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Electrophysiology Discordant Pacing Manoeuvers and a Narrow QRS Tachycardia – What is the Tachycardia Mechanism? Hussam Ali, 1 Pierpaolo Lupo, 1 Sara Foresti, 1 Guido De Ambroggi, 1 Gianluca Epicoco 1 and Riccardo Cappato 1,2 1. Arrhythmia & Electrophysiology Unit II, Humanitas Gavazzeni Clinics, Bergamo, Italy; 2. Arrhythmia & Electrophysiology Research Center, IRCCS Humanitas Research Hospital, Rozzano (Milan), Italy Abstract A 24-year-old female underwent an electrophysiological study because of recurrent episodes of drug-refractory, paroxysmal supraventricular tachycardia. During adrenergic stress, a narrow QRS tachycardia with eccentric atrial activation was reproducibly inducible. The response to premature ventricular extrastimulation during tachycardia suggested the presence of a slowly conducting accessory pathway. However, a comprehensive appraisal of the electrophysiological study delineated the tachycardia substrate as an atypical atrioventricular nodal reentrant tachycardia in the presence of a bystander nodofascicular pathway. Careful analysing of the basal pacing manoeuvers during sinus rhythm (para-Hisian and differential ventricular pacing) was crucial to establish the correct diagnosis and to avoid unnecessary left-side access to ablate this tachycardia. Keywords Narrow QRS tachycardia, para-Hisian pacing, atypical AVNRT, nodofascicular pathway Disclosure: Hussam Ali, Pierpaolo Lupo, Sara Foresti, Guido De Ambroggi, Gianluca Epicoco and Riccardo Cappato have no conflicts of interest to declare. Compliance with Ethics Guidelines: Informed consent for the procedure was obtained from the patient. Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any non-commercial use, distribution, adaptation and reproduction provided the original author(s) and source are given appropriate credit. Received: 27 June 2015 Accepted: 20 July 2015 Citation: European Journal of Arrhythmia & Electrophysiology, 2015;1(1):32–4 Correspondence: Hussam Ali, Arrhythmia & Electrophysiology Unit II, Humanitas Gavazzeni Clinics, Via M. Gavazzeni 21, 24125 Bergamo, Italy. E: firstname.lastname@example.org Case Presentation This case concerns a 24-year-old female with recurrent episodes of paroxysmal palpitations. Her physical exam, 12-lead electrocardiogram (ECG), 24-hour Holter monitoring and echocardiogram did not reveal any abnormality. In the last 2 years, she had several admissions to the emergency department where paroxysmal, adenosine responsive, supraventricular tachycardia was documented. Anti-arrhythmic drug therapy (beta-blocker, flecainide) was ineffective, and the patient was referred to our centre for electrophysiological evaluation. After obtaining informed consent, an electrophysiological study was performed under conscious sedation. Multipolar diagnostic catheters were positioned in the coronary sinus (CS), His bundle (HB) region and right ventricle (RV). Baseline atrio-His (AH) and His-ventricular (HV) intervals were 60 and 55 ms, respectively. RV pacing showed slow, decremental and eccentric ventriculoatrial (VA) conduction (earliest atrial activation at the level of mid-CS). The response to para-Hisian pacing manoeuver is shown in Figure 1. During atrial pacing, there was no evidence of ventricular preexcitation or antegrade dual atrioventricular (AV) node physiology. No tachycardia was inducible at the baseline state. After isoproterenol infusion (2 mcg/minute), and with aggressive atrial burst pacing (240 ms), a narrow QRS tachycardia was reproducibly inducible. Tachycardia cycle length had an average of 320 ms; AH and 32 HA intervals were 75 and 245 ms, respectively. Notably, tachycardia induction was independent of critical prolongation in the AH interval. Figure 2 demonstrates ECG and intracardiac recordings during tachycardia and the effect of a premature ventricular extrastimulus (PVE) (S 2 ). Based on these data, what is the most likely mechanism of this tachycardia? Discussion The retrograde conduction pattern (slow-decremental-eccentric) raised the suspicion of a left-sided, Coumel-type accessory pathway (AP). However, eccentric retrograde conduction over the nodal slow pathway (SP), particularly over the left posterior extension, has been described. 1 The para-Hisian pacing manoeuver is helpful when elucidating the retrograde conduction pattern and to explore the presence of occult APs. 2 In Figure 1, the loss of direct HB capture was associated with a significant increase in the stimulus-atrium (Stim-A) intervals without any change in retrograde atrial activation sequence (RAAS). This response confirms that retrograde conduction is His-dependent (i.e., nodal conduction) excluding an extranodal, septal, AP. Although this manoeuver may have limited utility with slowly conducting APs, this is usually due to the dominate retrograde conduction over the nodal fast pathway. In this case, retrograde conduction was constantly slow with long VA intervals (i.e., no retrograde conduction over the fast pathway), making such a masquerading effect unlikely. Additionally, retrograde conduction over a nodo-fascicular pathway (NFP) may still give a nodal response during this manoeuver because of its direct connection to the Tou c h ME d ica l ME d ia