Background: The response to premature atrial complexes (PACs) during tachycardia has been shown to differentiate atrioventricular nodal re-entrant tachycardia (AVNRT) from focal junctional tachycardia (JT). His refractory PACs (HrPACs) advancing the next His are considered diagnostic of AVNRT based on entrainment/resetting principles. Early PACs (ePACs) advancing the immediate His with continuation of tachycardia suggests JT but can also occur in AVNRT due to simultaneous conduction through the AV nodal fast and slow pathways (2 for 1 response, TFOR).
Objective: To evaluate the incidence, mechanism and differentiation of TFORs during AVNRT from the response occurring during JT.
Methods: Typical AVNRT cases were diagnosed using standard criteria. We evaluated the responses to scanning PACs delivered during tachycardia in 79 patients undergoing AV node slow pathway modification for AVNRT. Junctional tachycardia was excluded if HrPACs perturbed the next His. The responses to ePACs were documented as follows: 1) advancement of the immediate His with continuation of tachycardia, 2) advancement of the immediate His with termination of tachycardia, and 3) no change in immediate His (with continuation or termination of tachycardia).
Results: HrPACs were delivered in all cases and ePACs were delivered in 49 patients. We could not deliver ePACs in 30 patients because of atrial refractoriness or HrPACs terminating the tachycardia. In 11 patients (13.9% of the total cohort), ePACs advanced the immediate His with continuation of tachycardia. In all 11 cases, HrPACs advanced the next His, confirming AVNRT as the mechanism, and indicating a TFOR response. Only 3 of the 79 patients (3.8%) had a 2 for 1 response during sinus rhythm or atrial pacing maneuvers.
Conclusion: A TFOR to early PACs occurs relatively frequently during AVNRT. HrPACs always perturbed the next His in these cases, confirming the diagnosis of AVNRT and allowing for differentiation from JT. A TFOR is observed more frequently during AVNRT than during sinus rhythm. We propose that during AVNRT, the slow pathway may be refractory to retrograde penetration, thus facilitating a TFOR more frequently during AVNRT than during sinus rhythm.