LETTERS TO THE EDITOR To the Editor— Why AHH? Another explanation for the AHH phenomenon in the case Ho et al1 reported a narrow complex tachycardia with atrioventricular dissociation and made an excellent analysis. In the present case, atrial pacing and extrastimulus were delivered to differentiate junctional tachycardia (JT) and atrioventricular nodal reentrant tachycardia (AVNRT). However, contradictory results, which directly caused the confusion, were reported: atrial overdrive pacing showed an atrialHis-atrial (AHH) response, which indicated JT, while perturbation of the tachycardia was caused by a Hisrefractory atrial extrastimulus, which supported AVNRT. We agree that the most likely diagnosis should be AVNRT. However, we have a different interpretation of the AHH phenomenon. The AHH response was interpreted as a “double fire” phenomenon on the basis of the article. To our knowledge, the following potential explanations may contribute to the interpretation of the AHH response: (1) the tachycardia was terminated by stimuli, and the subsequent atrial pacing captured the ventricle through the fast pathway, with the last stimulus causing “double fire” and inducing a new series of tachycardia; or (2) a “pseudo”-AHH response was due to slow pathway conduction, which means that the first His wave after pacing was caused by the last stimulus through the slow pathway. Meanwhile, there should be multiple antegrade slow pathways because of various tachycardia cycle lengths shown in Figure 2 (middle panel), which can explain why the interval from the last stimulus to the first postpacing H wave was longer than other S-H intervals. According to the study by Fan et al,2 the above can influence interpretations, and more attention should be paid on AH interval changes during pacing and baseline stimulation to anticipate the 2 scenarios. Although previous studies reported several methods to differentiate AVNRT and JT, there is still no criterion standard for this differentiation. This topic is worth discussing to expand our experience.

References 1. Ho RT, Pietrasik G, Greenspon AJ. A narrow complex tachycardia with intermittent atrioventricular dissociation: what is the mechanism? Heart Rhythm 2014;11:2116–2119. 2. Fan R, Tardos JG, Almasry I, Barbera S, Rashba EJ, Iwai S. Novel use of atrial overdrive pacing to rapidly differentiate junctional tachycardia from atrioventricular nodal reentrant tachycardia. Heart Rhythm 2011;8:840–844.

Reply to the Editor— Why AHH? Another Explanation for the AHH Phenomenon in the Case We appreciate the interest and insightful comments by Dr Sun and colleague in our case.1 We agree that “double fire” after coronary sinus entrainment might not have been the result of entrainment per se but rather tachycardia termination and subsequent reinitiation upon pacing cessation. Alternatively, “double fire” with true entrainment could happen if the atrioventricular nodal circuit has a sufficiently large excitable gap that allows collision between n orthodromic (slow pathway [SP]) and n þ 1 antidromic (fast pathway [FP]) wavefronts to occur in the SP rather than in the FP. Both scenarios could perturb tachycardia similarly and result in acceleration of the His bundle/ventricle to the pacing rate, making differentiation between the two difficult. While a pseudo–atrial-His-atrial response with a different SP input driving the last and longer AH interval is conceivable, its occurrence accompanying only the last pacing stimulus when all other previous AH intervals are constant seems too coincidental. Reginald T. Ho, MD, FACC, FHRS [email protected] Grzegorz Pietrasik, MD Arnold J. Greenspon, MD Thomas Jefferson University Hospital Philadelphia, Pennsylvania

Huan Sun, MD Ping Yang, MD, PhD [email protected] Cardiology Department China-Japan Union Hospital of Jilin University Changchun, China

1547-5271/$-see front matter B 2015 Heart Rhythm Society. All rights reserved.

Reference 1. Ho RT, Pietrasik G, Greenspon AJ. A narrow complex tachycardia with intermittent atrioventricular dissociation: what is the mechanism? Heart Rhythm 2014;11:2116–2119.

http://dx.doi.org/10.1016/j.hrthm.2014.11.030