FEATURED ARRHYTHMIA Long-RP supraventricular tachycardia with 1:1 AV relationship: What is the mechanism? Robert Kim, MD,* Fred Kusumoto, MD, † Steve Hsu, MD,* Mark L. Greenberg, MD ‡ From the *Section of Cardiac Electrophysiology, University of Florida Health Sciences Center, Jacksonville, Florida; † Section of Cardiac Electrophysiology, Mayo Clinic, Jacksonville, Florida; ‡ Section of Cardiac Electrophysiology, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire. Case summary A 24-year-old woman was seen in the clinic with a 5-year history of intermittent palpitations. Ambulatory monitoring recorded frequent episodes of regular narrow-complex su- praventricular tachycardia (SVT) at a rate of 170 beats/min. After informed consent was obtained, she was taken to the electrophysiologic laboratory for diagnostic evaluation and treatment. Four catheters were placed in the standard loca- tions: high right atrium, His bundle position, right ventric- ular outflow tract, and coronary sinus. All baseline intervals were within normal limits. Retrograde atrial activation dur- ing ventricular pacing was decremental, with the coronary sinus os activated earliest, just ahead of the atrial electro- gram in the His bundle catheter. No evidence for dual atrioventricular nodal pathways could be demonstrated. SVT with a long RP interval and cycle length (CL) of 310to 420 ms was reliably induced by rapid atrial pacing. The retrograde atrial activation sequence during tachycardia was the same as during right ventricular pacing. Entrainment from the right ventricle was performed at pacing CL just shorter than the tachycardia CL (Figures 1 and 2). What is the mechanism of this tachycardia? Commentary The differential diagnosis of a long RP tachycardia with earliest atrial activation in the septum includes atrial tachy- cardia, orthodromic reciprocating tachycardia (ORT) utiliz- ing a slowly conducting accessory pathway (AP), and atyp- ical atrioventricular nodal reentrant tachycardia (AVNRT). SVT was successfully entrained from the ventricle, with the acceleration of the atrium to the paced CL and continuation of tachycardia upon termination of pacing. A “pseudo V-A- A-V” response is seen, ruling out atrial tachycardia as the mechanism (Figure 1). Vijayaraman et al 1 previously dem- onstrated the importance of accurately identifying the atrial activation caused by the last paced ventricular beat during tachycardia entrainment in the setting of a long retrograde conduction time. The numerical difference between tachycardia CL and postpacing interval is 170 ms, and the difference between S-A interval during pacing and V-A interval during SVT is also 170 ms (Figure 1), suggesting a relatively long distance between the pacing site and the tachycardia circuit. Values over 115 ms and 85 ms respectively for these parameters suggest atypical AVNRT as the diagnosis. There was no significant increment in the first postpacing A-H interval compared with the A-H interval during SVT. 2 Recently, 2 groups 3,4 have reported a novel method to distinguish AVNRT from ORT during tachycardia entrain- ment from ventricle. The number of fully paced (or fixed fusion) QRS beats before the atrium is accelerated to the paced CL is counted. Atrial entrainment before or simulta- neously with the appearance of the first fully paced QRS complex supports the diagnosis of ORT. Atrial entrainment more than 1 beat after the appearance of fully paced QRS complex suggests that the tachycardia circuit is relatively far away and protected from the pacing site, favoring AVNRT as the mechanism. One strength of this diagnostic method is that the result is valid even if the tachycardia is terminated by pacing. During tachycardia entrainment in this case, the atrium sped up to the rate of the paced CL on the 4th ventricular beat that demonstrated a fully paced QRS complex (Figure 2). Atrial timing perturbation (Rosman et al 4 ) is evident on the first beat after the first fully paced QRS complex (Figure 2). Both of these findings provide strong support for the diagnosis of atypical AVNRT rather than ORT. However, when a His-refractory ventricular premature beat (VPB) was delivered during the tachycardia, the atrium was reproducibly postexcited by more than 20 ms (Figure 3). This finding is diagnostic of ORT utilizing a slowly conducting AP because the retrograde route to an atypical AVNRT circuit would be inaccessible during His bundle refractoriness. KEYWORDS Supraventricular tachycardia; Entrainment; Orthodromic re- ciprocating tachycardia; AV bypass tract ABBREVIATIONS AP = accessory pathway; AVNRT = atrioventricular nodal reentrant tachycardia; CL = cycle length; SVT = supraventricular tachycardia; ORT = orthodromic reciprocating tachycardia; VPB = ventric- ular premature beat (Heart Rhythm 2012;9:2083–2085) Address reprint requests and correspondence: Dr. Robert J. Kim, Uni- versity of Florida Health Sciences Center, Section of Cardiac Electrophys- iology, 655 West 8th Street, 5th Floor (ACC Building), Jacksonville, FL 32209. E-mail address: Robert.kim@jax.ufl.edu. 1547-5271/$ -see front matter © 2012 Heart Rhythm Society. All rights reserved. doi:10.1016/j.hrthm.2011.09.060