tachycardia (AVNRT) from atrioventricular reciprocating tachycardia (AVRT) as well as the localization of accessory pathways (APs) is necessary to guide the catheter ablation of supraventricular tachycardia (SVT). However, existing techniques may prove challenging in differentiating atypical forms of AVNRT from AVRT using a septal AP; or localizing AVRT with different septal AP insertion sites. There are many diagnostic maneuvers during electrophysiological study eg ventriculo atrial interval (VA) during tachycardia, postpacing interval tachycardia cycle length (PPI-TCL) and stimulus atrial ventriculaoatrial interval (SA-VA). It is important to recognize that as with most diagnostic tests, no single observation or maneuver is 100% sensitive or specific. Therefore, it is important to obtain data from multiple observations and maneuvers to verify the diagnosis. Also, utility of these techniques usually depends on the tachycardia to be sustained, however sometimes tachycardia is rapidly terminating. VA interval at initiation of tachycardia is usually variable and becomes fixed after several beats. This may be explained by differences in retrograde conduction between AVRT and AVNRT which may be better exposed at the time of tachycardia induction, So measuring the number of beats until VA becomes fixed may theoretically help in differentiating AVNRT from AVRT. Although several reports have demonstrated the usefulness of the SA-VA using right ventricular (RV) apical stimulation (SA-VAapex) to distinguish AVNRT from AVRT, there remains significant overlap in the SA-VA complicating the distinction between these 2 arrhythmias. Theoretically, stimulation from the RV basal septum (SA-VAbase) would be expected to shorten the SA interval in AVRT because atrioventricular pathways insert in the ventricular base. On the other hand, the SA-VAbase should have the opposite effect on AVNRT because the impulse must first pass from the RV base to the apex to access the right bundle.